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LABORATOIRE NATIONAL DES CHAMPS MAGNÉTIQUES INTENSESANNUAL REPORT2009

Laboratoire National des Champs MagnétiquesIntensesGrenoble–ToulouseCentre National de la Recherche Scientifiquehttp://lncmi.cnrs.fr/Annual Report2009LNCMI is formally associated with the “Université Joseph Fourier”, Grenoble, the “UniversitéPaul Sabatier”, Toulouse and the “Institut National des Sciences Appliquées”, Toulouse.Editors: Fabienne Duc, Rachel Graziotti and Duncan Maude

TABLE OF CONTENTSPreface 1Carbon Allotropes 3Investigation of the dynamic alignment of single walled carbon nanotubes in pulsed high magnetic fields . . 5Charge transport mechanisms in arrays of multi-walled carbon nanotubes . . . . . . . . . . . . . . . . . . 6Propagative Landau states and Fermi level pinning in carbon nanotubes . . . . . . . . . . . . . . . . . . . 7Aharonov-Bohm modulation of the high energy subbands in carbon nanotubes . . . . . . . . . . . . . . . . 8Edge fingerprints and magneto-conductance in graphene nanoribbons . . . . . . . . . . . . . . . . . . . . 9Using Landau quantization to suppress Auger scattering in graphene . . . . . . . . . . . . . . . . . . . . . 10Observation of the half-integer quantum Hall effect in epitaxial graphene . . . . . . . . . . . . . . . . . . . 11Integer quantum Hall effect in epitaxial graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Improving graphene’s cleanliness for high field magneto-transport . . . . . . . . . . . . . . . . . . . . . . 13Metal-insulator transition for filling factor ν = −2 to ν = 0 in graphene . . . . . . . . . . . . . . . . . . . 14How perfect can graphene be? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Effect of a magnetic field on the two-phonon Raman scattering in graphene . . . . . . . . . . . . . . . . . 16Tuning the electron-phonon coupling in graphene with magnetic fields . . . . . . . . . . . . . . . . . . . . 17Thermal conductivity of graphene in Corbino membrane geometry . . . . . . . . . . . . . . . . . . . . . . 18Electric field doping of few-layer graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Low temperature magneto-transport in natural graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Magnetotransport to extract the spin gap for charged excitations in graphite . . . . . . . . . . . . . . . . . 21Magneto-transmission spectroscopy of graphite in high magnetic fields . . . . . . . . . . . . . . . . . . . . 22Graphite from the viewpoint of Landau level spectroscopy: An effective graphene bilayer and monolayer . . 23Magneto-transmission of multi-layer epitaxial graphene and bulk graphite: A comparison . . . . . . . . . . 24Two-Dimensional Electron Gas 25The surprisingly fragile quantum Hall ferromagnet at filling factor ν = 1 . . . . . . . . . . . . . . . . . . . 27Dispersive line shape of the resistively detected NMR on either side of filling factor ν = 1 . . . . . . . . . . 28Spin splitting enhancement of fully populated Landau levels . . . . . . . . . . . . . . . . . . . . . . . . . 29Spin polarisation of a disordered GaAs 2D electron gas in a strong in-plane magnetic field . . . . . . . . . 30High-order fractional microwave induced resistance oscillations . . . . . . . . . . . . . . . . . . . . . . . 31Crossover between distinct mechanisms of microwave photoconductivity in double quantum wells . . . . . 32Emergent fractional quantum Hall effect in a triple quantum well . . . . . . . . . . . . . . . . . . . . . . . 33Reentrant fractional quantum Hall states in a triple quantum well . . . . . . . . . . . . . . . . . . . . . . . 34Magneto-intersubband oscillations in multilayer electron systems . . . . . . . . . . . . . . . . . . . . . . . 35

Interference of fractional microwave induced resistance oscillations with magneto-intersubband oscillationsin a bilayer system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Microwave photoconductivity in multilayer systems: triple quantum wells . . . . . . . . . . . . . . . . . . 37Hole cyclotron resonance in a two-dimensional semimetal . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Electron-Phonon Interactions in a single modulation doped Ga 0.24 In 0.76 As/InP Quantum Well . . . . . . . . 39Temperature effect on Coulomb pseudogap in electron tunneling between Landau-quantized twodimensionalgases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Semiconductors and Nanostructures 41On the trigonal field acting at the Cr 3+ ( 2 E states) in Ruby . . . . . . . . . . . . . . . . . . . . . . . . . . 43Mobility spectrum analysis in InN:Mg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Cyclotron effective mass measurements in Indium Nitride . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Oscillatory magneto-absorption under pressure in Indium Selenide . . . . . . . . . . . . . . . . . . . . . . 46Magnetoresistance in β-FeSi 2 on n-type Si substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Two-dimensional weak localization in polycrystalline granular SnO 2 films . . . . . . . . . . . . . . . . . . 48Magnetoresistance mobility extraction in FinFET triple gate devices . . . . . . . . . . . . . . . . . . . . . 49Spin polarization of carriers in a GaAs/GaAlAs resonant tunneling diode . . . . . . . . . . . . . . . . . . . 50Spectroscopy and optical manipulation of a single Mn spin in a CdTe-based quantum dot in high magneticfield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51InP/GaP self-assembled quantum dots under extreme conditions . . . . . . . . . . . . . . . . . . . . . . . 52Metals, Superconductors and Strongly Correlated Systems 53Thermo-electric study of Fermi surface reconstruction in YBa 2 Cu 3 O y . . . . . . . . . . . . . . . . . . . . 55Anomalous criticality in high-T c cuprates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Magnetic field dependence of the superconducting energy gap in Bi 2 Sr 2 CaCu 2 O 8+δ . . . . . . . . . . . . . 57Magnetoresistance anisotropy and AMRO in the electron doped superconducting cuprate Nd 2−x Ce x CuO 4 . 58Transport measurements of H c2 and its anisotropy in FeSe 1−x Te x single crystals . . . . . . . . . . . . . . . 59Coexistence of magnetic order and superconductivity in iron pnictides . . . . . . . . . . . . . . . . . . . . 60High-field metamagnetism in the antiferromagnet CeRh 2 Si 2 . . . . . . . . . . . . . . . . . . . . . . . . . 61Field Evolution of Coexisting Superconducting and Magnetic Orders in CeCoIn 5 . . . . . . . . . . . . . . 62Fermi surface study of the hidden order state in URu 2 Si 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 63High field resistivity measurements on single crystalline UPt 2 Si 2 . . . . . . . . . . . . . . . . . . . . . . . 64Evolution of the Fermi surface of BaFe 2 (As 1−x P x ) 2 on entering the superconducting dome . . . . . . . . . 65Angular dependence of the Nernst effect in elemental bismuth . . . . . . . . . . . . . . . . . . . . . . . . 66Magnetic field-induced electronic instability in bismuth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Magnetic oscillations in a linear chain of compensated orbits . . . . . . . . . . . . . . . . . . . . . . . . . 68

Coexistence of closed orbit and quantum interferometer with the same cross section in the organic metalβ”-(BEDT-TTF) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Metal-non-metal transition in the charge transfer salt (BEDT-TTF) 8 [Hg 4 Br 12 (C 6 H 5 Br) 2 ] . . . . . . . . . . 70Magnetic torque experiments on the magnetic-field-induced organic superconductor λ-(BETS) 2 FeCl 4 . . . 71Temperature and magnetic field dependence of domain wall width and period of Condon domain structurein Ag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Intrinsic diamagnetic length scales of Condon domain phase in Be . . . . . . . . . . . . . . . . . . . . . . 73Magnetic Systems 75Y b 3+ → Er 3+ up-conversion luminescence under high pressure and pulsed magnetic fields . . . . . . . . . 77Nd 3+ crystal-field studies of weakly doped Nd 1−x Ca x MnO 3 . . . . . . . . . . . . . . . . . . . . . . . . . 78Magnetotransport in a disordered Zn 1-x Mn x GeAs 2 alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Anomalous Hall effect in (Ge,Mn)Te-(Sn,Mn)Te spin-glasslike crystals . . . . . . . . . . . . . . . . . . . 80High field torque magnetometry on a molecular Dysprosium triangle . . . . . . . . . . . . . . . . . . . . . 81NMR evidence for long zero-quantum coherence in antiferromagnetic molecular wheels NaFe 6 and LiFe 6 . 82Nuclear magnetic resonance determination of spin-superlattice structure of magnetization plateaus inSrCu 2 (BO 3 ) 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83Magnetic structure of the half magnetization plateau phase in CdCr 2 O 4 . . . . . . . . . . . . . . . . . . . . 84Structural analysis with pulse-length exposures up to 30 Tesla at ID06, ESRF . . . . . . . . . . . . . . . . 85Magnetization at low temperatures and high magnetic fields on LuFe 2 O 4 . . . . . . . . . . . . . . . . . . . 86Enhancement magnetic moment in the single phase nanostructure Gd 3 Fe 5 O 12 . . . . . . . . . . . . . . . . 87Effect of the M/Co substitution on magnetocrystalline anisotropy and magnetization in SmCo 5−x M x compounds(M=Ga; Al) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88Investigation of the intrinsic magnetic properties of the ThCo 4 B compound . . . . . . . . . . . . . . . . . 89Magnetic properties of Y 0.7 Er 0.3 Fe 2 (H, D) 4.2 compounds up to 35 T . . . . . . . . . . . . . . . . . . . . 90Field-induced transitions in RECo 0.50 Mn 0.50 O 3 (RE = Dy, Eu) . . . . . . . . . . . . . . . . . . . . . . . . 91High magnetic field study in chromium-based Mn 1−x Cd x Cr 2 S 4 thiospinels . . . . . . . . . . . . . . . . . . 92Magnetic properties of ErCo x Mn 1−x O 3 perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Ferromagnetic domains in nanosized erbium perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Biology, Chemistry and Soft Matter 95Concerted spin crossover and symmetry breaking yield three thermally- and one light-induced crystallographicphases of a novel molecular material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Magnetostructural correlations in Tetrairon(III) single-molecule magnets . . . . . . . . . . . . . . . . . . . 98Applied Superconductivity 99Magnetic field behaviour of ex-situ processed MgB 2 multifilamentary wires . . . . . . . . . . . . . . . . . 101Superconductivity of C and TiC doped multi-filamentary MgB 2 wires . . . . . . . . . . . . . . . . . . . . 102

Phthalocyanine doping to improve critical current densities in MgB 2 tapes . . . . . . . . . . . . . . . . . . 103Critical current measurements on Bi-2212 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Magneto-Science 105Changes in the microstructure resulting from a high cooling rate in Fe-xC-Mn alloys in strong magnetic field 107Study of the influence of magnetic forces on the mass transfer of paramagnetic particles in electrochemistry 108Large Alfvén Waves in liquid sodium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Magnetohydrodynamic effect on electrodeposition of nickel alloys-catalysts for hydrogen evolution . . . . 110Diffusion behavior of Al/Cu diffusion interface under a high magnetic field . . . . . . . . . . . . . . . . . 111Magnet Development and Instrumentation 113High field helix development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Magnets for neutron and x-ray scattering and absorption experiments . . . . . . . . . . . . . . . . . . . . . 116A new cooling loop for thermo-hydraulic magnet studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Pulsed energy supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Pulsed high-field coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119High strength conductors for pulsed magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120Megagauss magnetic field generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121SEISM: A 60 GHz electron cyclotron resonance (ECR) ion source prototype . . . . . . . . . . . . . . . . . 122Towards developing a high T c superconducting magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123Status report of the 42+ Tesla hybrid magnet project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124Measuring the vacuum magnetic birefringence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Special purpose NMR probe for spectroscopy of quadrupolar nuclei at 30 T . . . . . . . . . . . . . . . . . 126New rotating sample holder for broad-band quasi-optical HF-EPR spectroscopy . . . . . . . . . . . . . . . 127A new magnetometer for use in dc magnetic fields in excess of 28 T. . . . . . . . . . . . . . . . . . . . . . 128Proposals for Magnet Time – Projects Carried Out in 2009 129PhD Theses 2009 141List of Publications 2009 142Contributors of the LNCMI to the Present Report 147Collaborating External Laboratories 148

2009PrefaceDear Reader,You have before you the first annual report of the Laboratoire National des Champs Magnétiques Intenses (LNCMI),created on the 1st of January 2009 through the merger of the Laboratoire des Champs Magnétiques Intenses (Grenoble)and the Laboratoire National des Champs Magnétiques Pulsés (Toulouse). The LNCMI is a “Unité Propre de Recherche”of the Centre National de la Recherche Scientifique (UPR3228) and is part of the French “Très Grands Equipements”.The laboratory is associated with the Université Joseph Fourier de Grenoble, the Université Paul Sabatier de Toulouseand the Institut National des Sciences Appliquées de Toulouse. Its activities are also supported by the EC FP7 “Largeinfrastructures” program, where it is the coordinator of the EuroMagNET2 Integrating Activity.This report intends to provide a complete overview of the in-house and collaborative scientific and technical activities in2009 of the LNCMI.The 24MW power supply of the Grenoble site, dedicated to the generation of static fields, allows either to operate simultaneouslytwo resistive 10MW magnets or to operate one 24 MW magnet. The highest field generated at the LNCMI-G is35 T in a 34 mm bore diameter. A hybrid magnet, capable of generating 43 T is currently under construction.The 14 MJ capacitor bank generator of the Toulouse site, dedicated to the generation of pulsed fields, allows to generatefields up to 80 T. An installation to generate destructive pulsed fields up to 300 T is nearing its completion.This report and the list of publications show the importance and the interest of results obtained in magnetic fields, eitheron the basis of in-house research or as a result of a close collaboration with several research groups from many differentcountries.In addition, the LNCMI is proud to contribute to the training of young scientists by giving them the opportunity to preparetheir thesis work and/or to participate to European research activities.Finally, a word of thanks to the scientific, technical and administrative staff of the laboratory (altogether 90 people),our post docs and PhD students (almost 30) and the numerous visitors (over 200 this year) for their contributions to theimprovement of the installation and for the quality of their work.Geert RikkenDecember 20091

2009Carbon Allotropes3

2009 CARBON ALLOTROPESInvestigation of the dynamic alignment of single walled carbon nanotubesin high pulsed magnetic fieldsThe different carbon allotropes have attracted renewed attentionin recent years with the successful fabrication of thelower dimensional forms of graphite e.g. fullerenes, carbonnanotubes, graphene and graphene nano-ribbons. Carbonnanotubes are unique nano-objects with highly anisotropicelectrical, magnetic and optical properties. Over the lastfew years our understanding of the physics of carbon nanotubeshas made considerable progress and the detailedcomprehension of the complex physical properties of carbonnanotubes has significantly increased. Depending onthe diameter, and the helical arrangement of the hexagonallyarranged carbon atoms, carbon nanotubes can be eithermetallic or semiconducting.In this work we have investigated the magnetic propertiesof single walled carbon nanotubes. Semiconducting nanotubesare diamagnetic both along and perpendicular totheir long axis, however, the magnitude of the perpendicularsusceptibility is higher. Metallic nanotubes are paramagneticalong their long axis and diamagnetic perpendicularto it. This constrains both types of single walled carbonnanotubes to align parallel to a magnetic field. The samplesinvestigated here are composed of individually suspendedsingle walled carbon nanotubes in an aqueous solution. Theaim is to study the dynamic alignment of the single walledcarbon nanotubes during the application of a pulsed magneticfields via the absorption of polarized light.difference between the measured and the simulated lengthdistribution.Further measurements with length sorted samples are beingundertaken. The ultimate aim is to identify the role ofthe single walled carbon nanotubes in the dynamic processand determine the influence of the chirality on the magneticanisotropy.Figure 1: The time and field dependent alignment of a carbonnanotubes suspended in aqueous solution.To measure the alignment of single walled carbon nanotubeswe make use of the fact that nanotubes absorb lightonly if it is polarized parallel to the axis of the nanotube.We thus refer to linear dichroism spectroscopy: The absorptionratio between light polarized parallel and perpendicularto the applied magnetic field directly reflects the degree ofalignment of the ensemble on nanotubes under the influenceof this magnetic field. Measuring the integrated absorptionspectra of a white light source at different times during themagnetic field pulse allows us to determine the time dependentalignment of the single walled carbon nanotubes(figure 1).Our data can be understood with the aid of a theoreticalmodel based on rotational diffusion of rigid rods [Shaveret al., ACS Nano 3,1 131 (2009)]. A typical result of thesimulation is shown in figure 2. The length distribution ofthe single walled carbon nanotubes in the sample was determinedusing an atomic force microscope. The simulationwhich best describes our experimental data shows a slightFigure 2: Time dependent alignment and best match fit (red).Insert: The matching length distribution (red) is larger than themeasured distribution (blue).N. Ubrig, S. George, O. PortugallJ. Kono, M. Pasquali (Rice University)5

CARBON ALLOTROPES 2009Charge transport mechanisms in arrays of multi-walled carbon nanotubesUnlike the case of individual nano-objects, in these arraysthe inter-tube barriers and defects play an essential role inthe electrical transport properties of the carbon nanotubesarrays. In this work, we demonstrate the role of the surfacedefects of the nano tube by its spins. In thin layersof multi-walled carbon nanotubes (MWCNT) the variationof the resistance as a function of temperature range [4-60K] follows a Mott’s law of two dimensional (2D) VariableRange Hopping (VRH). Figure 3(a) shows the relative magnetoresistance(MR), ∆R/R 0 , in the temperature range inwhich the VRH is observed.where Ω B = 4DeBh≪ 1/τ ϕ , τ ϕ being the phase coherencetime, and ψ the digamma function. The obtained curvesfollows the 2D WL model, which demonstrates the role ofthe surface defects of the nano tubes by its spins.The minimum position of negative magneto-resistance(NMR) shifts to higher fields as the temperature rises. Bothlow-field NMR due to changing of phase between alternatehopping paths enclosing a magnetic flux (quantum interference)and high-field positive magneto-resistance (PMR)due to electronic orbit shrinkage are predicted for systemswith hopping conductivity mechanism; ie MR = NMP +PMR. In this model, the amplitude of the NMR declinesas the temperature is rising. However, in our samples, acontrario, the NMR amplitude increases as the temperatureincreases in the temperature range in which VRH canbe responsible of the charge transport mechanism. Fromthe other side negative MR is inherent for the systemswhere conductivity can be described in the frame of weaklocalisation(WL) theory [Lee and Ramakrishnan, Rev.Mod. Phys. 57, 287 (1985)]. Therefore, we made the assumptionthat MR data are the sum of the positive and negativecontribution due to MR effects in the VRH and WLregimes, respectively. We find that high-field positive partof MR can be approximated in frame of Kamimura modelfor spin-dependent VRH conductivity (due to the surfacedefects of the nano tubes) [Modern problems in condensedmatter sciences Chap. 7, Vol. 10, Efros A.L. & Pollak M.ed., North Holland 1985]:∆GG = −A KKHKK 2 − (1)H2where H KK is the characteristic field for spin alignment,A KK the saturation value of the magneto-conductance. Usingthis formula and values of parameters H KK and A KK obtainedfrom the approximation data of high field MR data,we calculated the positive magneto-conductance for lowfieldregion (figure 3(b)).H 2The pure negative contribution to MR according our assumptionis calculated by subtracting positive MR from theexperimental data and shown in figure 3(c). These data canbe fitted reasonably well by the equation for 2D WL:∆G = e2Πh α[Ψ(1 2 + 1Ω B τ ϕ+ ln(Ω B τ ϕ )] (2)Figure 3: (a) Relative magnetoresistance in the temperaturerange in which 2D VRH is observed. (b) Kamimura’s model forPMR (spin dependent VRH). (c) NMR obtained by subtraction ofthe calculated PMR from the experimental results.T.A. Dauzhenka, J. GalibertV.K. Ksenevich (Belarus State University, Dept Phys. SC & nanoelectronics, BY-Minsk), D. Seliuta (SC Institute LI-Vilnius), V.A. Samuilov (Garcia center Polymers at Engineered Interfaces, Stony Brook Univ. NY)6

2009 CARBON ALLOTROPESPropagative Landau states and Fermi level pinning in carbon nanotubesCarbon nanotubes (CNT), isolated graphene sheets andgraphene nano-ribbons (GNR) have revealed remarkableelectronic properties. The massless dispersion bands atthe charge neutrality point (CNP) drive spectacular phenomenalike an anomalously low backscattering and relatedlong mean free path in metallic nanotubes, or thehuge charge carriers mobility in graphene layers. CNTand GNR also have in common a large magnetic field dependenceof their 1-D subbands. In graphene, the highmagnetic field behavior of Dirac fermions has been shownto induce an half-integer quantum Hall effect. In singleand multi-walled carbon nanotubes (MWCNT), whileAharonov-Bohm phenomena were investigated in-depth foraxial magnetic fields, the exploration of Landau states inpresence of high transversal magnetic fields has been facingoverwhelming technical challenges. On the theoreticalside, a drastic change of the 1-D dispersion bands hasbeen predicted once the cyclotron radius equals the tuberadius. The calculations show that the resulting magneticbands remain dispersive along the tube axis at large k wavevectorssuggesting an inhomogeneous chiral current flowingat the flanks of the tube. At low k-vectors, a Landau levelsspectrum is derived with the √ nB magneto-fingerprintof graphene, whatever the CNT chirality. Notwithstanding,an experimental spectroscopy of Landau states in CNTshas not yet been achieved and the longitudinal magnetoconductancein the quantum regime remains unexplored.In this work, MWCNTs of diameters in the range of 10 nmto 20 nm are selected to reach the high magnetic fieldregime under 60 T, corresponding to a dimensionless parameterν = r/l B larger than 1 (r is the tube’s radius andl B = √ /eB, the magnetic length). In the following, wefocus on two MWCNTs whose external shells, mainly contributingto the conductance, have been identified respectively,as behaving as semiconducting and metallic shells,in the light of their Aharonov-Bohm magneto-fingerprints[Nanot et al., C. R. Physique 10, 268 (2009)].The contribution of propagative Landau states to magnetotransportin semiconducting and metallic MWCNTshells is experimentally unveiled [Nanot et al., submitted].For semiconducting shells, the occurrence of a zeroenergyLandau state associated with the energy gap closureis found to generate strongly delocalized states closeto the Dirac point, despite the presence of disorder andlow dimensionality, and irrespective of the electrostaticdoping strength. For doped metallic shells, the magnetoconductancealso exhibits an upshift of the massive 1Dbandsin agreement with the formation of Landau states.At the CNP, the re-introduction of the backscattering inthe metallic bands, clearly evidences the onset of the zeroenergyLandau state (figure 4). Even more spectacular isthe pinning of the Fermi level at the CNP in high field(figure 5). These remarkable features are supported byLandauer-Büttiker simulations of the magnetoconductanceof weakly disordered semiconducting and metallic CNTs.Figure 4: High magnetic field perpendicular magneto-conductanceobtained at 100 K on a individually connected MWCNT forvarious Fermi energies. The drastic decrease of the conductanceabove 25 T results from the re-introduction of the back-scatteringalong with the onset of the first Landau level at zero energy.Figure 5: Landauer-Büttiker simulation of the perpendicularmagneto-conductance on a (10,10) NTC in presence of weak disorderfor various magnetic field dependent electrostatic potentials.The magenta curve with cross marks is the calculated magneto–conductance at a fixed Fermi energy. In inset, the magnetic fielddependence of the electrostatic potential illustrating the so calledpinning of the Fermi energy at the charge neutrality point.S. Nanot, W. Escoffier, J-M Broto, B. Raquet,S. Roche, R. Avriller (Commissariat à l’Energie Atomique, INAC, SP2M, Grenoble)7

CARBON ALLOTROPES 2009Aharonov-Bohm modulation of the high energy subbands in carbon nanotubesIn this work, we demonstrate the usefulness of combiningthe nano-probing of individual carbon nanotubes and verylarge magnetic fields. We show that the magneto-fingerprintin the conductance under a parallel magnetic field is an unequivocalsignature of a metallic or a semiconducting behaviorof the external shell from which the locations of thedifferent 1-D subbands are deduced.Under 60 T threading the carbon nanotube along its axis,the electronic density of states undergoes quantum-fluxmodulations yielding to a giant Aharonov-Bohm oscillationof the conductance, orders of magnitude larger thanthe standard Aharonov-Bohm effect in metallic rings orcylinders. Here, we study the sub-bands modulation bythe Aharonov-Bohm flux within a large diameter MWCNT(d = 18 nm) with a distance L T = 150 nm between contacts(atomic force microscope (AFM) observation). Inthis case, we expect to observe more than 3 modulationperiods within 55 T. Meanwhile, a thin oxide thickness(t ox = 40 nm) is used to enhance the gate efficiency and allowshigher doped regime. As shown in figure 6, the pulsedfield magneto-conductance exhibits three periods and a π-dephasing of the modulation when changing the gate voltage.The period of 17 T corresponds to a quantum fluxthreading a 17.3 nm diameter cylinder, in consistence withthe AFM estimation.Figure 6: Left: Experimental magneto-conductance measuredon an 18 nm diameter MWCNT (AFM estimate) at 100 K, for variousback-gate voltages. Curves are shifted for clarity. Right: Calculationof the magneto-conductance for a (219,0) nanotube withdifferent locations of the Fermi energies. Comparison between theexperimental and calculated curves yields a direct assignment ofthe locations of the CNP and the vHs.The data are directly compared to the conductance calculationfor a metallic (219,0) nanotube in the frame of theLandauer-Buttiker formalism (figure 6) at 100 K. We assumea ballistic regime and, for a first qualitative approach,Schottky barriers are neglected. Without any fitting parameter,the agreement between the experimental curves andthe modelling based on the band structure modulation, evenin the highly doped states where several bands are carryingthe current, is convincing. When changing the gate voltagefrom +10 to −10 V, successive weakening and π-phasechange of the magneto-conductance are experimentally observed.Interestingly, the holes and electrons energies canbe deduced at any gate voltage without any analytical estimationby directly comparing the (magnetic) phase andrelative amplitude of the effect to the calculated curves. Infact, while the magnetic flux induces successive gap openingand closing, sub-bands at higher energies are split andshifted to lower (E − i )(or higher, E + i ) energies dependingon their clockwise (respectively counter-clockwise) movementwith respect to the applied magnetic field. As a consequence,at a given energy, the number of sub-bands carryingthe current is modulated. When the Fermi energy isbetween the CNP and E 1 /2 (0 and 35 meV in our case, redcurves figure 6), the number of sub-bands passes from 2 to0 during the first half-period and then rises back to 2 duringthe second half-period. The magneto-conductance is firstlynegative and then positive. At the same time, the magnitudeof the oscillations decreases as the Fermi energy increases,and vanishes at E F = E 1 /2 where the number of sub-bandsis magnetic field independent (green and cyan curves). BetweenE 1 /2 and E 1 (35 and 70 meV, black curve), the firstvan Hove singularity splitting to lower energies induces firstan increase of the number of sub-bands from 2 to 4 beforeΦ 0 /2 and then returns to 2 between Φ 0 /2 and Φ 0 . Thiscorresponds to a π-dephasing compared to the magnetoconductanceat low energies. A new vanishing, followed bya new dephasing, is consistently observed when the Fermienergy reaches and goes beyond the first vHs at E 1 (magentaand blue curves).Finally, we conclude that the magneto-fingerprints of theAharonov-Bohm effect is an unique tool to both identify themetallic or semiconducting behavior of the external shelland to assign the location of its charge neutrality point andthe van Hove singularities.S. Nanot, W. Escoffier, J-M Broto, B. Raquet,A. Magrez, L. Forro ()8

2009 CARBON ALLOTROPESEdge fingerprints and magneto-conductance in graphene nanoribbonsThe control of the current flow in gated graphene nanoribbons(GNR) constitutes a fascinating challenge for the futureof carbon-based electronic devices. The combinationof the extraordinary electronic properties of the Diracfermions in graphene, like their outstanding mobility atroom temperature, with a sizeable energy gap, opens a routeto outperform the ultimate scaling of silicon field effecttransistors [Wang et al, Phys. Rev. Lett. 100 206803(2008)]. However, the lateral confinement goes along witha problematic decay of the mobility. Low temperaturetransport measurements performed on nano-lithographedGNR also unveil a ubiquitous energy gap, irrespective ofthe edges orientation and exceeding the expected confinementgap.Drawing a parallel between GNR and carbon nanotubes(CNT) is certainly tempting. The presence of edges in GNRimposes Dirichlet boundary conditions and results in a lossof one conducting channel at the Charge Neutrality Point(CNP), compared to armchair CNT.obtained on high quality GNR is sorely lacking. Applyinga large transverse magnetic field is also a suitable tool toboth explore the expected 1D-electronic band structure andthe disorder induced localization.In this work, we present evidence of 1D-subbands signatureson the electronic transport on an 11 nm wide GNR.Comparison to band structures calculations of a set ofGNRs in a width window of 11±0.3 nm allows an assignationof the carbon atoms arrangement at the edges and thenumber of dimers (figure 7).The application of a 60 T perpendicular magnetic field inducesa drastic increase of the conductance, whose magnitudedepends on the doping level (figure 8(left). At zeroenergy, we demonstrate that the large positive magnetoconductanceoriginates from the onset of the first Landaustate accompanying the closing of the energy gap (figure8(right). Landauer-Buttiker simulations of the conductanceof GNR including bulk and edge disorder (and currentlyin progress) demonstrate that the magnetoconductanceresults from a subtle combination of magnetic bandformations and quantum interference effects in presence ofdisorder.Figure 7: Conductance versus back-gate voltage at 80 K measuredon the 11 nm wide GNR for two V bias , 50 mV and 1 mV(respectively red and blue curve). Superimposed is the calculateddensity of states (black curve) as a function of V g , for four distinctGNRs : (a,b,c) aGNR, N = 90,91,92 and (d) zGNR, N = 52.Non-perfect edges are also source of short range potentialdisorder responsible for inter-valley backscattering. Drasticconsequences on the electronic transport have been theoreticalanticipated with the formation of a mobility gap at zeroenergy, even in the presence of an ultra-smooth edge roughness.However, a direct comparison with experimental dataFigure 8: (Left) Magnetoconductance measured on the 11 nmwide GNR for various back gate voltages at 80 K. For comparison,the black dashed curve is the magnetoconductance measuredon a 90 nm GNR varying from 0.6 to 0.4G 0 under 55 T. (Right)Magnetoconductance at the CNP measured at 80,50 and 20 K.The black dashed curves are the simulated magnetoconductanceassuming a thermally activated regime and a magnetic field inducedclosing of the energy gap along with the onset of the firstLandau level at zero energy.J-M Poumirol, W. Escoffier, M. Goiran, J-M Broto, B. RaquetS. Roche, A. Cresti (Commissariat à l’Energie Atomique, INAC, SP2M, Grenoble), X. Wang, H. Dai (Physics departmentand Chemistry Laboratory, Stanford, US)9

CARBON ALLOTROPES 2009Using Landau quantization to suppress Auger scattering in grapheneCarrier-carrier scattering due to the Coulomb interactionis the dominant process which governs the dynamics ofhot carriers in solids at very short time scales. Thisis true for conventional (semiconductor) two-dimensionalsystems, even when their energy bands are quantized intodiscrete Landau levels by the application of a magneticfield. This is because Auger-type scattering processes betweenequidistant Landau levels, formed from bands withparabolic dispersions, are extremely efficient. Indeed,Auger scattering has long been considered as the main obstaclefor the fabrication of tunable far-infrared laser basedon inter-Landau level emission. However, the application ofa magnetic field should considerably influence the electronelectronscattering process in strongly non parabolic electronicsystems such as graphene.Here we investigate the dynamics of the non-equilibriumcarriers in graphene measured using a degenerate pumpprobetechnique which directly probes the occupancy ofstates well above the Fermi level. The differential transmission∆T /T as a function of delay between the pumpand probe pulses, is presented in figure 9(a-d) for magneticfields in the range 0 − 6 T. Two characteristic relaxationtimes are observed, a fast process (∼ 50 fs) which broadensthe photo-created distribution and a slower process (∼ 4 ps)due to thermalization. At low temperature (T = 18 K), themagnetic field has a considerable influence on both the fastand slow relaxation processes.This can be seen more clearly in figure 10 which plotsln(∆T /T ) versus delay time to highlight the exponentialcharacter of the relaxation. The characteristic time of thedecay (τ r ) can be extracted from the slope of such a plot.For the slow relaxation at zero magnetic field there is littledifference between the high and low temperature datawith a relaxation time of τ r ∼ 4 ps at both 18 K and 250 K.The relaxation becomes slower by a factor of 3–4, changingfrom τ r ∼ 4 ps at B = 0 T to τ r ∼ 12–14 ps for magneticfields above 3 T. For the fast decay, without magnetic fieldthere is little difference between the high and low temperaturedata with a relaxation time of τ r ∼ 55 fs at 18 K andτ r ∼ 70 fs at 250 K. However, at low temperatures, the applicationof a modest magnetic field ∼ 3–6 T doubles therelaxation time to τ r ∼ 110 fs, providing direct experimentalevidence that the electron-electron scattering is significantlyless effective in the presence of Landau quantization.This slow down, which is common for both the slowand fast relaxation processes, can be seen as a proof that inboth cases the electron–electron scattering or thermalizationof the hot plasma with the cold electrons is reduced inthe presence of Landau quantization which limits the possibleenergy of the initial and final states.This is interpreted as a reduction of electron-electron(Auger) scattering due to the unusual Landau quantizationof Dirac fermions in graphene. Our measurements, whichprobe Landau levels with a high index (n ≈ 100), suggestthat for lower Landau levels, Auger processes may be completelysuppressed. This makes graphene a promising systemfor the implementation of the long ago proposed tunablefar infrared Landau level laser [Plochocka et al. Phys.Rev. B 80, 245415 (2009)].Figure 9: The differential transmission ∆T /T as a function of thedelay between the pump and probe pulses measured at a temperatureof (a-b) 250 K and (c-d) 18 K for magnetic fields in the range0 − 6 T.Figure 10: (a-b) Natural log of the differential transmission as afunction of a delay between pump and probe pulses measured formagnetic fields (0 − 6 T) at 18K. The B = 0 T data measured at250 K is plotted using open circles. The relaxation times τ r extractedfrom linear fits are indicated. The inset of (a) shows thecalculated energy mismatch for Auger processes for carriers in then = 10 and n = 100 Landau level versus change in Landau levelindex mP. Plochocka, P. Kossacki, M. PotemskiA. Golnik, T. Kazimierczuk (Institute of Experimental Physics, University of Warsaw), C. Berger, W. A. de Heer (GeorgiaTech., Atlanta, USA)10

2009 CARBON ALLOTROPESObservation of the half-integer quantum Hall effect in epitaxial grapheneGraphene, a monolayer of sp 2 -bond carbon atoms, has recentlyattracted considerable interest due to its extraordinaryelectronic properties such as high charge carrier mobilitiesand a novel quantum Hall signature. These properties,that were first observed in exfoliated graphene, are adirect consequence of: (i) the linear band structure and (ii)the pseudo-spin, arising from the two sub-lattices. However,we use an epitaxial process to grow graphene monolayerson the Si-face of a semi-insulating silicon carbidesubstrate. The epitaxy and the sample fabrication is describedin [Emtsev et al. Nature Materials 8, 203 (2009)].As we can grow epitaxial graphene with a high and reproduciblequality (transport properties vary only about30% from sample to sample [figure 11(a) and (b)]) wewant to address an important question; are the quasirelativisticproperties of free-standing graphene also presentin graphene on SiC? Theory predicts distinct magnetotransportproperties for monolayer, bi-layer and multi-layergraphene that are nicely reproduced in exfoliated samples.In order to clarify if these predictions also hold for epitaxialgraphene, we have measured magneto-transport up to 28 Tin two different systems.The as prepared Hall bars show Shubnikov-de Haas (SdH)oscillations in the resistance R xx and emerging plateausin the Hall resistance R xy [see figure 12(a)]. The valuesof these plateaus, that are precursors to the quantumHall effect (QHE), follow the scheme of monolayergraphene: R xy = e 2 /((4n + 2)h) where n is an integer. Ifthe SdH extrema n are plotted against the inverse field1/B a linear dependence can be recognized [see inset infigure 12(b)]. The axis intercepts of 0 (1/2) for minima(maxima) yield a Berry phase of π as predicted for singlelayer graphene and confirmed in exfoliated samples.To probe the most interesting point of the band structure,the charge neutrality point, we evaporated ≈ 5 Å oftetrafluorotetracyanoquinodimethane (F4-TCNQ). This resultsin a reduced carrier density of n ≈ 5 × 10 11 cm −2 . Insuch a sample the half-integer QHE is visible [figure 12(b)].An evaluation of the SdH oscillations (barely visible below4 T) with the above mentioned procedure further confirmsthe picture of unperturbed graphene.We conclude that the close contact to the SiC substrate doesnot change the magneto-transport properties noticeable andepitaxial graphene reproduces the unique features observedin exfoliated graphene, while remaining a system which iscertainly more suitable for large scale production.Figure 11: (a) Histograms of the charge carrier density n, and (b)mobility µ, of samples with different geometries and sizes from5 mm to 400 nm. Some Hall bars are patterned on atomicallyflat terraces of the SiC substrate and therefore consist of perfectmonolayer graphene. (c) The step edges are clearly visible in theelectron micrograph.Figure 12: (a) R xx and R xy in a Hall bar on a single substrate terrace[cf. figure 11(c)]. SdH oscillations and plateaus in the Hallresistance are clearly visible. (b) The same quantities in a samplegated close to charge neutrality with F4-TCNQ. Half-integerQHE is visible above 7 T. The inset shows the evaluation of theSdH oscillations [open (closed) symbols: minima (maxima)]. Thelines are best fits to the data [black: Hall bar on a single substrateterrace, red: Hall bar covering several substrat terraces,blue: F4-TCNQ covered (plotted against 0.1/B for clarity)] thatyield a Berry phase of π as expected for monolayer graphene.D.K. MaudeJ. Jobst, D. Waldmann, H.B. Weber (Applied Physics Department, University of Erlangen-Nürnberg)11

CARBON ALLOTROPES 2009Integer quantum Hall effect in epitaxial grapheneIn graphene, a single sheet of hexagonally arranged carbonatoms, electrons behave as Dirac fermions while stillexhibiting the quantum Hall effect when subjected to a normalmagnetic field. Epitaxial graphene grown on SiC showsmost of the properties of single layer graphene (e.g. Landaulevel quantization), nevertheless, the quantum Hall effectremains elusive in this material. Very recently, the productionof high mobility (∼20 000 cm 2 /Vs) single-layergraphene on the C-face of SiC single crystalline wafershas enable the observation of quantum Hall plateaus atthe predicted values for graphene ( σ xy = νe 2 /h, with ν =4(N + 1/2) where N is the Landau level index, unambiguouslydemonstrating the graphene like nature of epitaxialgraphene [Wu et al. Appl. Phys. Lett. 95, 223108 (2009)].single terrace) or over steps, to investigate the influence ofsubstrate and defects on well-characterized samples. Conversely,these studies will bring insights on the potentialof epitaxial graphene as a scalable platform for graphenebasedelectronics, with potential applications in metrologyat room temperature.In this work, we extend this observation to magnetic fieldsup to 32 T where we observe the quantum Hall effect atfilling factor 1. This quantum Hall state is shown to be particularrobust against temperature in the epitaxial graphenesamples investigated here.In figure 13, the ν = 1 quantum Hall effect can be identifiedby a true zero-resistance state in the longitudinal resistivityρ xx . At the lowest temperatures a minimum is seen in ρ xxaround B ∼ 7.3T, probably related to the emergence of a gapat ν = 4. In this temperature range, ρ xx also exhibits sharpreproducible features coexisting with the quantum Hall effect,which are tentatively assigned to mesoscopic effects.Due to the spin and pseudo-spin degrees of freedom of electronsin graphene, the Landau levels are four-fold degenerateand the quantum Hall effect is thus observed for fillingfactors ν = 4(N + 1/2) (where N is the Landau level index)when this degeneracy is not lifted. The origin (spinor pseudo-spin) of the observed ν =1 (and ν =4) quantumHall states is currently the subject of debate [Z. Jiang et al.Phys. Rev. Lett. 99, 106802 (2007)].The temperature robustness of the quantum Hall effect observedin exfoliated graphene [K. S. Novoselov et al. Science315, 1379 (2007)] is also present in epitaxial graphene.In figure 14, the longitudinal R xx and Hall resistance R xy areplotted as a function of the magnetic field. The R xy quantizationat ν = 2 persists spectacularly up to room temperature.This behaviour, which primarily emanates from thevery large value of the cyclotron gap in graphene, also reflectsthe weak temperature dependance of the Landau levelbroadening.The observation of the quantum Hall effect in epitaxialgraphene opens new avenues for graphene physics: epitaxialgraphene for the first time offers the possibility to studygraphene layers on a atomically flat surface (device on aFigure 13: Longitudinale magnetoresistivity at T ∼ 20mK temperature.The filling factors corresponding to the observed QHstates are indicated.Figure 14: Temperature robustness of the quantum Hall effect.Longitudinale magnetoresistivity ρ xx at T = 4K (dashed line, rightscale). Hall resistance R xy as a function of the magnetic field fordifferent temperatures: T = 4K (thick line), and T = 300K (thinline).B.A. Piot, C. Faugeras, M. Potemski, D.K. MaudeC. Berger, W. A. de Heer (School of Physics, Georgia Institute of Technology, Atlanta, USA)12

2009 CARBON ALLOTROPESImproving graphene’s cleanliness for high field magneto-transportPulsed magnetic field magneto-transport on exfoliatedgraphene has been performed over the last few years andpreliminary results have shown anomalous Integer QuantumHall effect in disordered samples. The use of veryhigh magnetic field is required to investigate on the fundamentalelectronic states of 2D graphene, where Landaulevel degeneracy is expected to be lifted by the field. Foreven stronger magnetic field, the system eventually evolvesinto the fractional quantum Hall effect regime [Bolotin etal. Nature 462, 196 (2009)]. However, to make these investigationspossible, clean graphene devices (that is freeof defects or impurities and displaying very high mobility)are necessary. Actually, graphene devices are inescapablyheavily contaminated through the fabrication and contactingprocesses, as well as by direct exposure to air. Therefore,efficient cleaning procedures have to be found in orderto recover the intrinsic properties of graphene.Figure 16: Resistance of the graphene sample as a function ofgate voltage after several annealing processes.Figure 15: (a) Example of home-made graphene device mountedon a pulsed-field friendly sample holder (b) Optical microscopeimage of the central part of the device.Successive cleaning processes have been initiated in thelaboratory. Samples are first annealed at 250 ◦ C in secondaryvacuum. Investigations have shown that this processdrastically improves the quality of the samples which,however, quickly degrades when exposed to air. The devicesare then connected inside the experimental measurementsetup and annealed at 90 ◦ C under vacuum. This secondannealing helps in recovering high quality samples. Finally,electrical annealing [Moser et al. Appl. Phys. Lett.91, 163513 (2007)] is performed just before the magnetotransportmeasurements take place. All the aforementionedprocedures require specific know-how and often lead tosample’s destruction if not properly performed. These necessarytests (which are both time and sample consuming)have been done with home-made graphene samples (see figure15). Micro-mechanical cleaving of graphite (to obtaingraphene on a substrate) followed by electron-beam lithography,etching and metal deposition have been performedusing clean-room facilities at LAAS (CNRS).Figure 16 illustrates the influence of two of the above mentionedcleaning methods on a graphene sample. The resistanceis measured as a function of gate voltage before andafter annealing. The position of the resistance maximum aswell as the HWFM can be used to estimate the cleanlinessof the device. Further improvements have been achievedon similar home-made devices and high magnetic field experimentsare now under progress. These annealing methodshave also been successfully tested on another graphenesample (provided by the University of Manchester) and adirect comparison of the results could be made between thedisordered and clean regime. In the close vicinity of thecharge neutrality point, in the disordered case, transport isdominated by the formation of electron and hole puddlesin the sample over a large energy range. Accordingly themean Hall resistance is weak (contribution of electrons andholes to the Hall resistance cancel each other) and displayslarge fluctuations. Actually, disorder prevents the occurrenceof Landau level degeneracy lifting which could notbe observed in magnetic field as high as 60 T. A weak annealingof the device drastically changed its transport properties;at very high magnetic field the Hall resistance increases(contrary to the disordered case) although withoutdisplaying a new quantized resistance plateau. This effecttogether with the divergence of the longitudinal resistanceat very high magnetic field are interpreted as a first sign ofLandau level degeneracy lifting. Further experiments withimproved annealing processes are currently in progress toaddress the clean high field magneto-transport regime ingraphene.J.M. Poumirol, R. Ribeiro, A. Kumar, W. Escoffier13

CARBON ALLOTROPES 2009Metal-insulator transition for filling factor ν = −2 to ν = 0 in grapheneIn the framework of the scaling theory of the integer quantumHall effect (IQHE), plateau-plateau (PP) and plateauinsulatortransitions (PI) are interpreted as quantum phasetransitions with an associated universal critical exponentκ. The underlying physics is the Anderson localizationdelocalization.We investigate the quantum Hall effect(QHE) in a graphene sample at high magnetic fields, discoveringa metal-insulator (MI) quantum phase transitionpassing from filling factor ν = −2 to ν = 0. The analysisof the temperature dependence of the longitudinal resistancegives the value κ = 0.57 for the critical exponentof the MI (i.e PI) transition. This value is the same asthat found experimentally on the PI transitions in semiconductorsin quantum Hall regime [D. Shahar et al., SolidState Comm. 107, 19 (1998)] while is diverse from thePP transition exponent κ = 0.42 [A.J.M. Giesbers et al.arXiv:0908.0461v1]).For this purpose we have used graphene onto a Si waferwith a 300 nm SiO 2 top layer. It was processed depositing50Å Ti /500Å Au contacts in a Hall-bar geometry by usinge-beam nanolithography. The sample was annealed in-situprior to insertion into the cryostat at 100C for 90 minutesin a low pressure He exchange gas atmosphere. The insetwas placed in a 4 He bath cryostat while four probe measurementswere carried out using standard ac lock-in techniqueswith excitation currents of 1−10 nA and frequenciesof 1.3 − 13 Hz. The Dirac point appeared at V Dirac = −3V. In Fig.17 we show the longitudinal (R xx ) and Hall (R xy )resistances as a function of the magnetic field from 1.4to 4 K, with V gate = −8V. The hole density was n 2D =4.1×10 11 cm −2 and the mobility µ = 1.3×10 4 cm 2 V −1 s −1 .R xx grows exponentially and appears to exhibit a T independentcritical point at B c = 16.7T, while R xy closely followsthe ν = 0 quantized value, deep into the insulating phase.Unfortunately, the diverging behavior of R xx seriously hindersthe determination of R xy , as even a small Hall-contactmisalignment will result in a dominant contribution fromR xx this is why we show in the inset the R xy for +B and −Bin solid lines and R a xy = 1 2 [R xy(B)−R xy (−B)] in dashed line.The experimental characterization of quantum Hall PP andPI quantum phase transitions, relies on the behavior of thesystem as a function of T for magnetic fields sufficientlyclose to the transition point. In the vicinity of the criticalpoint (B c in our case) the longitudinal resistance is expectedto follow the empirical law:R xx = exp[−∆ν/ν 0 (T )], (3)with ∆ν = 1/B − 1/B c and B c = 16.7T (ν here should notbe confused with the Landau level filling factor). Fittingto Eq.(3) the experimental R xx measured in function of Twe can obtain the associated critical exponent κ by usingν −10∝ T −κ . [D.C. Tsui, Rev. Mod. Phys. 71, 891-895(1999)]. In Fig(18) we plot the R xx as a function of ∆ν with∆ν = 1/B - 1/B c T. Using the scaling procedure we obtainν 0 and a critical exponent κ = 0.57 (shown in the upper insert).We believe that this MI transition belongs to the sameuniversality class as the conventional PI transition from thelast quantum Hall liquid to the quantum Hall insulator.Figure 17: Longitudinal (R xx ) and Hall (R xy ) resistances as afunction of B for T = 1.4, 2, 3 and 4K with a V gate = −8V.Figure 18: Longitudinal resistance (R xx ) as a function of ∆ν. Inthe lower inset we show log(R xx ) as a function of B while in theupper the linear fit to obtain κD.K. MaudeM. Amado and E. Diez (Laboratorio de bajas temperaturas, Universidad de Salamanca, Spain), D. López-Romero (CT-ISOM, Universidad Politécnica de Madrid, Spain) J.M. Caridad, F. Rossella and V. Bellani (Universitá degli studi diPavia, Italy)14

2009 CARBON ALLOTROPESHow perfect can graphene be?We have identified the cyclotron resonance (CR) responseof purest graphene ever investigated, which can be found innature on the surface of bulk graphite, in form of decoupledlayers from the substrate material, and which have been recentlydiscovered in scanning tunnelling experiments [G.Li et al., Phys. Rev. Lett. 103, 176804 (2009)]. Probingsuch flakes in the THz range at very low magnetic fields, wedemonstrate a superior electronic quality of these ultra lowdensity layers (n 0 ≈ 3×10 9 cm −2 ) expressed by the carriermobility in excess of 10 7 cm 2 /(V.s) or scattering time ofτ ≈ 20 ps. These values significantly exceed those reportedin any kind of manmade graphene samples.The cyclotron resonance of electrons in these grapheneflakes has been measured in a high-frequency electron paramagneticresonance setup in double-pass transmission configuration,using the magnetic-field-modulation technique.A flake of natural graphite was placed in the variable temperatureinsert of the superconducting solenoid and viaquasi-optics exposed to the linearly polarized microwaveradiation emitted by a Gun diode.A typical example of our experimental finding is illustratedin figure 19a, where the derivative of the magnetoabsorptionspectrum of decoupled graphene on the surfaceof a natural graphite specimen at T = 25 K, measured asa function of the magnetic field at a fixed microwave frequency.The interpretation is schematically illustrated inpart (b). The observed spectral lines are assigned to cyclotronresonance transitions between adjacent Landau levelswith energies: E n = sign(n)˜c √ 2eB|n|, characteristicof massless Dirac fermions in graphene sheets with an effectiveFermi velocity ˜c. This velocity is the only adjustableparameter required to match the energies of theobserved and calculated CR transitions and reaches ˜c =(1.00 ± 0.02) × 10 6 m.s −1 .Since the well-defined Landau level quantization in ourgraphene flakes is spotted down to |B| = 1 mT we obtainvia the semi-classical quantization condition µB > 1 thecarrier mobility µ > 10 7 cm 2 /(V.s), almost two orders ofmagnitude higher in comparison to any other reported values.The scattering time τ ≈ 20 ps derived from the typicalCR width in our spectra also significantly exceeds valuesreported in any kind of graphene samples. This scatteringtime gives an independent estimation for the mobilityµ = eτ ˜c 2 /E F ≈ 3 × 10 7 cm 2 /(V.s) in good agreementwith the estimate above. Interestingly, bearing in mindthis exceptional quality, one may conclude that Landaulevel quantization should survive in studied graphene layersdown to the field of B = (/(E 1 · τ)) 2 ≈ 1 µT. Hence,the magnetic field of the Earth of ∼ 50 µT is no longer negligiblysmall. Instead, the energy gap up to ∆ ≈ 0.3 meVshould appear at the Dirac point, depending on the sampleorientation.Figure 19: (Derivative of) a typical magneto-absorptionspectrum measured at T = 25 K and microwave frequencyω = 1.171 meV (a) in comparison with the Landau level fanchart (b), where the observed CR transitions are shown by arrows.The occupation of individual levels is given by the Fermi-Diracdistribution plotted in the part (c). For simplicity, we consideredonly n-type doping with E F = 6.5 meV. The dashed lines showpositions of resonances assuming ˜c = 1.00 × 10 6 m.s −1 .Moreover, the estimated mobility should not decrease withtemperature, as no broadening of CRs is observed up toT = 50 K, when CR intensities become comparable withthe noise. This extremely high value of mobility combinestwo effects: the long scattering time τ and a very small effectivemass m = E F / ˜c 2 ≈ 2 × 10 −4 m 0 E F [meV]. Remarkably,the same scattering time in a moderate density sample(n 0 = 10 11 cm −2 ), would imply the mobility still remaininghigh, around µ ≈ 5×10 6 cm 2 /(V.s), and comparable to bestmobilities of two-dimensional electron gas in GaAs structuresat these densities.To conclude, our measurement significantly shifts the currentlimits of intrinsic mobility in graphene and poses aquest for further development in the technology of its fabrication.For details see, P. Neugebauer et al., Phys. Rev. Lett. 103,136403 (2009).P. Neugebauer, M. Orlita, C. Faugeras, A.-L. Barra, M. Potemski15

CARBON ALLOTROPES 2009Effect of a magnetic field on the two-phonon Raman scattering in grapheneAmong the different features that can be observed in theRaman scattering spectrum of graphene or graphite, theso-called 2D band feature is of particular interest. It is afully resonant second order Raman scattering process anddirectly involves the electronic band structure. Its observationas a single Lorentzian shaped contribution is a uniquefingerprint of the monolayer character of a graphene specimenor of the decoupled nature of the graphitic planes inmultilayer epitaxial graphene samples. As a fully resonantsecond order Raman scattering process, it shows a dispersionwith excitation laser energy which was used to tracethe phonon band structure of different carbon allotropes.We have measured the evolution of the 2D band feature ofa multilayer epitaxial graphene sample as a function of themagnetic field with the experimental set-up and in the sameconfiguration as is described in the preceding report. As canbe seen with the black dots in the left part of figure 20, fromB=0 to 33 T we observe first a quadratic and then a linearcontinuous red shift of the energy of this feature (8 cm −1 )together with a broadening of 20%.As can be seen in the schematics in the right part of figure20, in the semi-classical real-space picture of the Ramanprocess, the incident photon creates an electron and thehole with opposite momenta at an arbitrary location withinthe laser spot. They subsequently propagate along the classicaltrajectories, and emit phonons. If they meet at someother location, again with opposite momenta, they can recombineradiatively producing a scattered photon. In theabsence of magnetic field, the trajectories are straight lines,so that in order to meet at the same point with opposite momentaand contribute to the Raman signal, during phononemission the electron and the hole must necessarily be scatteredbackwards. This fixes the phonon momentum q(measured from the K or K ′ point) as q = p + p ′ , wherep = ω L /(2c∗) and p ′ = (ω L − 2ω ph )/(2c∗) are theelectronic momenta (also measured from the Dirac points)before and after the phonon emission, ω L and ω ph are respectivelythe excitation laser and the phonon frequencies,and c∗ is the electron velocity.In a magnetic field, the electron and hole trajectories areno longer straight lines but, because of the Lorentz forcethat acts on charged particles in a magnetic field, they correspondto Larmor circles. As a result, (i) phonons withsmaller momenta, q = pcosϕ + p ′ cosϕ ′ , can be emitted,and (ii) since each phonon can be emitted at an arbitraryinstant in time, the length of the arc describing the electrontrajectory is random [not exceeding the electron meanfree path c ∗ /(2γ), where γ is the electron scattering rate],and so are the angles ϕ,ϕ ′ . Tuning the magnetic field is,in this sense, equivalent to changing the resonant conditionsof the Raman scattering process at a fixed excitationwavelength and allows spanning part of the phonon bandstructure closer to the K point. Fact (i) results in an overallred shift of the Raman peak, while fact (ii) introduces anadditional spread in q, and contributes significantly to thebroadening of the peak as observed through Raman scatteringmeasurements.The solid red lines in figure 20 are the result of a calculationof the maximum Raman scattering intensities as a functionof magnetic field following our model calculating the Ramanmatrix element with an applied magnetic field and withthe assumption that Landau levels at the excitation laser energyare not separated (non-quantizing magnetic fields forhigh energy charge carriers). Experimental results are wellreproduced with only two adjustable parameters, the electronscattering rate deduced to be γ = 27 meV and σ, abroadening parameter introduced to take into account scatteringmechanisms other than electronic scattering and responsiblefor half of the observed total broadening. Thisfield induced 2D band feature evolution is not typical forgraphene and should also be observed in graphite.Figure 20: Left: Black dots are the 2D band feature Raman shiftand line width as a function of the magnetic field and solid redlines are calculated with our model. Right: Real space schematicsof the second order Raman scattering process responsible for the2D band at B=0T and B≠0T. The lightning is the excitation laserwith energy E L , black arrows represent the electron and hole trajectorieswith momentum p (-p) before the phonon emission andp’ (-p’) after, dashed green arrows are the K point optical phononswith momentum q and energy E(q), the flash is the emission of theRaman photon.C. Faugeras, P. Kossacki, M. Amado, M. PotemskiD. Basko (LPMMC-CNRS, Grenoble, France), M. Sprinkle, C. Berger, W.A. de Heer (Georgia Institute of Technology,Atlanta, USA)16

2009 CARBON ALLOTROPESTuning the electron-phonon coupling in graphene with magnetic fieldsLattice vibrations in solids, phonons, can be effectivelymodified via their coupling to electronic excitations givingrise to many phenomena such as the Köhn anomaly in metals,different phonon modes observed in semiconducting orin metallic carbon nanotubes or coupled phonon-plasmonmodes in doped polar semiconductors.In graphene which is not a polar material, the zone centerE 2g optical phonons, the so-called G-band, efficientlycouple to low energy, direct electronic transitions (∆ k = 0)through the deformation potential interaction. Low energydirect electronic transitions exist in graphene thanks to itspeculiar band structure in which the conduction and thevalence bands touch at the Dirac point (gapless material)and exhibit a linear dispersion. These transitions renormalizethe E 2g phonons energy observed for instance throughRaman scattering experiments. As a consequence, a modificationof the electronic excitation spectrum can modifythe E 2g phonon energy, a situation unique for a condensedmatter system and not encountered in traditionalsemiconductor-based systems for which a band gap of energymuch larger than the phonon energy is present and imposesa low energy onset for such direct electronic transitions.A method to severely alter the electronic excitation spectrumis to apply a magnetic field perpendicular to thegraphene plane. Under such conditions, the electronic excitationspectrum characteristic for graphene which is continuousat zero magnetic field, becomes discrete and composedof a series of optical transitions among Landau levelsas observed through magneto-infrared experiments. In multilayerepitaxial graphene samples grown on the carbon faceof a SiC substrate, the electronic scattering rate is extremelylow and Landau levels are well separated at magnetic fieldsas low as 40 mT. We have performed high field magneto-Raman scattering experiments with such high quality samplesin the back scattering configuration to observe possiblemodifications of the E 2g phonon energy.The observed Raman scattering signal arising from thislong wavelength phonon is composed of two distinct contributions,a field independent contribution and a magnetooscillatorycomponent which field evolution is presented infigure 21 as a false color plot. We observe a well definedseries of avoided crossings each time an optical transition istuned in resonance with the E 2g phonon energy. When allpossible electronic transitions are far from the E 2g phononenergy, the effective interaction is very weak as revealed bythe minimum line width of ∼ 7 cm −1 of the Raman scatteringsignal of the E 2g phonon, imposed in these conditionsby scattering mechanisms other than the electron-phononscattering.This effect had been nicely anticipated theoretically [Ando,J. Phys. Soc. Jpn. 76, 024712 (2007)] and the pink linesin figure 21 are the results of this calculation for a neutralgraphene system with a dimensionless electron-phononcoupling constant λ = 4.5 × 10 −3 and a broadening parametercharacteristic for electronic transitions δ = 90cm −1 .This experiment allows to directly extract the strength ofthe electron-phonon interaction and gives an insight into therich physics of electron-phonon coupling in graphene.Figure 21: Energy color map and evolution of the Half Widthat Half Maximum (HWHM) of the magneto-oscillatory componentof Raman scattering spectra of E 2g (G band) phonons asa function of the square root of the magnetic field measured atT = 4.2 K under λ = 720.7 nm laser excitation. The extractedpeak position and line width of this contribution are shown withfull dots. Their size is proportional to the line amplitude. Solidlines T k represent the energies of the series of inter Landau leveltransitions:L k;(k−1) → L k+1;(k) which couple to E 2g phonon (c* =1.02 × 10 6 m.s −1 is assumed). The solid pink lines are the resultsof the model describing the magneto-phonon resonance ingraphene.C. Faugeras, M. Amado, P. Kossacki, M. Orlita, M. PotemskiM. Sprinkle, C. Berger, W.A. de Heer (Georgia Institute of Technology, Atlanta, USA)17

CARBON ALLOTROPES 2009Thermal conductivity of graphene in Corbino membrane geometrySystems of fewer than three dimensions are consideredto be very efficient heat spreaders as their intrinsic thermalconductivity may eventually diverge for infinite specimens,following a power or logarithmic law for one- ortwo-dimensional systems, respectively. This conjecture isnow being confronted with experimentations on graphene -a single sheet of graphite, the closest archetype of a twodimensionalcrystal. Carbon crystallites such as diamondand graphite are known as exceptional heat conductors andan extremely efficient thermal conductivity has been alsoreported for graphene [Balandin et al., NanoLett 8, 902,(2008)]. This conclusion calls, however, for confirmationbecause the experimental methods applied to draw it arenot fully straightforward. A precise (contactless) temperaturereadout, an accurate sample geometry and an exactestimation of the absorbed laser power are among subtle issueswhich may significantly influence the apparent valuesof the extracted 3D thermal conductivity coefficient.We have performed a study of the thermal properties of alarge area graphene membrane which fully covers a 44 µmdiameter pinhole made in the 2 mm thick plate of copper(see photograph in the inset of figure 22). With the use ofsilver epoxy, the edges of the membrane (which extend outsidethe pinhole) are thermally short circuited to the copperplate which serves in our experiments as a room temperatureheat sink. The groundwork of our experiments consistsin using laser excitation to locally generate heat and measuringthe Raman scattering spectra to extract the actualtemperature of the membrane within the laser spot fromthe intensity ratio of Stokes to anti-Stokes Raman scatteringsignals. Our Corbino-like experimental configurationtogether with the direct temperature readout largely simplifiesthe data analysis. We present in figure 22 typicalStokes and anti-Stokes Raman scattering spectra measuredat different location along the radius of the membrane. Asexpected and seen in this figure, the laser excitation heatslocally the membrane, most efficiently when the laser spotis in the center of the membrane and significantly less whenit is placed closer to the copper plate heat sink. After carefulcalibration of the response of the experimental set-up,we read the temperature directly from the intensity ratio ofthe Stokes to anti-Stokes signals : I(ω exc−ω G )I(ω exc +ω G ) = exp( ω Gk B T ),where ω exc,G are the excitation laser and the phonon frequencyand T is the lattice temperature under the laser spot.Figure 22: Stokes and anti-Stokes Raman scattering spectra ofthe the G-band graphene membrane measured under 6.2 mW oflaser excitation focused at different points on the membrane (atdifferent distances r from the center of the membrane). Note thechange in the ratio of Stokes to anti-Stokes signal, which depictsthe drop of local temperature within the laser spot (2µm in diameter)when approaching the edge of the membrane (in thermal contactwith room temperature sink). Inset : Photograph of the 44 µmdiameter membrane as seen through a ×100 optical objective.When the laser spot is located in the center of the membrane(r = 0), the heat equation describing our experiment can besolved analytically and one can establish the following relation:T (0)−T edge∼ = 1.689α, relating the temperature differencebetween the center and the edge of the membrane assumedto be a heat sink at room temperature, to α = P/κdπ,P being the absorbed optical power recently measured to be2.3% of the total excitation power, d the thickness of thegraphene membrane and κ its 3D thermal conductivity coefficient.In this particular case, the temperature deducedwith a total excitation power of 6.2 mW focused on a 2 µmdiameter spot is T = 660 K and a resulting α coefficient of216 K.The 3D equivalent thermal coefficient is κ ≈ 630 W/m·K,i.e., somewhat less then previously claimed. The differencebetween the present and previous estimations of κ is mainlydue to different assumptions regarding the efficiency of thegraphene’s optical absorbance. Finite elements computationsperformed to deduce the values for α in the case ofexcitations at other locations along the radius of the membraneconfirm the validity of our approach.C. Faugeras, M. Orlita, M. PotemskiB. Faugeras (Université de Nice, Nice, France), R.R. Nair, A.K. Geim (University of Manchester, Manchester, UnitedKingdom)18

2009 CARBON ALLOTROPESElectric field doping of few-layer grapheneThe electronic properties of few-layer graphene have beeninvestigated at low temperature and high magnetic field.Few-layer graphene systems consist of a few tens of planesof carbon atoms stacked upon each others and exhibit acomplex electronic band structure. Regarding their transportproperties, they are natural candidates to study thecross over from 3D graphite to 2D graphene. So far, manystudies have focused on graphene, where a linear dispersionrelation at the K and K ′ points of the first Brillouin zoneleads to transport properties governed by massless quasiparticles.Such charge carrier dynamic makes graphenea unique 2D system and is responsible, among other, tothe anomalous quantum Hall effect. On the other hand,graphite supports the presence of different groups of chargecarriers (electron-like and hole-like quasi-particles), amongwhich massless Dirac fermions have already been reported[Luk’yanchuck et al. Phys. Rev. Lett., 93, 166402 (2004)].The experimental investigation of few-layer graphene hasbeen undertaken in an attempt to determine the relative contributionsof the different types of charge carriers to electronictransport, in a system where a small number of carbonlayers makes its electronic properties half-way betweenthose of graphite and graphene.+70 V shows a monotonously increasing function with nohint of resistance maximum. We therefore infer that thesample is intrinsically p-doped and that the charge neutralitypoint is out of the experimental range. Contaminantsand/or defects are responsible for shifting the charge neutralitypoint away from V g = 0 V.Figure 24: 3D plot of the Hall resistance R xy as a function ofmagnetic field B and gate voltage V gFigure 23: Oscillatory part of R xx (B,V g ) after subtracting asmooth background function. For clarity, the curves are shiftedvertically by 1.5 kΩThe few-layer graphene sample is obtained by exfoliationof bulk graphite and deposited onto a heavily dopedSi/SiO 2 substrate with 300 nm oxide thickness. Fourelectrodes have been fabricated on the sample using photolithographyso that the two-probe longitudinal and Hallresistance could be simultaneously measured during a pulseof the magnetic field. A gate voltage is applied thoughthe SiO 2 dielectric in order to continuously tune the chargecarrier density. As compared to the simple plane capacitormodel usually accepted for graphene, the few-layergraphene system displays reduced gate efficiency, mostprobably due to large screening effects. The gate voltagedependence of the resistance in the range −70 < V g

CARBON ALLOTROPES 2009Low temperature magneto-transport in natural graphiteHistorically, graphene forms the starting point for the Slonczewski,Weiss and McClure (SWM) band structure calculationsof graphite. In graphite, the Bernal stacked graphenelayers are weakly coupled with the form of the in-planedispersion depending upon the momentum k z in the directionperpendicular to the layers. The carriers occupy a regionalong the H − K − H edge of the hexagonal Brillouinzone. At the K point (k z = 0), the dispersion of the electronpocket is parabolic (massive fermions), while at the H point(k z = 0.5) the dispersion of the hole pocket is linear (masslessDirac fermions). A clear signature of Dirac fermionsat the H point of graphite has recently been reported usingfar-infrared magneto-absorption and ARPES measurements.Such measurements probe the very close vicinity ofthe H and K points where there is a maximum in the jointdensity of states.Typical low temperature R xx versus magnetic field for naturalgraphite, is shown in Fig. 25(a). The quantum oscillations,superimposed on a large magneto-resistance background,can be better seen in the background removed data∆R xx plotted in Fig. 25(a-c). Quantum oscillations are observedfor both majority electrons and holes with orbitalquantum number up to almost N = 100. These oscillationsare fully consistent with the presence of majority electronand hole pockets within the three dimensional SWM bandstructure calculations for graphite [Schneider et al. Phys.Rev. Lett. 102, 166403 (2009)]. At low magnetic fields,a perfect linear behavior in N(1/B) is observed for bothelectrons and holes. For high magnetic fields, clear deviationsfrom the linear behavior are observed for the electronfeatures (see Fig. 26(b-c)). This deviation from a periodicin 1/B behavior at high magnetic fields is due to theFermi level moving as the quantum limit is approachedin graphite. Clearly, the high field data should not beused to extract the phase of the oscillations. Instead weuse the complex Fourier transform ˆf (B) of the low magneticfield ∆R xx (1/B). The phase shift function K(ϕ,B) =Re[e −iϕ ˆf (B)] has maximum in the ϕ − B plane which canbe used to extract both the frequency (B) and phase (ϕ) ofthe oscillations. K(ϕ,B) is plotted in Fig. 26(d-e) in theregions of the hole and electron features. From the maxima,the determined frequency and phase are B fh = 4.51 T,ϕ h = −(0.56±0.1)π and B fe = 6.14 T, ϕ e = −(0.86±0.1)πfor the hole and electron features respectively.We therefore conclude that we have no evidence for theexistence of masseless Dirac fermions with a Berry phaseγ = 0. Transport measurements are sensitive to the densityof states at E F , which is modulated with increasing magneticfield, as the Landau bands cross the Fermi energy. Forholes, maxima in the density of states correspond to Landaubands crossing E F for k z < 0.5, away from the H point,where the dispersion is no longer linear and a priori there isno reason to expect the carriers to behave as Dirac fermions.Figure 25: (a) Resistance R xx versus B measured at T = 10 mKfor natural graphite. (a-c) Background removed data ∆R xx showingquantum oscillations measured over different magnetic fieldregions. The arrows indicate spin split electron and hole features.Figure 26: (a) Fourier transform of the low magnetic field∆R xx (1/B). (b-c) Orbital angular momentum quantum numberN, as a function of the reciprocal magnetic field positions of theelectron and hole features. (d) and (e) Contour plot of the phaseshift function K(ϕ,B) in the vicinity of the hole and electron features.Maxima in K(ϕ,B) determines the frequency and phase ofthe oscillations.J. M. Schneider, M. Orlita, M. Potemski and D. K. Maude20

2009 CARBON ALLOTROPESMagnetotransport to extract the spin gap for charged excitations in graphiteIn a magnetic field the properties of graphite are remarkablywell described by the Slonczewski, Weiss and Mc-Clure (SWM) band structure calculations. While orbitaleffects have been extensively used to caliper the Fermi surface,the more subtle spin effects have received less attention.Indeed, the well documented movement of the Fermienergy in a magnetic field seriously complicates the extractionthe spin gap (g-factor) from the magnetotransport data.Recent advances in experimental techniques, in particularthe vastly increased desktop computing power available fordiagonalizing the SWM Hamiltonian, makes it timely torevisit this problem, extending previous measurements tohigher magnetic fields and lower temperatures.The oscillatory component of longitudinal resistance ∆R xxas a function of the magnetic field from B = 0 − 28 T forvaries orientations between θ = 0 ◦ and θ = 90 ◦ is shownin figure. 27(a). Quantum oscillations due to the majorityelectrons and holes Fermi surfaces are clearly observed.For increasing tilt angles the quantum oscillations shift as1/cos(θ) to higher magnetic field demonstrating the quasi2D nature of graphite. The experimentally observed splitting∆B = B ↓ − B ↑ , where B ↑ and B ↓ are the magneticfield positions of the spin up and spin down features, isplotted as a function of the mean total magnetic field positionB m = (B ↓ + B ↑ )/2 for the n = 1 electron and holefeatures (figure.27(b)). The failure of ∆B to follow a simplequadratic behavior is an experimental signature that themovement of the Fermi energy must be taken into accountwhen extracting the g-factor.Figure 27: (a) ∆R xx as a function of the magnetic field for differentangles (0 ◦ ≤ θ ≤ 90 ◦ ). (b) Magnetic field splitting ∆B as afunction of the total magnetic field B m . The thick line is calculatedfor the n = 1 electron Landau band with g s = 2.53 takinginto account the movement of the Fermi energy. The thin line isthe expected parabolic behaviour if E f is constant and requiresand unreasonably large value of g s = 6.5In order to extract the g-factor, we use the SWM band structuremodel.The effect of the in-plane magnetic field can beincorporated into the SWM model through an effective spingap ∆ s = g e f f µ B B ⊥ where the real g-factor g s = g e f f cos(θ).In graphite, E f moves with the applied perpendicular magneticfield as carriers are transferred between the electronand hole pockets. The Fermi level has to be calculated selfconsistentlyassuming the sum of the electron and hole concentrationsis constant, n− p = n 0 . At each angle, the effectivespin gap is found for which the SWM model gives thecorrect magnetic field position for the crossing of the spinup and spin down Landau band with the Fermi energy.Figure.28 shows the result for the spin gap ∆ s extractedfrom the SWM calculations as a function of the total magneticfield for the n = 1 hole and the n = 1 − 4 electronLandau bands. The spin gap is similar for both the electronand holes Landau bands at a given total magnetic field.∆ s increases linearly with magnetic field and a linear fit to∆ s = g s µ B B m (solid line) for both electron and hole Landaubands gives g s = 2.53 ± 0.07. The enhancement comparedto the electron spin resonance value can be attributedto electron-electron interactions.Figure 28: (a) Spin gap ∆ s for the electron and hole Landaubands, obtained by fitting the SWM model to ∆B, as a functionof the magnetic field. (b) The corresponding SWM g-factors foreach data point. The thick dashed line corresponds to g s = 2.53while the thin dashed lines correspond to the anisotropic electronspin resonance g-factors.J. M. Schneider, M. Orlita, M. Potemski and D. K. MaudeN. A. Goncharuk, P. Vasek, P. Svoboda and Z. Vyborny (Academy of Science, Prague)21

CARBON ALLOTROPES 2009Magneto-transmission spectroscopy of graphite in high magnetic fieldsThe recent discovery of relativistic-like, zero effective restmass particles in graphene, an atomically thin layer of carbonhas caused a dramatic increase of interest in this material.Graphite, which is composed of Bernal stackedgraphene layers, is a far more complex system and is notfully understood despite nearly seventy years of research.In particular, spectroscopic studies have revealed the existenceof massive electrons at the K-point together with massless fermions near the H-point [Orlita et al. Phys. Rev. Lett.102, 166401 (2009)].In this work, we present the results of transmission measurementsthrough a thin layer of graphite in magnetic fieldsup to 56 T. We explore the higher energy range of themagneto-transmission in the near visible light range usinga tungsten halogen lamp. Transmission measurementswere circular polarization resolved and spectra recordedfor both polarities of the magnetic field. Measurementswhere performed in dc magnetic fields up to 32 T in Grenobleand in pulsed magnetic fields up to 56 T in Toulouse.Data has been taken at different temperatures in the range4.2 − 300 K. A representative transmission spectra as afunction of the temperature measured at 56 T is shown infigure 29. In the low temperature data we observe a splittingof the absorption lines (marked as ∆ in figure 29) whichvanishes with the increasing temperature.To identify the optical transitions we plot the position ofeach absorption line as a function of the magnetic field.The results are presented in figure 30. The experimentalposition of each absorption line is marked by symbols. Simultaneouslythe theoretical prediction for the expected opticaltransitions in graphite are marked by lines. The dashedlines correspond to the optical transitions in the vicinity ofK point with an energy ∝ B, while the solid lines correspondto the optical transitions in the vicinity of H pointwith an energy ∝ √ B as for massless fermions in graphene.As the electronic structure of graphite is more complex thanfor graphene the optically allowed transitions (dipole interband) can be assigned to transitions between L −m(−n) andL n(m) LLs, where m,n enumerates the Landau levels withthe following change of the LL index by 2 (marked in grey),by 0 (marked in black) and by 1 as for graphene marked bydifferent colors.All of the transitions characteristic for graphite are wellreproduced by the theoretical predictions. However, forthe graphene-like transitions and additional splitting of thelines is clearly observed. This is even more remarkablesince such a splitting is completely absent in magnetotransmissionmeasurements on graphene [Plochocka et al.Phys. Rev. Lett. 100, 87401 (2008)]. Intriguingly, polarizationresolved measurements (not shown) demonstratethat the splitting is not polarization dependent and thereforenot linked to the asymmetry of the Dirac cone.Figure 29: Representative differential transmission spectra (theB = 0 T spectra has been subtracted) measured at 56 T for differenttemperatures.Figure 30: Position of the absorption lines as a function of themagnetic field for spectra taken at T = 4 K. The dashed and solidlines correspond to the calculated optically allowed dipole interbandtransitions in graphite.P. Plochocka, N. Ubrig, P. Kossacki, M. Orlita, O. Portugall, G.L.J.A. Rikken22

2009 CARBON ALLOTROPESGraphite from the viewpoint of Landau level spectroscopy: An effectivegraphene bilayer and monolayerThe unusual properties of massless Dirac fermions observedin graphene monolayers, multi-layer epitaxialgraphene or bulk graphite have been extensively studied usingvarious optical and magneto-optical techniques. In contrast,few experiments have addressed the optical responseof a graphene bilayer in spite of the considerable theoreticalinterest into this system. Here we show that some part ofthe physics of the graphene bilayer can be, perhaps surprisingly,studied when investigating bulk graphite.We report on an infrared transmission study of a thin layerof bulk graphite prepared by a multiple exfoliation of anatural graphite crystal, performed in magnetic fields upto B = 34 T. All magneto-transmission spectra were takenin the Faraday configuration with the magnetic field appliedalong the c-axis of the graphite, using technique ofthe Fourier transform spectroscopy.Two series of absorption lines whose energy scales as √ Band B were found in the spectra and identified as contributionsof massless holes at the H point (figure 31a)and massive electrons in the vicinity of the K point (figure.31b), respectively. We have shown that the infraredmagneto-absorption spectra of graphite, measured over awide range of the energy and magnetic field, can be interpretedin a very simple, transparent and elegant manner, i.e.that graphite can be viewed as an effective graphene monolayerand bilayer. Our results thus confirm recent theoreticalmodel [B. Partoens and F. M. Peeters, Phys. Rev. B75, 193402 (2007); M. Koshino and T. Ando, Phys. Rev.B 77, 115313 (2008)], which drastically simplifies the standardSlonczewski-Weiss-McClure model considering onlytwo tight-binding parameters γ 0 and γ 1 , which describe theintra- and inter-layer tunnelling, respectively.In this picture, the dominant contribution to the optical responseis provided by the H point, where electron statesclosely resemble graphene but with an additional doubledegeneracy, and by the K point, where the energy spectrumresembles a graphene bilayer, but with an effective couplingof 2γ 1 , twice enhanced compared to a real bilayer system.Remarkably, using this simple graphene monolayer plus bilayerview of graphite, we are able to correctly reproducethe magnetic field evolution of all observed inter-Landauleveltransitions using only the SWM parameters γ 0 and γ 1 ,with values which perfectly match those derived from studiesof real graphene monolayer and bilayer systems (see figure.31).To conclude, the electronic states at K point of graphite areinterestingly found to mimic those of the graphene bilayer,but with a doubled value of the effective mass. It should benoted that as the validity of the model is limited in the vicinityof the Fermi level, it is not useful, for example, for theinterpretation of magneto-transport experiments. Nevertheless,bulk graphite remains a material of choice to studymagneto-optical phenomena in systems with both masslessas well as massive Dirac fermions.For details see, Orlita et al., Phys. Rev. Lett. 102, 166401(2009) and also Orlita et al., ibid 100, 136403 (2008).Figure 31: (a) Positions of the absorption lines related to theH point as a function of √ B. The solid and dashed lines representexpected positions of absorption lines for ˜c = 1.02 × 10 6 m/s(γ 0 = 3.2 eV). (b) Positions of absorption lines related to the Kpoint as a function of B. The solid lines show expected dipole allowedtransitions in a graphene bilayer with an effective coupling2γ 1 for γ 0 = 3.2 eV and γ 1 = 0.375 eV. The inset schematicallyshows the observed inter-band transitions in the effective bilayer.The gray points in the part (b) have been measured on highly orientedpyrolytic graphite.M. Orlita, C. Faugeras, J. M. Schneider, G. Martinez, D. K. Maude, and M. Potemski23

CARBON ALLOTROPES 2009Magneto-transmission of multi-layer epitaxial graphene and bulk graphite:A comparisonThe fabrication of multi-layer epitaxial graphene on thecarbon-terminated surface of SiC crystal [see e.g. W. A.de Heer et al., Solid State Commun. 143, 92 (2007)], withthe characteristic rotational stacking of graphene sheets [J.Hass et al., Phys. Rev. Lett. 100, 125504 (2008)], naturallyinduced live discussions about differences in electronicband structures of this system and well-known bulkgraphite, which has been intensively studied within lastsixty years.Here we report on magneto-transmission measurements inthe far infrared spectral range, performed to help in understandingof electronic band structures of both these materials.In our study, we have compared magneto-optical responseof two kinds of samples: Multi-layer graphene sample,which was epitaxially grown by thermal decompositionon the carbon-terminated surface of a 4H-SiC substrate andcontains around ∼100 graphene layers, and a thin layer ofbulk graphite (single-crystal natural graphite), prepared bya simple exfoliation using an adhesive tape. Typical lowmagnetic-fielddata obtained in our experiment are shownin Fig. 32.In our magneto-transmission measurements, we have focusedon spectral features evolving linearly with √ B, whichrepresent a hallmark of massless Dirac fermions presentaround K and H points in graphene and graphite, respectively.The magneto-optical response of both materialswas found to be surprisingly similar and practically thesame Fermi velocity has been extracted from data, ˜c =1.02 × 10 6 m.s −1 . Nevertheless, the obtained data still allowedus to distinguish between (strongly anisotropic) 3Dbulk graphite and 2D multi-layer epitaxial graphene. The3D nature of bulk graphite was mainly demonstrated viaappearance of an additional set of absorption lines (denotedby Greek letters in Fig. 32b) in comparison to multilayergraphene. Hence, whereas the results obtained onmulti-layer graphene fully corresponded to expectations fordipole-allowed transitions in a 2D gas of massless Diracfermions, the transmission spectra taken on bulk graphiteappeared to be more complex. The standard Slonczewski-Weiss-McClure model of the bulk graphite band structurewas found in latter case to be sufficient to explain existenceof all absorption lines scaling as √ B as well and their individuallineshapes.For details see, M. Orlita et al., Solid State Commun. 149,1128 (2009).Figure 32: Transmissions spectra of multi-layer epitaxialgraphene (a) and bulk graphite (b) for selected magnetic fields atT = 2 K. The absorption lines corresponding to dipole-allowedtransitions in graphene are denoted by Roman letters. Greek lettersare used for additional transitions which scale as √ B and areonly found in spectra taken on bulk graphite. For clarity, the spectrain part (a) and (b) were shifted by amount of 0.18 and 0.10,respectively.M. Orlita, C. Faugeras, G. Martinez, D. K. Maude, J. M. Schneider, M. PotemskiM. Sprinkle, C. Berger, W. A. de Heer (Georgia Institute of Technology, Atlanta, USA)24

2009Two-Dimensional Electron Gas25

2009 TWO-DIMENSIONAL ELECTRON GASThe surprisingly fragile quantum Hall ferromagnet at filling factor ν = 1In two dimensions many body interactions often dominateover the single particle physics giving rise to novel collectiveground states. A striking example is the quantum Hallferromagnet. At filling factor ν = 1, the lowest Landau levelis half empty so that all the electrons have the same orbitalquantum number and only the spin degree of freedom remains.The ground state is predicted to be an extremelyrobust itinerant quantum Hall ferromagnet with a gap forspin excitations greatly exceeding the single particle Zeemanenergy. On the other hand, either side of ν = 1 thesystem depolarizes more rapidly than predicted by the singleparticle picture, due to the formation of spin textures(Skyrmions or anti-Skyrmions) in the ground state.wave) excitation spectrum. This prediction, shown by thedot-dashed line largely overestimates the robustness of theν = 1 quantum Hall ferromagnet. Intriguingly, the predictedspin polarization calculated for two spin levels separatedby the bare Zeeman energy, gµ B B ≃ 2.1 K, plottedas a dashed line coincides much better with our data. Ourresults suggest that the itinerant quantum Hall ferromagnetat ν = 1 is not robust, and collapses, whenever possible,to a lower energy ground state with a large number of reversedspins [Plochocka et al. Phys. Rev. Lett. 102, 126806(2009)].To measure the absorption spectrum of a single GaAs quantumwell (QW) at low temperatures we have used a structurewhich forms a half-cavity for the incoming light withthe QW located at the anti-node of the standing waveformed by the optical field. In figure 33(a) we plot the integratedintensity of the absorption to the n = 0 Landau levelfor both circular polarizations as a function of filling factor.For both polarizations (σ + and σ − ) the absorption is nonzero over almost all the range 0 < ν < 2. This implies thatthe upper Zeeman (UZ) and lower Zeeman (UZ) levels arealmost never fully occupied. However, for the absorptionto the lower Zeeman (LZ) level a sharp minimum (zero) atν = 1 is observed.The spin polarization of the 2DEG can be obtained directlyfrom the optical dichroism shown in figure 33(b). The measuredpolarization at T = 1.6 K is comparable with previousabsorption and reflectivity measurements, the spin polarizationsaturates at approximately 0.8 and the depolarizationon both sides of ν = 1 is roughly symmetric and compatiblewith the formation of spin textures (Skyrmions or anti-Skyrmions) in the ground state of size S ≈ A ≈ 3. At verylow temperature (T = 40 mK), the system does indeed fullypolarize within experimental error (∼ 0.97) at exactly fillingfactor ν = 1, and that this feature is extremely sharpwith a width of only δν ≈ 0.01 (figure.33(c)). The fully polarizedstate is a signature of the quantum Hall ferromagnetat filling factor ν = 1 while the sharp depolarization observedeither side of ν = 1 corresponds to ≈ 15 spin flipsper magnetic flux quanta added or removed from the system,consistent with the formation of a lower energy spintexture (skyrmion or anti-skyrmion) ground state either sideof ν = 1.It is extremely surprising that a temperature of a few hundredmK is already sufficient to suppress full spin polarization(figure.33(d)). The thermodynamics of the ν = 1quantum Hall ferromagnet should be governed by thermalactivation to the continuum of the spin-exciton (spinFigure 33: (a) Integrated intensity (I σ ±) of the absorption tothe n = 0 Landau level measured for both σ + and σ − polarizationsas a function of filling factor. (b) optical dichroism,(I σ − − I σ +)/(I σ − + I σ +) (spin polarization). The calculated depolarizationfor finite size skyrmions/anti-skyrmions (A=S=3) and(A=S=15) is shown. (c) spin polarization around filling factorν = 1 measured at T=40, 500 mK and 1.6 K. (d) detailed temperaturedependence of the spin polarization at exactly ν = 1 (thesolid line is drawn as a guide to the eye). Broken lines show thepredicted temperature dependence of the polarization for a spinwave excitation and for two spin levels separated by the singleparticle Zeeman energy.P. Plochocka, J. M. Schneider, D. K. Maude, M. PotemskiM. Rappaport, V. Umansky, I. Bar-Joseph (Weizmann Institute, Israel), J. G. Groshaus, Y. Gallais, A. Pinczuk(Columbia University, New York)27

TWO-DIMENSIONAL ELECTRON GAS 2009Dispersive line shape of the resistively detected NMR on either side of fillingfactor ν = 1The resistively detected nuclear magnetic resonance (RD-NMR) technique is a tool which allows the detection of theresonant excitation of nuclear spins using electrical measurement.This technique is especially well suited to probeNMR in high mobility two-dimensional electron gases inthe quantum Hall regime, over a wide range of filling factors.The underlying effect is the hyperfine interactionwhich couples the nuclear and electronic spins. At the resonantradio-frequency (RF) field, the nuclear spin magnetizationis reduced which, through the hyperfine interaction,leads to a change in the apparent magnetic field seen bythe electron spins. The resulting variation of the electronicZeeman energy leads to a change in the sample resistance.Early measurements performed on GaAs/AlGaAs heterostructuresrevealed anomalous RDNMR lines characterizedby a dispersive-like shape which occur on each sideof filling factor ν = 1 [Desrat et al. Phys. Rev. Lett. 88,256807 (2002)]. Such a line shape, composed of a negativeand a positive response of the resistance cannot be explainedby a simple uniform Zeeman energy change only.The origin of the anomalous line shape remains the subjectof debate. Two main ideas have arisen linking the line shapeto, (i)the contribution of domains with different electronicpolarizations or (ii) the role of thermal effects (heating).The latter was supported by the inversion of the RDNMRdispersive lineshape observed between ν = 1 and ν = 2/3which coincided with the change in sign of dR xx /dT .temperature increase leads to a negative (positive) ∆R xy onthe low (high) field side of ν = 1. For R xx a sign changeoccurs on each side of ν = 1. The magnetic fields forwhich ∆R xx has extrema are the fields previously introduced(B = 5.3, 5.525, 7.7 and 8 T) and labelled a, b, c and d,respectively. The comparison between the dispersive RD-NMR shapes and the sign of dR/dT shows that the minmax(max-min) shapes always correspond to positive (negative)dR/dT . This is verified for both the longitudinal andthe Hall resistance.This result strongly suggests that thermal effects play a rolein the anomalous RDNMR lineshape and that the Zeemanenergy change is not the unique contribution at the resonantRF field.Here we report similar measurements but extended to bothsides of filling factor ν = 1 and to both longitudinal and Hallresistances. The eight bottom graphs in figure34 representRDNMR lines measured either in R xx or in R xy at four differentincreasing magnetic fields, B = 5.3, 5.525, 7.7 and8 T, labeled a, b, c and d, respectively. Cases a and b areon the low field side of ν = 1 (i.e. between ν = 4/3 and1), while cases c and d are on the high field side, towardsν = 2/3. All spectra show dispersive-like shapes but someconsist of a resistance minimum followed by a maximumwith increasing RF, or inversely a maximum followed bya minimum with increasing RF. The min-max shape is observedin R xx for cases b and c and in R xy for cases c andd.Now we turn to the temperature dependence of the R xx (B)and R xy (B) curves traced in the top graph of figure34. Onesees that a slight increase of the temperature, from T = 60mK to T = 70 mK, leads to a narrower plateau in R xy anda narrower dissipationless region in R xx . The resulting resistancechanges ∆R xx and ∆R xy are plotted in the middlegraph by dashed and solid lines respectively. We see that aFigure 34: (Top) R xx and R xy as a function of B for two differenttemperatures T = 60 mK (solid) and T = 70 mK (dash). (Middle)Resistance change ∆R = R 70 mK − R 60 mK vs B for the longitudinal(dash) and Hall resistances (solid). (Bottom) RDNMR spectrameasured in R xx and R xy at the magnetic fields indicated above,respectively). All spectra are plotted over a range of 100 kHz.B.A. Piot, D.K. MaudeW. Desrat (GES-UM2, Montpellier), Z. R. Wasilewski (Institute of Microstructural Sciences, NRC, Ottawa)28

2009 TWO DIMENSIONAL ELECTRON GASSpin splitting enhancement of fully populated Landau levelsThe effect known as “g-factor enhancement” is the prominentmanifestation of electron-electron interactions in thestudy of a two-dimensional electron gas (2DEG) in theregime of the integer quantum Hall effect. This effect isroutinely apparent in magneto-transport experiments. Theyshow that thermal activation of charged carriers across theFermi energy located in between spin split Landau levels(2DEG at odd filling factors) is ruled by the gap which cansignificantly surpass the so called bare gap expected fromsingle particle band structure models. As far, the effect of g-factor enhancement (at odd filling factors) has been mostlyapparent in experiments which probed the vicinity of theFermi level. As illustrated in figure. 35, the motivation ofour work was to check whether g-factor is also enhancedfor spin split Landau levels located deep below the Fermilevel.We have probed such states with magneto-luminescenceexperiments performed on 2DEG confined in aCdTe/CdMgTe quantum well. Focusing on the results oflow temperature experiments (100 mK) we investigate theenergy positions of σ + and σ − luminescence peaks relatedto fully occupied electronic 0th Landau level. The difference∆ between energies of these peaks is clearly notmonotonic with the magnetic field in contrast to the lineardependence expected when neglecting the effects ofelectron-electron interactions. As can be deduced fromfigure 36, we find that the oscillatory component of ∆ isproportional to the spin polarization of the 2DEG. Notably,∆ = g e f f µ B B when spin polarization vanishes (at even fillingfactors). g e f f = 1.3 corresponds well to the combinedelectron-hole bare g-factor in our structure. Effects ofelectron-electron interactions are therefore concluded toaffect the spin splitting of fully occupied Landau levels locateddeep below the Fermi level and not only those in thevicinity of the Fermi energy.Figure 35: Scheme of Landau levels of a two-dimensional electrongas at even (6) and odd (5) filling factors. Electron-electroninteraction is known to enhance the spin splitting at the Fermienergy if Landau level filling factor is odd. Enhanced or notenhanced spin splitting of fully occupied levels, deep below theFermi level, can be probed with polarization resolved magneto-luminescenceexperiments.Figure 36: a,b) Luminescence spectra of a 2DEG confined ina 20 nm-wide modulation doped CdTe quantum well at two distinctmagnetic field corresponding to Landau level filling factor 5(a) and 4 (b). Note that spin splitting of the main peak is larger atsmaller field. c) Energy positions of the main σ + and σ − luminescencepeaks and their separation, ∆ as a function of the magneticfield. Note the increase of ∆ each time the Landau filling is odd.J. Kunc, K. Kowalik, F.J. Teran, P. Plochocka, D.K. Maude, M. PotemskiG. Karczewski, T. Wojtowicz (Institute of Physics, Polish Academy of Sciences, Warsaw, Poland)29

TWO-DIMENSIONAL ELECTRON GAS 2009Spin polarisation of a disordered GaAs 2D electron gas in a strong in-planemagnetic fieldAn in-plane magnetic field couples to the spins of a 2Dsystem, and can thus be used to probe the impact of manybody effects on the ground state of the 2D electron gas (2-DEG). For example, the longitudinale resistance under parallelmagnetic field generally exhibits a saturation or a kinkfor a magnetic field B = B p signaling the complete spin polarizationof the 2D system [T. Okamoto et al., Phys. Rev.Lett. 82, 3875 (1999)].present in disordered systems and should under no circumstancesbe interpreted as evidence for a ferromagnetic instability.Using high magnetic fields up to 32 T, we report on the particularlyrich parallel field physics occurring in 2D electrongas in GaAs, revealing an interplay between these spin effects,disorder, and orbital effects. The magneto resistancekink usually associated with the 2-DEG complete spin polarizationis observed up to B p ∼ 30 T, and shifts down continuouslyon more than 20 T as the electron density (andconsequently mobility) is lowered in the sample (see figure37(a). This reduction of B p with decreasing electrondensity is consistent with the predicted electron-electron interactionenhancement of the spin susceptibility at low density,favoring the 2-DEG polarization. However, the absolutevalue of B p remains 2-3 times smaller than the one calculatedat T = 0K for a disorder-free system, and extrapolatesto B p = 0 at a relatively “high” electron density, whereno ferromagnetic states have ever been observed. We arguethis behaviour is due to the localization of electrons whichare subsequently subtracted from the total free carrier density[B.A. Piot et al. Phys. Rev. B 80, 115337 (2009)]. Inthis situation the Fermi sea appears effectively smaller andless magnetic field is required to fully polarize the system.This approach is motivated by the strong mobility dropmeasured as the electron density is decreased in this veryregion. The temperature dependence of B p , which dependson the effective size of the Fermi sea, corresponds to thepolarization of a smaller Fermi sea, and indicates that thefraction of localized states increases as the electron densityis reduced. From this temperature behavior, a density of delocalizedelectron can be estimated, and used to re-plot theexperimental B p of figure 37(b) (triangles), giving a betterquantitative agreement with theory.In summary, the magnetic field at which complete spin polarizationoccurs is dramatically reduced both by electronlocalization and electron-electron interaction enhanced atlow carrier density. The large value of the extrapolatedcarrier density for full spin polarization at zero magneticfield simply reflects the large number of localized electronsFigure 37: (a) Magneto resistance for different electron densities.(b) Field associated with the complete spin polarization B pas a function of the electron density (circles and squares), and asa function of the “corrected” electron density (triangles). TheoreticalT = 0 K calculations of the magnetic field for full spin polarizationincluding many-body effects: Random Phase Approximation(RPA) [Y. Zhang and S. D. Sarma, Phys. Rev. Lett. 96,196602 (2006)], and Quantum Monte Carlo Calculation (QMC)[A. L. Subasi and B. Tanatar, Phys. Rev. B 78, 155304 (2008)].B.A. Piot, D. K. MaudeU. Gennser, A. Cavanna, D. Mailly (Laboratoire de Photonique et de Nanostructures, Marcoussis, France)30

2009 TWO-DIMENSIONAL ELECTRON GASHigh-order fractional microwave induced resistance oscillationsResonant transitions of electrons between Landau levels(LLs) lead to microwave-induced resistance oscillations(MIROs) whose period is governed by the ratio of the radiationfrequency ω to the cyclotron frequency ω c = eB/m ∗ ,where m ∗ is the effective mass of electrons. For high microwaveintensity, the resistance has resonant features associatedwith the fractional ratios ε = ω/ω c = n/m where nand m are integers (m ≥ 2). These fractional microwaveinducedresistance oscillations (FMIROs) have been observedup to ε=1/4 and explained by multiphoton absorptionprocesses which appear in the strongly nonlinear regime.The FMIROs are described by two theoretical models: asimultaneous absorption of several photons and a stepwisesingle-photon absorption. In both cases the FMIROs correspondingto larger fractional denominators m (higher-orderFMIROs) appear progressively with increasing MW power.In contrast to the previous studies with low electron densityand ultrahigh mobility, we investigate high-density electrongas with moderate mobilities. The samples are 14 nmwide GaAs quantum wells from different wafers. In this reportwe focus on the results for samples with a carrier concentrationn s = 7.8 × 10 11 cm −2 and mobility of 1.2 × 10 6cm 2 /Vs. Figure 38 first presents photoresistance for twochoosen frequencies at 1.4 and 6.5 K at a constant MWelectric field. We observed that FMIRO features are betterpronounced when they are not hindered by Shubnikov-deHaas (SdH) oscillations. Second, FMIROs up to denominatorm=8 are visible for low frequencies and all FMIROs arequenched above 85 GHz. Power dependence for FMIROsis close to linear behavior for low MW power and amplitudesaturates with increasing MW power [S. Wiedmann etal., Phys. Rev. B 80, 035317 (2009)].observation of high-order FMIROs, see figure 39. Therefore,high-order FMIROs require another explanation.Figure 38: (left) FMIROs for 75 and 66 GHz at temperatures1.4 and 6.5 K at a constant electric field of E=6 V/cm. (right)Frequency dependent photoresistance: for 90 GHz, all FMIROfeatures are quenched.Theoretical analysis is based on both competing FMIROmodels. The simultaneous absorption model can only accountfor ε = n/2, but is negligible small for high denominatorm. The stepwise absorption model has constraintsfor experiments of FMIROs. For the observations of commensurabilityresonances n/m associated with the singlestepwisetransitions between adjacent Landau levels (n=1)and in the regime of separated Landau levels [α

TWO-DIMENSIONAL ELECTRON GAS 2009Crossover between distinct mechanisms of microwave photoconductivity indouble quantum wellsDouble quantum wells (DQWs) or bilayer electron systemsexposed to microwave (MW) irradiation exhibit a peculiarphotoresistance which can be explained by an interferencebetween magneto-intersubband (MIS) oscillationsand microwave induced resistance oscillations (MIROs).The microscopic mechanism describing the MIROs at lowtemperatures T is associated with a MW-generated nonequilibriumoscillatory component of the electron distributionfunction. The corresponding contribution to magnetoresistanceis proportional to the inelastic scattering timeτ in due to electron-electron scattering and decreases withincreasing temperature according to τ in ∝ T −2 [Wiedmannet al., Phys. Rev. B 78, 121301(R) (2008)].For temperature dependent measurements we have choosenthe frequency range between 55 and 140 GHz and we focushere on measurements with a fixed frequency (85 GHz) at aconstant MW electric field E ω , see figure 40. For these temperatures,Shubnikov-de Haas (SdH) oscillations are completelysuppressed. The amplitude of enhanced MIS peaksis not saturated yet for this electric field to ensure that weare far away from the saturated regime. The estimated electricfield is E ω ≃ 2.0 V/cm. Figure 40 presents the analysisof experimental data for different temperatures up to8 K. The temperature-dependent quantum lifetime τ q andtransport time τ tr are extracted from dark magnetoresistancemeasurements if T e ≃ T . The comparison of experimentalmagnetoresistance, see model in [Wiedmann et al.,Phys. Rev. B 78, 121301(R) (2008)], at constant ω andE ω shows that the theoretical temperature dependence ofinelastic scattering time τ in ∝ Te−2 fits well up to 3.5 K. Assumingthat τ in = ε F /λ in Te 2 , where λ is a numerical constantof order unity, we can find λ in ≃ 1. Note that theelectron temperature T e is at least 2.8 K (for this choosenMW electric field) because of heating of electrons due tomicrowave irradiation (see also figure 41), but for the latticetemperatures T > 3 K the heating effect is negligible,T e ≃ T . By comparing experiment and theory, e.g. figure40(c) at T = 6.0 K for 0.1 T < B < 0.2 T, we see thatthe inverted MIS oscillations are not resolved in the theoreticalplot and we have to introduce an enhanced inelasticscattering time τ ∗ in with τ∗ in = 3.5τ in to fit the experimentalresults.The deviation starts at T c = 4 K, and for T c > 4 K we obtaina nearly constant τ ∗ in ≃ 190 ps [figure 41]. Similar saturationof τ in with increasing temperature at T ≃ 2 K has beenrecently observed in a one-subband system and attributed toa crossover to the displacement mechanism of photoresistance[Hatke et al., Phys. Rev. Lett. 102, 066804 (2009)].This explanation is not applicable to our samples becausethe quantum lifetime, according to our estimates, is stillmuch smaller than τ in at T e ∼ 4 K. Our theoretical model,based on inelastic mechanism of MW photoresistance, failsto describe the obtained oscillation picture for T e > 4 K andfurther theoretical studies of MW photoresistance, possiblyinvolving new mechanisms, are necessary.Figure 40: Temperature dependence at 85 GHz up to 8.0 K.Experimental traces (bottom, black) are fitted to the theoreticalmodel (top, red). With increasing temperature, an enhanced inelasticscattering time τ ∗ in (middle, blue) is necessary to fit experimentaltraces. SdH oscillations are suppressed for these temperatures.Figure 41: Inelastic scattering time τ ∗ in as a function of electrontemperature T e . Deviation from inelastic mechanism occurs forT e > 4 K, where T -dependence of τ in is saturated.S. Wiedmann, J.C. PortalG.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute of SemiconductorPhysics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk, Russia)32

2009 TWO-DIMENSIONAL ELECTRON GASEmergent fractional quantum Hall effect in a triple quantum wellFractional quantum Hall (FQH) effect in a two-dimensional(2D) electron gas is a consequence of the existence of incompressiblestates at certain fractional filling factors ν ofLandau levels. Bilayer and trilayer systems possess an extradegrees of freedom and this leads to the appearance ofnew FQH states which are not present in single layer systems.Such correlated states occur if the interlayer electronelectroninteraction, controlled by the ratio of layer separationto the magnetic length, is comparable to the intralayerinteraction. Correlated states have already been discoveredin bilayer systems. In trilayer systems (triple quantumwells, TQWs), correlated states should also exist in acertain interval of parameters determined by the interlayerseparation, electron density and the magnetic length. Experimentsin low-density TQWs have not revealed the statespredicted in [MacDonald, Surf. Sci. 229, 1 (1990)]. A furtheradvance in fractional quantum Hall physics is based onnew many-body ground states in multilayer electron systemswhich are different from already studied bilayer systems.In our experiments we use symmetrically doped GaAsTQWs with a 230 Å wide central well and equal 100 Åwide lateral wells coupled by tunneling. The total electronsheet density is n s = 9·10 11 cm −2 and the mobility is 4·10 5cm 2 /V s. Here we present results with a barrier thicknessof 20 Å. The estimated density in the central well is 30%smaller than in the side wells. Experiments have been carriedout in a diluation refrigerator at low temperatures downto T ≃ 50 mK.of the tunneling gap is expected. Now electron motion isconfined into a single layer and correlation of eletrons innearby layers can lead to new states. In fact, when the sampleis tilted to an angle Θ > 45 ◦ , three new minima occurin R xx and Hall resistance exhibits precursors of the correspondingquantized plateaus. In figure 42(b) we presentedthe situation where three new fractional states are developed.Within an accuracy of 2%, the plateau at B ⊥ ≃10.2 Tcorresponds to the filling factor ν=10/3. The same effectoccurs for filling factor ν=2 [Gusev et al., Phys. Rev. B 80,161302(R) (2009)].To summarize, the observation of the collapse of integerfilling factors ν=4 and ν=2 and the emergence of new FQHstates with increasing in-plane magnetic field can be attributedto new correlated states in a trilayer electron systembecause these states occur when the in-plane magneticfield suppresses tunneling and multilayer many-body correlationsbecome possible.In order to observe many-body correlations one have toincrease the localization of electrons by applying an inplanemagnetic field. This in-plane magnetic field adds anAharanov-Bohm phase to the tunneling amplitude whichcauses oscillations of the tunnel coupling between electronstates in the layers and a suppression of this coupling forlow Landau levels (LLs) [G.M. Gusev et al., Phys. Rev. B78, 155320 (2008)]. This effect, which is a single-particlephenomenon, can be seen in figure 42(a) in the plot of R xxin the tilt angle - perpendicular magnetic field plane for thesecond LL at filling factors ν=7, 9 and 10 which vanish andreappear with increasing tilt angle. In this sample, fractionalquantum Hall states also occur with filling factorsν=17/3, 16/3 and 8/3 up to 15 T.Now, we focus on integer filling factor ν=4 where a completesuppression of the resistance is observed at Θ ≃ 40 ◦whereas the state at ν=5 remains robust with increasingtilt angle. The corresponding parallel magnetic field correspondsto the situation where the exponential suppressionFigure 42: (a) Longitudinal magnetoresistance in the tilt angle–magentic field plane for a TQW with a barrier width of 2.0 nm.Minimum for ν=4 is suppressed and three new FQH states occur.(b) Longitudinal and Hall resistance at Θ = 0 ◦ (dashed line) andΘ = 49 ◦ (solid) for T =50 mK.S. Wiedmann, J.C. PortalG.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute of SemiconductorPhysics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk, Russia)33

TWO-DIMENSIONAL ELECTRON GAS 2009Reentrant fractional quantum Hall states in a triple quantum wellTriple quantum wells (TQWs) consist of three quantumwells separated by thin barriers and can be considered asthree parallel two-dimensional (2D) electron layers coupledby tunneling. The corresponding Landau level (LL)fan diagram for TQWs consists of spin-split LLs separatedby energy gaps which are described by the expressionω c (N + 1/2) ± ∆ Z /2 + E j , where ω c is the cyclotron energy,∆ Z the Zeeman energy, and E j ( j = 1,2,3) the energiesof quantization in the TQW potential. Within thetight-binding model [Hanna et al., Phys. Rev. B 53, 15981(1996)] these energies as well as the corresponding singleelectronwave functions can be estimated. An in-plane magneticfield adds an Aharonov-Bohm phase to the tunnelingamplitude which causes oscillations of the tunnel couplingbetween electron states in the layers and suppresses the tunnelcoupling for low LLs. Here, we investigate fractionalquantum Hall (FQH) around total filling factor ν=5/2.An interesting behavior is observed in the region betweenfilling factors 2 and 5/2, see figure 44. Several new plateausoccur which are absent for B ‖ =0 T. The exact origin of theemergent and reentrant plateaus at fractional filling factorsis still not clear. We believe that this phenomenon involvescorrelation of electron states in several (three) partially populatedsubbands. Further studies are needed to understandthe nature of FQH effect in TQWs.We have studied symmetrically doped GaAs TQWs with a230 Å wide central well and equal 100 Å wide lateral wells.The total electron sheet density is n s = 9 × 10 11 cm −2 andthe mobility is 4 × 10 5 cm 2 /V s. The estimated density inthe central well is 30% smaller than in the side wells. Allmeasurements have been carried out in a resistive magnetat a temperature of T ≃ 100 mK up to 34 T.Figure 43 presents our main observation: the FQH stateν =7/3 first disappears with increasing tilt angle, is then replacedby an emergent ν = 12/5 state and exhibits a reentrancefor Θ = 55 ◦ with a very wide plateau. We explainthis behavior by the influence of perpendicular and parallelmagnetic fields. The perpendicular field leads to a consecutivedepopulation of the subbands whereas the parallelcomponent is responsible for a decrease of the subbandgaps due to suppression of tunnel coupling. If tunnel couplingis present, we have always gaps between subbands.When tunnel coupling is cut off by the in-plane field, thedepopulation of the upper subband is accompanied by a decreaseof the separation between the upper and the lowersubbands, as a result of modification of the TQW potentialprofile owing to electron redistribution, until subbands startto overlap. The overlap effect is essential for total fillingfactors ν < 5/2. We estimate that the depletion of the uppersubband down to partial filling factor ν 3 = 1/3 correspondsto a strong overlap whereas the depletion to ν 3 = 2/5 correspondsto a weak overlap. This gives rise to a suppressionof ν = 7/3 and the emergence of a more favourable plateauat ν = 12/5 but the reentrance of ν = 7/3 with increasing tiltangle cannot be attributed within this model, and is possiblyrelated to enhancement of electron-electron correlations bythe parallel magnetic field [Gusev et al., Phys. Rev. B 80,161302(R) (2009)].Figure 43: Longitudinal and Hall resistance for Θ = 0 ◦ (dashed),Θ = 46.3 ◦ (red/gray) and Θ = 55 ◦ (blue/dark grey) present reentranceof the FQH state ν=7/3 and the appearance of the emergentFQH state ν=12/5.Figure 44: Hall resistance R xy as a function of B ⊥ at 100 mKfor different tilt angles. Plateau ν=7/3 fist dissappears and is replacedby ν= 12/5 with increasing tilt angle. For Θ = 49.5 ◦ , ν=7/3exhibits reentrant behavior. Several new FQH states occur.S. Wiedmann, J.C. PortalG.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute of SemiconductorPhysics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk, Russia)34

2009 TWO-DIMENSIONAL ELECTRON GASMagneto-intersubband oscillations in multilayer electron systemsTwo-dimensional (2D) electron systems with several occupiedsubbands exhibit magneto-intersubband (MIS) oscillationsdue to the modulation of intersubband scatteringprobability when Landau levels of different subbands arealigned. In contrast to Shubnikov de Haas (SdH) oscillations,the MIS oscillations are only weakly damped with increasingtemperature. Recently, MIS oscillations have beenobserved and investigated in double quantum well systemswhich consist of two quantum wells coupled by tunneling[Mamani et al., Phys. Rev. B 77, 205327 (2008)]. The finalstep is now a theory generalized to the multi-subband casewith N layers which is presented in this report, experimentallyverified in triple quantum wells (TQWs).We have studied symmetrically doped GaAs TQWs witha total electron sheet density n s = 9 × 10 11 cm −2 and mobilitiesof 5 × 10 5 cm 2 /V s. In a perpendicular magneticfield, Landau levels (LLs) pass sequentially the Fermi energyand the LL staircase associated with each subband issketched in figure 45(a) as well as a FFT analysis allowingus to extract the corresponding energy gaps. We havefound ∆ 12 = 4.0 meV, ∆ 13 = 5.4 meV and ∆ 23 = 1.4 meVfor a TQW with d b = 14 Å . The theory is generalized tothe multi-subband case [Wiedmann et al., to be publishedin Phys. Rev. B (2009)]. Here we present the MIS contributionto resistivity (second order quantum contribution).We use a simplified approximation that partial subband occupationsn j , transport scattering rates ν trj j ′ , and quantumlifetimes τ j are equal to each other (τ j = τ q ). Due to hightotal density and strong tunnel coupling, this approximationis reasonable for our system. In the regime of classicallystrong magnetic fields, we obtain the magnetoresistance forN=3 in the formρ d (B)ρ d (0) = 1 + 2 [3 d2 1 + 2 ( )3 cos 2π∆12ω c+ 2 ( )3 cos 2π∆13+ 2 ( )]ω c 3 cos 2π∆23, (5)ω cto electron-electron scattering which becomes essential forT >2.0 K. The result is in good agreement with experimentalobservations in single or bilayer systems, indicating thatthe sensitivity to electron-electron scattering is the fundamentalproperty of magnetoresistance oscillations originatingfrom second-order Dingle factor.Figure 45: Landau level staircase for a three-subband system andpicture of a TQW with three occupied subbands (1,2,3). (b) FFTspectra for the TQW with a barrier thickness of d b = 14 Å .where d = exp(−π/ω c τ q ) is the Dingle factor. This theoreticalmodel is in a good agreement with our observations andconfirms also the value of the energy gaps ∆ j j ′. The magnetoresistancein equation (5) is calculated assuming that thescattering potential is essential only in the side wells sincegrowth technology implies that most of the scatterers residein the outer barriers.In figure 46 we present temperature dependence of MISoscillations and the extracted averaged quantum lifetime.SdH oscillations are suppressed for T >4.2 K. The behaviorof τ q follows a T 2 -dependence and this is attributedFigure 46: (a) Temperature dependence of MIS oscillations and(b) extraction of averaged quantum lifetime as a function of temperaturefor samples with d b = 14 Å . For 4.2 K, SdH oscillationsare completely suppressed for B ≤0.6 T.S. Wiedmann, J.C. PortalN.C. Mamani, G.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute ofSemiconductor Physics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk,Russia)35

TWO-DIMENSIONAL ELECTRON GAS 2009Interference of fractional microwave induced resistance oscillations withmagneto-intersubband oscillations in a bilayer systemMicrowave induced resistance oscillations (MIRO) whichevolve into “zero resistance states” (ZRS) in high mobilitysamples and for a sufficiently high microwave power arecurrently both under experimental and theoretical investigation.They are governed by the ratio of the radiation frequencyω to the cyclotron frequency ω c = eB/m ∗ , wherem ∗ is the effective mass of the electrons. With increasingmicrowave (MW) power, fractional MIROs (FMIROs)are observed which can be described by ε = ω/ω c = n/mwith integer n and m and m ≥2. If a double quantum well(DQW) is exposed to microwave irradiation, photoresistancediffers from the single-subband case because of additionalintersubband scattering. MIS oscillations are increased/decreasedor show an inversion (peak flip) for lowfrequencies. The oscillating magnetoresistance is caused byan interference of the physical mechanisms responsible forthe MIS oscillations and conventional MIRO, strongly correlatedwith microwave frequency. It has also been foundthat the inelastic mechanism explains photoresistance measurementsin a two-subband system at low temperatures[Wiedmann et al., Phys. Rev. B 78, 121301(R) (2008)].In this contribution we report on experimental and theoreticalstudies of interference between MIS oscillations andFMIROs. In a certain temperature range (8 K

2009 TWO-DIMENSIONAL ELECTRON GASMicrowave photoconductivity in multilayer systems: triple quantum wellsTwo-dimensional (2D) systems exposed to microwave(MW) irradiation in the presence of a weak perpendicularmagnetic field have attracted much experimental andtheoretical interest over the past years. In systems withone occupied subband, the magnetoresistance exhibitsmicrowave-induced resistance oscillations (MIROs), whichare governed by the ratio of the radiaton frequency ω tothe cyclotron frequency ω c and evolve into “zero-resistancestates” for elevated MW intensity and ultrahigh mobility.MIROs have also been investigated in systems with two occupiedsubbands (double quantum wells, DQWs), wherethe magneto-intersubband (MIS) oscillations are observedin the absence of microwave irradition. The MIS resonancesoccur when different Landau levels of the two subbandsare sequentially aligned by the magnetic field. Themost interesting aspect of two-subband systems is the interferencebetween MIROs and MIS oscillations, which leadsto a peculiar magnetoresistance pattern [Wiedmann et al.,Phys. Rev. B 78, 121301(R) (2008)].We present in this contribution the interference of MIS andMIROs which is theoretically studied in multilayer electronsystems and experimentally confirmed in high-densitytriple quantum wells (TQWs). The dissipative resistivity inthe presence of MW irradiation is given byA ω = P ω(2πω/ω c )sin(2πω/ω c )1 + P ω sin 2 . (8)(πω/ω c )In figure 49 we compare symmetrically doped GaAs TQWswith a total electron sheet density of n s = 9 × 10 11 cm −2 , amobility of 5×10 5 cm 2 /V s with a central well width of 230Å and side wells widths of 100 Å to the theoretical model inequation (7). The barrier thickness is d b = 14 Å. The MWelectric field is slightly varied around 3.2 V/cm.The MIS oscillation picture in TQWs is more complicatedbecause of the presence of three subbands with energiesε j ( j = 1,2,3). The periodicity of the MIS oscillationsis determined by several subband separation energies∆ j j ′ = |ε j −ε j ′|. Therefore, if such a TQW is exposed to microwaveirradiation, MIS oscillations also change its oscillationpicture, correlated with the radiation frequency. Thetheoretical model generalized to the multi-subband case isin agreement with experimental results for N=3.ρ MWd = ρ d + ρ in + ρ di , (6)where ρ d is the dark resistivity. ρ in and ρ di are themicrowave-induced contributions due to inelastic [Dmitrievet al., Phys. Rev. B 71, 115316 (2005)] and displacementmechanism with the dominance of the inelastic contributionat low temperature because of τ in ∝ T −2 . In a simplifiedapproach and for the case of three layers (N=3), we assumethat the partial densities n j , transport scattering rates ν trj j ′ ,and Dingle factors d j = e −π/ω cτ j , where τ j is the quantumlifetime for subband j, are equal to each other (d j = d).Then, neglecting the SdH oscillations, one can write themagnetoresistance in the form:ρ d (B)ρ d (0) = 1 + 2 [3 (1 − A ω)d 2 1 + 2 ( )3 cos 2π∆12ω c+ 2 ( )3 cos 2π∆13+ 2 ( )]ω c 3 cos 2π∆23, (7)ω cwhich describes both the contribution of MIS oscillationsgoverned by the intersubband energy gaps ∆ j j ′ and modificationof the quantum part of the resistivity by the MWirradiation, given by a dimensionless factorFigure 49: Measured (upper panel) and calculated (lower panel)photoresistance of a TQW with a barrier thickness of 14 Å exposedto MW irradiation at a lattice temperature of 1.4 K. Thecuvres for 70, 110 and 170 GHz are shifted up for clarity.S. Wiedmann, J.C. PortalN.C. Mamani, G.M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O.E. Raichev (Institute ofSemiconductor Physics, NAS of Ukraine, Kiev, Ukraine), A.K. Bakarov (Institute of Semiconductor Physics, Novosibirsk,Russia)37

TWO-DIMENSIONAL ELECTRON GAS 2009Hole cyclotron resonance in a two-dimensional semimetalRecently a two-dimensional (2D) semimetal was discoveredin undoped HgTe quantum wells (QWs) with (013)surface orientation. This semimetal exists due to the featureof size quantization in (013) surface oriented HgTequantum wells with inverted band structure with a thicknessof thickness 18-21 nm. It was established that the energyspectrum of this system is determined by the overlap(about 10 meV) of the electron energy dispersion curve witha minimum in the center of the Brillouin zone and the valenceband with two maxima along [0-31] direction [Kvon,et al., JETP Lett. 62, 502 (2008)]. It was also shown thatin such QWs a simultaneous existence of 2D electrons andholes with densities of about 10 11 cm −2 and high mobilityof both 2D electrons µ n = (3 − 6) × 10 5 cm 2 /V·s and 2Dholes µ p = (3 − 10) × 10 4 cm 2 /V·s can be realized. So inthis report we present the results of the first experimentalstudy of hole cyclotron resonance (CR) in a 2D semimetal.Figure 50: Cyclotron resonance of holes: a, photoconductivityG ph vs magnetic field B for three different frequencies of microwaveradiation and b, photoconductivity peak position BCR h vsfrequency of microwaves.In the present work CR was studied in photoconductivitymeasurements. The samples were in Hall bar geometryfabricated on the basis of undoped (013)-grown 20 nmthick Cd x Hg 1−x Te/HgTe/Cd x Hg 1−x Te QWs with x = 0.75.Electron and hole densities (N s , P s ) and their mobility (µ n ,µ p ) were found from Hall data to have the following valuesin the samples studied: N s = (4 − 5) × 10 10 cm −2 andµ n = (5 − 6) × 10 5 cm 2 /V·s; P s = (1 − 1.3) × 10 11 cm −2and the mobility µ p = (3.5 − 6.6) × 10 4 cm 2 /V·s. TheseHall bars were exposed to microwave irradiation in the frequencyrange from 80 to 170 GHz. Photoconductivity G phwas measured using modulation technique and in magneticfields up to 6 T and at temperatures from 1.4 K to 4.2 K.Cyclotron resonance in the system studied arises becauseof the heating of 2D electrons or holes caused by a resonantincrease of the absorption of microwave radiation at photonenergies equal to the distance between Landau levels.So the dependencies of the photoconductivity on magneticfield G ph (B) should be a curve with a sequence of maximaand minima where the position depends on photon energyand type of carries. In the studied range of microwave frequenciesand magnetic fields, we resolved cyclotron resonancesfor holes only. CR of electrons was not observedbecause of a considerably smaller effective mass and CRwas situated too close to zero magnetic field. For observationof electrons CR one should use higher microwavefrequencies.The results of photoconductivity G ph measurements areshown in figure 50(a). One can see that measured signalG ph (B) is the wide resonance with a certain maximum atB = 0.83 T for a frequency of f = 120 GHz. The dependenceof the position this maximum on the frequency ispresented in figure 50(b). The hole effective mass foundfrom the positions of the CR peaks is m p = (0.19±0.01)m 0 .This is the first measurements of hole effective mass in 2Dsemimetal in HgTe QWs. It is interesting to compare measuredhole mass with electron mass m e = 0.025 × m 0 inthe HgTe metallic state [Kvon, et al., Physica E 40, 1885(2008)]. As expected, hole effective mass is ≈ 7 − 8 timesbigger than electron effective mass.S. Wiedmann, J.C. PortalD. A. Kozlov, Z. D. Kvon, N. N. Mikhailov, S. A. Dvoretsky (Institute of Semiconductor Physics, RAS, Novosibirsk,Russia)38

2009 TWO-DIMENSIONAL ELECTRON GASElectron-Phonon Interactions in a single modulation doped Ga 0.24 In 0.76 As/InPQuantum WellAbsolute magneto-transmission experiments, as a function decreases an additional interaction sets in (figure 51) andof the magnetic field B up to 13 T, have been performed clearly follows the expected interaction by polaronic effectson a series of single modulation doped Ga 0.24 In 0.76 As/InP with the LO modes. The relative strength of this interaction,Quantum Well (QW) with a width dw = 10 nm. The carrierconcentration N s is monitored by experimental condi-mimicked by Im(−1/ε phonon ), is displayed in figure 52.tions from (2 − 4.2) × 10 11 cm −2 with mobilities rangingfrom 10 5 cm 2 /Vs to 2×10 5 cm 2 /Vs respectively. In terms ofphonons the mixed compound Ga x In 1−x As is a two modesystem with two transverse optical (TO) phonons varyinglinearly with the x content between those of InAs and GaAswhereas the two longitudinal optical (LO) phonons are coupledby the macroscopic electric field. As such the “longitudinaloscillator strength” related to the Fröhlich interactionis partly transferred from the lower energy LO mode to thehigher one (Nash et al. Semicond. Sci. Technol. 2,329(1987)).The transmission spectra are simulated for different valuesof B with a multi-dielectric model (Bychkov et al. Phys.Rev. B 70,85306 (2004)). In the simulation process theFigure 51: Variation of δdielectric function of the doped QW is expressed as:0 with the energy (ω c ) for differentcarrier concentrations N s in units of 10 11 cm −2 .ω 2 p/dwε xx (ω,B) = ε phonon −ω[ω − (ω 0 − Re(Σ)) + ı(η + Im(Σ))]where ω 2 p, the square of the plasma frequency is a functionof N s . ω c = ω 0 −Re(Σ) is the observed cyclotron resonance(CR) frequency and δ 0 = η + Im(Σ) the effective dampingparameter. Besides the known dielectric parameters enteringinto the expression of phonons for all layers, the only fittingparameters for each value of B are ω c and δ 0 . In the absenceof any specific interaction, which will give rise to theself energy Σ(ω), the spectra are well simulated with ω 0 (B)which takes into account non-parabolicity and with thedamping parameter η reflecting the non-resonant interactionwith background impurities. Im(Σ(ω)) and Re(Σ(ω)) Figure 52: Fit of δ 0 − η (blue line) for data with N s = 4.2 × 10 11should be related by the Kramers-Krönig (KK) transformation.We focus this report on the results obtained forcm −2 (red stars). The Imaginary part of the ε phonon and−1/ε phonon are displayed as green and magenta lines respectively.the fitted values of δ 0 as a function of the energy ω c .These results are displayed in Figure 51 for a given sampleWhereas the simulation with the TO interaction is ratherand different carrier concentrations. Whereas δ 0 remainssmall for CR energies lower than the phonon energies oftrivial, that with the polaronic effects requires to fit the evolutionof δ 0 at energies much higher than the LO energiesGa 0.24 In 0.76 As, it increases noticeably when entering theenergy range of these phonons.(C. Faugeras et al. Phys. Rev. B 80, 073403 (2009))whichcorrespond to data obtained at higher fields.For the higher N s values, the data can be fitted withIm(ε phonon ) as shown in figure 52 if we increase the broadeningof phonons due to strain effects. This clearly demon-appears to be unique to identify and even quantify the dif-Nevertheless, without any further information, this systemstrates that the interaction occurs at the TO modes with ferent types of electron-phonon interactions occurring in athe mechanism of the deformation potential. When N s quasi-two dimensional electron gas.M. Orlita, C. Faugeras, G. MartinezS. Studenikin, P. Poole, G. Aers, Institute of Microstructural Sciences, NRC, Ottawa, Canada39

TWO-DIMENSIONAL ELECTRON GAS 2009Temperature effect on Coulomb pseudogap in electron tunneling betweenLandau-quantized two-dimensional gasesThe pseudogap is common effect for the tunnel structureswith two-dimensional layers. It is revealed as in high-T Csuperconductors so in semiconductor heterostructures. Experimentallya pseudogap is observed as a suppression ofthe tunnel current at the low bias voltage [Eisenstein etal., Phys. Rev. Lett. 69, 3804 (1992)] or an additionalhigh voltage shift of the resonant current peak in the I-Vcurve [Popov et al., JETP, 102, 677 (2006)]. In this caseno signature of the gap is revealed in the lateral electrontransport in the two-dimensional electron gas (2DEG). Theorigin of the pseudogap is still under investigation. For examplethere is an inhomogeneneous model [Fogler et al.,Phys. Rev. B, 54, 1853 (1996)], in which a 2DEG is segregatedon two phases with different integer filling factorsdue to the screened Coulomb interactions of the electrons.The electron spectrum is very different in the each phasedue to the different value of exchange enhancement of spinsplittingof the Landau levels (LL). Hence in the averagetunnel spectrum one should to observe two maxima correspondedto the spin-split LL. This model can explain theresonance splitting but not the high-voltage shift of the resonanceobserved in the tunnel junction between 2DEGs withdifferent electron concentrations. In other models the electrontunneling is considered as an instant event comparedwith the energy relaxation of the whole 2DEG. This meansthe tunneling electron should have some extra energy or energyof the pseudogap to organize its relaxation. Severaltypes of the collective relaxations had been considered suchas composite fermion scattering and magnetoroton excitations.The schematic conduction-band-bottom diagram of the tunneldiode is shown in the insert (a) in figure 53 with thequantum subband levels in the 2DEGs. The parametersof the 2DEGs are the following: the concentration of the2DEG with the level E 01 is n 1 = 4×10 11 cm −2 ; the concentrationof the 2DEG with level E 02 is n 2 = 6 × 10 11 cm −2 .The tunnel characteristics were measured at liquid 3 He temperaturesranged from 0.5 K up to 1.5 K. Current peakor the second derivative minimum corresponds to the firstcoherent resonance at the bias voltage V r = 6.5 mV, i.e.,E 01 (V r ) = E 02 (V r ). The second derivative maximum atV s = −14 mV associates with the second resonance, i.e.,E 01 (V s ) = E 12 (V s ). In the magnetic field directed perpendicularthe 2DEG planes the resonant features had beenshifted in voltage position (see square symbols showing positionsV r in figure 53). This voltage shift consists of twopart: first is the single-particle one that can be described inthe single-particle model (see sections of lines in figure 53)and the second part is the pseudogap shift ∆V . The pseudogapshift have been studied at temperature from 0.5 K up to10 K and unexpectable strong temperature dependence hasbeen revealed at the high magnetic field B > 12 T at lowtemperatures T < 2 K. This can be seen from insert (b) infigure 53 where the pseudogap shifts are shown by squaresymbols for T = 1.6 K and by circles for T = 0.5 K. It isinteresting to note that the temperature dependence takesplace at the fields when the cyclotron energy exceeds theintersubband one. This observation is accompanied by thedecreasing of the pseudogap growth (see insert (b) in figure53) and decreasing of the pseudogap shifts of the elasticreplicas. All this facts point on that the psedogap is forminginfluenced by intersubband plasmons. To date there isno theory considering such kind of effects.Figure 53: Topography map of the second current derivative asa function of a bias voltage and magnetic field. The experimentalvalues of the resonant voltage V r are shown as squares. Thevalues calculated in the single-particle model are shown as sectionsof lines. In the insert (a) the diagram of the conduction-bandbottom are shown with levels in the quantum wells. The diagramis shown for zero bias and zero energy corresponds to the Fermilevel. In the insert (b) the pseudogap shift ∆V is plotted versusmagnetic field as squares for T = 1.5 K and circles for T = 0.5 K.S. Wiedmann, J.-C. PortalV.G. Popov (Institute of Microelectronics Technology of RAS, Chernogolovka, Moscow district, Russia)40

2009Semiconductors and Nanostructures41

2009 SEMICONDUCTORS AND NANOSTRUCTURESOn the trigonal field acting at the Cr 3+ ( 2 E states) in RubyAs the magnetic field breaks the time-reversal symmetry,magneto-optical spectroscopy allows to unravel details intransition-metal ions electronic structure by producing efficientlifts of degeneracy. In particular, interaction betweenelectronic spin and orbital magnetic moments with an appliedmagnetic field B yield a splitting of electronic states,namely the well-known Zeeman effect.Representative results are shown in figure 54. In particular,the observed splitting in very high field above 50 T unveilsthe existence of two parameters ∆ 1 and ∆ 2 ; the deviation ofthe excited state Landé factor from the value of the groundstate factor g 0 (see figure 55). Under high pressure, thesecoefficients exhibit a significant linear increase that unveilsan unexpected increase of the trigonal field under pressure.The relative enhancement of the trigonal field matrix element< k| =< t 2g |V tr |t 2g > is 6.2 × 10 −3 /GPa.In fact there are two contributions to the pressure dependenceof the trigonal crystal-field energy. The first one isexplicit and positive and scales with the trigonal coordinateQ t , whereas the second term depends implicitly on the evolutionof the system sensitivity to the trigonal distortion i.e.on the changes of the coupling parameter with pressure at aconstant Q t value. Crystallographic data under pressure onpure Al 2 O 3 allows to assume that the crystal is homotheticallycontracted and therefore the distortion is reduced as itscales with the a lattice parameter.Figure 54: Typical magneto-photoluminescence spectrum at highpressure (5 GPa) and high field (56 T). Eight major peaks (A-H)are recorded. Logarithmic intensity scale enhances the visibilityof the four weaker peaks (a-h). (b) Zeeman energy splitting fanchart highlighting the two pairs of peaks (BC) and (FG) giving directaccess to ∆ 1 and ∆ 2 the intrinsic splitting factor of the excitedstates related to the trigonal field component.We have reported high-field magneto-optical measurementsunder high pressure on ruby single crystals up to 10 GPaand 56 T [Millot et al., Phys. Rev. B 78, 155125 (2008)].Figure 55: Zeeman splitting of ground state and first excitedstates of ruby. Observed emission lines are described by allowed(solid arrows) and forbidden (dashed arrows) dipolar absorptiontransitionThen the observed variation is consistent with the presentmodel and reflects the increase of the trigonal coupling parameterwith pressure, its dependence being proportional toR −n with n = 5. This value agrees with the exponent derivedfrom crystal-field theory but in the present case is obtainedempirically from magneto-optical spectroscopy, andconstitutes the first experimental evidence of a R −5 law.M. Millot, S. George and J.M. BrotoJ. Gonzalez and F. Gonzalez (DCITIMAC, Santander, Spain)43

SEMICONDUCTORS AND NANOSTRUCTURES 2009Mobility spectrum analysis in InN:MgThe mechanism of electrical conductivity and the possibleoccurrence of superconductivity in InN remains the subjectof considerable controversy. All recent Hall measurementsof InN samples doped with Mg or Be acceptors give a signfor the Hall voltage appropriate for electrons. This anomalousresult is presumably due to the presence of an n-typesurface inversion layer. One of the method used to detectthe expected presence of multi-carrier conduction in theInN system is the measurement of the resistivity tensor as afunction of magnetic field to perform a quantitative mobilityspectrum analysis. In the case of InN:Mg such an analysisshould in particular give the signs of the contributionsoriginating from electrons and holes if both are present.However, in the case of InN:Mg this is a challenging measurementsince the mobility of holes is probably very lowand comparable with that of the surface electrons. In addition,the samples are inhomogenous and unfortunately, theycan be measured only in the van der Pauw configurationbecause etching of InN samples to produce Hall bars stillposes a problem. All this makes it difficult to obtain preciseabsolute values of the ρ xx and ρ xy resistivity components,which is indispensable for a reliable mobility spectrumanalysis. Moreover, even for strictly n-type samplesthere often appears in the mobility spectrum the so called“ghost holes” which can lead to errors in the interpretationof the results. Thus, an appearance of a hole peak in themobility spectrum is not necessarily a reliable signature ofthe real presence of holes in a sample.peak is higher than electron peak revealed a positive signfor the thermopower). It also agrees very well with the conclusionthat only for moderate Mg content can free holesreally be detected in InN:Mg, while for higher doping theformation of donor-like defects (in agreement with amphotericdefects model) makes the sample progressively moreand more n-type.Figure 56: Magnetic field dependence of a resistivity tensor componentsρ xx and ρ xy of the E1053 sampleWe have performed a systematic investigation of 11InN:Mg samples with Mg-doping from 6 × 10 18 cm −3 to7.3 × 10 20 cm −3 together with 3 undoped (n-type) InN referencesamples. Our previous preliminary measurementswith two van der Pauw configurations and magnetic fieldup to 19 T showed hole peaks in mobility spectrum analysisbut it was problematic to find a reasonable correlationbetween the mobility spectra and the nominal Mg-doping.Our recent measurements show that only all possible permutationsof configurations of current and voltage probesand both polarities of the magnetic field up to 28 T givesreliable results. Figure.56 presents typical ρ xx and ρ xytraces as a function of the magnetic field. In figure 57 wepresent the results of mobility spectrum analysis for a seriesof samples with increasing Mg content (from 6 × 10 18to 4.2×10 20 cm −3 ) for the samples E1003 to E1041 respectively).The obtained series of spectra correlates well withthe measurements of thermoelectric power performed onthe same set of samples (only sample E1053 for which holeFigure 57: Mobility spectrum analysis of a series of InN:Mg sampleswith increasing Mg content (from E1053 to E1041, respectively).D.K. MaudeL. Dmowski (Institute of High Pressure Physics, Warsaw), L.Konczewicz (GES-UM2, Montpellier), M.Baj (Institute ofExperimental Physics, University of Warsaw)44

2009 SEMICONDUCTORS AND NANOSTRUCTURESCyclotron effective mass measurements in Indium NitrideAmong the group III-nitride materials, the electronic structureof InN is matter of continuing debate despite significantprogress during the last decade in understandingthe band structure, as the revision of its band gap energyfrom 1.8 − 2.1 eV to 0.7 eV. So far, band parameters aremostly derived from indirect methods of limited accuracysuch as infrared reflectivity measurements. For instance,the values of the effective mass remain scattered in a widerange from 0.044 m 0 to 0.093 m 0 . Moreover, a stronglyanisotropic electronic structure of the bulk crystal has beenclaimed to explain Shubnikov-de-Haas (SdH) oscillationsand magneto-optical properties. To date, the synthesis ofhighly pure InN single crystals remains a challenge and, inaddition, most measurements are affected by the presenceof an intrinsic low mobility surface and/or interface electronaccumulation layer that opens parallel conduction channels.an isotropic electron-LO phonon coupling constant in InN,α = 0.22, the polaron mass ism ∗ P = m∗ P= (1 + α/12)/(1 − α/12). (10)m∗ One finds a 4% correction that finally gives the bare massat the bottom of the conduction band equal to m ∗ 0 = 0.055±0.002 m 0 . The band parameter E p in the dispersion relation(1) thus becomes E p = 12 eV , with E g = 0.69 eV.We have recently performed the first measurement of thebulk electron cyclotron effective mass by Landau levelsspectroscopy: the most direct approach to measure effectivemasses. To derive the mass, we have used the temperaturedependence of the SdH oscillation amplitudes measuredunder magnetic field up to 60 T in the temperaturerange 2 − 70 K. A set of InN samples 1 µm thick havingHall concentrations 2 − 3 − 6 × 10 18 cm −3 has been investigated.We found an isotropic electron cyclotron effectivemass equal to 0.062 ± 0.002 m 0 but the highest doped sampleexhibits a puzzling anisotropy.In the present study, we find a single SdH series (see figure58) for magnetic field parallel to the c-axis in contrastwith [Inushima et al, Phys. Rev. B 72, 085210(2005)] where an additional SdH series at higher frequencyis reported for a sample with a similar Hall concentrationn H = 2.2 × 10 18 cm −3 . Our single series behaves like themain series of this previous study and keeps the same periodwhen the magnetic field is tilted towards a direction perpendicularto the c-axis. Consequently, this series accounts foran isotropic bulk Fermi surface as stated in this work. Onthe other hand since 2D-surface accumulation series are notevidenced, one may suggest that the surface electrons havea low mobility that broadens the 2D-Landau levels and/orinhomogeneous electron concentration causing Fermi levelfluctuations that washes out the oscillations.Taking into account non-parabolicity corrections the bottomband effective mass is[m ∗ 0 = m ∗ 1 − E ]F( m∗− 1) 2 , (9)E g m 0and takes the value m ∗ 0 = 0.057 m 0. Another correction tobe taken into account is the polaron contribution. AssumingFigure 58: (a) Resistance versus magnetic field for the three samplesS1, S2 (left scale) and S3 (right scale) measured at 2 K. (b)SdH oscillations versus reciprocal magnetic field obtained fromthe magnetoresistance curves; a parabolic back-ground contributionhas been subtracted and all curves have been shifted verticallyfor clarity. (c) Temperature dependence of the oscillation amplitudefor SdH peaks with N = 3 and N = 4 Landau level index(sample S1)To summarize, electron cyclotron effective mass of InN onc-sapphire substrate is obtained from the temperature dependenceof Shubnikov-de Haas oscillations. An isotropiccyclotron effective mass equal to 0.062 ± 0.002 m 0 is measuredfor samples having bulk electron concentration in therange 1 − 4 × 10 18 cm −3 . After non-parabolicity and polaroncorrections, the effective mass at the bottom of theband is found to be m ∗ 0 = 0.055 m 0 ± 0.002.J.M. Poumirol, M. Millot, M. Goiran and J. LeotinW. Walukiewicz (Lawrence Berkeley National Laboratory, Berkeley, USA) and I. Gherasoiu (RoseStreet Labs Energy,Phoenix, USA)45

SEMICONDUCTORS AND NANOSTRUCTURES 2009Oscillatory magneto-absorption under pressure in Indium SelenideIndium selenide is a layered semiconductor widely investigatedin the last decades for its potential optoelectronic applicationscharacterized by an anisotropic anomalous bandstructuredue to the balance between covalent and van derWaals bonds. Various experiments have revealed a puzzlingbehavior for the electron effective mass.Modern ab-initio band structure calculations by NAO-DFTcombined with optical measurements under high pressurehave explained this anomaly and have also unveiled newinteresting features such as the onset of a ring-shaped valenceband maximum above 2 GPa. In fact, a specific k.pmodel has recently been proposed for this class of layeredsemiconductors [Segura et al., Phys. Stat. Solidi B 235(2),267276 (2003)].Oscillatory magneto-absorption spectroscopy measurementsunder high pressure yield a very deep insight intothe unusual electronic properties of this compound. We obtainedfor the first time clear signature of the Landau quantizationin a wide energy range above the fundamental gap,giving access to the conduction band non-parabolicity (figures59 and 60). In addition, the high field regime for theexcitonic feature has been studied.The pressure dependence of the magneto-fingerprints unveilan increase of the reduced effective mass similar withthe evolution of the second gap transition, which confirmsthe main hypothesis of the specific k.p model. Moreover,the reentrant behavior of the pressure-quenched excitonpeak under high magnetic field (figure 61) unveils anunusual ring-shaped valence band maximum with a smallhole effective mass as suggested by previous transport experimentsunder pressure and ab-initio calculations.Figure 59: Oscillatory magneto-absorption spectroscopy colorplot of InSe measured at 4 K up to 53 T. The excitonic groundstate feature is clearly distinguishable around 1.34 eV as well asinterband Landau level transition which energy increases linearlywith the applied magnetic field.Using a toy model taking into account a linear Landau gapopening and the reported pressure dependence of the valenceband maxima and conduction band minima form previousoptical measurements under pressure we can estimatethe hole mass on the secondary maximum to be ∼ 0.03 m 0 .Figure 60: Interband Landau fan chart recorded in the diamondanvil cell.Figure 61: Reentrant behavior of the pressure-quenched excitonpeak under high magnetic field. This unveils an unusualring-shaped valence band maximum with a small hole effectivemass.M. Millot, S. George and J.M. BrotoA. Segura (ICMUV, Valencia, Spain)46

2009 SEMICONDUCTORS AND NANOSTRUCTURESMagnetoresistance in β-FeSi 2 on n-type Si substratesHall bars consisting of 30nm of FeSi were deposited by DCmagnetron sputtering at 30 W in a high vacuum onto nativeoxide Si (001) on to n-type phosphorous doped Si substrateswith 1-10 Ω resistivity. Post deposition the films were annealedfor 10 hours at 850 ◦ C . At these temperatures withthe breakdown of the native oxide, the film harvested theunderlying Si. X-ray diffractometry confirmed β-FeSi 2 asthe final stable phase upon the substrate. Once this wasconfirmed, Cu top electrodes were deposited, also definedusing shadow masking during sputtering. The resistance ofthe bilayer structure was measured using standard 4 wirelock-in techniques in magnetic fields, H, up to 22 T. Themagnetoresistance, MR=∆R/R, was then calculated fromthe following equation:A similarly large positive MR has been observed in borondoped Si [Schoonus et al., Phys. Rev. Lett., 100, 127202(2008)]. This effect has been attributed to autocatalytic impactionisation in the Si. They observe isotropic MR unlikethe MR in our system. All data displayed is in the transversefield (magnetic field perpendicular to the current direction),when the current and magnetic field direction concura smaller effect is measured.∆RRR(H) − R(0)= , (11)R(0)Figure 63: MR at the crossover between conduction through thefilm and substrate. Above 20 K an external magnetic field increasesthe substrate resistance until it becomes comparable to thefilm at which point the relatively smaller MR in the film is observed.At 14.4 K it is assumed that all conduction is through theβ-FeSi 2 and 11 % MR is measured.Figure 62: MR of the bilayer, the peak MR, arising from the Sioccurs at ∼45K. Below this temperature the β-FeSi 2 layer resistanceis lower than the Si and with the change in current path theMR tends to plateau.At high temperatures the resistance of the film far exceededthat of the substrate, therefore the majority of the currentshunted through the substrate. At these temperatures themagnetoresistance of the doped Si substrate is apparent.This large positive MR has recently observed in phosphorousdoped silicon [Delmo et al., Nature, 457, 1112 (2009)],explained in terms of breaking of the space charge effect.The extraordinarily large MR saturates as the substrate resistancecompares to that of the film. Figure 62 illustratesthe MR in the doped Si. This MR increases with decreasingtemperatures until the majority of the conduction is throughthe β-FeSi 2 film. At low temperatures (∼25 K) the Si substrateresistance exceeds the film and it can be assumed thatconduction is predominantly directed through the β-FeSi 2layer. This is demonstrated in figure 63, suggesting thecrossover between the two regimes. The large MR of theSi underlayer is increased by the external magnetic field.As this resistance exceeds begins to exceed that of the filmonly the MR inherent to the β-FeSi 2 layer is observed. Theexternal magnetic field creates a non linear Hall effect thatwas measured simultaneously to the MR. The non-linearityis most pronounced below 30K where the high magneticfields, and large MR of the doped Si causes the conductionpath to change.D. K. MaudeN. A. Porter, C. H. Marrows, (School of Physics and Astronomy, University of Leeds, Leeds, UK, LS2 9JT),47

SEMICONDUCTORS AND NANOSTRUCTURES 2009Two-dimensional weak localization in polycrystalline granular SnO 2 filmsThe phenomenon of quantum interference in disorderedconductors is well known and its effects on the electricalconductance are widely used to determine the inelasticscattering time of charge carriers, and thus, the mechanismsof inelastic scattering. Because of the prominenteffects of weak localization in 2D systems they became anobject of intensive studies. However, the reduced dimensionalityand the presence of disorder lead not only to enhancementof interference effects, but also to the necessityto take into account the electron-electron interaction. It appearedthat such characteristics as density of states, temperatureand magnetic field dependencies of electrical conductancecould be described only if one takes into accountthe effects of electron-electron interaction in a disorderedlow-dimensional system [Altshuler and Aronov, Modernproblems in condensed matter sciences Vol 10, Efros andPollak. ed., North Holland 1985]. Moreover, the dephasingmechanisms in weak localization are strongly relatedto interaction effects; inelastic electron-phonon scattering,quasi-elastic electron-electron scattering with small energytransfer. The interplay of interference and interaction effectsremains a puzzling problem that is still far from beingsolved.(2006)] where the weak localization model is extended beyondthe diffusion limit. In the frame of this model, onlysmall closed loops are believed to give contribution to theeffect of weak localization at such high magnetic fields, theminimum number of collisions in each loop being 3. Onecan suppose that relative to electron-electron interaction effects,these triangles should be easier to study rather thanany complicated electronic path with a large number of collision.As in many materials the dephasing in the weak localizationeffect was found to be due to electron-electroninteractions, the study of these materials in high magneticfields should provide important information about these interactions(single electron’s wave function interference isdestroyed, so the interaction effects should give the maincontribution to magnetoconductance).In our SnO 2 polycrystalline films the low transverse magneticfield dependence of the conductance is positive andcan be described in the frame of a 2D weak localizationmodel, the phase breaking mechanism being electronelectronscattering with small energy transfer.In figure 64, the dependence of the magnetoconductance onthe normalized magnetic field B/B ϕ , measured on the samesample, are presented over the full range of applied magneticfield up to 50 T. Here, B ϕ is the value of magnetic fieldat which the flux of magnetic field through an area enclosedby the electron’s paths becomes equal to the flux quantumh/2e. From both the inset, where the curves are presentedin linear coordinates, and from the main part of figure 64,one can see that at high fields the curves exhibit differentbehaviour and no longer overlap any more. We thus suggestthe necessity to take into account the anisotropy ofthe scattering potential in order to describe the observedresults. Different particularities of single scattering eventsmay have influence at different temperatures in this highfield region.The experimental curves in figure 64 resemble those derivedby Zduniak et al. [Phys. Rev. B 56, 1996 (1997)]and by Germanenko et al. [Phys. Rev. B 73, 233301Figure 64: The magnetic field dependencies of magnetoconductanceas a function of normalized magnetic field. Inset shows fielddependencies of magnetoresistance in linear coordinates measuredon the same sample.The analysis of high-field magnetoresistance data, obtainedon our samples of SnO 2 polycrystalline thin films, will provideanother one possibility to justify the mechanism ofelectron-electron interaction in disordered systems. The advantageof our samples is in highly controllable degree ofdisorder, which provides possibility to tune the range of occurrenceof quantum interference under applied magneticfields.T. A. Dauzhenka, J. GalibertV. K. Ksenevich (Belarus State University, Dept Phys. SC & Nanoelectronics, BY-Minsk), I.A. Bashmakov (BelarusState Univ. Research Institute of Physicochemical Problems, BY-Minsk)48

2009 SEMICONDUCTORS AND NANOSTRUCTURESMagnetoresistance mobility extraction in FinFET triple gate devicesWe have applied the geometric magnetoresistance (MR)technique to FinFET devices. The geometric µ MR mobilityis give by ρ(B) = ρ 0 (1 + µ 2 MR B2 ) where ρ 0 is the zeromagnetic field (B = 0) resistivity. The devices studied inthis work are triple-gate FinFET from IMEC in Belgium.The devices characteristics used are: oxide thickness of2 nm (EOT), fin height of 60 nm, channel width (W f in )of 10µm, the channel length (L) varies from 90 − 910 nm.The electrical characteristics were acquired using a semiconductorparameter analyzer (HP 4156A) measuring thedrain current-voltage I D (V G ) curves at different magneticfields, low temperatures and for low drain bias (linear region:V DS = 50 mV) in order to compute the µ MR . Thismethod works for transistor length much smaller than thewidth (typically W/L > 5).Another problem with the use of short devices at low temperaturesis the series resistance effect. Figure66(b) showsthat there is a reduction of the mobility with decreasingchannel length, while for long channels, the phonon scatteringis dominant. However, it is noted that the mobility increasesis attenuated for shorter devices, owing to the largeseries resistance, which masks the mobility enhancement atlow temperature.In order to obtain the µ MR , we plot the µ MR resistance ratioR(B)/R 0 for various values of V G as a function of B 2as shown in figure 65 for typical transistor with a channellength of 410 nm, fin width of 10µ m at T = 100 K. Theother devices measured present the same behaviour.From the slope of this curve, for each applied gate voltageV G , the mobility µ MR can be calculated. The mobility µ MR isplotted as a function of V G in figure 66(a). In order to makea comparison, we also plott the variations of effective mobilityµ e f f and field-effect mobility µ FE . These curves havebeen calculated by taking µ FE = gm/((W/L)C ox V D ) andµ e f f = √ µ 0 − µ FE at B = 0. The low electric-field mobilityµ 0 were extracted using Y (V G ) = I D / √ gm(V G ), whichallows us to eliminate the first-order mobility attenuationfactor and source-drain series resistance effects [Ghibaudo,Electron Lett. 24, 543 (1988)]. While here we present resultsfor one transistor at one temperature, for all the transistorsmeasured, at three different temperatures (77K, 100Kand 200K), same behaviour was observed.Figure 65: µ MR ratio R(B)/R 0 in function of square of magneticfield for different values of gate bias. The symbol line representsthe “intrinsic” mobility ratio µ 0 (0)/µ 0 (B) extracted from the Y–function. For L = 410 nm and T = 100 K.In figure 65 we plot the low-field mobility ratioµ 0 (0)/µ 0 (B) as a function of B 2 that was extracted by thestandard relation µ 0 (B) = µ 0 /(1+µ 2 0,MR B2 ). From the slopeof these lines it is possible to extract the low-field MR mobilityµ 0,MR (figure 66 (b) that is independent of both thegate bias and the magnetic field.Figure 66(b) shows the µ 0,MR and µ 0 for transistors as afunction of the channel length, for different temperatures.µ 0,MR is higher than µ 0 for all temperatures. As the temperaturedecreases there is an increase in the mobility dueto the reduced phonon scattering [Ohata, ESSDERC 2004,109 (2004)].Figure 66: a)Variations of µ MR mobility, effective mobility andfield effect mobility (corresponding to a zero magnetic field) versusgate voltage. For L = 410 nm and T = 100 K. b) Low-field mobilityµ 0 and low-field, MR mobility µ 0,MR versus channel lengthat different temperaturesD.K. MaudeC. D. G. dos Santos , J. A. Martino (EPUSP- Escola Politécnica da Universidade de São Paulo, São Paulo, Brazil), S.Cristoloveanu (IMEP, Grenoble, France)49

SEMICONDUCTORS AND NANOSTRUCTURES 2009Spin polarization of carriers in a GaAs/GaAlAs resonant tunneling diodeSpin polarized carrier systems are of interest particularlywith a view to polarized spin injection for giant magnetotoresistance(GMR) devices and spintronics applications.However, for various physical reasons, spin polarized injectionis in general difficult to achieve in real devices. Herewe show that it is possible to monitor the spin polarizationof carriers throughout a resonant tunneling diodes using polarizationresolved micro-photoluminescence.We have investigated the spin polarization of carriersin asymmetrical n-i-n GaAs/GaAlAs resonant tunnelingdiodes (RTD) by analyzing the polarized resolved photoluminescencefrom both GaAs quantum well (QW) and contactlayers under applied voltage and magnetic field. Forthe measurements a micro-photoluminescence setup with alow temperature x − y − z displacement stage was used tosimultaneously perform transport and photoluminescenceon single RTD etched mesa structure. Figure 67 shows aschematic band diagram of our device under forward biasvoltage, light excitation, and magnetic field parallel to thetunnel current. Under applied bias, the photo-generatedholes from the top contact can tunnel through the structureand recombine with electrons into the QW and contact layers.Figure 68 presents typical polarized resolved photoluminescencespectra at 0.195 V bias voltage. The higher energyemission peak (∼ 1.605 eV) is attributed to the fundamentalquantum well (QW) transition whereas the emissionaround 1.52 eV is due to various optical transitions in thecontact layers including the indirect recombination betweenfree holes and electrons in the two dimensional electron gas(h-2DEG) formed at the accumulation layer (∼ 1.512 eV).charge density, filling factors and spin polarization of thetwo-dimensional gas formed in the accumulation layer.Figure 67: Schematic band diagram of our device under forwardbias, light excitation and magnetic field parallel to the tunnel current.Figure 68: A representative photoluminescence spectra from theQW and contact layers measured at B = 19 T and T = 4.2 K.In a magnetic field the photoluminescence from the QWand contact layers shifts to higher energies and shows oscillationsin intensity (not shown). Figure 69 shows themagnetic field dependence of circular polarization degreefrom the QW, contact layers (total PL spectra) and fromthe h-2DEG emission. The polarization of the carriers,P = (I σ+ −I σ− )/(I σ+ +I σ− ), can be obtained from the integratedintensity of the polarization resolved photoluminescence.We have observed that the QW emission presentsnegative polarization degree up to 55% which oscillateswith magnetic field. A similar behavior is observed for theemission from the GaAs contact layers.To further understand this data we are currently in the processof modeling the spin polarization of carriers in thestructure taking into account the carrier tunneling, carrierFigure 69: Magnetic field dependence of spin polarization of theQW, GaAs contact layers and from the h-2DEG emissions.J. Kunc, M. Orlita, D. K. MaudeY. Galvão Gobato, L.F. dos Santos (Federal University of São Carlos, Brazil), M. Henini (School of Physics and Astronomy,University of Nottingham, UK)50

2009 SEMICONDUCTORS AND NANOSTRUCTURESSpectroscopy and optical manipulation of a single Mn spin in a CdTe-basedquantum dot in high magnetic fieldQuantum dots containing single magnetic ions have recentlyattracted significant interest as systems close to theultimate limit of information storage miniaturization. Anefficient optical read-out of the Mn spin state has beendemonstrated [Besombes et al., Phys. Rev. Lett. 93,207403 (2004)] as well as the writing and storing of theinformation using the Mn spin state. It has been shown thatit is possible to optically manipulate the single Mn spin byinjecting the spin-polarized excitons into the quantum dot,either by direct quasi-resonant excitation of the dot with circularlypolarized light [Le Gall et al., Phys. Rev. Lett. 102,127402 (2009)] or by using a spin-conserving transfer ofthe excitons between two coupled quantum dots [Goryca etal., Phys. Rev. Lett. 103, 087401 (2009)].We have applied the micro-photoluminescence measurementsto directly probe states of a single Mn atom embeddedin a CdTe quantum dot in high magnetic fields.The sample was placed in a micro-photoluminescence setupconsisting of precise piezo-electric three-dimensional stageand a microscope objective. The micro-photoluminescencesystem was kept at the temperature of 4.2 K in a cryostatplaced in a resistive magnet producing magnetic field up to28 T. The field was applied parallel to the the growth axisof the sample (Faraday configuration).Figure 70: Color-scale plot of the photoluminescence intensityof a single Mn-doped QD as a function of emission energy andmagnetic field. The branches of emission lines can be assigned tobright (X) and dark (DX) exciton transitions.The photoluminescence of the QD’s was excited by circularlypolarized tunable dye laser in the range 570−610 nm.The excitation beam and the collected PL signal werepassed though a mono-mode fiber connected directly to themicroscope objective. Optical spectra have been recordedfor both circular polarization of light.An example of the PL measurement of a neutral excitonspectrum of a single Mn-doped QD as a function of themagnetic field is shown in figure 70. It reveals not only detailsof the optical transitions of the so called bright excitons(excitons with J = 1), but also transitions of dark excitons(J = 2) are clearly visible (5 lines in the low energy partof the spectra). This is due to the mixing of electron spinstates by electron-Manganese exchange interaction and opticalorientation of the spin of the Mn ion.Calculated excitonic transitions in a dot containing a singleMn ion is shown in figure 71. It is clearly visible that inthe case of dark excitons we should indeed observe only 5lines, in contrast to bright excitons (two upper branches oflines in figure 71), where 6 strong lines should be visiblein each circular polarization for detection. The number ofthese lines reflects the 6 possible spin projections of the Mnion (S = 5/2) onto the quantization axis.Figure 71: Calculated optical transitions of a single Mn-dopedQD. Line width reflects the oscillator strength, colors indicate circularpolarization of the emitted light (red - σ + , blue - σ − )M. Goryca, P. Plochocka, P. Kossacki, M. PotemskiP. Wojnar (Institute of Physics, Polish Academy of Sciences, Warsaw), J. A. Gaj (Institute of Experimental Physics,University of Warsaw)51

SEMICONDUCTORS AND NANOSTRUCTURES 2009InP/GaP self-assembled quantum dots under extreme conditionsSemiconductor quantum dots are promising for the fabricationof novel optoelectronic devices and a number of investigationshave been carried out to understand the electronicand optical properties of quantum dot systems. One powerfulmethod toward understanding the electronic states andshell structure is to investigate photoluminescence (PL) inmagnetic fields.5 nm is inferred, which is smaller than the radius of thequantum dots of about 10 nm. Thus, the measurement providesfurther evidence that the electrons belong to the InPvalleys and suggests a type-I band alignment for InP/GaPquantum dots [Dewitz et al., Appl. Phys. Lett. 95, 151105(2009)].The quantum dot material systems that have been the mostextensively investigated using magnetoluminescence areInGaAs/GaAs and InP/InGaP quantum dots. The InP/GaPquantum dot material system has received significantly lessattention. It is very interesting, however, for both a fundamentalunderstanding of quantum dots based on directindirectband gap semiconductors, as well as its potentialapplications for visible light emitters.Figure 73: Pressure dependence of the magneto-photoluminescencededuced values of the confinement length l 0 (squares) andthe exciton effective Bohr radius a ∗ (triangles) within the quantumdots. This reveals an increase of the quantum confinement in theplane of the dots.Figure 72: Pressure dependence of the energy of the two photoluminescencelines up to 1.2 GPa. These two lines correspond tothe direct radiative recombination between the ground and the firstexcited states of the dots. An unusually low pressure coefficientis measured (indicated by the slopes of the solid lines, which arelinear fits to the data).We have first investigated the radiative recombination inInP/GaP self-assembled quantum dots measured in highmagnetic fields. Using magneto-photoluminescence spectroscopythe reduced effective mass is determined to havea value of 0.094 m 0 expected for electron states associatedwith the InP valley. Using the determined effective massand the diamagnetic shift in the photoluminescence peakat low magnetic fields, an average exciton radius of aboutIn addition, we have also performed cryogenichigh-pressure photoluminescence and magnetophotoluminescenceexperiments in the 1 GPa range, inseveral cycles. As previously reported [Goñi et al., Phys.Rev. B 67, 075306 (2003)], the luminescence is quenchedat 1.2 GPa, and the intensity decreases continuously. However,our experiments do not show any signature of theΓ → X crossover reported to occur between ambient pressureand 0.3 GPa. We observe in fact a linear and continuousincrease of both the ground state and the first excitedexcitonic state photoluminescence features up to 1.2 GPawhere the emission is suppressed. A very low pressure coefficientis measured which may originate from a decreaseof the quantum confinement along z driven by a strong increaseof the effective mass. On the other hand, in the planeof the dots, an increase of the quantum confinement is evidenced[Millot et al., accepted in High Pressure Research].M. Millot, S. George, J. Leotin and J.M. BrotoC.v. Dewitz, F. Hatami and W.T. Masselink (Humboldt-Universitt, Berlin, Germany) and J. Gonzalez (DCITIMAC,Santander, Spain)52

2009Metals, Superconductors and StronglyCorrelated Systems53

2009 METALS, SUPERCONDUCTORS...Thermo-electric study of Fermi surface reconstruction in YBa 2 Cu 3 O yThe Seebeck and Nernst coefficients S and ν of the high-T c superconductor YBa 2 Cu 3 O y (YBCO) were measured ina single crystal with a hole concentration p = 0.12 in magneticfields up to H = 28 T. For temperatures down to 9 K, νbecomes independent of field by H ≃ 30 T, showing that bythen the Nernst signal due to superconducting fluctuationshas become negligible. In this field-induced normal state,S/T and ν/T are both large and negative in the T → 0 limit.The magnitude of S/T is consistent with the small Fermisurface pocket previously detected via quantum oscillationsin YBCO at a similar doping and its negative sign confirmsthat the pocket is electron-like. For more information see[J. Chang et al., arXiv:0907.5039].Our Nernst measurements in YBCO reveal that the quasiparticlecontribution in cuprates can be as large as the vortexcontribution, on which most of the attention has been focuseduntil now, see for example [Yayu Wang et al., Phys.Rev. B 73, 024510 (2006)]. We expect this quasiparticlecontribution, which can be of either sign, to dominate theNernst signal well above T c , as found in the hole-dopedcuprate Eu-LSCO [O. Cyr-Choiniere et al., Nature 458, 743(2009)] and the electron-doped cuprate Pr 2−x Ce x CuO 4 [P.Li and R.L. Greene, Phys. Rev. B 76, 174512 (2007)],where quasiparticle and vortex signals have also been disentangled.The sample was an uncut, unpolished, detwinned crystal ofYBCO grown, at UBC in Vancouver (Canada), in a nonreactiveBaZrO 3 crucible from high-purity starting materials.The dopant oxygen atoms (y = 6.67) were made to orderinto an ortho-VIII superstructure, yielding a superconductingtransition temperature T c = 66.0 K. Transport propertieswere measured via gold evaporated contacts (resistance< 1 Ω), in a six-contact geometry. The thermal gradient∆T was applied along the a-axis and the field H along thec-axis. For a detailed describtion of the experimental setupsee [O. Cyr-Choiniere et al., Nature 458, 743 (2009)].The Nernst and Seebeck coefficients are plotted as a functionof magnetic field in Fig. 1. At T >80 K, the Seebeckcoefficient S is essentially field independent. At T

METALS, SUPERCONDUCTORS... 2009Anomalous criticality in the electrical resistivity of a high-T c cuprateAn important theme in strongly correlated electron systemsis quantum criticality and the associated quantum phasetransitions that occur at zero temperature upon tuning anon-thermal control parameter g (e.g. pressure, magneticfield H or composition) through a critical value g c . Onefeature of such a system is the influence that critical fluctuationshave on the physical properties over a wide region inthe (T , g) phase diagram above the quantum critical point(QCP), inside which the system shows marked deviationsfrom conventional Landau Fermi-liquid behaviour. A numberof candidate non-Fermi-liquid systems have emerged,particularly in the heavy-fermion family, though there areothers, e.g. certain transition metal-oxides, that displaysimilar characteristics.The physics of copper-oxide high temperature superconductorsmay also be governed by proximity to a QCP.The generic temperature-doping (T , p) phase diagram resemblesthat seen in the heavy-fermions, with an apparentfunnel-shaped region that either pierces or skirts the superconductingdome. Above this region, cuprates display anin-plane resistivity ρ ab that varies linearly with temperatureover a wide temperature yet narrow doping range. ThisT -linear resistivity has been widely interpreted, in tandemwith other anomalous transport properties, as a manifestationof scale-invariant physics borne out of proximity to theQCP. This viewpoint has remained untested, largely due tothe high upper critical field H c2 values in hole-doped high-T c cuprates that restrict access to the important limiting lowtemperatureregion below T c (p).In our experiment, we employed a combination of persistentand pulsed high magnetic fields to expose the normalstate of La 2−x Sr x CuO 4 (LSCO) over a wide dopingand temperature range and studied the evolution of ρ ab (T )with carrier density, from the slightly underdoped (p =0.15) to the heavily overdoped (p = 0.33) region of thephase diagram. Our measurements revealed the preservationof the T -linear resistivity in all superconducting samplesto temperatures as low as 1.5 K [Cooper et al., Science323, 603 (2009)]. Indeed, for all dopings, the ρ ab (T )curves for T < 200 K could be fitted either to the expressionρ ab (T ) = α 0 + α 1 T + α 2 T 2 or to a parallel-resistorformalism1/ρ ab (T ) = 1/[α 0 + α 1 T + α 2 T 2 ] + 1/ρ max withρ max (= 900 ± 100µΩcm). The inclusion of ρ max helps toaccount smoothly for the escalation of ρ ab (T ) to highertemperatures and makes the values of α 1 and α 2 insensitiveto the temperature range of fitting.Figure 75: Doping evolution of the temperature-dependent coefficientsof ρ ab (T ). (A) Doping dependence of α 1 , the coefficientof the T -linear resistivity component. (B) Doping dependenceof α 2 , the coefficient of the T 2 resistivity component. Inboth panels, solid squares are coefficients obtained from least--square fits of the ρ ab (T ) curves for T < 200 K to the expressionρ ab (T ) = α 0 + α 1 T + α 2 T 2 , whilst the filled circles are obtainedfrom fits over the same temperature range to a parallel-resistor-formalism1/ρ ab (T ) = 1/[α 0 +α 1 T +α 2 T 2 ]+1/ρ max withρ max = 900± 100 µΩcm. The open symbols are obtained fromcorresponding fits made to the ρ ab (T ) data of [Ando et al., Phys.Rev. Lett. 93, 267004 (2004)] between 70 K and 200 K. Thedashed lines are guides to the eye.As shown in the top panel of figure 75, for both sets ofdata and analysis, α 1 is seen to grow rapidly with decreasingp, attaining a maximum value of around 1 µΩcm/K atp c = 0.185 ± 0.005. The preservation of T -linear resistivityin LSCO over such a wide doping range is wholly unexpectedand contrasts markedly with what is observed inother candidate quantum critical systems. Our analysis alsoreveals (figure 75) that the magnitude of the T -linear termscales monotonically with T c on the strongly overdopedside but saturates, or is maximal, at a critical doping levelp crit ∼ 0.19 at which superconductivity itself is most robust.The observation of a singular doping concentration inLSCO close to p = 0.19 at which a bulk transport propertyundergoes a fundamental change at low T lends support toearlier thermodynamic studies that showed the pseudogaptemperature T ∗ or energy scale ∆ g vanishes inside the superconductingdome, rather than at its apex. The magnitudeof α 1 at p = p crit , coupled with other features of the data,suggests that the opening of the pseudogap coincides withthe loss of quasiparticle coherence for states near the zoneboundaries.B. Vignolle, C. ProustR. A. Cooper, Y. Wang, N. E. Hussey (University of Bristol, Bristol, UK)56

2009 METALS, SUPERCONDUCTORS...Magnetic field dependence of the superconducting energy gap inBi 2 Sr 2 CaCu 2 O 8+δIn a conventional BCS materials superconductivity is suppressedby a magnetic field since the superconducting energygap 2∆ decreases continuously with increasing fieldand vanishes at a critical field H c2 . The reported behaviorof 2∆ in high-T c cuprates studied by a tunnelingmethod ranges from almost magnetic field independentin hole-doped cuprates to a strong H-dependence inelectron-doped cuprates. Interlayer tunneling spectroscopyof Bi 2 Sr 2 CaCu 2 O 8+d (Bi2212) shows that a peak in thetunneling conductance dI/dV associated with 2∆ broadensand shifts towards higher voltages with increasing magneticfield. Such a behavior of 2∆ has never been observed inBCS superconductors. The superconducting gap in optimallydoped and overdoped Bi2212 is close in size to thepseudogap questioning if the gap of the superconductingstate is detected at all in the tunneling measurement. Recentangle-resolved photoemission spectroscopy (ARPES)studies of Bi2212 showed that the spectral gap in the nodaland antinodal regions of momentum space has a distinctlydifferent temperature dependence [Lee et al. Nature 450, 81(2007)]. This suggests that the magnetic field dependenceof the energy gap at different points of the Fermi surfaceshould also very different. Most experiments are carriedout in the c-axis tunneling configuration where the tunnelingsamples an angular average over the ab-plane density ofstates. In the case of ab-plane tunneling, the tunneling occursalong the CuO 2 planes and the shape of spectra can bequite different depending on the tunneling direction. Wehave performed ab-plane tunneling experiments on threeslightly underdoped Bi2212 single crystals (T c =84 K and∆T c = 1.5 K) using break junctions in magnetic fields upH = 20 T. Mechanically retuning the break junction repeatedlyin liquid helium, we were able to fabricate a large numberof tunnel junctions at different places along the initialbreak of the crystal where tunneling occurs in the ab-plane.Figure 76(a) displays the tunneling conductances dI/dV asa function of the bias voltage V . All curves have a shapetypical for SIS tunnel junctions with sharp peaks in the conductanceat the voltage ±V = 2∆/e. 2∆ is 45 meV at 4.2 Kwhich, is reasonable when compared with our previous results.In contrast to previous measurements on Bi2212,where the position of the gap peak remained almost unchangedin an applied magnetic field, the superconductingconductance peaks in figure 76(a) decrease in magnitudeand shifts to lower voltages with increasing magnetic field.The only possible interpretation of such a behavior of thetunnel spectra is the suppression of 2∆ by magnetic field.To our knowledge, this is the first unambiguous observationof the suppression of the superconducting gap by magneticfield in tunneling investigations of the Bi2212 system.In figure 76(b), we present the magnetic field dependenceof 2∆ using a semilog scale for the sample No. 1 extractedfrom the gap spacing in the dI/dV curves in figure76 (a).Data for break junctions No. 2 and No. 3 formed on twoother Bi2212 single crystals are also included to demonstratethe reproducibility of the field dependence of 2∆.This figure suggests that 2∆ shows a logarithmic decreasewith magnetic field (dashed lines), although we cannot excludeand exponential dependence which also reproducesthe data reasonably well (not shown).Figure 76: (a) Tunneling conductances dI/dV as a function ofthe bias voltage V for a tunnel break junction measured at variousmagnetic fields applied parallel to the c axis of the crystal (sampleNo. 1). All dI/dV curves except the lowest one are offset verticallyfor clarity. (b) The magnetic field dependence of 2∆ plottedusing a semilog scale for the three break junctions formed on eachof the three Bi2212 single crystals.B. A. Piot, D. K. MaudeS. I. Vedeneev (P.N. Lebedev Physical Institute, Russian Academy of Science, Moscow, Russia)57

METALS, SUPERCONDUCTORS... 2009Magnetoresistance anisotropy and AMRO in the electron dopedsuperconducting cuprate Nd 2−x Ce x CuO 4Recent discoveries of magnetic quantum oscillations inhole- and electron-doped cuprate superconductors clearlydemonstrate the importance of high-field magnetotransportexperiments for exploring the Fermi surface inthese materials. Besides quantum oscillations, semiclassicalangle-dependent magnetoresistance oscillations(AMRO) are known to be a very efficient method forFermi surface studies of layered systems, such as organicconductors [Kartsovnik, Chem . Rev. 104, 5737(2004)]. AMRO have also been observed in hole-overdopedTl 2 Ba 2 CuO 6+δ [Hussey et al., Nature 425, 814 (2003)].Aiming to find AMRO in the electron-doped superconductorNd 2−x Ce x CuO 4 (NCCO), we have performed detailedstudies of the angular dependence of its interlayer magnetoresistance.The experiments were performed on crystalswith doping levels in the range 0.13 ≤ x ≤ 0.17, in a 28 Tresistive magnet. The samples were mounted on a homemadetwo-axes rotating stage allowing an in situ rotation ata fixed B, at temperatures down to 1.4 K. The interlayer resistanceR c was measured as a function of angle θ betweenthe field direction and [001] axis of the crystal, at differentfixed angles ϕ, see inset in Fig. 77(a).0.17. Hence, the relevant Fermi surfaces should be identicalfor all these compositions. On the other hand, ShubnikovdeHaas data [Helm et al., Phys. Rev. Lett. 103,157002(2009)] suggest a reconstruction of the Fermi surface to occurbetween x = 0.16 and 0.17. The apparent discrepancy isresolved by taking into account a possible magnetic breakdownbetween the hole- and electron-like parts of the reconstructedFermi surface. In this case, one should consider apossibility that the Fermi surface is reconstructed over theentire superconducting doping range. Further experimentson AMRO and magnetoquantum oscillations at fields above30 T are essential to clarify the details of the Fermi surfaceevolution in overdoped NCCO and its implications for superconductivity.As shown in figure 77(a), underdoped NCCO crystals displayan anomalous dome-like shape of R c (θ), which is mostlikely associated with spin dependent scattering in a magneticallyordered system. Notably, this behavior is observedeven for superconducting compositions, up to the optimaldoping level. The presence of ordered spins readjusted atchanging the magnetic field orientation is manifested in ahysteresis between up and down θ-sweeps.At increasing Ce concentration to the optimal and, further,to the overdoped regime, the anomalous contribution tomagnetoresistance weakens, giving way to the conventionalmechanism associated with the orbital effect of magneticfield on charge carriers. This gives rise to the positive slopeof the angular dependence, dρ c /d|θ| > 0 in an extendedangular range, 30 ◦ < |θ| 80 ◦ , as shown in Fig. 77(b) fora sample with x = 0.165. The most interesting feature inthis range is a shallow hump superposed on the monotonicslope around ±52 ◦ . The same feature has been found onsamples with x = 0.16 and 0.17 (the latter being the highestaccessible doping level for NCCO). The position of thehump stays constant at changing temperature or magneticfield strength. Such a behavior is characteristic of AMRO.At this stage, a rigorous quantitative analysis of the AMROfeatures is difficult due to their small amplitude. Nevertheless,already from the existing data one can draw an importantqualitative conclusion. The positions of the AMROturn out to be the same, at all ϕs, for x = 0.16,0.165, andFigure 77: Angle-dependent interlayer magnetoresistance ofNd 2−x Ce x CuO 4 at B = 28 T, T = 1.4 K. (a) Underdoped sample,x = 0.13, exhibits an anomalous dome-like shape with a hysteresisbetween up and down θ-sweeps. At |θ| > 70 ◦ the resistancesharply drops due to the onset of superconductivity. Inset showsthe geometry of the experiment. (b) For the overdoped sample,x = 0.165, a conventional magnetoresistance dominates at least at30 ◦ < |θ| < 70 ◦ . Arrows point to AMRO features whose positionsdepend on azimuthal angle ϕ.I. SheikinM. V. Kartsovnik and T. Helm (Walther-Meissner-Institut, Garching, Germany)58

2009 METALS, SUPERCONDUCTORS...Transport measurements of H c2 and its anisotropy in FeSe 1−x Te x single crystalsExploring the temperature dependence of H c2 and itsanisotropy in the new Fe superconductors is an importantexperimental tool to help revealing the mechanism of superconductivity.For example in LaFeAsO 0.89 F 0.11 , the upturnin curvature for H c2 was observed, which suggeststwo-band superconductivity [Hunte et al, Nature 453, 903(2008)]. The high transition temperatures imply high valuesof H c2 , meaning that it is not easy to obtain the full curvedown to low temperature, and many low temperature valuesof H c2 are in fact extrapolations. However, in order to comparethe real behavior to theoretical models, measurementsdown to low temperature, and thus at very high field, arenecessary.A serious limitation at present is that the homogeneity ofmany samples that are studied is highly questionable, andthe highest T c values are obtained on doped, probably inhomogeneous,systems. From this aspect the recent discoveryof superconductivity in FeSe and FeTe 1−x Se x suggestsa promising new direction as these compounds are close tobeing stoichiometric and have a relatively simple structure.We have recently grown single crystals of FeSe [Braithwaiteet al, J. Phys.: Condens. Matter 21, 232202 (2009)],which so far are not homogeneous, and FeTe 1−x Se x whichshow homogeneous bulk superconductivity, with T c around14 K, characterized by specific heat measurements (figure78), and seem suitable for more detailed studies.In the LNCMI-Grenoble, we have measured the anisotropyof H c2 on our FeTe 1−x Se x single crystals up to 28 T. Thisgives a very precise determination of a large part of thecurve. These measurements have been completed by measurementsin pulsed field in LNCMI-Toulouse where thelow temperature part of the curve was obtained, showingthat H c2 reaches almost 50 T at low temperature (figure 79).Analysis of the data is now in progress but a preliminarystudy suggests that although H c2 significantly exceeds thePauli limit, some Pauli limiting is occurring, and this isquite anisotropic. Refinements of the analysis and preparationof a publication are in progress.Figure 78: Specific heat of the single crystal of FeSe 1−x Te xgrown in CEA/Grenoble used in these measurements. Inset showsthe electronic part of C/T after subtraction of the phonon contribution.Figure 79: Upper critical field of a FeSe 1−x Te x single crystalgrown in CEA/Grenoble, combining DC field measurements upto 9 T made in CEA/Grenoble, DC field measurements up to 28T made in LNCMI/Grenoble, and pulsed field measurements upto 55 T made in LNCMI/Toulouse. Lines show fits made using aBCS weak coupling model.I. Sheikin, W. KnafoD. Braithwaite , G. Lapertot, C. Marin (CEA/Grenoble, Grenoble, France),59

METALS, SUPERCONDUCTORS ... 2009Coexistence of magnetic order and superconductivity in iron pnictidesHow long range magnetic order can coexist with bulk superconductivityis a central question in a number of unconventionalsuperconductors. Both in the copper oxideand in the ”new” iron pnictide family of high temperaturesuperconductors the superconducting phase is obtained byadding charge carriers into a phase with antiferromagneticorder. Observation of phase coexistence in such a situationimmediately raises two important questions. The firstone is about the intrinsic character of the coexistence. Thisquestion is obviously linked to a rather subtle issue in theseoff-stoichiometric materials: to which extent is chemicalinhomogeneity of the samples intrinsic or not, i.e. unavoidableor not? The next important question is: Do the phasesoverlap in space, or do they occupy mutually exclusive volumes?In short, on which length scale do the two phasescoexist? This is an equally delicate problem because unambiguousdirect experimental proofs are in most cases difficultto obtain, even for true local probes such as nuclearmagnetic resonance (NMR), muon spin rotation (µSR) orscanning tunneling microscopy (STM). As a matter of fact,the debate is still going on in the cuprates. In the ironpnictides, magnetic order (a spin density wave state) hasrecently been found to coexist with superconductivity inseveral (but not all) families. Yet, a global picture of theconditions for this coexistence in the pnictides is still lacking.We have measured the NMR properties of 75 As nucleiin single crystals of Ba(Fe 1.95 Co 0.05 ) 2 As 2 andBa 0.6 K 0.4 Fe 2 As 2 , both of which show 100% Meissner volumefraction [Julien et al., EPL 87, 37001 (2009)]. Ourresults demonstrate that both samples show the coexistenceof bulk superconductivity with magnetic (SDW) order. Yet,the details appear different:- In Ba(Fe 1.95 Co 0.05 ) 2 As 2 , the fact that the NMR signalat the paramagnetic resonance frequency vanishes abruptlyand completely below TSDW onset = 56 K indicates that the twophases coexist at the microscopic scale probed by NMR, asituation which is often referred to as ”homogeneous mixing”in the literature. This implies either that both types oforders are simultaneously defined at each Fe site (owing tothe multiple bands present at the Fermi level), or that theyare mixed on the scale not greater than one or two latticespacing. A nanoscale coexistence involving superconductingislands (without magnetic order) of typical size definedby the coherence length ξ ≃ 2.8 nm appears to be unlikely.In this case, some paramagnetic NMR signal from regionsas large as ten times the Fe-Fe distance should have beenobserved.- In Ba 0.6 K 0.4 Fe 2 As 2 , on the other hand, the NMR signalhas decreased but remains finite in the whole temperaturerange, including the superconducting state. Thus, the magneticregions do not occupy the full sample volume. Inthis sense, the coexistence might be qualified as ”inhomogeneous”.Figure 80: Top panel: In-plane resistivities. Bottom panel: NMRsignal intensities (renormalized by temperature). Excessive signalloss is due to the spin-density-wave transition in all (Co-dopedmaterial) or part (K-doped material) of the samplePerhaps unexpectedly, the inhomogeneity is considerablystronger in the potassium-doped sample. Actually, the factthat substitutions at the Fe site, unlike substitutions at theCu site in the cuprates, improve conductivity and even inducesuperconductivity is one of the most remarkable surprisesof these new superconductors. It is difficult to fullyunderstand the Ba 1−x K x Fe 2 As 2 system from the presentNMR data only. The important inhomogeneity/disordercould be due to inhomogeneity of K + concentration and/orto a particularly strong impact of these ions on the localelectronic structure in FeAs layers. No evidence for phaseseparation could be detected in the K-doped sample, andthe single crystal exhibits 100% Meissner fraction. Wepoint out that T SDW vs. x is extremely steep near x = 0.4.Phase separation or inhomogeneous coexistence could thusoriginate from K-doping inhomogeneity around this particularconcentration. Therefore, they might not reflect theproperties at somewhat lower x values. In other words,Ba 1−x K x Fe 2 As 2 should perhaps be simply viewed as a disorderedversion of Ba(Fe 1−x Co x ) 2 As 2 . Anyhow, the coexistenceof magnetic and superconducting phases clearlyemerges as a cornerstone of the iron-pnictide physics.M. Horvatić, C. BerthierM.-H. Julien, H. Mayaffre (Laboratoire de Spectrométrie Physique, Université J. Fourier, Grenoble, France), X. D.Zhang, W. Wu, G.F. Chen, N.L. Wang, J.L. Luo (Beijing National Laboratory for Condensed Matter Physics andInstitute of Physics, Chinese Academy of Science, Beijing, China)60

2009 METALS, SUPERCONDUCTORS...High-field metamagnetism in the antiferromagnet CeRh 2 Si 2CeRh 2 Si 2 is a heavy-fermion antiferromagnet which canbe driven to a magnetic instability either by applying pressureor in a high magnetic field. It orders antiferromagneticallyat a second-order phase transition T N = 36 K, afirst-order phase transition occurring at a lower temperatureT 1,2 = 26 K, below which the antiferromagnetic structure ismodified [Graf et al., Phys. Rev. B 57, 7442 (1998)]. Applicationof hydrostatic pressure induces a quantum phasetransition to a paramagnetic Fermi liquid regime at around11 kbar, and unconventional superconductivity was evidencedin the vicinity of the quantum phase transition,below a critical temperature going up to TSCmax ≈ 0.4 K[Movshovich et al., Phys. Rev. B 53, 8241 (1996)]. Here,we present a careful study of the magnetic field-temperaturephase diagram of CeRh 2 Si 2 when a magnetic field is appliedalong the easy-axis c. This study was made combiningtransport, torque and dilatometry experiments using thepulsed magnetic fields generated at the LNCMI-Toulouse.Figure 81 shows a plot of the field-derivative of the torqueversus magnetic field of CeRh 2 Si 2 , at temperatures between4.2 and 24 K. The torque signal is proportional to MH sinθ,where M is the magnetization and θ the small angle betweenthe magnetic field H and the easy axis c of the sample.The field-induced polarization of the system is accompaniedat 4.2 K by two successive minima in the fieldderivativeof the torque, which are the characteristics oftwo first-order transitions, separated by 0.5 T, at H 2,3 andH c ≃ 26 T. From figure 81, it is clear that the two transitionsH 2,3 and H c merge at about 20 K into a single first-ordertransition H c . In our data, a first-order-like anomaly at H ccan be seen up to 23 K, which is characterized by a symmetricpositive anomaly in the field-derivative of the torque.Above 24 K, a second-order-like anomaly has replaced thefirst-order-like anomaly, and the torque versus field data arecharacterized by an asymmetric step-like anomaly in thefield-derivative of the torque.Our torque data are in good agreement with our transport,thermal expansion, and magnetostriction measurements(not shown here), but also with studies performedby Settai et al. [J. Phys. Soc. Jpn. 66, 2260 (1997)], Abeet al. [J. Phys. Soc. Jpn. 66, 2525 (1997)], and Demuer,Sheikin et al. [to be published]. This study permitted todraw the magnetic field-temperature phase diagram of thesystem, indicating the presence of three distinct antiferromagneticphases (see figure 82). The possibility of a tetracriticalpoint at around (24 T, 20 K), where the four antiferromagnetictransition lines could merge, was suggestedhere. It should be further checked using continuous highmagnetic fields. The temperature dependence of the resistivity(not shown here) was also extracted from our pulsedfield scans. This permitted to show that the quadratic coefficientA of the resistivity is enhanced in a rather large Hwindow, of about 10 T, which contrasts with the additionalsharp enhancement of A through the first-order metamagnetictransitions H 2,3 and H c , which are only separated by0.5 T. Finally, a drop of resistivity observed at H c is compatiblewith the idea of a Fermi surface reconstruction.Figure 81: Magnetic field-derivative of the torque versus magneticfield of CeRh 2 Si 2 , for temperatures T ≤ 24 K and magneticfields along c.Figure 82: Magnetic field versus temperature phase diagram ofCeRh 2 Si 2 , with H ‖ c, obtained from resistivity, torque, and thermalexpansion. The insert focuses on the low-temperature part ofthe phase diagram.W. Knafo, D. Vignolles, B. Vignolle, Y. Klein, C. Jaudet, C. ProustD. Aoki, A. Villaume, J. Flouquet (Commissariat à l’Energie Atomique, Grenoble)61

METALS, SUPERCONDUCTORS ... 2009Field Evolution of Coexisting Superconductingand Magnetic Orders in CeCoIn 5Heavy fermion compound CeCoIn 5 is one of the most intriguingexamples of a manifestation of the coexistence ofmagnetic and superconducting (SC) orders. In the SC stateof this compound application of a magnetic field (H 0 ) inducesa long range magnetic order (LRO), restricted to asmall high-field low-temperature region of the phase diagramjust below H c2 . What is more, this particular region ofthe phase diagram was initially identified as the first realizationof the long-sought Fulde, Ferrell, Larkin, and Ovchinnikov(FFLO) state, a superconducting state with a nonzeropair momentum and a spatially modulated order parameter.However, important questions regarding the truenature of this SC phase, the details of the magnetic orderand its field dependence, and the potential driving mechanismsof their coexistence remain unanswered. Therefore,CeCoIn 5 provides a strikingly rich ground to study thecomplex interplay between exotic SC and magnetic degreesof freedom. Experimentally, nuclear magnetic resonance(NMR), as a microscopic probe sensitive to both magneticand SC degrees of freedom, provides a powerful tool for theinvestigation of these puzzles.Here we report detailed low temperature (T ) 115 In NMRmeasurements on the three distinct In sites in CeCoIn 5 , forH 0 ||[100] [Koutroulakis et al., Phys. Rev. Lett. in press(arXiv:0912.3548)]. The axially symmetric In(1) is locatedin the center of the tetragonal Ce planes, while In(2 ac ) andIn(2 bc ) sites correspond to In atoms located on the lateralfaces (parallel and perpendicular to the applied field, respectively)of the unit cell. In figure 83 the H 0 evolutionof the In(2 ac ) spectra at T ≈70 mK is plotted. Loweringthe field below ≈ 11.7 T establishes magnetic LRO, whichis evident from the broadening of the In(2 ac ) line into aspectrum with two extrema/peaks and finite signal weightin between them. Such spectra are characteristic of incommensurate(IC) LRO along one spatial dimension. For9.2T H 0 10.2 T, the spectra of all In sites consist of asingle peak, i.e. no signature of the IC state is observed.However, these spectra remain significantly broader thanthe ones for H 0 9.2 T, where the linewidth of all sites canbe adequately described by the spatial distribution of magneticfields resulting from the vortex lattice of an Abrikosov(low-field) SC state (lfSC).By NMR we have thus established that at T ≈ 70mK aphase with static magnetic LRO is stabilized for fieldsabove ≈ 10.2T in the SC state. By analyzing the spectraof the different In sites, we deduce that the LRO is an incommensuratespin density wave (IC-SDW) with momentsoriented along the ĉ-axis, independent of the in-plane H 0orientation. We fit the data to simulated spectra for the IC-SDW order with magnitude of the local magnetic moment(µ 0 ) as a fitting parameter. This allows us to map the detailedfield evolution of the moment. From the analysis ofthe field dependence of the NMR shift we show that this IC-SDW coexists with a novel SC state, that is likely an FFLOphase, characterized by an enhanced spin susceptibility.Figure 83: Low temperature NMR spectra of In(2 ac ) for variousH 0 ‖â. The frequency scale is defined by subtracting ω 0 , the zeroNMR shift frequency. N denotes the normal phase, IC the LROphase, eSC the state with strong spin fluctuations, and lfSC theAbrikosov SC state. The LRO is evident in the fact that the In(2 ac )line broadens into a spectrum with two extrema. Such spectra arecharacteristic of IC LRO along one spatial dimension. Red solidlines are simulated spectra for the IC-SDW order.By consideration of the spectral lineshapes and spin decoherencetime of different In sites for 9.2T H 0 10.2Tat low-T , we conclude that this corresponds to an ‘exotic’SC (eSC) phase characterized by strong AF fluctuations.This might be a ‘true FFLO’ phase, unperturbed by LROmagnetic order. This phase is separated from the above describedhigh-field, low-T phase by the 2 nd order phase transitionpreviously identified at H ∗ ≈ 10.2 T from the NMRlineshift data. Such a two step phase transition from lfSCto FFLO (assuming it exists in eSC) and then to the IC statewas theoretically predicted when spin fluctuations are considered.The IC magnetism can arise in the FFLO state ina d-wave SC as a consequence of the formation of Andreevbound states near the zeros of the FFLO order parameter.The formation of the IC LRO is triggered by a large DOSin these bound states.M. Horvatić, C. BerthierG. Koutroulakis, V. F. Mitrović (Brown University, Providence, U.S.A.), G. Lapertot, J. Flouquet (INAC, SPSMS, CEAGrenoble, France)62

2009 METALS, SUPERCONDUCTORS...Fermi surface study in the hidden order state of URu 2 Si 2The heavy fermion superconductor URu 2 Si 2 has attractedmuch attention for the past two decades because of the socalledhidden order (HO) phase below T 0 = 17.5K, wherea tiny magnetic moment of 0.03µ B with the wave vectorQ 0 = (1,0,0) appears. The associated entropy at T 0 is,however, too large. That is why many theoretical modelshave been proposed, such as, spin- and/or charge-densitywave, higher orbital ordering, helicity order, etc. Furthermoreit was recently reported that a new phase transition ora crossover appears at H ∗ = 23T below 2K [Shishido et al.Phys. Rev. Lett. 102, 156403 (2009)]. Above H ∗ , a newdHvA frequency with relatively light effective mass was detectedin a high quality sample. Nevertheless, there are stillundetected Fermi surfaces, considering the large specificheat coefficients and carrier numbers. It is important to determinethe electronic states from the microscopic point ofviews. One of the most powerful experimental probe is thede Haas-van Alphen (dHvA) or Shubnikov-de Haas (SdH)effects. Recently, we succeeded in growing high qualityURu 2 Si 2 single crystals (RRR > 200) in order to performthe SdH experiments. Here we report the recent results ofSdH experiments using high quality single crystals at highfields up to 34T and at low temperatures down to 30mK.Figure 84(a) shows the magnetoresistance for the currentalong [100] at various field directions in URu 2 Si 2 . Thelarge magnetoresistance and clear SdH oscillation indicatethe high quality of our sample. The kink is clearly detectedfor H ‖ [001] at H ∗ = 24T, which shifts to the higher fieldwith increasing the field angle from [001] to [100]. Theangular dependence of H ∗ is shown in Fig. 84(b). H ∗ approximatelyfollows the 1/cosθ-dependence. We show infigure 85 the typical SdH oscillation for H ‖ [001] and forthe field tilted 52deg from [001] to [100]. As clearly seenin figure 85, the SdH amplitude abruptly increases aboveH ∗ for H ‖ [001], while the SdH amplitude at field angle52deg smoothly increases, following the Lifshitz-Kosevichformula. Below H ∗ , the fast Fourier transform (FFT) analysisshows three kinds of SdH branches, namely α, β andγ. The cyclotron effective masses for H ‖ [001] below H ∗are 12m 0 and 18m 0 for branch α and β, respectively. Althoughthe number of wave is not enough to analyze thedata, the SdH frequency for branches α and β shows nochange above H ∗ , however, the cyclotron mass for branchβ seems to decrease above H ∗ . These results clearly indicatethat the electronic state is changed above H ∗ in the HOstate. Further experiments, such as thermoelectric power,are required to confirm this point.Figure 84: (a)Magnetoresistance at various field directions from[001] to [100] at 30mK in URu 2 Si 2 . (b)Angular dependence ofH ∗ , which is defined as a kink of magnetoresistance. The solidline corresponds to the 1/cosθ dependence.Figure 85: Typical Shubnikov-de Haas oscillations for the fieldalong [001] and for the field tilted 52deg from [001] to [100].I. SheikinD. Aoki, E. Hassinger, V. Taufour, J. Flouquet (CEA-Grenoble),63

METALS, SUPERCONDUCTORS... 2009High field resistivity measurements on single crystalline UPt 2 Si 2Tetragonal UPt 2 Si 2 has recently been characterized as amoderately mass enhanced antiferromagnet (T N = 32 K),which in various physical properties closely resembles thehidden order material URu 2 Si 2 [Süllow et al., J. Phys. Soc.Jpn. 77, 024708 (2008); Johannsen et al., Phys. Rev. B 78,121103 (2008) ; Kim et al., Phys. Rev. Lett. 91, 256401(2003); Amitsuka et al., Physica B 177, 173 (1992) ].are clearly to be seen, supporting the notion of the similaritybetween the two systems. Presently, high field magnetizationmeasurements at 3 He temperatures are underway toextend the determination of these phase boundaries to verylow temperatures.Similar to URu 2 Si 2 , high field phase transitions at low temperatureshave been observed in UPt 2 Si 2 , that is in fieldsof 30 T to 45 T along the a direction and 20 T to 33 Talong c [Amitsuka]. Recent high field measurements revealadditional structure in the magnetic phase diagram, emphasizingthe close resemblance to URu 2 Si 2 .Based on these findings, the goal of the present set of experimentswas to verify and more accurately define the highfield phase boundaries observed in previous resistivity andmagnetization measurements. Therefore, high field resistivityexperiments at low temperatures along the crystallographica and c axes of UPt 2 Si 2 were initiated and carriedout.We have measured the high field resistivity along the crystallographica and c axes in field and temperature sweeps upto 28 T and 40 K. From our data we calculated the differentialresistivities ∂R/∂B and ∂R/∂T , in which the transitionsappear as local maxima (∂R/∂B) or minima (∂R/∂T ).In the temperature sweeps the transition to the antiferromagnetphase is clearly visible for both directions. In fieldsweeps with B applied along the a axis the high field phaseis not accessible but for B//c the phase boundary to thelowest high field phase occurring at ∼ 25 T below 18 K isvisible.Together with the data taken previously, we construct themagnetic phase diagram of UPt 2 Si 2 , as shown in figure 86.In these diagrams, field induced phases similar to URu 2 Si 2Figure 86: Proposed magnetic phase diagrams for the crystallographica and c directions derived from previous and actual magnetizationand resistivity measurements.I. SheikinD. Schulze Grachtrup, S. Süllow (TU Braunschweig, Institute for Physics of Condensed Matter, Braunschweig, Germany)64

2009 METALS, SUPERCONDUCTORS...Evolution of the Fermi surface of BaFe 2 (As 1−x P x ) 2on entering the superconducting domeThe Fermi surface topology of the iron-pnictides is widelythought to play a key role in determining the type of magneticor superconducting order that occurs in these materialsat low temperature. Measurements of the de HaasvanAlphen (dHvA) effect provide a unique way of measuringthe Fermi surface of these materials. Prior to thepresent work, measurement had only been possible in thestoichiometric end members of the various families. However,to gain an understanding of how the bulk Fermi surfaceevolves as the correlation effects responsible for superconductivitybecome strong, it is necessary to measurethe highest T c superconducting samples. With the notableexception of the low T c phosphide material LaFePO, otheriron-pnictides need to tuned, either chemically or with pressurein order for them to become superconducting. Chemicallytuning often adds disorder, for example replacing Feby Co in the series Ba(Fe 1−x Co x ) 2 As 2 . This decreases dramaticallythe dHvA signal, making it unobservable even inhigh pulsed fields. Recently, it was discovered that substitutingAs for P in the series BaFe 2 (As 1−x P x ) 2 producedhigh T c superconductivity without changing the carrier concentration(both As and P are in the 3+ state so this is anisoelectric substitution). Importantly, this substitution doesnot appear to produce significant disorder in the conductingplane, making this an excellent candidate system for adHvA study.Measurements were made in Toulouse on BaFe 2 (As 1−x P x ) 2samples with three different values of x, with T c rangingfrom 12 K to 30 K. These measurements were supplementedwith other data obtained using superconductingmagnets in Osaka (17 T), the hybrid magnet in Tallahassee(45 T), and a resistive magnet in Nijmegen (33 T).Figure 88: Variation of the size of the electron orbits (α andβ) and the effective mass of quasiparticle moving on the β orbitas a function of phosphorous fraction x in BaFe 2 (As 1−x P x ) 2 .The variation of T c with x is also shown. The data show that theelectron Fermi surfaces shrink and the effective mass increasesas x decreases and T c becomes larger. Data for samples withx = 0.64,0.72, and 1.0 were obtained using dc fields in Osaka,Nijmegen and Tallahassee.Band-structure calculations suggest that the Fermi surfaceof these materials consists of quasi-two-dimensional electronand hole sheets. The total volumes of the electron andhole sheets are exactly equal (compensated metal). In ourexperiment only the electron Fermi surfaces were observed.This follows a trend found for other iron-phosphides wherethe mean free path on the electron Fermi surface is muchlonger than on the hole sheets.Figure 87: Measured de Haas-van Alphen data for samplesof BaFe 2 (As 1−x P x ) 2 . The left panels show raw torque data atT = 1.5 K. The right panels show the oscillatory part of the torqueand the corresponding fast Fourier transforms, for samples withx = 0.41 and 0.56. No oscillations were observed for the highestT c = 30 K sample (x = 0.33).The main conclusion is that the data show that the volumeof the electron sheets (and via charge neutrality also thehole sheets) shrink linearly and the effective masses becomestrongly enhanced with decreasing x. Calculationsshow that it is unlikely that these changes are a simpleconsequence of the one-electron bandstructure but insteadthey likely originate from many-body interactions. Thesechanges may be intimately related to the high T c unconventionalsuperconductivity in this system. These results arereported in detail in Shishido et al. arXiv:0910.3634.D. Vignolles, B. Vignolle, C. ProustA. Carrington, A.F. Bangura, A.I. Coldea, P.M.C. Rourke (Bristol University), A. McCollam (HMFL, Nijmegen), H.Shishido, S. Tonegawa, K. Hashimoto, S. Kasahara, H. Ikeda, T. Terashima, Y. Matsuda, T. Shibauchi (University ofKyoto), R. Settai, Y. Ōnuki (University of Osaka)65

METALS, SUPERCONDUCTORS... 2009Angular dependence of the Nernst effect in elemental bismuthThe Fermi surface of bismuth consists of one pocket ofhole-like carriers with an ellipsoid shape whose long axisis along the trigonal direction, and three electron pockets,arrayed symmetrically around the hole ellipsoid (see figure89(a)) for a sketch). The electron pockets are muchmore anisotropic than the hole pocket. It is known thatin two directions the electronic bands are described by theDirac Hamiltonian [Wolf, J. Phys. Chem. Solids 25,1057(1964)]. The full volume of the Fermi surface occupies10 −5 of the volume of the Brillouin zone. One consequenceof this remarkable property is that the quantum limit can bereached for a magnetic field as low as 9 T, oriented alongthe trigonal axis. It has recently appeared that the Nernst effect(the transverse voltage induced by a longitudinal temperaturegradient, in the presence of a magnetic field) is avery sensitive probe of quantum oscillations in the vicinityof the quantum limit. Above 9 T, i.e. beyond the holequantum limit (QL), three unexpected anomalies were seenin the Nernst response of bismuth [Behnia et al., Science11, 1729 (2007)]. These anomalies were interpreted as signaturesof many-body effects. However, it has recently appearedthat the one-particle spectrum of bismuth is complex,and a small misalignment off the trigonal axis woulddrastically change the field position of the electron Landaulevels. In absence of an angular-resolved study, the additionalNernst anomalies could be explained in one-particlepicture with a small misalignment [Sharlai Mikitik, Phys.Rev. B 79, 081102(R) (2009)].In order to clarify the origins of these unexpected Nernstanomalies, we studied the angular dependence of the Nernsteffect in bismuth. For this purpose, we built our ownthermoelectric single and double rotator set-up based on apiezoelectric positionner (typical angular accuracy: 0.01 ◦ ).The orientation of the sample in the magnetic field was determinedby Hall probes. Figure 89(b)) presents the Nernstvoltage at T = 1.3 K for a magnetic field tilted in the trigonalbinary plane. The typical angular step is 0.5 ◦ . At lowfield, quantum oscillations with a main period of 0.15 T −1 ,corresponding to the hole ellipsoid can be seen. Over theangular range investigated here the QL of the hole pocketdoes not change significantly. Above the QL, we can resolvetwo quasi vertical lines in the (B, θ) plane, which aresymmetrical about θ = 0. These lines are more obvious onthe (B, θ) color map of the high magnetic Nernst voltage reportedin figure 89(c)) (deduced from figure 89(b))). Thesetwo lines define a cone which is reminiscent to the cone observedby torque [Li et al, Science, 321, 547 (2008)] andtransport measurement [Fauqué et al, Phys. Rev. B 79,245124 (2009)]. According to the recent calculation on theLandau levels spectrum of bismuth performed by [Aliceaand Balents, Phys. Rev. B 79, 081102(R) (2009)], this conecorresponds to the 0 + electron Landau level.Figure 89: (a) Sketch of the Fermi surface of bismuth: the holeand the electron pockets are respectively in red and green. (b)Nernst voltage as a function of B at T = 1.3 K for a magnetic fieldtilted in the trigonal binary plane. θ, the angle between the trigonalaxis and the magnetic field direction, varies from −8.4 ◦ to 8.5 ◦ .The curves are shifted for clarity. (c) Color map of the Nernstvoltage between 10.5 T and 28 T for a magnetic field tilted in thetrigonal binary plane. (d) Nernst voltage for θ=0 as a function ofthe magnetic field for various temperatures T = 1.3,2.3,3.4,5.5and 8.5 K.In addition to these lines, we can identify at least three additionallines inside and outside the cone. Each of these linesseems to be characterized by their own angular dispersionin the (B, θ) plane. Figure 89(d)) presents the evolution ofthe Nernst response with temperature, for a magnetic fieldoriented along the trigonal direction(θ=0). As seen in thefigure the anomalies fade away when the temperature exceeds3.4 K.In conclusion, our angular-dependent study reveals that: (i)the Nernst effect can reveal hole and electron Landau levelsspectrum (ii) the unexpected Nernst anomalies cannotbe explained by the electron Landau levels and are characterizedby their own angular dependence. The additionalNernst anomalies are unexpected in single-particle theoryand point to collective effects, which are yet to be understood.A.B. Antunes, L. MaloneH. Yang, B. Fauqué, K. Behnia (LPEM/ESPCI, Paris, France)66

2009 METALS, SUPERCONDUCTORS...Magnetic field-induced electronic instability in bismuthIn the presence of a reasonably large magnetic field, theelectric resistivity of bismuth is enhanced by five orders ofmagnitude with no sign of saturation. This feature, discoveredas early as 1928 by Kapitza, has escaped wide theoreticalattention [Abrikosov, J. Phys. A 36, 9119 (2003)].When a magnetic field exceeding 9 T is applied along thetrigonal axis of a bismuth crystal, the carriers are confinedto their lowest Landau level. In the case of electron-like carriers,this level has two sub-levels of opposite spins. Holelikecarriers , on the other hand, are spin-polarized. Recentexperimental studies of bismuth [K. Behnia et al., Science11, 113012 (2009); L. Li et al. Science, 321, 547 (2008);B. Fauqué et al., Phys. Rev. B 79, 245124 (2009)] haveuncovered a number of enigmatic field scales beyond thisquantum limit.We have extended previous measurements of longitudinaland Hall resistivity of bismuth to 55 T. Our study has uncovereda new field scale in the vicinity of 40 T pointing to anunidentified electronic instability. The signatures of electronicreorganization at this field in the experimental data,namely, a minimum in field-dependence of electric resistivity(see figure 90) and a peak in the Nernst response (foundusing the dc hybrid magnet in NHMFL-Tallahassee), are almostas drastic as the crossing of the quantum limit at 9 T.Our angular-dependent resistivity studies establish that thisfield scale persists even when the field is strictly parallel tothe trigonal axis and, thus cannot be attributed to the oneparticleenergy spectrum. Thus, this result constitutes themost solid experimental evidence available until now for afield-induced electronic instability caused by electronic interactionsin bulk bismuth.In bismuth, quantum oscillations of resistivity (theShubnikov-de Haas effect) are superimposed on a hugemonotonous background. The main period of oscillation(0.15 T −1 ), clearly visible in dρ/dB plots (figure 90(b)), isassociated with the cross section of the hole ellipsoid. Thepresence of the 40 T anomaly was checked in five Bi singlecrystals from different sources.According to recent theoretical calculations [Alicea and Balents,Phys. Rev. B 79, 241101 (2009); Sharlai and Mikitik,Phys. Rev. B 79, 081102 (2009)], the one-particle spectrumof bismuth for a field oriented close to the trigonal axis isremarkably complex. This is due to the implications of thecharge neutrality in a compensated system, the particularFermi surface topology and the relatively large Zeeman energy.According to these calculations, the field scales ofthe three electron ellipsoids are expected to present a verysharp angular dependence when the field is slightly tiltedaway from the trigonal axis. On the other hand, if the fieldis strictly oriented along the trigonal axis, above B ∼ 10 T,no other field scale is expected in the one-particle picture.We have verified that tilting the field slightly away from thetrigonal axis has little effect on the field position of the 40 Tanomaly. The high-field anomaly is present even when thefield is parallel to the high-symmetry axis and thus it cannotbe attributed to one of the three sharply anisotropic electronellipsoids.This field-induced electronic reorganization leads to a betterconductivity and an enhanced metallic behaviour, insharp contrast to the expected signatures of a density-wavetransition. A version of such a transition is widely believedto occur in graphite in a similar ultra-quantum configuration[Yaguchi and Singleton, J. Phys.: Condens. Matter 21,34 (2009) for a review].Figure 90: Left: Magnetoresistance of three bismuth single crystals.Sample (a) was measured in a dc hybrid magnet and the twoothers in the pulsed magnet at LNCMI-Toulouse. The high-fieldminimum in resistivity occur far beyond the quantum limit. Right:Field-derivative of resistance a s function of B −1 . Quantum oscillationsare visible in addition to the newly discovered field scalemarked by arrows.Figure 91: Longitudinal and Hall resistivity as a function of B −1in a bismuth single crystal for different tilt angles. The θ = 0curves are in red. The high-field anomaly shows little variationwith tilt angle.B. Vignolle, C. ProustB. Fauqué, K. Behnia (ESPCI, Paris)67

METALS, SUPERCONDUCTORS... 2009Magnetic oscillations in a linear chain of compensated orbitsDue to their rather simple Fermi surface, organicmetals provide a rich playground for the investigationof quantum oscillations in physics. In that respect,the most well known example is provided by κ-(BEDT-TTF) 2 Cu(NCS) 2 (where BEDT-TTF stands for bisethylenedithio-tetrathiafulvalene)which can be regarded asthe experimental realization of the Fermi surface consideredby Pippard in the early sixties for his model. In theextended zone scheme, such a Fermi surface is composedof closed hole orbits and quasi-one dimensional sheets coupledby magnetic breakdown. This kind of Fermi surfaceyields quantum oscillations spectra with numerous frequencycombinations that cannot be accounted for by thesemi-classical model of Falicov and Stachowiak. This phenomenonwhich has generated great interest is generally attributedto either the formation of Landau bands and/or oscillationsof the chemical potential in a magnetic field.In the case where the effective masses linked to electronandhole-type orbits are the same (m ∗ e = m ∗ h), calculationsdemonstrate that chemical potential oscillations vanish forsuch a Fermi surface. More generally, in the case wherem ∗ e and m ∗ hare different, these oscillations are significantlydamped, all the more if the magnetic breakdown field issmall, as evidenced in figure 93.Figure 92: Calculated Fermi surface of the Bechgaard salt(TMTSF) 2 NO 3 in the temperature range below the anion orderingand above the spin density wave condensation, according toKang et al. [EPL 29 635 (1995)]. Solid blue and red lines arecompensated hole and electron orbits, respectively. This Fermisurface achieves linear chains of compensated orbits.Contrary to the above mentioned example, the Fermi surfaceof numerous organic metals is composed of compensatedelectron- and hole-type closed orbits, yieldingmany frequency combinations as well, as far as ShubnikovdeHaas oscillations are concerned. We have computedthe field and temperature dependence of the de Haas-vanAlphen oscillations spectra of an ideal two-dimensionalmetal whose Fermi surface achieves a linear chain ofsuccessive electron- and hole-type compensated orbits.Such a topology is realized e.g. in the Bechgaard salt(TMTSF) 2 NO 3 (where TMTSF stands for tetra-methyltetra-seleno-fulvalene)in the temperature range in-betweenthe anion ordering temperature and the spin density wavecondensation (see figure 92).Figure 93: Field dependence of the chemical potential for Fermisurface such as in figure 92 for m ∗ e = m 0 and m ∗ h = 2.5m 0 (m ∗ e andm ∗ h are the electron and hole orbit effective mass, respectively; m 0is the free electron mass) at a reduced temperature t = 10 −4 (t = T× k B m 0 A 0 /2π 2 , where A 0 is the unit cell area). b is the reducedmagnetic field (b = B × eA 0 /2π), b 0 is the reduced magneticbreakdown field. The inset compares the chemical potential oscillationsfor two electron orbits and two compensated orbits with,respectively, the same effective masses as in the main panel, in theabsence of magnetic breakdown (b 0 →∞).It appears from the analysis of the numerical resolutionof Landau levels, including the electron-hole band interaction,that the Lifshits-Kosevich semiclassical formalismcan be applied for the first harmonic, provided magneticbreakdown orbits, although with higher effective masses,are taken into account. The resulting high order terms canlead to apparent temperature-dependent effective mass forclean crystals in the high B/T limit in the case where onlyone effective mass is considered for the data analysis, as itis usually done. For example, in the absence of magneticbreakdown, m ∗ = min(m ∗ e, m ∗ h) in the low field range whilem ∗ = √ m ∗ em ∗ hat high field.On the contrary, strong deviation from the Lifshits-Kosevich behavior is observed for the second harmonic.The main feature of this latter component being the zeroamplitude occurring at a B/T value depending on the ratioof the two effective masses (m ∗ h /m∗ e), only, independent ofthe magnetic breakdown field value.A. AudouardJ.-Y. Fortin (Institut Jean Lamour, Nancy)68

2009 METALS, SUPERCONDUCTORS...Coexistence of closed orbit and quantum interferometer with the same crosssection in the organic metal β”-(BEDT-TTF) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2The family of quasi-two-dimensional charge transfer saltsβ”-(BEDT-TTF) 4 (A)[M(C 2 O 4 ) 3 ]Solv (where BEDT-TTFstands for bis-ethylenedithio-tetrathiafulvalene, A is amonovalent cation, M is a trivalent cation and Solv is asolvent) have raised great interest in particular because ityielded, more than ten years ago, the first organic superconductorat ambient pressure with magnetic ions.due to the relatively narrow field range in which these latteroscillations can be observed (B > 20 T).The Lifshitzs-Kosevich formalism accounts for the fieldand temperature dependence of both the SdH and dHvAdata over all the explored range. However, a very weak thermaldamping of the Fourier component F b , with the highestamplitude, is evidenced for SdH spectra above about 6 K(see figure 95). As a result, magnetoresistance oscillationsare observed at temperatures higher than 30 K. Taking intoaccount the temperature dependence of the scattering rate,this feature, which is not observed for dHvA oscillations(recorded up to 15 K), is in line with the coexistence, at leastin the temperature range around 6 K, of a closed orbit b anda symmetric (i.e. with a zero effective mass) quantum interferencepath with the same area (keeping in mind that dHvAoscillations are only sensitive to the density of states). Thisresult, which cannot be interpreted in the framework of theFermi surface displayed in figure 94(d), points to a Fermisurface reconstruction in this compound. For details, see[Vignolles et al. Eur. Phys. J. B 71 203 (2009)].Figure 94: (a) Field-dependent interlayer resistance ofβ”-(BEDT-TTF) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2 for θ = 0 ◦ (θ is theangle between the field direction and the normal to the conductingplane). (b) Fourier analysis deduced from the oscillatory part ofthe magnetoresistance displayed in the inset. The field range is18-54 T. Marks are calculated with F a = 74 T and F b = 348 T. (c)Magnetic torque at θ = 29 ◦ . Corresponding Fourier analysis aredisplayed in the inset. The field range is 30-53.5 T and 38-53.5 Tbelow and above 9 K, respectively. (d) Textbook case of Fermisurface accounting for the frequencies a, b and b-a.Magnetoresistance and magnetic torque of the salt with A= H 3 O + , M = Fe 3+ and Solv = C 6 H 4 Cl 2 have been investigatedin pulsed magnetic fields of up to 54 T. Shubnikov-deHaas (SdH) oscillations reveal three basic frequencies F a ,F b and F b−a , which, in line with band structure calculations,can be interpreted on the basis of three compensated closedorbits originating from a hole orbit with an area equal to thatof the first Brillouin zone (see figure 94). Only F a and F bare observed in de Haas-van Alphen (dHvA) spectra, likelyFigure 95: Temperature dependence of the amplitude of the boscillations for dHvA and SdH data. Empty and solid symbolscorrespond to a mean field value of 44.6 T and 30 T/cos(θ), respectively(θ is the angle between the field direction and the normal tothe conducting plane). Solid lines are best fits of the Lifshitzs-Kosevichformula. A zero-effective mass and a temperature-dependentscattering rate are considered for the SdH data in the hightemperature range.D. Vignolles, A. AudouardV.N. Laukhin, E. Canadell (ICMAB, Barcelona, Spain), E.B. Yagubskii (IPCP, Chernogolovka, Russian Federation)69

METALS, SUPERCONDUCTORS... 2009Metal-non-metal transition in the charge transfer salt(BEDT-TTF) 8 [Hg 4 Br 12 (C 6 H 5 Br) 2 ]At room temperature and ambient pressure, all the membersof the family of charge transfer salts (BEDT-TTF) 8 [Hg 4 X 12 (C 6 H 5 Y) 2 ] (where X, Y = Cl, Br and BEDT-TTF stands for bis-ethylenedithio-tetrathiafulvalene) areisostructural. According to band structure calculations,their Fermi surface is composed of one electron and onehole compensated orbit, coupled by magnetic breakdown(MB). However, even though a metallic ground-state is observedfor X = Cl, a metal-non metal transition occurs as thetemperature is lowered for X = Br, as displayed in figure 96.(2003), Audouard et al. Euro. Phys. Lett. 71 783 (2005)]),only few MB orbits are observed for X = Br. This is due toa larger MB gap between electron and hole orbits in the lattercase, in line with band structure calculations. The mostsalient feature is the sizeable decrease of the effective masslinked to the compensated orbits observed as the appliedpressure increases (roughly a factor of two in the exploredrange). In addition, the effective mass scales with the coefficient(A) of the T 2 law. Such a behaviour is reminiscent ofa Brinkman-Rice scenario which predicts the divergence ofthe effective mass as approaching a Mott transition. In thatrespect, no structural phase transition can be inferred fromX-ray data down to 100 K at ambient pressure, for X = Br.Figure 96: Temperature dependence of the interlayer resistivityfor (a) X = Cl and (b) X = Br. Even though the two compounds areisostructural at room temperature, a metal-non metal transition isobserved for X = Br at low pressure. T 2 dependence of the interlayerresistivity at low temperature for (c) the metallic compoundwith X = Cl and (d) the pressure-induced metallic state of X =Br. The Fermi surface for X = Br is displayed in the inset of (b).Electron (red) and hole (blue) orbits are compensated.We have studied the interlayer magnetoresistance of the twocompounds with Y = Br under applied pressure of up to 1.1GPa. For X = Cl, a metallic ground-state is observed in allthe pressure range explored while for X = Br, a pressureinducedmetallic state is observed at a few tenth of GPa.For both compounds, a T 2 variation of the zero-field resistance(ρ = ρ 0 + A×T 2 ), typical of correlated Fermi liquids,is observed in the metallic state. Shubnikov-de Haas (SdH)oscillations are observed for both X = Cl (see figure 97)and X = Br. While many frequency combinations, typicalof coupled orbits networks are observed in the former case(for more details see [Vignolles et al. Eur. Phys. J. B 31 53Figure 97: (a) Field-dependent resistance for X = Cl at 1.1 GPaand (b) corresponding Fourier spectra. Label a stands for the compensatedorbits (see figure 96), the other labels corresponds to eitherfrequency combinations or Fermi surface pieces located in--between the orbits. (c) The coefficient (A) of the T 2 law of thezero-field resistivity scales with the square of the effective mass(m ∗ ) deduced from SdH oscillations.D. Vignolles, A. Audouard, F. Duc, M. NardoneR.B. Lyubovskii, R.N. Lyubovskaya (IPCP, Chernogolovka, Russian Federation), E. Canadell (ICMAB, Barcelona,Spain)70

2009 METALS, SUPERCONDUCTORS...Magnetic torque experiments on the magnetic-field-induced organicsuperconductor λ-(BETS) 2 FeCl 4Layered organic superconductors have upper critical fieldsexceeding the theoretical Pauli limit for superconductivitywhen the magnetic field is applied parallel to the layers.Theory predicts then the possibility of a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) superconducting state[Fulde and Ferrel, Phys. Rev. 135, A550 (1964);Larkin and Ovchinnikov, Sov. Phys. JETP 20, 762(1965)]. Besides κ-(BEDT-TTF) 2 Cu(NCS) 2 , the two dimensionalfield-induced superconductor λ-(BETS) 2 FeCl 4 ,where BETS is bis(ethylenedithio)tetraselenafulvalene, is acandidate which fulfils all necessary conditions. When themagnetic field is applied parallel to the metallic layers, asuperconducting phase appears above 17 T below 1 K.In our first experiment we focused on the critical field at17 T. Figure 98 shows torque data taken at different temperaturesduring field sweeps. Surprisingly, a broad kinklikeanomaly appears only above 22 T indicating the transitioninto the field-induced superconducting state. Below200 mK a pronounced hysteresis appears. The experimentsshow no evidence for additional phase transitions withinthe superconducting state. The phase diagram clearly differsfrom that reported by Uji et al. A misalignment of thesample can be ruled out. A possible explanation may befound in the rather rapid cooling conditions in our experimentwhich is known to cause anion-disorder effects in 1Dorganic superconductors. The result stimulates further experimentsunder more controlled experimental conditions.This field-induced superconductivity is well understoodin the framework of the Fischer theory, based on theJaccarino-Peter effect. Uji et al. reported possible FFLOstates at the two critical fields based on dip structures inthe resistivity [Uji et al., Phys. Rev. Lett. 97, 157001(2006) and references therein]. This motivated us to lookfor a thermodynamic phase transition between the homogeneoussuperconducting phase and the FFLO states bymeans of magnetic torque experiments using a capacitancecantilever technique. The probe was equipped with a rotator,which allowed us to align the layers parallel to the fielddirection with a precision of 0.001 ◦ . Single crystals of λ-(BETS) 2 FeCl 4 were grown by the standard electrochemicaloxidation technique.Figure 98: Magnet torque of λ-(BETS) 2 FeCl 4 is plotted as afunction of field at different temperatures.I. SheikinR. Lortz (The Hong Kong University of Science and Technology, Kowloon, Hong Kong), Y. Nakazawa ( University ofOsaka, Osaka, Japan), B. Zhou, A. Kobayashi, H. Kobayashi (Nihon University, Tokyo, Japan)71

METALS, SUPERCONDUCTORS... 2009Temperature and magnetic field dependence of domain wall width and periodof Condon domain structure in AgDiamagnetic instability of electron gas in normal metals underquantizing magnetic field and low temperature is a resultof strong electron correlations induced by the magneticfield. It gives rise to a phase transition with formation ofcomplex domain patterns (Condon domains).hysteresis in magnetization curves, existence of persistentcurrents which results in a discontinuity of magnetic inductionalong the interface boundaries of regular domainpatterns. Still, there remain open fundamental questionsrelated to the formation of Condon domain phase. As anexample, the important information about the size of thedomains, the domain wall width and surface energy of theinterface boundaries is still lacking. In our studies [N. Logoboyand W. Joss, Solid State Comm. 149, 2007 (2009)]we offer a way to calculate the expected values of domainwall width and period of the domain structure by means ofmeasurement of the value of the jump of magnetic inductionat the domain wall δB (figure 99 and figure 100).Figure 99: Temperature dependence of the DW width δ (a) andperiod of the domain structure D (b) at the conditions of the experiment[Kramer et al., Phys. Rev. Lett. 95, 267209 (2005)].The solid lines correspond to the theory, the circles are calculatedfrom the temperature dependence of the measured jump of magneticinduction at the interface boundaries.The diamagnetic phase transition has received recentlymuch attention due to a number of unusual phenomena forthe physics of diamagnetism, e.g. formation of complexbranch structures, strong dependence of magnetic phase diagramson Fermi-surface topology, presence of diamagneticFigure 100: Magneticfield dependence of the DW width δ (a)and period of the domain structure D (b) at the conditions of theexperiment [Kramer et al., Phys. Rev. Lett. 95, 267209 (2005)].The solid lines correspond to the theory, the circles are calculatedfrom the temperature dependence of the measured jump of magneticinduction at the interface boundaries.W. JossN. Logoboy (The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, Israel and The Instituteof Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel)72

2009 METALS, SUPERCONDUCTORS...Intrinsic diamagnetic length scales of Condon domain phase in BeMagnetic quantum oscillations which are the result of Landauquantization of the quasiparticle spectrum are a powerfultool in investigation of Fermi surfaces of wide spectrumof substances, including traditional normal metals, lowdimensionalsystems and superconductors. The increasein the amplitude of the dHvA oscillations at low temperatureunder quantizing magnetic field gives rise to the magneticinstability with formation of nonuniform diamagneticphase (Condon domains). The low curvature of the quasi-2D ’cigar’-like part of Fermi surface of beryllium results inrelatively high amplitude of dHvA oscillations and makesit favorable to observe the diamagnetic instability in beryllium.We show that the properties of correlated electrons innormal metals at the conditions of diamagnetic instabilitywhen the differential magnetic susceptibility a =µ 0 max{∂ ˜M/∂B} ≥ 1 ( ˜M is oscillating part of the magnetizationand B is magnetic induction of the sample) can bedescribed by the effective energy functionalthe size and the shape of the sample, and short-range electroninteraction on the scale of r c which gives rise to positiveenergy of interface boundaries.Thus, for a plate-like sample of thickness L, the minimizationof total free energy of periodic domain structureG PDS = G d−d + G σ , containing two terms, e. g. dipoledipoleenergy G d−d = (7/π 3 )ζ(3)y 2 0D, where ζ(3) is zetafunction,and surface energy of separation of two domainsG σ = (2L/D)σ (σ is the surface energy of a single domainwall), allows to calculate the steady-state period of the domainstructureD = 25/2 π(r c L) 1/2[7ζ(3)] 1/2 (sec 2 y2 02 − a)1/4 . (14)We calculate the expected values of domain wall width andperiod of the domain structure by use of measured valuesof the jump of magnetic induction at the domain wall δB(figure 101).G = − 1 2 K sin2 Θ + 1 2 A(∂ ζΘ) 2 . (12)In Eq. (12) we use a variable Θ = πy/2y 0 where y = 4πkMis a reduced magnetization, k = 2πF/(B a ) 2 = 2π/∆H, F =(F h +F w )/2 is average fundamental frequency of the dHvAoscillations (F h = 970.9 T and F w = 942.2 T are two fundamentalfrequencies corresponding to two extremal crosssectionsof ’cigar’-like Fermi surface of Be), ∆H is dHvAperiod. The uniform magnetization y 0 = y 0 (a) is given inexplicit form by equation y 0 = asin y 0 . Parameters K andA in Eq. (12) are defined asK = 4asin 2 y 02 (1 − acos2 y 02 ), A = a(2r cy 0 /π) 2 . (13)The first term in RHS of Eq. (12) is analogous to the easyaxiscrystallographic anisotropy (K > 0), while the gradientterm (ζ is coordinate) accounts for the short-range correlationson the scale of the cyclotron radius r c and correspondsto the exchange interaction in the physics of spinmagnetism. It follows from Eq. (12) that there is a closeanalogy between easy-axis anisotropy ferromagnetic sampleand the system which shows the diamagnetic instability.Thus, the problem of the diamagnetic length scales can besolved by the standard methods of the physics of magneticmaterials.We assume the existence of periodic domain structure withalternative magnetization ±y 0 in neighboring domains. Periodof the domain structure D is defined by competitionbetween long-range dipole-dipole interaction dependent onFigure 101: (a) Temperature dependence of the domain wallwidth δ = δ(T ) and (b) period of the Condon domain structureD = D(T ) at fixed value of B a = 2.642 T and Dingle temperaturesT D = 1.9 K in beryllium plate-like sample with the width L = 1.8mm. Circles (triangles) represent the diamagnetic length-scalescalculated from the data [G. Solt and V. S. Egorov, Physica B 318,231 (2002)] when heating (cooling).W. JossN. Logoboy (The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, Israel and The Instituteof Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel)73

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2009 MAGNETIC SYSTEMSY b 3+ → Er 3+ up-conversion luminescence under high pressure and pulsedmagnetic fieldsEr 3+ -doped materials are essential for laser application,displays, infrared detectors as well as in telecommunicationusing it in optical fibre amplifiers or other planar waveguides.Therefore, the understanding of the effect of an externalmagnetic field on the photoluminescence (PL) bandsof Er 3+ is important in order to obtain information aboutthe electronic structure, particularly, the Zeeman sublevelstructure forming the excited and ground state manifold.We have investigated the Zeeman splitting of the 4 S 3/2 → 4I 15/2 Er 3+ transition, which is responsible of the greenluminescence of high-quality single-crystal thin layers ofKY (WO 4 ) 2 : Er 3+ ,Y b 3+ in pulsed magnetic fields as afunction of hydrostatic pressure by resonant up-conversionspectroscopy (figure 102). The green Er 3+ PL was resonantlyexcited via up-conversion processes after Y b 3+ andEr 3+ excitation in the near infrared around 980 nm usinga tunable titanium-sapphire laser. Under the applied magneticfield, the energy resonance between the Y b 3+ levelsand the Er 3+ changes by the Zeeman effect. The upconversionprocess can be completely suppressed and therefore,a precise tuning of the laser excitation wavelength tocompensate the Zeeman splitting of Y b 3+ ions was necessaryin order to assure the resonance at high magnetic fields.Up-conversion process is a way to transform low energyphotons into higher energy photons different from secondharmonicgeneration which is commonly used in solid-statelasers. The most efficient up-conversion system is that onebased on Er 3+ and Y b 3+ . It is possible to observe, by thenaked eye, visible emission from Er 3+ after Y b 3+ excitationin the near-infrared.Figure 103: Pressure effect on the green photoluminescence of4 S 3/2 → 4 I 15/2 Er 3+ transition at two different external magneticfields.Figure 102: Low temperature Zeeman splitting of Er 3+ photoluminescencespectra in KY (WO 4 ) 2 : Er 3+ ,Y b 3+ for a 5.0 T magneticfield applied parallel to the crystallographic b axis correspondingto the lowest lying Kramers’ doublet of the 4 S 3/2 to the splitKramers’ doublets of the 4 I 15/2 ground state. The insets showdetails of some relevant peaks.The results indicate that pressure induces a linear redshiftfor all peaks at the two selected magnetic fields (figure 103).The pressure induced shift rates are almost independent ofthe magnetic field making this material suitable as sensor.In fact, the magnetic-field splitting mainly depends on thefield intensity and thus can be used as magnetic probes andthe highest energy peak position which depends on bothmagnetic field and pressure can be used in combinationwith the Zeeman splitting to unambiguously determine thepressure and magnetic field.Hence, KY (WO 4 ) 2 crystals doped with Y b 3+ and Er 3+ areexcellent systems for using as probes of high magneticfield and high pressure conditions through spectroscopicproperties of the Er 3+ green photoluminescence via upconversion.The PL spectrum at low temperature consistsof a series of Zeeman split peaks, which are distinctly sensitiveto the magnetic field intensity and pressure. This behaviorallows us to identify selected peaks by energy andintensity that provides an unambiguous determination of Band P simultaneously through the peak position E (B, P).The measured shift rates make it suitable for using as doublymagnetic and pressure sensor [Valiente et al., acceptedin High Pressure Research].M. Millot, S. George and J.M. BrotoR. Valiente, J. Gonzalez, F. Rodriguez (DCITIMAC, Santander, Spain), S. Garca-Revilla ( ESI, Bilbao, Spain), Y.Romanyuk (EMPA, Dübendorf, Switzerland) and M. Pollnau (MESA+, Enschede, Netherland)77

MAGNETIC SYSTEMS 2009Nd 3+ crystal-field studies of weakly doped Nd 1−x Ca x MnO 3Manganites RMnO 3 compounds based on lanthanides (R)are antiferromagnetic systems with important Jahn-Tellerdistortions. Substitution of lanthanides by Ba, Sr, or Caleads to appearance of double exchange interactions, reductionof Jahn-Teller-type distortions and simultaneous observationof metallic and ferromagnetic character with increasingmolar fraction x (R 1−x A x MnO 3 , A = Ba, Sr, orCa). In this work, Nd 3+ ions crystal field (CF) excitationsin Nd 1−x Ca x MnO 3 (x = 0.025, 0.05 and 0.1) singlecrystals have been investigated using infrared transmissionspectroscopy at different temperatures and external magneticfields and compared to undoped NdMnO 3 .Our study reveals the presence of a magnetic phase separationin the doped samples. As a consequence of dopingby calcium, we report the detection of two sets ofCF levels, as already observed in Nd 1−x Sr x MnO 3 [see S.Jandl et al., Phys. Rev. B 71, 024417 (2005) and S.Jandl et al., ibid 72, 024423 (2005)]. One is associatedwith unperturbed sites related to the NdMnO 3 antiferromagnetismwith its typical Zeeman splitting below Neeltemperature, and a second one is linked to perturbed sitesin the vicinity of the Ca 2+ cations where local A-typeantiferromagnetism is suppressed. While the energy differencesbetween the two sets are within the uncertaintyvalues of the CF parameters that describe the NdMnO 3CF Hamiltonian and predict the CF levels, their detectionstrengthens the role of local probe the rare-earth CF levelsplay in manganite compounds. Finally, under appliedexternal magnetic field, Zeeman splittings are observed inNdMnO 3 and Nd 0.975 Ca 0.025 MnO 3 , while they are maskedin Nd 0.95 Ca 0.05 MnO 3 and Nd 0.9 Ca 0.1 MnO 3 . The latter resultsare due to doping-induced band broadening and possibletwining.The Zeeman splitting well-resolved at lower doping densitiesis demonstrated in Fig. 104, where the middle-infraredtransmission spectra of investigated samples taken withoutand with the externally applied magnetic field (parts A andB) are presented. For details see, S. Jandl et al., J. Magn.Magn. Mater. 321, 3607 (2009).Figure 104: Transmission spectra showing 4 I 9/2 → 4 I 11/2 Nd 3+transitions in Nd 1−x Ca x MnO 3 at T = 1.8 K: x = 0 (a), x = 0.025(b), x = 0.05 (c) and x = 0.1 (d) measured at (A) B = 0 T andB = 12 T. ∗ indicates the new CF excitations that are due to thedoping. The Zeemen splitting is well-resolved at lower concentrations(x = 0 and 0.025) but masked at higher doping densities.M. OrlitaS. Jandl (University of Sherbrooke, Canada), A. A. Mukhin, V. Yu. Ivanov (General Physics Institute of the RussianAcademy of Sciences, Moscow, Russia), A. Balbashov (Moscow Power Engineering Institute, Moscow, Russia)78

2009 MAGNETIC SYSTEMSMagnetotransport in a disordered Zn 1-x Mn x GeAs 2 alloyMagnetotransport studies in high magnetic fields are a veryeffective tool for studying the interplay between the electronicand magnetic properties of magnetic materials. Inthis work the results of the magnetoresistance and Hall effectstudies in Zn 1-x Mn x GeAs 2 (0.053 ≤ x ≤ 0.182) are presented.The investigated compound Zn 1-x Mn x GeAs 2 belongsto the II-IV-V 2 group of diluted magnetic semiconductors.Our recent studies of magnetic properties of thisalloy [L. Kilanski, et al., Acta Phys. Pol. A 114, 1151(2008)] showed that the transition from a Curie-Weiss paramagnetinto a ferromagnetic material with Curie temperaturesexceeding 300 K occurs in this system for x ≥ 0.06.The high Curie temperature observed in this alloy is probablyconnected with the presence of nano-sized MnAs grainsin the crystal.The results of Hall effect investigations in the form of magneticfield dependence of the off-diagonal resistivity tensorcomponent ρ xy are presented in figure 106. It is interestingto note that in the Zn 1-x Mn x GeAs 2 crystals we do notobserve the anomalous Hall effect, a phenomenon widelyobserved in ferromagnetic semiconductors. The ρ xy (B)dependence in the case of ferromagnetic Zn 1-x Mn x GeAs 2(x ≥ 0.06) crystals showed the presence of large nonlinearitiesat temperatures lower than 9 K. The bending of theρ xy (B) curve is the most prominent in the case of the samplewith the highest concentration of Mn ions. Such behaviormay indicate the presence of two types of conductingcarriers influencing the carrier transport in this material.It is well established that the magnetic properties of crystalshave a significant influence on the electron transport inZn 1-x Mn x GeAs 2 samples with different Mn content. Apartfrom changes in the basic transport properties such as carrierconcentration and resistivity, we have also observed asignificant differences in the high field magnetoresistanceand the Hall effect. The magnetic field dependence of theresistivity tensor component ρ xx recorded at T ≈ 1.45 K arepresented in figure 105.In the case of the paramagnetic Zn 0.947 Mn 0.053 GeAs 2 samplewe observe a small negative magnetoresistance saturatingat moderate magnetic fields of about B ≈ 50 kOe. Theorigin of this behavior of the magnetoresistance curve is thespin-disorder scattering of the conducting carriers on theMn magnetic moments embedded inside the semiconductorhost. It may be added that the negative magnetoresistance isobserved only at temperatures lower than 5 K. The positivecontribution to the magnetoresistance observed clearly athigher temperatures results from the orbital carrier movementin the presence of the external magnetic field. Thispositive contribution competes at low temperatures with theeffect of spin disorder scattering causing the curves to showa minima.In the case of the two ferromagnetic Zn 1-x Mn x GeAs 2 sampleswith x > 0.06 we observed negative magnetoresistancewith an amplitude much larger than in the case of the paramagneticcrystal. The observed effect is interpreted as agiant magnetoresistance due to the polarization of conductingcarriers inside ferromagnetic grains (similar to that observedin the case of granular ferromagnets). The amplitudeof the giant magnetoresistance is highly compositiondependent due to changes in the concentration of magneticinclusions in the material.Figure 105: Magnetoresistance curves obtained at T ≈1.45 K forZn 1-x Mn x GeAs 2 crystals with different chemical composition x.Figure 106: Magnetic field dependence of the Hall resistivity ρ xymeasured at T ≈ 1.45 K for Zn 1-x Mn x GeAs 2 crystals with differentchemical composition x.D. K. MaudeL. Kilanski, W. Dobrowolski (Institute of Physics Polish Academy of Science, Warsaw, Poland),S. A. Varnavskiy, S. F. Marenkin (Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russia)79

MAGNETIC SYSTEMS 2009Anomalous Hall effect in (Ge,Mn)Te-(Sn,Mn)Te spin-glasslike crystalsThe anomalous Hall effect shows a direct interplay betweenmagnetic and electronic properties of solid. Inthis contribution, we show the results of the anomalousHall effect studies in the series of Ge 1-x-y Sn x Mn y Tecrystals with chemical composition 0.090≤x≤0.142 and0.012≤y≤0.115 in the temperature range 1.4≤T ≤200 Kand magnetic fields up to 130 kOe. Recent investigation ofmagnetic properties of this alloy [Kilanski, et al., J. Appl.Phys. 105, 103901 (2009)] showed the presence of the spinglasslikestate with transition temperatures T SG ≤60 K.R S is probably more complex function of both magneticand electrical properties than just chemical compositionof the alloy. It may be noted that the obtained anomalousHall constants are similar to the ones observed in the literaturefor other IV-VI based diluted magnetic semiconductorslike Sn 1-x Mn x Te and Ge 1-x-y Mn x Eu y Te [Brodowska, et al.,Journal of Alloys and Compounds. 423, 205 (2006)].The magnetic field dependence of the resistivity tensorcomponent ρ xy for Ge 0.815 Sn 0.091 Mn 0.094 Te crystal at temperaturerange 4.3≤T ≤50 K is presented in figure 107. Thesharp increase of ρ xy (B) dependence visible in the low fieldregion is the manifestation of the anomalous Hall effect (attemperatures lower than T SG ). The off-diagonal resistivityρ xy (B) is a sum of ordinary R H and anomalous R S contributiondescribed by the following equationρ xy = R H B + µ 0 R S M(H), (15)where µ 0 is a magnetic permeability of vacuum. In orderto determine the anomalous Hall constant the isothermalmagnetization curves M(H) were measured at thesame temperatures in which magnetotransport effectswere investigated. The magnetization curves for selectedGe 0.850 Sn 0.119 Mn 0.031 Te crystal are presented in figure 108.The M(H) dependencies (see figure 108) shows nonsaturatingbehavior even at magnetic fields as high as 100 kOe.The magnetization curves observed at temperatures lowerthan spin-glass freezing temperature showed behavior differentthan that for spin-glass system, namely the largespontaneous magnetization. The experimental M(H)curves are more complex than that of a Weiss ferromagnetwhat may be interpreted that short range interactionsplay an important role in this system. The maximum valuesof magnetization obtained in each crystal were slightlysmaller than the estimated using chemical composition ofthe alloy. It indicates, that possibly a fraction of Mn ionsremained in a spin state reducing their magnetic moment orthere exists antiferromagnetic clusters.The equation 1 was fitted to the experimental data in orderto extract values of ordinary R H and anomalous R SHall coefficients. The obtained values of R S varies betweencrystals with a different amount of both alloyingelements by about an order of magnitude in the range of2.7×10 −5 cm 3 /C and 30×10 −5 cm 3 /C. The chemical compositionof the alloy has significant effect on the R S i.e. itdecreases with the amount of Sn in the alloy and increasewith the Mn content. It must be noted, however that theFigure 107: Magnetic field dependence of the off-diagonal resistivityρ xy obtained for selected Ge 0.815 Sn 0.091 Mn 0.094 Te crystal.Figure 108: Magnetetization curves obtained at different temperatures(see legend) for selected Ge 0.850 Sn 0.119 Mn 0.031 Te crystal.A. B. AntunesL. Kilanski, W. Dobrowolski (Institute of Physics Polish Academy of Science, Warsaw, Poland), V. E. Slynko, E. I.Slynko (Institute of Materials Science Problems, Ukrainian Academy of Sciences, Chernovtsy, Ukraine)80

2009 MAGNETIC SYSTEMSHigh field torque magnetometry on a molecular Dysprosium triangleLanthanide based molecular nanomagnets have been attractingconsiderable attention due to their interesting magneticproperties. Mononuclear complexes of 4f ions haveshown slow relaxation of the magnetization at very hightemperatures compared to those observed in transition metals.Despite their out of phase ac signal observed above 40K, hysteresis curves were observed only at very low temperatures[Ishikawa et al., Angew. Chem. 117, 2991 (2005)].On cooling, deviations from the Arrhenius law predictedfor single molecule magnet behavior become more important,which indicates that tunnelling plays a crucial role inthe relaxation of the magnetization. It is therefore of crucialimportance to obtain information on the low lying sublevelsof the 4f electronic systems, in order to understand the magnetism,and especially the relaxation mechanisms of thesecompounds.We have performed high field torque magnetometry measurementson the recently investigated molecular Dy triangle[Luzon et al., Phys. Rev. Lett. 100 247205(2008)]. This compound shows an unprecedented magneticbehaviour having a non-magnetic ground doublet whichoriginates from the noncollinearity of the single-ion easyaxes of the Dy 3+ ions that lie in the plane of the triangleat 120 ◦ one from each other. This gives rise to a peculiarchiral nature of the ground nonmagnetic doublet and toslow relaxation of the magnetization which exhibits abruptaccelerations at the crossings of the discrete energy levels.The ground (|J = 15/2,m J = ±15/2〉) and the first excited(|J = 15/2,m J = ±13/2〉) doublets, were considered to describethe energy levels of the single ion assuming that theother excited states are very high in energy and do not contributeto the magnetic properties at low temperatures. Thesystem therefore mimics the behaviour of an S=3/2 spin.We have used the spin Hamiltonian approach to describethe low lying energy levels. The expression of the Hamiltonianused is,anisotropy in the system. The performed high field torquemeasurements at 50 mK enabled us to quantify the crystalfield splitting between the 15/2 and the 13/2 doubletsin Dy 3+ . Figure 109 (a) shows the measured torque signalat 50 mK up to 32 Tesla for the magnetic field applied atdifferent angles to the plane of the triangle. A peak in thetorque is evident at around 28 Tesla. Figure 109 (b) showsthe calculated torque signals for δ = 250 cm −1 , j = 0.064cm −1 , and g = 1.35 at 50 mK, which reproduce fairly wellthe experimentally observed curves. The observed peak inthe torque signal at around 28 Tesla for transverse magneticfields points towards the high anisotropy in the system. Thecrystal field splitting of δ = 250 ± 10 cm −1 is close to thatexpected from previously performed ab initio calculations[Chibotaru et al., Angew. Chem. Int. Ed. 47, 4126 (2008)].In a more general picture, this study has contributed to abetter understanding of lanthanide based systems. In particular,we have proven that high field torque magnetometrycan be a good substitute to spectroscopy in systems whichare spectroscopically inactive.Ĥ = − j(Ŝ 1 · Ŝ 2 + Ŝ 2 · Ŝ 3 + Ŝ 3 · Ŝ 1 )− gµ B ∑ B · Ŝ i + δ oi=1,314 ∑ (( 15i=1,32 )2 − Ŝ 2 z i)(16)The first term is the isotropic exchange between the Dy 3+ions, the second is the Zeeman term, and the last termdescribes the single-ion anisotropy where δ o is the zerofield splitting between |J = 15/2,m J = ±15/2〉 and |J =15/2,m J = ±13/2〉 states of each Dy 3+ ion. We have particularychosen torque magnetometry as it is a strong tool toinvestigate the anisotropy in high-spin clusters, especiallyin systems which are not accessible by spectroscopy. Apeak in the torque signal occurs at a characteristic fieldwhich depends on the temperature as well as the magneticFigure 109: (a) Torque signals for the magnetic field applied atdifferent angles close to 90 ◦ from the plane of the triangle at 50mK. (b) Similar calculated torque curves with the best fit parameters(δ = 250 cm −1 , j = 0.064 cm −1 , and g = 1.35). The temperatureand anisotropy dependent peak in the torque signal is evidentat around 28 Tesla. The inset shows the molecular structure of theDy 3 cluster.A. B. Antunes, I. SheikinF. El Hallak, J. van Slageren, M. Dressel (University of Stuttgart, Germany), M. Etienne, J. Luzon, R. Sessoli (Universityof Florence, Italy), C. Anson, A. Powell (University of Karlsruhe, Germany)81

MAGNETIC SYSTEMS 2009NMR evidence for long zero-quantum coherence in antiferromagneticmolecular wheels NaFe 6 and LiFe 6Molecular magnetic clusters of nanometer size have receivedenormous attraction because of their spectacularquantum phenomena. A unique class of these clusters arethe antiferromagnetic (AFM) molecular wheels, in whichmagnetic metal ions are assembled in a ring-like structure.The dominant AFM Heisenberg interaction J between themagnetic metal ions leads to a nonmagnetic S = 0 groundstate and a first excited S = 1 state in zero magnetic field.In strong magnetic fields the Zeeman splitting induces levelcrossings (LCs), where the ground state of the moleculechanges from S = 0, M = 0 to S = 1, M = −1, and furtherto S = 2, M = −2, etc. Due to the hyperfine couplingbetween the nuclear spin and the Fe III ions, the nuclear relaxationrate T −11is very sensitive to the spin state and thespin dynamics of the ferric wheel. Most NMR studies havebeen carried out by means of 1 H NMR, since protons providea very strong signal, and their T −11is sensitive to thedynamics of the Néel vector ⃗n = ∑ 6 i=1 (−1)i ⃗s i . However, amajor drawback is a huge amount of inequivalent protonson each ring complicating the analysis of the spin dynamics,especially around the level crossings. In order to overcomethis problem we performed NMR measurements onthe nuclei which are located in a single-site position of highsymmetry, in the center of the ferric wheel–here 23 Na or 7 Linuclei. Their hyperfine interaction is proportional to the totalspin of the ferric wheel ⃗S = ∑ 6 i=1 ⃗s i leading to importantdifferences as far as T −11is concerned.Here we present the first central alkali NMR study atlow temperature and at high field, focusing on the magneticfield dependence at the level crossings [L. Schnelzeret al., submitted to EPL]. NMR measurements on 23 Naand 7 Li nuclei were carried out on single crystals of [Na/Li⊂Fe 6 {N(CH 2 CH 2 O) 3 } 6 ]Cl·5CHCl 3·0.5H 2 O (Na/LiFe 6in short). They were performed in 17 and 20 T superconductingmagnets in Grenoble and Karlsruhe. Single crystalswere mounted in the mixing chamber of a dilution refrigeratorfor very low temperatures and in pumped 4 He for measurementsat 2 K. Figure 110 shows the measured T −11ratesat 220 mK and 2 K. The low temperature measurements ofNaFe 6 at 220 mK reveal a strong increase of T1 −1 towardsthe LC at 12 T. The LC is not characterized by an additionalenhancement of T1 −1 as expected for proton NMR,but by a small reduction of T1 −1 (inset to figure 110a). Thisis attributed to the insensitivity of the central alkali nucleito the fluctuations of the Néel vector. At 2 K T1 −1 shows abroad maximum around LC and, surprisingly, a reductionby three orders of magnitude of T −11in-between the 1 st andthe 2 nd LC. Similar results have been obtained for LiFe 6 ;plotted on a reduced field scale (B/J) the measurements onLiFe 6 and NaFe 6 are almost identical. Measurements of23 Na T1 −1 at the first LC, at temperatures down to 80 mK,reveal the existence of a very small gap 0.06 K, in spiteof high symmetry of the molecule. This implies the existenceof a small perturbation reducing the symmetry of themolecule. The most striking phenomena is a spectaculardecrease by three orders of magnitude of T1 −1 occurring inthe middle between the first and the second LC.The relaxation data have been described by calculating thecorresponding spectral density of spin fluctuations by amethod of moments. It turns out that the observed strongextinction of relaxation is not at all visible in the intensity ofthe zero frequency resonance (the zero moment), but is entirelydue to its width, calculated as the second moment m 2 .For the calculation of m 2 we used the secular (i.e., energyconserving) part of the dipolar intermolecular interaction.Between the first and the second LC there is a broad andvery deep minimum of m 2 . In this field range the groundstate is a S = 1 state whereas the first excited state changesfrom S = 0 to S = 2 through a broad anti-LC. Since theminimum of m 2 results from the balance of the matrix elementsof these states, it extends over the same field range asthe anti-LC. It is remarkable that √ m 2 can become smallerby two orders of magnitude than the nuclear Larmor frequency.This quite unusual situation leads to the reductionof T1 −1 by three orders of magnitude. One should also realizethat the correlation time τ c = 1/ √ m 2 corresponds tothat of a zero quantum coherence. τ c can be as long as ≃0.2 µs, which is very unusual for an electronic spin system.Figure 110: (a) Field dependence of 23 Na T1 −1 in NaFe 6 atT = 220 mK (white squares) and 2 K (black squares), and (b) 7 LiT1 −1 in LiFe 6 at T = 2 K (black squares) and 1 H T1 −1 (dots). Theresults of the calculations are given by solid lines. Inset displays azoom on the field dependence close to the LC at 220 mK.M. Horvatić, C. BerthierL. Schnelzer, B. Pilawa, M. Marz, H. von Löhneysen (Physikalisches Institut, Universität Karlsruhe,Germany)82

2009 MAGNETIC SYSTEMSNuclear magnetic resonance determination of spin-superlattice structureof magnetization plateaus in SrCu 2 (BO 3 ) 2Quantum spin systems are nowadays in the focus of considerableexperimental and theoretical efforts, in particularfor the magnetic field induced exotic quantum states. Ofspecial interest are interacting systems of Heisenberg, antiferromagnetic,S=1/2 spin dimers in a magnetic field whichcloses the gap between the singlet and one of three Zeemansplittriplet levels of each dimer. At low temperature, whenonly these two states per dimer are relevant, the systemcan be described by the corresponding S=1/2 pseudospins,or the equivalent hard-core bosons. Interacting dimers arethen equivalent to a system of interacting bosons where boson(triplet) density can be tuned by the magnetic field. Thissystem generally undergoes a Bose-Einstein condensation(BEC) and therefore provides an exceptional access for experimentalstudies of this phenomenon. In the hard-coreboson representation the balance of the kinetic and the interaction(repulsion) energy is determined by the degree offrustration of interdimer spin coupling, where frustrationstrongly reduces kinetic energy. In this latter case, insteadof being itinerant and undergo BEC, bosons can be localizeddue to mutual repulsion into a charge ordered state,that is a Wigner crystal. Since this state is gapped, the bosondensity will be magnetic field independent and the systemwill present a plateau of magnetization.Perfect frustration occurs in the geometry of orthogonaldimers in 2D, given by the Shastry-Sutherland Hamiltonian,and the SrCu 2 (BO 3 ) 2 compound is its first recognizedrealization. The discovery of the magnetization plateaus at1/8, 1/4 and 1/3 of the saturation magnetization in this compoundwas followed by extensive experimental and theoreticalinvestigation. In particular, NMR study performed atLNCMI proved that the 1/8 plateau, as predicted, indeedcorresponds to a commensurate spin superstructure. Subsequentstudies showed that a superstructure persists at highermagnetic field [Takigawa et al., Phys. Rev. Lett. 101,037202 (2008)], and that there are other, yet undiscoveredplateaus. One of these, adjacent to the 1/8 plateau on thehigh field side, was indeed confirmed by the torque measurements[Levy et al., EPL 81, 67004 (2008)]. At the sametime appeared several contradictory predictions for the existenceof many other magnetization plateaus in this system,calling for experimental verification. However, theoreticaldescription of SrCu 2 (BO 3 ) 2 is very difficult, because the interdimerinteraction is too strong to be properly describedby a perturbation theory, while exact numerical methods arelimited to only very small 2D systems.We have therefore continued our NMR investigation to observeevolution of the spin superstructure through magneticfield dependence of 11 B NMR spectra in the 27-34 T range,at 0.43 K. From the observed spectra we clearly identifyplateau phases where the NMR spectra, and thus the spinsuperstructure, is magnetic field independent. In additionto the long known 1/8 and 1/4 plateau and the recently discoveredplateau adjacent to the 1/8 plateau, a new plateauwas discovered half way up towards the 1/4 plateau (seefigure 111). Between these plateaus NMR spectra evolvecontinuously with magnetic field, and no other plateau wasdetected. The same sequence of plateaus was confirmedby the new, “differential” torque measurements, performedat ∼0.1 K. This method determines the magnetization withconsiderably enhanced precision, allowing us to find thatthe magnetization of the first three plateaus scales as 1/8: 2/15 : 1/6, which confirms some of the theoreticallyproposed values and excludes others. Detailed analysis ofthe 11 B NMR spectra allows us to make complete determinationof the spin-polarization superstructures. We findthat previously identified “extended triplets” are always arrangedin stripe structures, specific to each plateau, oftendifferent from what is proposed theoretically [Takigawa etal., unpublished].Figure 111: The distribution of the internal field at the 11 B sites,obtained by deconvoluting the NMR spectra from the quadrupolesplitting. Presented spectra are representative of 4 magnetizationplateaus, attributed to fractions 1/8, 2/15, 1/6 and 1/4 of saturationmagnetization.M. Horvatić, C. Berthier, S. Krämer, I. SheikinM. Takigawa, T. Waki, Y. Ueda (ISSP, University of Tokyo, Japan), H. Kageyama (Kyoto University, Japan), F. Mila(EPFL, Lausanne, Switzerland)83

MAGNETIC SYSTEMS 2009Magnetic structure of the half magnetization plateau phase in CdCr 2 O 4CdCr 2 O 4 belongs to the well-known family of cubic Crbasedspinels ACr 2 O 4 (A=Hg, Cd, and Zn) which have attractedmuch attention because of the highly frustrated pyrochlorelattice formed by their magnetic Cr 3+ (S = 3/2)ions. Furthermore, due to the direct overlap of t 2g orbitalsof neighboring Cr 3+ ions (3d 3 ), the spin Hamiltonianhas dominant isotropic antiferromagnetic nearest neighborinteractions. The resulting strong frustration suppressesthe system from ordering down to a much lower temperaturethan the Curie-Weiss temperature Θ CW . In case ofCdCr 2 O 4 , the system remains paramagnetic up to T N =7.8 K, far below |Θ CW | = 88 K. The ordered state is accompaniedby a cubic to tetragonal structural transition, and isnot a simple collinear antiferromagnet, but an incommensurate(IC) helical magnetic order with a single characteristicwave vector of Q m = (0,δ,1) or (δ,0,1) where δ ∼ 0.09[Ueda et al. Phys. Rev. Lett. 94, 047202 (2005); Chunget al., Phys. Rev. Lett. 95, 247204 (2005)]. Upon applicationof an external magnetic field, CdCr 2 O 4 undergoes aphase transition into a half-magnetization plateau phase atH c1 = 28 T suggesting that each tetrahedron has three upand one down spins (3:1 constraint). Under this restriction,two spin arrangements, one with the rhombohedralR3m symmetry and one with the cubic P4 3 32, are possible,depending on the sign of the next nearest neighbor interaction.Thanks to the recent combination of a 30 T portable miniaturepulsed magnet and the world highest flux neutronsource of the Institut Laue-Langevin (ILL) [Yoshii et al.,Phys. Rev. Lett. 103, 077203 (2009)], we have succeededin following the field dependence of selected magneticBragg reflections, which allowed to distinguish betweenthese two possible magnetic structures. The experimentwas carried out on the thermal neutron tripleaxisspectrometer IN22. The single crystal (a thin plate(∼ 4 × 4 × 0.2 mm 3 ) of ∼ 40 mg) was mounted with the[111] and [110] axes in the horizontal scattering plane. Thepulsed measurements were performed more than 100 timesat each reflection to obtain reasonable statistics.Figure 112(a) shows the time dependence of the elastic neutronscattering intensity measured at the IC magnetic peakof (1.0675, -1.0125, 0.0275) at 2.5 K. The peak intensitygradually decreases to background level and then remainszero between 3 and 4.6 ms (H > 28 T) after which the intensityincreases back to the intermediate level but not tothe original intensity because of magnetic domain orientation.In order to find out where the magnetic intensity ofthe IC peak was transferred to, we performed similar measurementsat a commensurate Q = (1,-1,0) position. Asillustrated in figure 112(b), when a magnetic field was applied,no signal was initially observed at (1,-1,0) for 3 msat which point the intensity suddenly increased due to thefirst-order nature of the field-induced phase transition. Thecommensurate magnetic intensity remained non-zero overexactly the same range of time (and field) over which theIC magnetic signal went down to zero. Our results indicatethat as CdCr 2 O 4 enters the half magnetization plateaustate, the magnetic structure changes from the IC spiral to acommensurate collinear spin structure with Q m = (1,0,0).Figure 112: Time dependence of the magnetic field (solid redlines) and neutron counts (filled circle) measured at (1.0675,-1.0125, 0.0275) and (1,-1,0) reflections at T = 2.5 K.Figure 113: Magnetic field dependence of the peak intensity ofthe (2,-2,0) reflections measured at T = 2.5 K with the ascending(filled circles) and descending (open circles) field.To distinguish between the two possible models, we alsoperformed similar pulsed field measurements at (2,-2,0) atwhich point the R3m structure should produce magneticBragg scattering while the P4 3 32 structure would not. AtH = 0 T, nuclear Bragg intensity is observed at (2,-2,0). Asshown on figure 113, the (2,-2,0) intensity does not changeas the system enters the half-magnetization phase. Thus, weconclude that the half-magnetization spin state of CdCr 2 O 4has the P4 3 32 spin structure.F. Duc, P. Frings, B. Vignolle, G.L.J.A. RikkenH. Nojiri, K. Ohoyama, S. Yoshii (Institute for Materials Research, Tohoku University, Japan), M. Matsuda (JAEA,Tokai, Japan), L-P. Regnault (CEA, DRFMC-SPSMS-MDN, Grenoble)84

2009 MAGNETIC SYSTEMSStructural analysis with pulse-length exposures up to 30 Tesla at ID06, ESRFAs topical tests of the newly commissioned experimentalsetup at ID06, ESRF, synthetic powder samples of high T cNdFeAsO and of a natural atacamite, Cu 2 Cl(OH) 3 , havebeen measured in pulsed fields of up to 30 Tesla, usingpulse-simultaneous exposure times of less than 10 ms attemperatures down to 150 K (NdFeAsO) and 6 K (atacamite).temperatures and fields (Rp ∼ 0.06-0.07). The lower panelshows there is a measurable, and isotropic, effect on the inplanecell parameters with applied field. Preliminary analysisof cell parameter have given us direct information on thelattice strain in the sample. This taken with variable temperatureand field data should allow us to establish how the anycoupling parameters at the transition are field-affected. Theinternal parameters would give information on characteristicstructural features; such as bond lengths and perhapsmore critically, bond angle behaviour under variable fieldand temperature.Figure 114: Lattice distortion in NdFeAsO. The top panel shows,in blue, the variation in b/a, or the level of in-plane orthorhombicdistortion and in red the level of pseudo-tetragonal [2c/(a + b)]distortion at all temperatures and fields of 3 - 30 T . The lowerpanel indicates the effect on in-plane cell parameters at a fixedtemperature of 90 K, while augmenting pulse strength to 30 T.The green line shown indicates an increasing trend of 0.04 ÅT −1 .The two samples offer different challenges; not only froma point of view of disturbing their inherent magnetic propertiesat reduced temperature, but also from our ability toextract reliable structural information from powder sampleswith contrasting symmetries and structural complexities.For NdFeAsO, it is evident from figure 114 that lattice distortionmarkers show most variance in the range of measuredtemperatures between 120−205 K. Above and belowthese points there is little scatter, despite the same numberof data at each temperature and equivalent quality fits at allFigure 115: Rietveld refined, to R Bragg = 0.06, structure of atacamiteat T = 7 K, B = 30 T from 7 ms data collection on mar345image plate.The second study was designed to test the limits ofwhat structural analysis would be possible; using an altogethermore complicated orthorhombic structure, at basetemperatureof the cryostat and with our pulse-length limitedexposure times. Again here, the aim would be to determineinternal structural parameters to estimate if it wouldbe possible to extract any affect of high-field applicationon the frustration of the Kagomé-like lattice. We havebeen successful in refining atacamite to R Bragg of 0.06 atsub 10 K, with field strengths of up to 30 Tesla and pulselockedexposure times. Figure 115 shows the refined structure.Detailed analysis of the full dataset will determineif any field-induced frustration can give rise to a tractableresponse at these extreme conditions, though with experiencegained in setup, control and running this demandingmeasurement; we are confident that these methods offerconsiderable scope for future in situ diffraction research.F. DucT. Roth, C. Detlefs, W. A. Crichton (ESRF, Grenoble, France), S. Margadonna (University of Edinburgh, U.K.)85

MAGNETIC SYSTEMS 2009Magnetization at low temperatures and high magnetic fields on LuFe 2 O 4LuFe 2 O 4 is thought to be a multiferroic [Ikeda et al. Nature436, 1136-1138 (2005)], with a novel ferroelectric mechanism,based on charge order. The electrically active Fe withaverage valence of 2.5+ is contained in trigonal Fe-O doublelayers, a highly frustrated arrangement. Below ∼ 320 Kthe Fe valences order, resulting in the double layers becomingpolar, apparently with an antiferroelectric stacking ofthe polarization of the individual double layers [M.Angst etal. Phys. Rev. Lett. 101, 227601 (2008)].The frustration also effects the spin ordering occurring below240 K [Christianson et al. Phys. Rev. Lett. 100,107601 (2008)]. Several indications for magnetoelectricityhave been observed, though the microscopic details ofthis coupling remain to be elucidate. Apart from magnetoelectriccoupling, the magnetism in LuFe 2 O 4 has also attractedattention due to a giant magnetic coercivity, recentlyattributed a freezing of nano-scale pancake-like (Ising) ferromagneticdomains. In low fields we have observed magnetictransitions of sharpness and clear 3D spin order. Bymagnetization, spectroscopic, neutron and synchrotron experimentswe identified a further transition at 170 K, involvinga disruption of magnetic order and a structural distortionwhich can be tuned by a magnetic field.In our experiment at the LNCMI with a 10MW-magnet weperformed hysteresis loops with H||c and a magnetic fieldup to 22 T, within a temperature range between 60 K andhelium base temperature of 3 K. All the magnetization datawas measured with the extraction method. These measurementswere done after cooling down the sample in a zerofield (ZFC). From this ZFC we were able to obtain a virgincurve of the magnetization. The hysteresis loop at basetemperature (inset figure 116) shows a plateau in its virgincurve at a magnetic field of 15T, what is in good agreementwith the data from optical reflectance contrast [Xu etal. Phys. Rev. Lett. 101, 227602 (2008)]. At this valuethe transition is not complete and it remains up to a valueof about 20T until the crystal structure is switched (fromhexagonal to monoclinic) and the magnetization is saturated.We observe only one step in the virgin magnetizationcompared to [Iida et al. Physica B 155 307, (1989)] whereseveral occur. This is may be due to a better crystal qualitywith less different grains. From the in loop magneticbehaviour we were able to complete the magnetic phase diagramfor LuFe 2 O 4 for low temperatures, as shown in figure117, where it is shown that there is a coexistence betweena ferromagnetic (FM) phase and a antiferromagnetic(AFM) phase at low temperatures.We also measured the magnetization in as high fields as feasibleperpendicular to the c-axis. In this part of the experimentthere was no indication of a lacking of the magnetocrystallineanisotropy. This means that the magnetic momentsare all aligned in c-direction.Figure 116: Magnetic field dependence from the magnetizationwith H||c in LuFe 2 O 4 at different temperatures. All measurementswhere taken in zero field cooling (ZFC).Figure 117: Low temperature H-T phase diagram for H||c inLuFe 2 O 4 determined from data shown in figure 116 (arrows indicatethe direction of the transition as a function of the appliedfield).A. B. AntunesJ. de Groot, M. Angst (Forschungszentrum Jülich GmbH, 52428 Jülich, Germany)86

2009 MAGNETIC SYSTEMSEnhancement magnetic moment in the single phase nanostructure Gd 3 Fe 5 O 12Rare earth iron garnets (REIG) are promising candidatesfor use in high performance microwave and electrochemicaldevices owing to its high resistivity, high Curie temperature,and high chemical stability and possess uniquemagnetic, optical, thermophysical and mechanical properties.Gd 3 Fe 5 O 12 (R 3+3 c[Fe 3+2 ] a (Fe 3+3 ) dO 2−12h) particles weresynthesized in polycrystalline form by the solid-state reactiontechnique from a mixture of α−Fe 2 O 3 and Gd 2 O 3 innominal compositions of 5 : 3. The X-ray diffractogramsshow that all the milled Gd 3 Fe 5 O 12 garnet particles retaintheir single phase structure (Ia3d space group). The averagegrain size decreases with milling and reaches 29 nm forthe 40 hours milled sample. The 57 Fe Mössbauer spectrawere recorded at 300 K and 77 K for the different samples.There is no evidence for the presence of Fe 2+ chargestate. On increasing the milling time, one observes the progressiveoccurrence of a central quadrupolar feature at both300 K and 77 K, in addition to three magnetic sextets withdecreasing intensity but increasing line width compared tothat of the bulk. The quadrupolar feature has to be decomposedat both temperatures into two broad line quadrupolardoublets: the presence of ferric and ferrous species isthen identified. The Fe 2+ content linearly increases withthe milling time (figure 118).The measured magnetization (M) for the as-prepared GdIGis found equal to 15.9 µBmol −1 at 4.2 K. When the grainsize is reduced below 100 nm, the M is strongly appliedfield dependent and no saturation is observed even underthe highest applied field of 320 kOe (figure 119). In the170 − 320 field range the magnetization curves of the differentmilled samples are very well (within 1%) describedby the approach law M = M sat (1 − b/H 2 ). According toNéel the b coefficient is determined by the magnetocrystallineanisotropy and given by b = 8K 2 /105(M sat ) 2 .18.For the 35h milled sample M sat is only equal to 14 µBmol −1and remains much smaller than the bulk value. The socalculatedvariation of K is reported in figure 120 versusthe Fe2+ content where a large increase of K is observedwhen the milling time increases. The modified formulaeof our GdIG particle is given by Gd 3+3(Fe 3 3(1−r) + Fe2+ 3r )[Fe 3 2(1−s) + Fe2+ 2s ] O2− 12−2.5ptaking into account the presenceof oxygen vacancies related to the Fe 2+ content (p). Assumingthat only the Fe 3+ contribute to the ferrite magnetizationr and s values were calculated using the p and M satvalues deduced from the Mössbauer spectra and high fieldbehavior analysis respectively. The particular behavior ofthe 35h milled samples is then explained by the fact thanthe octahedral site contains the largest part of Fe 2+ ion. Wenote that (i) the T comp values of the nanogarnets are few degreeshigher than that of the bulk; (ii) However, the Curietemperature (T C ) of the various sized nanocrystalline GdIGsamples are found to be significantly higher than that of thebulk.Figure 118:Figure 119:Figure 120:Fe 2+ content versus the milling time.Isothermal magnetization curves at 4.2K.Anisotropy constant versus the Fe 2+ content.M. GuillotC. N. Chinnasamy, B. Latha, T. Sakai, S. D. Yoon, C. Vittoria, V. G. Harris, (Center for Microwave Magnetic Materialsand Integrated Circuits, Northeastern University, Boston). J. M. Greneche (LPEC, Universit du Maine, Le Mans).87

MAGNETIC SYSTEMS 2009Effect of the M/Co substitution on magnetocrystalline anisotropy andmagnetization in SmCo 5−x M x compounds (M=Ga; Al)The RCo 5 compounds have been discovered a few decadesago but are still attracting much interest in order to understandtheir exceptional intrinsic magnetic properties. Theinfluence of substitution of p elements (Al, B, Ga...) forCo on the magnetic properties of the RCo 5 type phases isalso an active field of research nowadays. The samples formagnetization measurements were processed from powderswith a particle size smaller than 25 µm. The powder wasoriented at room temperature using an orientation field oftypically 10 kOe and fixed in slow setting epoxy resin. Themagnetic field was applied either parallel or perpendicularto the alignment direction. According to the X-ray analysis,the CaCu 5 type structure of the RCo 5 is preserved upon substitutionof Ga and Al for Co in the SmCo 4 Al and SmCo 4 Gacompounds. According to the neutron diffraction resultson these isotype compounds, the Al and Ga atoms havea pronounced preference for the Co 3g atomic position inthe hexagonal CaCu 5 structure. For both SmCo 4 Al andSmCo 4 Ga, at room temperature, the easy magnetization directioncoincides with the c-axis of the hexagonal lattice.Ga and Al for Co substitution induce a large reduction ofthe Curie temperature from typically 900 K to 1000 K forthe RCo 5 to around 500 K only for the RCo 4 Al or RCo 4 Gaphases. The RCo 4 M phases containing M=Ga and Al havesimilar Curie temperature.As can be seen from the isothermal curves in figures 121and 122, the use of high magnetic field is required dueto the large magnetocrystalline anisotropy of the samples.The spontaneous magnetization has been determined froma linear extrapolation of the M(H) curve to zero appliedfields. The saturation magnetization has been obtainedfrom extrapolation of the M(H) curve with H parallel tothe alignment direction and larger than 20 T according toM(H) = M sat + a/H2. The replacement of one Co atomby one Ga or Al atom induces a strong decrease of the saturationmagnetization from 8.46 µB/formula unit at 4 K toonly about 4 µB/f.u. Such dramatic decrease of the Co magnetizationis consistent with earlier reported results on isotypecompounds. SmCo 5 is well known for it exceptionallylarge magnetocrystalline anisotropy µ 0 H a ∼ 52 T at 4 K.The anisotropy field can be estimated by the field at whichthe two magnetization curves will merge for the two differentorientations. The anisotropy is found to be largerfor the Ga than for Al containing compounds; 89 T and83 T respectively. An effective magnetocrystalline coefficienthas been derived from the anisotropy field accordingto the relation H a = 2K e f f /M s . The values found for theSmCo 4 Al and SmCo 4 Ga are 16.3 and 19.3 MJ/m 3 respectively,that is to say of the same order of magnitude thanfor the SmCo 5 . Consequently, the huge anisotropy field observedfor the SmCo 4 Al and SmCo 4 Ga result from largeanisotropy constant and a strong decrease of the saturationmagnetization respectively. The presence of Al or Ga hasbeen reported to significantly reduce the Co sublattice contributionto the magnetocrystaline anisotropy in the YCo 4 Alor YCo 4 Ga isotype compounds. Thus we conclude thatthe huge anisotropy field reported here for SmCo 4 Al andSmCo 4 Ga results from a reinforcement of the Sm contributionto the magnetocrystalline anisotropy in comparison tothe SmCo 5 phase. Further studies are in progress to investigatethe thermal dependence of the magnetic features oftheses phases.Figure 121: Isothermal (4.2K) high field magnetization curve ofSmCo 4 Al recorded parallel to the easy magnetization direction inorder to obtain the spontaneous magnetization M spon . Inset; determinationof the saturation magnetization M sat .Figure 122: Isothermal hysteresis cycle recorded at 4 K on orientedSmCo 4 Al when applying the magnetic field perpendicularto the alignment direction (c axis).M. GuillotA. Laslo, C.V. Colin, O. Isnard (Institut Néel, CNRS and Université Joseph Fourier)88

2009 MAGNETIC SYSTEMSInvestigation of the intrinsic magnetic properties of the ThCo 4 B compoundWe have meaured the intrinsic magnetic properties ofThCo 4 B to investigate the influence of a tetravalent elementsuch as Th on the magnetic properties of the Co sublatticein comparison to the already well investigated trivalentrare-earth containing isotype compounds. The compoundis single phase (CeCo4B structure) with two different crystallographicsites for thorium (1a and 1b), two other sitesfor cobalt (2c and 6i) and one site for the boron atom (2d)(figure 123). Magnetic measurements were carried out inthe temperature range 4.2 − 300 K in a continuous magneticfield up to 230 kOe. No single crystals of ThCo4Bwas available, thus the samples were sieved down to a particlesize smaller than 25 µm. The powder was mixed withepoxy resin and subsequently aligned at room temperatureusing an orientation field of typically 10 kOe. The saturationmagnetization has been obtained from extrapolation ofthe M(H) curve with H parallel to the alignment directionaccording to the following equation M(H) = M sat + a/H 2 .As seen in figure 124 ThCo 4 B orders ferromagneticallyat 301 K temperature much smaller than that of isotypeRCo 4 B (R is a rare-earth element or yttrium).Assuming that T c results only from Co-Co exchange interactionsthe Co 4 B compound exhibit rather large exchangeinteraction between Co neighbors corresponding to an exchangefield of 175 T. Both neutron diffraction experimentsand electronic band structure calculations indicate the existenceof a large Co magnetic moment at the Co 2c position.For this site the magnetic moment 1.8 µB/formula unit isfound to be typically the same as those observed in Cometal or in well known RCo 5 type compounds, thus indicatingthat unlike the Co 6i site, the Co 2c site is almost insensitiveto the presence of Th. This confirms the significantdifference in the magnetic behavior of the two inequivalentCo magnetic sites in ThCo 4 B. The Co 2c site has a largemagnetic moment and localized character whereas the Co 6isite displays a nearly ferromagnetic state with a large degreeof delocalization. As can be seen from figure 125 hugemagnetocrystalline anisotropy is observed for the ThCo 4 Bcompound. The estimation of the 4 K anisotropy field givesa value of about 47 T. This large value can be consideredas surprising if one keeps in mind that the YCo 4 B isotypecompound exhibit a spin reorientation transition resultingfrom the competition of the Co 2c and Co 6i inequivalentatomic positions. The RCo 4 B type compounds arestructurally deriving from the RCo 5 type structure whereboth Co sites are competing in terms of magnetocrystallineanisotropy, the Co 2c and Co 6i preferring an alignment alongthe c-axis and within the basal plane respectively. However,as in ThCo 4 B the Co 6i magnetic moment is close tozero, its contribution to the magnetocrystalline anisotropyis expected to decrease significantly also. The ThCo 4 B unusuallylarge magnetocrystalline is related to the large Co 2ccontribution.Figure 123: Crystal structure of ThCo 4 B (Th atoms are occupyingthe sites labelled R).Figure 124: Thermal evolution of isofield magnetization curvesrecorded at the indicated temperatures.Figure 125: Magnetization isotherms of ThCo 4 B recorded atT = 4.2 with the field applied parallel and perpendicular to theeasy magnetization direction.M. GuillotO. Isnard, V. Pop, H. Mayot, J.C. Toussaint (Institut Néel, CNRS and Université Joseph Fourier)89

MAGNETIC SYSTEMS 2009Magnetic properties of Y 0.7 Er 0.3 Fe 2 (H, D) 4.2 compounds up to 35 TRFe 2 Laves phases can absorb hydrogen or deuterium upto 5H(D)/mol and this absorption modifies significantly thestructural, magnetic and electronic properties. The YFe 2 D xdeuterides are ferromagnetic with an increase of the meanFe moment and a decrease of T C for x ≤ 3.5 D/mol. Forx = 4.2, the monoclinic compound is ferromagnetic at lowtemperature, then undergoes a sharp first order magnetovolumictransition towards an antiferromagnetic structure at84 K. Surprisingly this transition is very sensitive to the Hfor D substitution, which increases the mean Fe moment at4.2 K and shifts the transition temperature extrapolated atzero field T M0 to 131 K (by 50%). Since the cell volumeof the hydride is 0.78% larger than the deuteride, this giantisotope effect has been related to the strong dependence ofthe itinerant electron metamagnetic behavior (IEM) behavioron the volume of one of the Fe sites among eight whichhas a different number of H(D) neighbors.Thermomagnetization curves deduced from cooling underlow magnetic field (300 Oe) for Y 0.7 Er 0.3 Fe 2 H 4.2 andY 0.7 Er 0.3 Fe 2 D 4.2 respectively show sharp transitions atT MO = (107±2) K and (61±1) K respectively. These transitiontemperatures were found equal to 131 and 84 K inthe YFe 2 hydride and deuteride. The higher T MO valuefor the ferromagnetic YFe 2 H 4.2 hydride is attributed toa larger cell volume. However, in Y 0.7 Er 0.3 Fe 2 H 4.2 andY 0.7 Er 0.3 Fe 2 D 4.2 the exchange couplings between the differentmagnetic atoms may also influence the transitiontemperatures. This is confirmed by the thermomagnetizationcurves (figures 126 and 127) where a large increaseof T MO is observed together with a metamagnetic behavior.The transition field (HTR) is determined from themaximum of (dM T /dH). For Y 0.7 Er 0.3 Fe 2 H 4.2 the transitionis observed in the 4.2 − 40 K and 125 − 175 K temperaturesranges On the contrary the transition exists inthe 4.2 − 55 K and 80 − 175 K temperature ranges forY 0.7 Er 0.3 Fe 2 D 4.2 . In the 4.2 − 40 K range the (HTR) valuesare identical. However, when the temperature increasesa large isotopic effect is revealed by the temperature variationof HTR (figure 128).The transition field increases linearlyversus temperature but with a smaller HTR/T slopefor the deuteride (2.48 kOe/K) compared to the hydride(2.58 kOe/K). These values are nevertheless larger than forYFe 2 H 4.2 (1.34 kOe/K) and YFe 2 D 4.2 (1.37 kOe/K), showingan additional influence of the Er substitution. This suggeststhat the Y for Er substitution modifies the structuraland magnetic properties of the hydrides and deuterides: (i)The cell volume of the hydride is 0.8% larger than the correspondingdeuteride; (ii) A spin reorientation of Er momentis induced at low temperature, independently to the H/Disotope effect; (iii) The values of T M0 are smaller than inthe YFe 2 H 4.2 and YFe2D 4.2 compounds, but remain sensitiveto the isotope influence. The sensitivity of T M0 to thechange of volume confirms the magnetovolumic characterof the transition. (iv) The B/T slopes are different for thehydride and deuterides, whereas there were similar in thenon substituted compounds.Figure 126: Thermomagnetization of Y 0.7 Er 0.3 Fe 2 (H, D) 4.2 .Figure 127:D) 4.2 .Figure 128:Isothermal magnetization of Y 0.7 Er 0.3 Fe 2 (H,Transition field versus temperature.M. GuillotV. Paul-Boncour, T. Leblond (CMTR, ICMPE, CNRS and Univ. Paris XII, Thiais)90

2009 MAGNETIC SYSTEMSField-induced transitions in RECo 0.50 Mn 0.50 O 3 (RE = Dy, Eu)One interesting family of perovskites is RE(TM,Mn)O 3 inwhich the Mn atom has been partially substituted by atransition-metal element TM like Co giving rise to Mn 4+ -Co 2+ ferromagnetic interactions. In addition, if the RE elementbears a large magnetic moment, then the RE sublatticemay interact with the ordered TM-Mn sublattice [Peñaet al., J. Magn. Magn. Mater. 312, 78 (2007); Antuneset al., J. Europ. Ceram. Soc. 27, 3927 (2007)]. Inthis work we present the magnetic properties of bulk ceramicsamples EuCo 0.50 Mn 0.50 O 3 and DyCo 0.50 Mn 0.50 O 3 .Nickel-containing samples of similar composition (Ni/Mn= 0.50/0.50) are measured for comparison. Samples wereprepared by solid state synthesis from the correspondingsubmicronic powder oxides. They were characterized byX-ray diffraction showing pure perovskite orthorhombicstructure (Pbnm). Magnetic measurements were performedon specimens cut from ceramic bulks.ZFC curve follows a similar behaviour as the Eu case, thatis canted antiferromagnetism since the magnetic propertiesof the Co-Mn sublattice predominate. When the sampleis field cooled the Co-Mn sublattice orders at T C = 85 K,a lower temperature as the Eu-case because of the smallerionic radius of the Dy ion compared to Eu.The magnetization loop is shown in figure 130(a) forEuCo 0.50 Mn 0.50 O 3 sample. We notice five steep transitionson the hysteresis curves. The magnetization valuemeasured at 20 T is lower than the theoretical value(M S = 3.87 µ B ) if we consider all the spins of Mn 4+and Co 2+ aligned and no contribution from Eu 3+ . ForEuNi 0.50 Mn 0.50 O 3 there are no step-like transitions. Themeasured value of the magnetization at 20 T is also lowerthan the calculated value of 3.35 µ B if all spins of transitionmetals are aligned on the field direction. This suggeststhat Eu 3+ in these systems is a classical case of VanVleck magnetism. For DyCo 0.50 Mn 0.50 O 3 (figure 130(b))we can see two field transitions (near 2 T and 5 T) andthe magnetization measured at 20 T reaches the theoreticalvalue (M S = 6.76 µ B ) if we consider a ferrimagneticmodel of two sublattices (rare earth and transition metalones) fully aligned, one in opposition to the other. For theDyNi 0.50 Mn 0.50 O 3 sample we do not see any field-inducedanomaly on the hysteresis curves up to 20 T.Figure 129: Thermal dependence of the ZFC-FC magnetizationfor EuCo 0.50 Mn 0.50 O 3 and DyCo 0.50 Mn 0.50 O 3 measured at0.025 T.Temperature dependence of the magnetization is shown infigure 129 for EuCo 0.50 Mn 0.50 O 3 and DyCo 0.50 Mn 0.50 O 3 .Samples were first cooled under no magnetic field, thenthe static field of 0.025 T was applied and samples allowedwarming until 300 K (ZFC). Then, they were subsequentlycooled under the same static field (FC). ZFC branch forEu sample shows that, upon warming, the Co-Mn sublatticeorders in an antiferromagnetic state with a small ferromagneticcomponent due to non-linearity of the spins. Thelow magnetic moment of Eu and the low external field appliedto the sample, result in almost no contribution fromEu. During the FC procedure, the transition metal sublatticeorders ferromagnetically at T C = 130 K. For the Dysample we can notice that the ZFC curve starts from a finitevalue at 2 K and decreases to almost zero when thetemperature increases. This is just due to the Curie-Weissbehaviour of Dy 3+ that follows 1/T law, typical of freespinnon-correlated moments. For higher temperatures, theFigure 130: Magnetization loop at2.5K for (a) EuCo 0.50 Mn 0.50 O 3 (insert: EuNi 0.50 Mn 0.50 O 3 ) and(b) DyCo 0.50 Mn 0.50 O 3 (insert: DyNi 0.50 Mn 0.50 O 3 ).A. B. AntunesO. Peña (Sciences Chimiques de Rennes, Université de Rennes 1, Rennes, France), C. Moure, A. Moure (Electrocerámicas,Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain)91

MAGNETIC SYSTEMS 2009High magnetic field study in chromium-based Mn 1−x Cd x Cr 2 S 4 thiospinelsMaterials with spinel AB 2 X 4 structure are widely studiedbecause of outstanding physical properties, e.g. halfmetallicityand colossal magnetoresistance. Among thesematerials, the MnCr 2 S 4 thiospinel presents a collinear ferrimagneticstructure at 65 K and a Yafet-Kittel transitionat ∼ 6 K toward a triangular non collinear state [Tsurkanet al., Phys. Rev. B 68, 134434 (2003)]. Application ofvery high magnetic fields produces a reorientation of themoments toward a ferromagnetic state.We investigate here a solid solution obtained by partial substitutionof magnetic Mn by non-magnetic Cd. Sampleswere prepared from high-purity elements, sealed in evacuatedquartz ampoules, using I 2 as transport agent. Ampouleswere slowly heated up to 800 ◦ C over a period of1 week. The resultant was reground, pelletized and firedat 900 ◦ C for 3 days. Samples were characterized by X-raydiffraction. Patterns were fully indexed in the Space GroupFd3m, with no secondary phases observed within the limitsof the experimental detection. Magnetic measurementswere performed on specimens cut from ceramic pellets.The ordered regime was investigated through ZFC/FC magnetizationcycles performed under low applied field (figure131). Samples were first cooled under non-magneticfield and then warmed from 2 up to 300 K under the appliedfield of 0.01 T (ZFC mode). Once in the paramagnetic state,samples were then cooled down to the lowest temperatureof 2 K under the same applied field (FC mode).into a stronger overall ferromagnetism [Barahona et al., J.Alloys Comp. 480, 291 (2009)].Figure 132: Magnetization at given temperatures for theMn 1−x Cd x Cr 2 S 4 solid solution.In order to obtain a better knowledge concerning spins reorientation,magnetization measurements were performedat fixed temperatures in fields up to 20 T (figure 132). Datashow a progressive loss of the antiferromagnetic interactionsbetween A and B sublattices, favouring the parallelalignment of moments pointing into the same direction, thatis, the ferromagnetism of the Cr 3+ network. To be noticedthat the threshold field for the moments realignmentis 5 times smaller (H C ∼ 6 − 8 T) than the one reported forpure MnCr 2 S 4 . However, for all samples, magnetizationsdo not attain full saturation of free spins at 20 T (see figure133).Figure 131: ZFC/FC magnetization for Mn 1−x Cd x Cr 2 S 4 solidsolution (ZFC : open symbols ; FC : filled symbols).In the parent compound MnCr 2 S 4 , the Cr 3+ and Mn 2+networks are ferromagnetic but point in opposite direction,creating a ferrimagnetic state, while in the solid solution(Mn 1−x Cd x )Cr 2 S 4 , the antiferromagnetic interactionbetween the Mn and Cr networks is progressively lost whenthe content of non-magnetic cadmium increases, resultingFigure 133: Normalized magnetization at 4 Kfor Mn 1−x Cd x Cr 2 S 4 . The normalization was done with respectto effective magnetic moment calculated from high temperaturedata of figure 131 fitted to Curie-Weiss law (not shown).A. B. AntunesP. Barahona (Instituto de Ciencias Básicas, Universidad Católica del Maule, Talca, Chile), A. Galdamez, V. Manriquez(Departamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago, Chile), C. M. Campos, O. Peña(Sciences Chimiques de Rennes, Université de Rennes 1, Rennes, France)92

2009 MAGNETIC SYSTEMSMagnetic properties of ErCo x Mn 1−x O 3 perovskitesA spin reversal phenomenon in perovskite manganitesABO 3 may appear when the rare-earth element, having alarge magnetic moment (e.g. Gd, Er), interacts with theMn 3+ -Mn 4+ sublattice. This has been found in manganitespartially substituted at the A-site by alkaline earth elements.B-site substitutions also drastically modify the physicochemicalproperties of these materials, since for each divalenttransition metal introduced in the lattice, a Mn 3+ ionwill transform into Mn 4+ . If the substitute is Co, the solidsolution may extend over a large range of concentrationssince cobalt may adopt a 2+ and a 3+ oxidation states, dependingon the synthesis conditions. In these materials, thetotal magnetization changes its sign when field-cooled dueto a negative polarization of the rare-earth moment in thepresence of the internal field created by the ordered manganesesublattice [Peña et al., J. Magn. Magn. Mater. 310,159 (2007)].The isothermal magnetization shows two interesting features:(i) an intersection of the decreasing and increasingbranches of the magnetization loop at low fields, related tothe spin reversal described above; (ii) a step-like increase athigh fields when the applied magnetic field increases [Peñaet al., J. Magn. Magn. Mater. 312, 78 (2007)]. Thesetwo anomalies have been observed only in the Er-Co-basedsystem, for materials close to the ErCo 0.50 Mn 0.50 O 3 composition(figure 134).Figure 135: High field magnetization for ErCo x Mn 1−x O 3 samplesnormalized for the value at 15 T.Important results were obtained: (a) the high field transitionshifts towards higher applied fields when temperaturedecreases; (b) the high field transition shifts towards higherapplied fields when the Co/Mn concentration changes fromthe particular 50/50 ratio; (c) the magnetization does notsaturate at the highest applied field, but the ferromagneticloop ends up at about 10 T; (d) no abrupt jumps are observedfor temperatures above 3 K; (e) magnetic relaxationis observed just before the transition occurs (figure 136).Magnetization loop for the ErCo 0.50 Mn 0.50 O 3 com-Figure 134:position.Figure 136: Magnetization as a function of time forErCo 0.50 Mn 0.50 O 3 . Insert: fit to a logarithmic behaviour.Furthermore, the high-field transition is of a dynamical naturesince both its amplitude and position depend on thesweep-rate of the applied-field. To get a deeper insight ofthis anomaly, high-field magnetization measurements wereperformed at 4 K. (figure 135).Based on these results, several scenarios are possible, suchas, dynamical movements of domain walls, magnetic relaxationof ferromagnetic clusters, metamagnetic-like interactionsbetween Er 3+ and Co 2+ -Mn 4+ moments.A. B. AntunesO. Peña (Sciences Chimiques de Rennes, Université de Rennes 1, Rennes, France), C. Moure (Electrocerámicas, Institutode Cerámica y Vidrio, CSIC, Madrid, Spain), S. de Brion (Institut Néel, Grenoble, France)93

MAGNETIC SYSTEMS 2009Ferromagnetic domains in nanosized erbium perovskitesCooperative phenomena constitute an important researcharea because of many applications at the frontiers of chemistry,physics and electronics. Perovskites manganiteshave received growing attention because of mixed-valenceMn 4+ /Mn 3+ and resulting magnetoresistance effect. Wepreviously reported interesting results in Er(Co,Mn)O 3 dueto an antiferromagnetic interaction between Er and |Co,Mn|sublattices, producing a reversal of the magnetic momentand formation of ferromagnetic domains. An avalanchemechanism occurs under moderate magnetic fields (∼35 kOe), leading to the rotation of the domains in one ormore steps, depending on the rate of variation of the appliedfield and on the grains formation [Peña et al., J. Magn.Magn. Mater. 312, 78 (2007)].In order to investigate the influence of the micro-structurewe have elaborated nanosized materials and correlated themagnetic response to the grains size and to boundaries percolation.ErCo 0.50 Mn 0.50 O 3 compound was prepared at700 ◦ C by a citrate method and further calcined at increasingtemperatures. Samples were characterized by X-raydiffraction both before and after the sintering conditions,confirming the presence of a pure perovskite orthorhombicPbnm structure and phase purity was checked by energydispersive analysis. The micro-structure, characterizedby scanning electron microscopy, consists of homogeneousspherical grains of 20 − 30 nm diameter for as-preparedsample synthesized at 700 ◦ C, which progressively growwith increasing sintering temperature, attaining 100 − 200nm and a very good percolation at 950 ◦ C, that is, whengrains fuse together and grain barriers almost disappear.Magnetic measurements were performed on the startingmaterial and sintered pellets.temperatures of 950 ◦ C and above, that is, when grain barrierstend to disappear. We correlate this sudden jump withthe reorientation of ferromagnetic domains. At the sametime, the critical field H c decreases with increasing annealingtemperature suggesting that ferromagnetic domains rotatemore easily inside a large homogeneous grain since nobarriers are present.To confirm the reorientation phenomenon, we have performedsubsequent runs at 2 K, measuring the full M(H)loop. The starting bulk corresponds to a piece of the pelletsintered at 950 ◦ C. This piece was then crushed into finepowder (sieved at 80 µm) and let free to rotate under theaction of the applied field. In a third run, this same powderwas homogeneously dispersed inside a drop of liquefiedvacuum grease, then frozen at 2 K in order to blockany rotation of the fine particles, and performed a M(H)loop. Finally, the gel-type solution was heated at 350 K insidethe cryostat, and a 50 kOe field was applied. The gelwas then cooled under the same static field down to 2 K,with all domains oriented parallel to the applied field. Figure138 shows the full M(H) loop under these 4 differentexperimental set-ups.Figure 138: M(H)-loops measured under 4 different conditionsfor ErCo 0.50 Mn 0.50 O 3 sample calcined at 950 ◦ C.Figure 137: Positive part of magnetization loops at T = 2 K forgiven calcination temperatures (in ◦ C).Figure 137 presents part of the M(H)-loops for selectedpellets. Below an annealing temperature of about 950 ◦ C,the magnetization increases smoothly with increasing field,until an inflexion point occurs at about 40 kOe. This inflexionpoint transforms into a sudden jump at H c , for sinteringIt can immediately be noticed that the most significantchange in the magnetization loop concerns the height of thejump, which increases by about 60% in the oriented powderwith respect to the bulk ceramics. It is also evident that,when the particles are homogeneously dispersed and not allowedto rotate (run 3), the magnetization jump is smoothedout and becomes just an inflexion point. In conclusion, wehave unambiguously shown that grains size and percolationare the most important mechanisms in the rotation of magneticdomains in this material.A. B. AntunesC. M. Campos, O. Peña (Sciences Chimiques de Rennes,Université de Rennes 1, Rennes, France), G. Pecchi (Facultadde Ciencias Químicas, Universidad de Concepción, Concepción, Chile)94

2009Biology, Chemistry and Soft Matter95

2009 BIOLOGY, CHEMISTRY AND SOFT MATTERConcerted spin crossover and symmetry breaking yield three thermally- andone light-induced crystallographic phases of a novel molecular materialSpin crossover (SC) complexes have been widely studiedover the last decades: the reversible low-spin(LS) ⇄ high-spin (HS) switching triggered by a changein temperature or by light irradiation, has attractedmuch interest for potential applications in informationstorage. A new SC material [Fe II H 2 L 2−Me ][PF 6 ] 2 ,1 where H 2 L 2−Me denotes bis[((2-methylimidazol-4-yl)methylidene)-3-aminopropyl] ethylene diamine, hasbeen synthesized.Figure 139 shows the variation of the χ M T product of 1,evidencing a two-step SC process. The INT ⇄ LS step centeredaround 97 K shows a thermal hysteresis loop of 6 Kwidth, characteristic of a first order transition. When 1 wasirradiated with green light, a quantitative light-induced excitedspin state trapping effect (LIESST) was clearly observed.When the irradiation was stopped, two steps relaxationto the fully LS state was observed upon increasing T.Figure 139: Temperature dependence of the χ M T product oncooling () then warming mode () and after irradiating the sampleat 10 K with a 532 nm laser light. ()at sweeping rate of 1 Kmin-1The crystal structures of 1 were determined by singlecrystalXRD. In the HS state (250 K, P22 1 2 1 ), the averageFe-N bond length 〈Fe-N〉 = 2.190 Å is typical of an HSFe II site with six N donors. The crystal structure is madeof hydrogen bonded cation layers and anion layers in the(a,b) plane (Figure 140a). In the INT phase, an orderingof spin-states occurs among the Fe II sites, as a cell doublingalong the crystalline axis. The space group decreasesto the non-isomorphic monoclinic subgroup P2 1 : there aretwo non equivalent cation sites (Figure 140b), one mainlyHS (〈Fe1-N〉 = 2.13(1) Å) and the other one is mainly LS(〈Fe2-N〉 = 2.04(1) Å). The ordering in the INT phase resultsin the LS-HS-HS-LS pattern. Below 97 K, in the LSphase (P2 1 2 1 2 1 ), the unit cell has changed to (4a,b,c) andincludes two independent LS cations per unit cell (〈Fe1-N〉= 2.012 Å = 〈Fe2-N〉) (Figure 140c). Because of the symmetrybreakings, a related distortion of each molecule occurs,accompanied with slight tilts as well as displacementsof the ions.A novel structural reorganization occurs upon generationof the PIHS state: the P22 1 2 1 space group is the same asin the HS phase but with different translation symmetry.The structure includes two independent complex cationsper unit cell (Figure 140d) with Fe-N bond lengths typicalof HS Fe II sites (〈Fe1-N〉 = 2.17(1) Å and 〈Fe2-N〉 =2.18(1) Å). This is the first-case of photoinduced SC involvingsymmetry breaking while previous report demonstratedisostructural reorganization. The occurrence of four differentphases with different symmetries is unique and shouldbe related to the strong intermolecular interactions and tothe specific packing of anion and cation layers.Figure 140: Projection of the crystal packing in the HS phase(a), for the INT phase (b), the LS state (c) and PIHS (d). [HS(blue) and LS (red) sites] Additional projections along the multipliedcrystals axis on the right show the motions of the ions.N. BréfuelL. Toupet, E. Collet (University of Rennes), H. Watanabe, K. Tanaka (University of Kyoto, Japan), J.-P. Tuchagues (Universityof Toulouse), N. Matsumoto (University of Kumamoto, Japan)97

BIOLOGY, CHEMISTRY AND SOFT MATTER 2009Magnetostructural correlations in Tetrairon(III) single-molecule magnetsSlow magnetic relaxation in molecular systems is a livelyresearch area which spans the interface between chemistry,physics and material science. This phenomenon is observedin two main families of compounds: single-molecule magnets(SMM) and single-chain magnets (SCM). The persistenceof magnetization in such systems is limited to lowtemperature (below about 4.2 K), but can be in principleexploited for applications in the field of magnetic storageand information processing. Tetrairon(III) complexes withformula [Fe 4 (L) 2 (dpm) 6 ],are providing a growing class ofSingle Molecule Magnets displaying unprecedented syntheticflexibility and ease of functionalization ( Hdpm =2,2,6,6 − tetramethyl-heptane-3,5-dione).Here we report on three novel derivatives prepared byusing as bridging ligands pentaerythritol monoethers,H 3 L = R’-O-CH 2 C(CH 2 OH) 3 with R’=allyl (1), (R,S))-2-methyl-1-butyl (2), and S-2-methyl-1-butyl (3) along witha new polymorph the complex containing 11-(acetylthio)-2,2-bis(hydroxymethyl) undecan-1-ol ligands (4b) [Gregoliet al., Chem. Eur. J. 15, 6456 (2009)]. High-FrequencyEPR (HF-EPR) spectra have been collected, at two frequencies(190 and 230 GHz) and several temperatures between 5and 30 K, on polycrystalline samples of the four complexesin order to determine the zero-field splitting (zfs) parametersin the ground spin state. The spectra obtained show thetypical behavior of systems with an S = 5 ground spin statecharacterized by an easy-axis type anisotropy (D < 0):H = µ 0 B.g.S + DS 2 z + E(S 2 x − S 2 y) + B 0 4 O0 4 (17)where O 0 4 is a Stevens operator, while D, E and B0 4 arethe crystal field parameters defining the anisotropy of thesystem. Setting an isotropic Landé factor g = 2.0, the bestsimulations of experimental spectra were obtained with thefollowing parameters (in cm −1 ): D = −0.417, E = 0.015,B 0 4 = +1.3 × 10−5 for (1a), D = −0.435, E = 0.009,B 0 4 = +0.9 × 10−5 for (1b), D = −0.449, E = 0.030,B 0 4 = +2.4 × 10−5 for (2), D = −0.442, E = 0.0312,B 0 4 = +1.65 × 10−5 for (3) and D = −0.412, E = 0.006,B 0 4 = +1.8 × 10−5 for (4b)(figure 1). (1a) and (1b) standfor the two structurally inequivalent molecules present inthe cell of crystal (1). Experimental and calculated spectrafor complex (2) at 230 GHz are displayed in figure 141.In (1-3) and (4b), and in other six isostructural compoundspreviously reported, a remarkable correlation is found betweenthe axial zfs parameter D and the helical pitch γof the propeller-like structure (figure 142). γ is definedas the average dihedral angle between the mean Fe4 planeand the three FeO 2 Fe ”blades”.The relationship is directlydemonstrated by (1), which features both structurally andmagnetically inequivalent molecules in the crystal. Thetwo polymorphs of (4) ((4a,b)) span the whole range ofanisotropies for [Fe 4 (L) 2 (dpm) 6 ] complexes, suggestingthat crystal packing effects may be largely responsible forthe observed structural and magnetic differences.The dynamics of the magnetization in the four complexeshas been investigated by AC susceptometry, and the resultsanalyzed by master-matrix calculations. The large rhombicityof (2) and (3) is found responsible for the fast magneticrelaxation observed in the two compounds. However, complex(3) shows an additional faster relaxation mechanismwhich is unaccounted for by the set of spin-Hamiltonianparameters determined by HF-EPR.Figure 141: HF-EPR experimental (bold) and simulated powderspectra recorded at 230 GHz on derivative (2).Figure 142: Axial anisotropy D versus helical pitch γ for thetwelve Tetrairon(III) propellers so far characterized. The redpoints correspond to the four derivatives discussed here. The solidline correspond to the best fit for the derivatives featuring twotripodal ligands.A.L. Barra, P. NeugebauerA. Cornia, L. Gregoli, C. Dianeli (University of Modena, Italy), R. Sessoli (University of Florence, Italy)98

2009Applied Superconductivity99

2009 APPLIED SUPERCONDUCTIVITYMagnetic field behaviour of ex-situ processed MgB 2 multifilamentary wiresIn order to make MgB 2 useful not only for dc but also for acapplications further conductor development and optimizationare still needed, in particular to reduce the ac lossescaused by magnetic hysteresis in the MgB 2 core, filamentcoupling and eddy currents flowing through the metallicmatrix. In this context research work should be focused onmultifilamentary strands with a large number of very finefilaments, twisted filaments and non-magnetic and high resistivitysheath.We have focused our work on obtaining multifilamentaryconductors with a large number of very fine filaments. Inthis context, the powder’s granulometry can play a crucialrole. In this experiment we have prepared two MgB 2 startingpowders which are either not milled (NM) or milled(M) with different granulometries (NM= 1.5 µm and M=450 nm) and by the ex-situ powder in tube (PIT) methodwe have realized multifilamentary wires with 19, 91 and361 filaments and an average size of each filament of 279,110 and 30 µm respectively. In figure 143 the cross sectionsof the three wire types are shown.We have studied the relationship between grain and filamentsize in terms of transport properties. The measuredcritical current densities (J C ) for the samples with NM powderand M powder are reported in figure 144. The criticalcurrent density improves with milling for all samples asreported in our previous work [A. Malagoli et al J. Appl.Phys. 104, 103908 (2008)]. Focusing on the behaviour ofthe not milled samples, passing from 19 to 91 filaments aremarkable critical current density degradation is evident,that is partially recovered going to 361 filaments. On thecontrary for the milled samples 19M and 91M have almostidentical critical current density - a slightly better behaviourin field being observed in 91M. When the number of filamentsincreases up to 361, critical current density decreasesstaying though above the 361NM.Such a behaviour of the critical current density in field cannotbe explained or well understood simply considering theeffects of milling. In these complex conductors several factorshave to be taken into account which have an effect onthe transport properties: the starting granulometry of theMgB 2 powders, the cold deformation force and the final filamentsize. Therefore, in the final analysis, the capabilityof these conductors to transport high critical currents cruciallydepends on a proper balance of these parameters. Inthis work we have obtained the best ratio filament size/grainsize on a 91 filaments wire with an average filament size ofabout 110 µm and a powder starting average grain diameterof about 450 nm. A finer MgB 2 granulometry seems to beneeded to realize very thin filaments (10−30 µm) with highcritical current density.Figure 143: Images of different cross sections through the wireswith 19, 91 and 361 filaments respectively.Figure 144: Transport critical current density (J C ) for differentmilled (M) and not milled (NM) samples measured up to magneticfields of 13 T using a wide bore resistive magnet and a Heliumbath cryostat (T = 4.2 K).E. MossangA. Malagoli, G. Romano, M. Vignolo, C. Ferdeghini, M. Putti (CNR-INFM LAMIA, Genova, Italy), S. Brisigotti, G.Grasso, A. Tumino (Columbus Superconductors S.p.A., Genova, Italy)101

APPLIED SUPERCONDUCTIVITY 2009Superconductivity of C and TiC doped multi-filamentary MgB 2 wiresRecently a great deal of both fundamental and technical researchhas been carried out on carbide-doped MgB 2 dueto its high upper critical field (H c2 ) which presents considerableinterest for the fabrication of practical magnets, especiallythe GM-cryocooled MRI magnet. For engineeringapplications, it is necessary to make MgB 2 superconductorsinto multi-filamentary wires or tapes. The in-situ and exsitupowder-in-tube (PIT) processes have become the dominantor standard methods due to their commercial potentialfor large-scale and low-cost production of MgB 2 wires andtapes.Figure 145: Cross section of the different multi-filamentMgB 2 /NbCu wires.In this work, various multi-filament MgB 2 /NbCu roundwires with carbon or TiC doping have been fabricated byin-situ PIT method at the Northwest Institute for NonferrousMetal Research (NIN). The typical diameter of the finalwire is 1 mm. The MgB 2 coil was fabricated using thewind and react method with a 1.5 m long wires and a heattreatment at 680 ◦ C for 1.5 h. The transport critical currentof the MgB 2 coils was measured with magnetic fields upto 10 T at various temperatures using a standard four probemethod. The critical current density, J c , of the coil was calculatedfrom the measured critical current I c divided by thecross sectional area of the MgB 2 core.Figure 145 shows the 6-, 12- and 36-filamentaryMgB 2 /NbCu wires with amorphous carbon doping. Thevolume of MgB 2 in whole wires is around 20%, 14% and14%, respectively. Figure 146 shows the transport criticalcurrent density J c values as a function of applied field at20 K for the multi-filamentary MgB 2 coils with amorphouscarbon or TiC doping. The 6-filament wires with carbondoping has the largest J c , as high as 4.5×10 4 A/cm 2 at 2 T.The large value of J c in this sample may be due to the goodgrain connectivity and strong flux pinning force. This resultindicates that carbon doping is beneficial for the fabricationof high performance long length multi filamentary MgB 2wires.Figure 146: Transport critical current density of different multi–filamentary MgB 2 coils at T = 20 K.Figure 147 shows the result of the transport J c versus Hmeasurements at temperatures from 4.2 to 30 K measuredfor 6-filament wires with carbon doping. This value islower than the best values found in the literature, but webelieve that the transport J c could be improved by optimizingvarious processing parameters.In summary, the J c of multi-filamentary long length MgB 2wires were enhanced by amorphous carbon doping. Thehighly reactive amorphous carbon can easily substitute intothe lattice of MgB 2 even with a heat-treatment at lower temperature.On the other hand, the lower heat-treatment temperatureresults in a smaller MgB 2 grain size, which introduceda high density of flux-pinning centers.Figure 147: Transport critical current density of a 6-filamentMgB 2 coil at various temperatures.E. MossangQ.Y. Wang, G. Yan (Northwest Institute for Non-Ferrous Metal Research, Xi’an, China), A. Sulpice (Institut Neel,Grenoble102

2009 APPLIED SUPERCONDUCTIVITYPhthalocyanine doping to improve critical current densities in MgB 2 tapesCarbohydrates show a prominent capability in the criticalcurrent, J c − B, enhancement of MgB 2 . It is claimed thatthese materials can decompose at relatively low temperature,and generate a lot of reactivate carbon atoms beforethe MgB 2 phase formation. The reactivate carbon atomsand small sized impurities are favorable for good superconductingproperties of MgB 2 . Compared to nano-sized C orSiC, carbohydrate doping can achieve a uniform dispersionwithin the MgB 2 matrix. As the insulated MgO particles arevery harmful to the MgB 2 grain connectivity, oxygen-freecarbon compounds, which will not produce MgO by reactionwith MgB 2 , are preferable to use as MgB 2 dopants.In this work, we tried phthalocyanine (C 32 H 18 N 8 ), whichhas carbon 72.8-76.6%, nitrogen 21.1-22.3%, and no oxygencontent, as MgB 2 doping material. The relationshipsbetween the critical current properties, crystallinity, irreversibilityfield (H irr ) and upper critical field (H c2 ) werestudied as a function of the doping level of phthalocyanine.Figure 148 shows the field dependence of transport J cat 4.2 K for undoped and phthalocyanine-doped MgB 2 /Fetapes. Clearly, the in-field J c properties of MgB 2 tapes weremuch improved by the phthalocyanine doping, suggestingthat H c2 was enhanced. The best J c − B properties were obtainedin 2.2 wt% phthalocyanine-doped samples. At 4.2 Kand 10 T, a J c value higher than 1.6 × 10 4 A cm −2 wasachieved, which is almost an order of magnitude higherthan that for pure ones. On the other hand, when the phthalocyaninedoping amount increased to 3 wt%, the J c valuesstarted to decrease, while the field dependence was almostnot affected. This suggests that the excess phthalocyanineaddition will have a negative effect on the grainconnectivity and thus the coupling of MgB 2 grains, dueto the non-superconducting and generally insulating secondphases introduced by phthalocyanine addition.Figure 148: Transport J c − B properties of Fe-sheathed undopedand doped tapes heated at 800 ◦ C for 1 hour. The measurementswere performed in magnetic fields parallel to the tape surface at4.2 K.The normalized temperature dependence of H c2 and H irrfor all samples is shown in figure 149. Although the actualcarbon contents in phthalocyanine-doped samples aresmall, the H c2 and H irr values of MgB 2 have been greatlyenhanced. For example, the H c2 and H irr of the 3 wt%phthalocyanine-doped sample at 24 K are 8.7 and 6 T, respectively,which is markedly higher than those of the undopedsamples.Figure 149: Normalized temperature dependence of H irr and H c2for undoped and phthalocyanine-doped samples sintered at 800 ◦ C.H c2 and H irr were defined as H c2 = 0.9R (T c ) and H irr = 0.1R (T c )from the R versus T curve.E. MossangX. Zhang, Y. Ma, D.Wang, , Z. Gao, L. Wang, Y. Qi (Institute of Electrical Engineering, Chinese Academy of Sciences,Beijing, China) S. Awaji, K. Watanabe (Institute for Materials Research, Tohoku University, Sendai, Japan)103

APPLIED SUPERCONDUCTIVITY 2009Critical current measurements on Bi-2212High critical temperature superconductors (HTS) open extremelyinteresting perspectives for high magnetic field applicationssuch as high field magnets for NMR or SMES,nuclear fusion, or future colliders. The demand is high for25−50 T magnets which is beyond the possibilities offeredby low critical temperature superconductors, i.e. Nb 3 Sn.The possibility of precisely measuring the rather elevatedcritical currents of superconductors in a high magnetic fieldenvironment is crucial for the development of HTS magnets.(length 30 mm) can be seen in figure 1. The results havebeen checked by performing cross characterizations at theSACM, CEA Saclay.Complete characterization of Bi-2212 and YBaCuO wiresand tapes at high magnetic fields and low temperatures(from 4.2 to 80 K) can be performed using this new VTI,which will be a very useful tool for the studies of HTS inthe frame of the SUPER-SMES project.Interest for Bi-2212 round wire for high magnetic field applicationsincreased recently due to the wires outstandingperformance concerning their intrinsic transport properties.The critical current density is higher than 1000 MA/m 2at 4.2 K and 45 T. Equally the second generation (2G)of YBaCuO coated HTS conductors, show very promisingperformances in terms of critical currents under very highmagnetic fields. In addition, their mechanical properties areexcellent for the ion beam assisted deposition (IBAD) route.The mechanical performances are of great importance forvery high field magnets. Significant progress has also beenachieved in terms of lengths, to the point where it it is nowpossible fabricate HTS magnets. The possibility to operateat higher temperatures than 4.2 K improves considerablythe stability of the magnet due to the rapid increase of thespecific heat at higher temperatures. The stability is one ofthe limitations of LTS magnets in term stored magnetic energyper unit mass. On the other hand the protection of themagnet is much more difficult since the propagation velocitiesare low, leading to a difficult detection of any quench.HTS magnet protection has been identified as an issue fortheir development.The HTS wires (BiSrCaCuO PIT or YBaCuO coated conductor)are produced by Nexans or other providers (suchas OST). The typical cross section of the tapes is 4 ×0.1/0.2 mm 2 and a diameter of 1 mm for the round wires.The critical current is of the order of ∼ 500 A in theself-field at 4.2 K. In the frame of the ANR “SUPERSMES” contract, a new Variable Temperature Insert hasbeen built in collaboration between LNCMI and the NéelInstitute. The available space has been optimized to maximizethe sample length: a 34 mm long sample can betested in the 39 mm diameter field bore. An investigationof the sample anisotropy is possible, since the sample canbe rotated through 90 ◦ . Critical currents are measured atthe LNCMI under fields up to 20 T. The sample holderhas been designed to enable measurements on a VAMASlikecoil sample. Preliminary measurements performed onBi-2212 VAMAS sample (length 1 m) and short samplesFigure 150: (a) Transport I c versus magnetic field at differenttemperatures in parallel orientation for a VAMAS Bi-2212 tape.(b) Transport I c versus magnetic field at different temperatures inparallel orientation for a short Bi-2212 sample.E. Mossang, F. Debray, J.P. Domps, S. DufresnesP. Brosse-Maron, O. Exshaw, P. Gandit, L. Porcar, P. Tixador (Institut Néel, CNRS, Grenoble, France), J.M. Rey(DSM-DAPNIA-SACM, CEA Saclay, France)104

2009Magneto-Science105

2009 MAGNETO-SCIENCEChanges in the microstructure resulting from a high cooling rate in Fe-xC-Mnalloys in strong magnetic fieldThe impact of magnetic field on the microstructures obtainedafter rapid cooling is observed in Fe-xC-1.5wt%Mnsteels with x = 0.2 and 0.3 wt%C. The experiments aredone in a furnace in which alloys are water cooled in magneticfield from the austenite state [T.Garcin et al., Patent955380, 2009]. After holding at 1173K for 5min, the poweris switched off and the sample is quenched. More than200 K/s is obtained by water quenching between 1173 Kand 373 K. The Figure show the SEM micrographs afterquenching under 0T and 16T.These modifications in the resulting microstructure by theapplication of a static magnetic field may be interestingwhen a mixture of ferrite, bainite and martensite is desired.Figure 1(a) shows the 0.2wt%C steel grades cooled under0T and 16T. In the 0T sample, the microstructure is composedof a mixture of bainite (α B ) and martensite(α’). Bainiteis a two-phase structure that contains ferrite-cementiteregions formed from the eutectoid decomposition of austenite(γ). Martensite is a product of diffusionless transformationand can occur in the form of thin, lenticular plateswhich often extend right across the parent gamma grains, oras packets of approximately parallel, fine laths whose sizeis generally smaller than that of the γ grains. Only severalislands of allotriomorphic ferrite are observed in the structure.The allotriomorphic ferrite (α) grows rapidly alongthe austenite grain boundary (which is an easy diffusionpath) but thickens more slowly. Application of 16T leads toa considerable increase in the allotriomorphic ferrite fractionalong the prior austenite grains boundaries. In addition,a large amount of Widmansttten ferrite (α W ) is detected bythe presence of thin-wedge plates which have grown fromthe prior austenite grain boundaries.Figure 1(b) shows the 0.3wt%C steel grades cooled under0T and 16T. In the 0T sample the microstructure iscomposed of a mixture of bainite and martensite. No allotriomorphicferrite is observed probably due to the relativelyhigh carbon content in alloy. When the transformationoccurs under 16T, the microstructure is predominantlymartensite but also has allotriomorphic ferrite, Widmanstttenferrite, bainite and even pearlite. The martensite structurecan be here referred to as lath martensite which consistsof martensite needles with different but well-definedorientations. Notice that the spherical shape of pearlitecolonies is obvious in this sample because of the lack ofimpingement with other pearlite colonies. Vickers hardnessmeasurements have been performed for the different alloyscomposition after thermomagnetic treatment. The applicationof a strong magnetic field results in a general decreasein the hardness values. This reduction of hardness is relatedto the enhancement of the ferrite precipitation in the alloystreated under static magnetic field.Figure 151: SEM micrographs showing the structure after waterquenching under 0 and 16 T for the Fe-0.2C-1.5Mn (a) and forFe-0.3C-1.5Mn (b)(wt%). Original magnification 4000x, 4% nitaletching sample.F. DebrayT. Garcin, S. Rivoirard, R. Morgunov, E. Beaugnon, (CRETA, CNRS, Grenoble, France)107

MAGNETO-SCIENCE 2009Study of the influence of magnetic forces on the mass transferof paramagnetic particles in electrochemistryThe context of our work is the characterization of the magneticforces which influence the processes of mass transferin electrochemistry. It appears experimentally that in thepresence of a magnetic field in solutions, where a gradientof concentration of paramagnetic ions is present, even if theaction of Lorentz forces is not effective, a driving force actingon the solution will arise. This force, referred to as theconcentration gradient force, is often written,−→ B 2−→ ∇CF m = χ m , (18)2µ 0where χ m (m 3 /mol) is the molar magnetic susceptibility,B(T) the magnetic flux density, C(mol/m 3 ) the bath concentrationand µ 0 (Hm −1 ) the permeability of vacuum. Theexperiment was performed using a 10 MW resistive magnetwhich generates a homogeneous a magnetic field of 6 T ina 286 mm diameter. The experimental device is a rectangularchannel with platinum electrodes on the upper and lowerwalls, between which a voltage drop is applied (figure 152).The channel is closed with insulating walls to eliminate theedge effects. The cell is immersed in an electrolytic bathprepared with an equimolar solution of 0.05 mol/m 3 Ferriferro-cyanide with 0.5 mol/m 3 and K 2 SO 4 as the supportingelectrolyte. The temperature condition was about 17 ◦ Cand an Ag/AgCl reference electrode was used to control theelectrodes potential.of the boundary layer. As a result, an additional mechanismof transport of the solution is generated. Correspondingly,the limiting currents of reactions proceeding in the electrochemicalsystem will become a function of the appliedmagnetic flux density. The influence of magnetic field onthe evolution of the mean limiting current density is plottedusing logarithmic coordinates in figure 153. We can showsthat for the two modes, anodic and cathodic, the limitingcurrent density follows a law in B 2/3 . Note that this lawstarts at 0.5 T for the anodic mode and at 2 T for the cathodicmode due to the paramagnetic forces which drivesa more important flow than the gravity. This phenomenologicalbehaviour was observed by many authors [Waskaas,Acta Chimica 50, 516 (1996)], Rabahand et al., Journal ofElectroanalytical Chemistry 571, 85 (2004)].Figure 152:Experimental configuration.The results were obtained by the polarographic method.When the two electrodes located on the faces of the channelare submitted to a voltage drop, controlled by a potentiostat,an electric current is imposed and in the presence ofan homogeneous magnetic field, the gradient of paramagneticions due to electrode reaction will cause a redistributionof velocities in the bath which then acts on the depthFigure 153: Evolution of the limiting current densities with thework electrode (WE) in the (a) cathodic mode and (b) anodicmode.F. DebrayD. Baaziz, A. Alemany, (EPM-SIMAP Laboratory, Grenoble, France), D. Kalache (Fluids Mechanics Laboratory,University of USTHB, Algiers)108

2009 MAGNETO-SCIENCELarge Alfvén Waves in liquid sodiumAlfvén waves are hydromagnetic waves involving fluid velocityand magnetic field. They are a key phenomenonin many geophysical and astrophysical area, such as theEarth’s liquid core, the magnetosphere, solar wind and interstellarplasma dynamics. They are difficult to observe inexperiments but it would be useful to reach a state of interactingAlfvén waves as a key example of weak turbulence.Here, we use liquid sodium (figure 154). Compared togalinstan (gallium/indium/tin liquid metal alloy) it is a betterelectrical conductor (ohmic dissipation of Alfvén wavesis reduced) and its density is much less (hence Alfvénwaves are faster). The geometry of the liquid sodium cavityis a cylinder of diameter 10 cm and of length 20 cm.This was inserted in an available diameter of 16 cm witha maximum value of 16 T. 7 coils are placed around thiscylinder at regular intervals along the length. In addition, aso-called ‘emission’ coil is placed at one end of the cylinderto provide a magnetic excitation of Alfvén waves. This is ashort electrical pulse current producing a poloïdal magneticfield. The Alfvén wave can then be observed on the signalof the 7 measuring coils. The characteristic dimensionlessLundquist number (propagation time divided by dissipationtime) changed from a maximum of 60 with galinstanto around 500 with liquid sodium. Hence we expect to observeAlfvén waves very clearly. In figure 155, the arrivalof an Alfvén wave is recorded at the end of the cylindricalcavity, while it was produced at the other end by a pulsein a coil. On the left-hand side, one can see the electricalpulse (black curve) followed by the arrival of a main oscillationwith a shorter delay when the applied B is stronger,according to the Alfvén propagation time. On the righthandside, time is made dimensionless using the theoreticaltime of propagation of an Alfvén wave from one end of thecylinder to the other: all arrivals collapse around a dimensionlesstime of unity. In figure 156, the 7 measured signalsare shown. It is possible to follow the signal from the endwhere it was created to the other end. There is however anadditional component to the signal which is the signatureof structural vibrations. The linearity of the response signal(figure 157) shows however that the excitation is not yetlarge enough to reach a turbulent state.Figure 155: Propagation of an Alfvén wave recorded on the coilfarthest from the ‘emission’ coil for different intensities of magneticfield. Same plot using the dimensionless time scale based ontheoretical Alfvén speed.Figure 156:a pulse.The 7 signals of the 7 coils are plotted together, afterFigure 154: These photographs show the process of filling thecontainer with sodium, the bare setup and finally the setup withinits thermal insulator equipped with pressure and temperature sensors.Figure 157: Linear response of electromotive force with respectto the intensity of the excitation pulse. Energy losses during reflections.F. DebrayTh. Alboussiere, P. Cardin, P. La Rizza, J.P. Masson, H.C. Nataf, F. Plunian, N. Schaeffer, D. Schmitt(LGIT/CNRS/OSUG/UJF, Grenoble)109

MAGNETO-SCIENCE 2009Magnetohydrodynamic effect on electrodeposition of nickel alloys-catalysts forhydrogen evolutionElectrodeposition is strongly affected by the addition ofnivellant agents or brighteners in the electrolytic bath.Many electroactive species can be used to modify the characteristicsof the deposits. However, the presence of organiccompounds in the baths gives alloys whose properties withrespect to the corrosion resistance are degraded considerably[Meguro et al. J. Electroch. Soc. 147 3003 (2000)].The magnetic forces generated by the passage of a currentin the presence of a magnetic field create additional convectionwhich affect the morphology of the deposits.field influences deposition by generation of additional convectionclose to the electrode surface and by reduction ofthickness of diffusion layer near cathode surface.In theory, the size of the grain of the deposits is a functionof the speed of nucleation and growth of the nucleus;the more numerous nucleuses are, the lower is the grainsize. A magnetic field applied parallel to the surface of theelectrode generates convection (magnetohydrodynamic effector MHD) of the electrolyte; it results in a laminar flowon the surface of the electrode which reduces the diffusionlayer and increases the concentration gradient. This resultsin change in the size of the grains. In principle, the grainsize is a function of the nucleation rate and the growth ofthe nuclei: the more the nuclei the smaller is the resultinggrain size.The influences of magnetic field on composition, structureand morphology of Cu-Ni alloys have been investigated.The XRD diffraction pattern of deposited alloys ispresented in figure 158. Alloys show a fcc structure ofCu 0 .81Ni 0 .19. There is no difference in the alloy structurewith increasing applied magnetic field while the grainsize of Cu-Ni deposit decreases. This could be explainedby the generation of additional convection close to the electrodesurface by the MHD effect. At the same time thenickel content of the alloy is increased with increasing appliedmagnetic field. This could be explained by magneticfield action on the hydrogen evolution. Nickel electrodepositionis always accompanied by violent hydrogen evolution.When a magnetic field is applied the size of hydrogenbubbles is smaller and they are more rapidly removed fromsurface of the electrode. In this way the nickel partial currentefficiency is increased and nickel content of the alloy ishigher. The magnetic field also influences the morphologyof Cu-Ni alloys. Hydrogen evolution enhanced by the appliedmagnetic field leads to a smooth surface of the alloys(figure 159). However, this effect is visible only for appliedmagnetic field of 6 T or more.In conclusion, a magnetic field changes the morphologyand grain size of deposited alloys. There is no influenceof applied magnetic field on structure of deposit. MagneticFigure 158: XRD diffraction pattern of Cu-Ni alloys depositedwith superimposed magnetic field parallel to the surface of electrode.Figure 159: Morphology of Cu-Ni alloys deposited with superimposedmagnetic field parallel to the surface of electrode.F. DebrayP. Zabinski, R. Kowalik, A. Jarek (AGH University of Science and Technology, Krakow, Poland)110

2009 MAGNETO-SCIENCEDiffusion behavior of Al/Cu diffusion interface under a high magnetic fieldThis work has investigated the effect of a 16 T magneticfield on the diffusion behavior of the Al/Cu diffusion interface.Figure 160 shows that the interface morphology ofthe Al/Cu diffusion couple heated to 615 ◦ C and held for5 hours, and then solidified with and without a 16T magneticfield. It can be observed that the surface of the samplefabricated in the case of no magnetic field [figure 160(a)] isirregular. This may be attributed to formation of Marangoniconvection so that natural flow has forced the liquid out ofthe Cu crucible. When a 16 T magnetic field is applied, aregular surface forms which can be attributed to the dampingof the Marangoni convection and natural flow.Figure 161 shows micrographs and aluminum concentrationprofiles obtained by measuring the content of Al atdifferent positions as a function the distance from the copperside to the aluminum side of the intermediate layers atthe Cu/Al diffusion interface fabricated with and without a16 T magnetic field. The Cu/Al interface consists of fourintermediate layers with a thin irregular edge and three flatlayers. The concentration profiles indicate that the compositionof every layer is constant. This is consistent with thephase diagram (according to the phase diagram and EPMAanalysis, the intermediate layers are Cu 3 Al 2 (δ), Cu 12 Al 9(ξ2), CuAl (η2) and CuAl 2 (θ), respectively).In addition, when the sample is placed in a magnetic field,it will become magnetized. For the materials with differentmagnetic property, the magnetization is different; for theAl/Cu diffusion interface fabricated under high magneticfield, the magnetization of the Cu and Al is different. As aconsequence, a gradient of the magnetic field is formed andthe magnetic force is produced at the interface between theCu and Al. This force may retard the diffusion of the Alatom to the Cu crucible and cause a change in the shape ofthe liquid interface (forms a convex surface).Figure 160: Effect of a 16 T magnetic field on the diffusion andthe formation of microstructure in the Aluminum/Copper diffusioncouple heated to 615 ◦ C and held for 5 hours, and then solidified(a) with B = 0 T and (b) with B = 16 T.This shows that an application of a high magnetic field hasnot affected the phase composition of the diffusion layers;however, the magnetic field has decreased the depth of thediffusion layers suggesting that the magnetic field has retardedthe diffusion. This may be attributed mainly to twoeffects of the magnetic field. Firstly, the damping of theconvection and secondly, the formation of a magnetic force.It is well known that magnetic field damps the flow and undera 16 T magnetic field, the convection may be dampedtotally. The effect of the flow on the diffusion has beenwidely investigated and it is generally accepted that flowenhances the diffusion. Thus, when a high magnetic fieldis applied during the diffusion process, the diffusion will beretarded owing to the damping of the convection, resultingin a decrease of the depth of the intermediate layers.Figure 161: Micrograph of the intermediate layers in the Al/Cudiffusion couple heated to 700 ◦ C and held for 0.5 hours and thensolidified with and without a magnetic field of B = 16 T as indicated.F. DebrayY. Fautrelle (SIMAP, EPM, CNRS, Grenoble), X. Li (University of Shanghai, Shanghai)111

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2009Magnet Development and Instrumentation113

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONHigh field helix developmentThe highest continuous magnetic fields at LNCMI are obtainedusing helix inserts surrounded by large Bitter coils.The 14 helix insert was made available on a regular basisto researchers in September 2006 and generates 32 T in a34 mm diameter bore. An upgraded version was developedand tested successfully in July 2007 at 34 T. A new versionwas constructed and tested at 35 T in March 2009. Aftertwo weeks of running we encountered an anomalous heatingof helices 9 and 10. This was proved to be related witha material imperfection (small cracks)on one of the helicesduring the production process. As a consequence, the 34 Tinsert was again into operation until the normal summerstop of the high field facility (end July). The 35 T insertwas reinstalled in September and has since been used undersevere NMR running conditions (continuous high fields formany hours) without problem (figure 162).In parallel we have designed and constructed two new50 mm inserts, one is a 12 helix insert directly derived fromthe 14 helix insert and the other is a first attempt to combinea central radially cooled helix (cooling between thepitches with a set of 9 longitudinally cooled helices (coolingwithin the annular spacing between adjacent helices).Both versions should produce magnetic fields in excess of31 T. One of the two inserts will be in operation in 2010,the second one in 2011. The priority will be defined in thefirst quarter of 2010 depending on the results of the tests ona dedicated 40 bar hydraulic loop.The performances of the 35 T magnet are summarized intable 1. The insert produce 25 T in a very compact manner(outer copper radius of 186 mm. The power is 11.5 MWfor an inlet water temperature of 14 ◦ C. An insert capable of26 T with the same power (with some loss of homogeneity)has been optimized by changing only the 4 innermost helices.It will be tested depending on the high field facilityplanning during 2010. The surrounding two Bitter stacksproduce an additional 10 T with a power of 10 MW forthe same inlet temperature of 14 ◦ C. This value correspondsroughly to a mean condition over the year for inlet temperature(from 10 ◦ to 20 ◦ ) depending on the season and onthe type of experiments (“sweepers” or “sitters”). Consequentlythe power at a given magnetic field can be = ±5%depending on both the period over the year and the type ofexperiments. A common work program is underway in thelaboratory to optimize the cost of such high field experiments.InsertoutsertType of coil 14 helices 2 Bitter StacksInner radius (mm) 19.3 200Outer radius (mm) 186 500Power (MW) 11.6 10.2Inlet temperature ◦ C 14 14Electric current (A) 29835 29835Field contribution (T) 25.14 9.86Table 1: Main specifications of the 35 T magnet.Figure 162: A cut through view of a 14 helix insert. The warmbore available for users is 34 mm.C. Auternaud, F. Debray, J. Dumas, M. Kamke, J. Matera, C. Mollard, R. Pfister, B. Pardo, D. Ponton, J. Spitznagel,C. Trophime, E. Verney, N. Vidal, S. VeysJ.M Tudela (SERAS, CNRS, Grenoble)115

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009Magnets for neutron and x-ray scattering and absorption experimentsThe maximum magnetic field available today in a split configurationis 15 T (at ILL, HZB in Berlin, Spring 8 inOsaka and few other places). The magnet conception isbased on Nb 3 Sn superconducting cable technology. Forhorizontal field configuration without specific radial access,commercial superconducting solenoids are availableup to 20 − 22 T. The European Synchrotron Radiation Facility(ESRF), and the Institut Laue Langevin neutron facility(ILL) intend to put into operation high field magnetsadapted for neutron scattering, x-ray scattering and x-rayabsorption experiments. During 2008 a design study forthe implementation of dc high magnetic fields was led byESRF and ILL in collaboration with the LNCMI. The studyaddresses the three critical technical aspects of the project:the cooling and electrical power capacities of the Grenoblesite, and the magnet designs. Magnet designs were based onthe availability of a power of 35 MW in the magnet. Twomain designs have been considered:1 - A horizontal field magnet suitable for back scatteringand absorption experiments. The design is derived from the35 T in 34 mm vertical field magnet in operation at LNCMI.The more compact design has an outer diameter of copperless than 400 mm and gives a magnetic field of 31 T in a34 mm for a power of 22 MW and an inlet temperature of20 ◦ C. It uses exclusively a set of 14 helices powered by acurrent of 36000 amperes. The second version (figure 163)uses in addition a stack of outer Bitter plates. Each of thetwo magnets are supplied by a current of 36000 amperes,the total power reaching 33 to 35 MW for 40 T. Further optimizationwill be carried out on the Bitter stack to optimizethis value.2 - A split magnet design. In this configuration, the efficientheat transfer required for split magnets is obtained usingradial channels arranged between the magnet turns. Consequently,the innermost windings are better cooled thanwhen using traditional longitudinally cooled windings. Additionally,the main cooling water flow is parallel to themid-plane and offers a larger flexibility for the design ofthe mechanical devices necessary to withstand the attractingforces existing between the two halves of the magnet.Each magnet half is made of 4 helices arranged concentrically.The outer diameter of the outer helices is smallerthan 400 mm. Using this design it was possible to optimizea magnet that could reach 30 T in a split configuration.Classical Colburn type correlation can be used for thethermo-hydraulic modeling to determine the heat transfercoefficients despite the fact that the hydraulic diameter ofthese channels are of the order of 0.15 to 0.6 mm. The mainmodeling effort has focused on withstanding the attractingforces between the two halves of the magnet. The maximumadmissible primary total stress in a normal duty situationis 880 MPa. Calculations show that the sector shapesolution for split magnet assembling that leaves a 10 mmair gap with an associated 2 × 3 ◦ take off angle, would sustainthe attractive force between the two half magnets up toa field of 28.8 T with 7 port accesses of 36 ◦ . Higher gapand or higher port angle could be obtained to satisfy userneeds by changing the contact part thickness resulting in adecrease of B and of the attracting forces. For neutrons,the ports are replaced by four aluminum rings. The symmetryof this structure allows to reach a higher field, 30 T,for the same primary total stress. Critical issues of the designsuch has high current densities on the inner windingswere studied by mean of the construction and test of prototypes.Electromagnetic and thermo-mechanical numericalsimulations were conducted in order to propose mechanicaldesigns capable of holding the attractive forces between thetwo halves of the split magnet.To reinforce and secure these ambitious designs, complementaryhydraulic and electromagnetic studies will be performedat the LNCMI during the period 2010 to 2012within a new partnership with the ESRF and the ILL in theframe of the EMFL FP7 program.Figure 163: A 40 T horizontal magnet with a conical access atleast equal to 2×10 ◦ . The warm bore available for users is 34 mm.F. Debray, J. Dumas, S. Labbe-Lavigne, R. Pfister, C. Trophime, N. VidalJ. Giraud (LPSC, CNRS, Grenoble), F. Wilhelm (European Synchrotron Radiation Facility, Grenoble), M. Enderle(Institut Laue Langevin, Grenoble)116

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONA new cooling loop for thermo-hydraulic magnet studiesHeat fluxes encountered in high field magnets can be ashigh as 500 W/cm 2 with submillimetric cooling channels.Consequently, heat transfers are, together with the materials,a key issue for magnet optimization.The LNCMI has a long term collaboration with the Laboratoiredes Ecoulements Geophysiques et Industriels (LEGI,CNRS INPG). Most of the experiments performed were todetermine either the threshold of cavitation in the coolingchannel or the heat transfer coefficient in a well defined geometry[Reynaud et al., Int. Journal of heat and mass transfer,48, 3197,(2005)]. Nevertheless, the hydraulic coolingloop was limited with a maximum pressure loss of 13 barswhich is roughly half of the pressure loss through our highfield magnets. In addition, high heat fluxes could not bereached on the test section.In 2008 and 2009, LNCMI and LEGI in the frame of theircollaboration have upgraded an existing 40 bars hydrauliccooling loop to study high field magnet thermo-hydraulics.The hydraulic cooling loop (photograph in figure 164) isnow operational for two main kinds of experiments. One isthe continuation of the precise modelling of heat transfer ina single flat thin channel. Figure 165 shows the related testsession that is now under construction for this purpose. Itis foreseen to hold a pressure of 40 bars and to characterizethe heat transfer with a velocity up to 40 m/s and heattransfer of at least 100 W/cm 2 , with a variable width of thechannel from 0.2 to 1 mm.Figure 164:assembly.View of the 40 bar hydraulic cooling loop underThanks to the high hydraulic power available on the newhydraulic cooling loop (80 KW instead of 2 KW for the oldhydraulic cooling loop) it is now possible to study directlythe thermo-hydraulics of an entire coil or a group of criticalcoils (helix or Bitter) up to a flow rate of 15 l/s. Thefirst study will concern the flow distribution of the radiallycooled helix that will be used at the center of the new highfield 50 mm, 31 T magnet. Consequently, an aluminiummodel has been prepared and will be tested at the end of theyear 2009 and it will help to detect possible weak pointsin the very constrained hydraulic design without using thehigh field facility for this purpose.This work has been partially supported by the ESRF upgradeprogramm and is supported now by the EuromagnetII FP7 program. In this context an engineer, B. Pardo,has been hired in October 2009 and will focus on thermohydraulicmodelling and comparisons with experimentaldata. The main goal of this study is to establish a numericalmodel of the radially cooled helix in order to optimizethe geometry of cooling channels. The work will permit toenhance the thermo-hydraulics performance of the magnet.Figure 165: A exploded view of the new hydraulic test section tomodel the heat transfers in one channel. Heat flux will be higherthan 100 W/cm, velocity up to 40 m/s and the thickness of thechannel can be changed without dismantling the apparatus. Itis expected to be operational mid 2010. Water flows from leftto right, the thickness of the channel can be varied from 0.2 to0.6 mm. The heaters are located in the red block equipped with aflat heat flux sensor.C. Auternaud, F. Debray, J.Dumas, M. Kamke, J. Matera, R. Pfister, B. Pardo, C. Trophime, E. Verney, N. VidalM. Deleglise (SERAS, CNRS, Grenoble), JP Franc and M. Riondet (LEGI CNRS, INP, UJF, Grenoble)117

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009Pulsed energy supplyThe 14 MJ capacitor bankThe 14 MJ capacitor bank (see figure 166) has functionedwithout any major problems. The financial contributionby the European Union based on the number of performedpulses requires a stricter accounting system for the use ofthe generator. A simple logging system was already inoperation. The arrival of Jean-Pierre Nicolin, a professionalinformatics engineer, permitted us to implement afull-featured database containing coils, users, pulses andother relevant data. He developed a system that includescontrolled access based on a password. Implementing thisin the Siemens Step7 software was possible but not straightforward,nevertheless, one can now load the user-rights andpassword via a file containing all the accepted proposals.use with the old control unit for the 150 kJ bank. The useof this central unit severely limits the versatility of the 1 MJbank since it can only be used at a repetition rate of 1 (fullenergy) pulse every 20 minutes and the commutation optionis not operational. Due to the professional informaticssupport we were able to greatly improve and stabilize thesoftware interface between the old (150 kJ) central moduleand the outside world (direct user interface or controlvia the ESRF “SPEC” system). An interface with the samelook and feel (but partly based on different hardware) isnow being written for the new (1 MJ) central module. Theassembly of the central module is reaching completion, butdue to unforeseen long delivery times of the optically firedthyristors (a delay of 1 year to get a viable offer with a deliverytime of 40 weeks!) we will be obliged to cannibalizethe 150 kJ unit (i.e. the thyristor stack of the 150 kJ unitwill be dismounted and temporarily used in the 1 MJ unit).The final test of the central (charging) unit is foreseen forJanuary 2010.Figure 166:basement.A view of the 14 MJ, 24 kV, 65 kA generator in theFigure 167: The developed in-house, rapid commutation switch,for changing automatically the polarity of the field.The construction of a 1 MJ transportable capacitorbankDriven by the success and the frequent use of the transportable150 kJ bank (ILL, ESRF) we decided to build a1 MJ capacitor bank. This new unit is not only a higherenergy version of the 150 kJ generator but it has two essentialmodifications. First of all it will have the possibilityto work with two polarities, and secondly the chargingand the commutation (see figure 167) will take only a fewminutes and will be controlled completely by a computerintegrated in the central charging unit. For budgetary reasonsit was decided to first build the two storage modulessince they could in principle be energized by the existingcharger unit (at the expense of a long charging time). Sincethe first quarter of 2009 the two capacitor modules are inOutlookThe most important and urgent investment is a moderatelysized (∼ 6 MJ) capacitor bank with a short pulse. Thisequipment will permit us to energize coils in the 80 T rangewithout making compromises in the design. It is foreseenthat the installation will be built as two completely independent3 MJ units. Each unit will consist of a mediumsize container. The use of containers has two advantages: ifrequired one can use more than 1 MJ at remote sites (ILL,ESRF, etc.) and secondly the delivery and use of the 6 MJgenerator can start before the extension of the building hasbeen finalized. The foreseen use of several generators imposesa review and a profound modification of the groundingstrategy. This long due modification needs to be finalizedin 2010.P. Frings, B. Griffe, J.-P. Nicolin, T. Schiavo118

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONPulsed high-field coilsThe production of user coilsAlso in 2009 the copper-zylon coils remain the workhorseof the laboratory. The production of these coils has nowbecome standard; parts are parameterized and can be producedin 2 weeks on the computerized milling machine,the actual winding takes less than a week and testing a fewdays. Since most of the time parts are in stock and at least4 members of the team have the experience to wind a coil,a standard coil can be produced in 10 days. Moreover, thepolicy is to have at least one standard coil in stock, in orderto avoid as far as possible any delay for users after a(finally unavoidable) coil failure. A first glidcop zylon coilwas tested up to 72 T and is now available for users duringthe time the 1 MJ transportable generator is available.Given the success of this small-volume, short-pulse prototype,we have constructed a longer pulse (t max = 33 ms,t total = 200 ms ) 72 T coil which can be energized by the14 MJ generator. This magnet has been successfully testedand is now available to users on a permanent basis.Figure 168:XXL coilSpecial coilsIn 2009 a special coil for optical experiments (BMV) wasdeveloped and successfully tested in house and to the highestfield at our colleagues in Dresden. This coil (nicknamedXXL - see figure 168) is an up-scaled version of the X-coil. This coil produced 300 T 2 .m and is an improvementof 1100% compared to the former X-coils. The split coil,specially designed for synchrotron X-ray experiments, wasfinally tested inside its cryostat after some problems due tothe complicated integration of the coil-body with the liquidnitrogen cryostat. Last, but not least, a mini-coil madefrom Cu-stainless wire was produced by Jérome Béard andhe was able to break with this coil (used as an insert in astandard 60 T coil) the record field of the Toulouse laboratorythat now stands at 81 T. This test was partly made tovalidate our multi-coil design code that was used to designa large two-coil magnet (ARMS4).Technical developmentsMost of the time in coil winding is spent on applying thezylon reinforcement, therefore we decided to improve thezylon spinhead by making it more reliable (new frictionbrakes) and easier to maintain (the spinhead modules canbe exchanged in 2 minutes, to be cleaned and maintained).Since the main progress in coil performance has to comefrom progress in materials it is of utmost importance tostudy the materials used in coil construction under realisticconditions (cryogenic temperature and high mechanicalloads). To test the efficiency of the fibre reinforcement wedecided to continue the explo-vessel studies (as first developedin at the Universiteit van Amsterdam). Unfortunatelyonce the setup was running (using the high-pressure rig atthe Clarendon Laboratory in Oxford) and 30 pressure vesselshad been produced, it turned out that this installationwas not longer available to us due to changing priorities inOxford. Another high-pressure rig was identified in Franceand we expect to restart the explo-vessel tests before theend of the year. These tests will be used to determine forthe first time the properties of zylon reinforcement underrealistic conditions, study the influence of various impregnatesand impregnation methods, and last but not least toinvestigate other high-strength fibres that could be used as afall back solution in case zylon would not be available anymore.Apart from the mechanical strength of the conductorand the fibre reinforcement the quality of the isolation underhigh mechanical load at cryogenic temperature is of greatimportance to have coils with a long lifetime. In order totest various types of insulations we developed an isolationtest on wires applying realistic loads to the isolation. Thisinstallation has started to work and the first results will beavailable before the end of the year.OutlookThe success of pulsed fields combined with large-scale neutronand X-ray sources has created a demand for more timeat high magnetic fields per 24 hour period. This will be realizedby improving the duty cycle of the coils by increasingthe pulse duration (to prolong lifetime) combined with a reductionof the cool down time. Some test with liquid nitrogenbased on techniques used up to now in dc fields lookedvery promising (cool down times of only a few minutes). Itis likely that by implementing these techniques in our coilswe can gain at least a factor of ten in the duty cycle.J. Béard, J. Billette, P. Frings, F. Giquel , J.-M. Lagarrigue, J. Mauchain119

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009High strength conductors for pulsed magnetsR&D of high strength composite conductors- for B > 80 T in the coilin/coilex system: the developmentof Cu/SS wires, for the coilin, with 60% of stainless steel(UTS(77 K)>1550 MPa) and a cross section of 2.00×1.25mm 2 allowed us to reach 81 Tesla (the LNCMI record) inspring 2009.- Cu alloys for magnets with optimized current distribution:conductors made of copper alloys like GlidCop(CuAl 2 O 3 ) or CuAg (with low silver content 0.08%) havebeen processed by industrial companies to be combinedwith Zylon fibers in the user’s magnet with optimized reinforcementdistribution.Ultra-high strength nanocomposite Cu/Nb conductorsThe development of reinforced conductors, with high electricalconductivity and high strength, is essential to providenon-destructive high pulsed magnetic fields over 80 Tesla:the best compromise is obtained with copper-based continuousnanofilamentary wires (UTS = 2 GPa and ρ =0.6 µΩcm at 77 K). The fabrication process of thesenanocomposite wires is based on severe plastic deformation(SPD) applied by accumulative drawing and bundling,leading to a multi-scale copper matrix containing up toN = 85 4 (∼ 50×10 6 ) continuous and parallel niobium nanotubes[Vidal et al., Scripta Mat., 60, 171 (2009)].In 2007, the extrusion of the nanocomposite conductorshas been stopped at the “Centre de Recherche” of TRE-FIMETAUX because of its closure. We decided to pursueour development in collaboration with the CEA/LTMEX inSaclay where a 575 tons extrusion press is available.Development of a new co-axial CuNbCuNb nanocompositewiresA new nanocomposite structure has been designed with asuperposition of Nb and Cu nanotubes (figure 169). Weadded nanometric phases remembering that the extraordinarystrengthening of the co-cylindrical structure is relatedto: (i) an increase of Cu-Nb interfaces surface acting asdislocations barriers; (ii) a rapid and controlled access tonanometre scale where size effect operates on the plasticitymechanisms; (iii) the contribution of an additional reinforcingphase: the Cu-f nanofibers embedded in the Nbnanotubes behave as whiskers with strong size dependence.A conductor containing 85 3 Nb nanowhiskers, Cu nanotubesand Nb nanotubes, embedded in a copper matrixhas been obtained. This Cu/Nb/Cu/Nb system is thereforemore efficient than Cu/Nb filamentary and Cu/Nb/Cu cocylindricalsystems for high-strength applications in magnets(figure 170): it exhibits a controlled microstructureand an efficient strengthening in the nanocomposite zones,where size and also geometry play major roles.Figure 169: Co-axial conductors: (a) N = 85; (b) N = 85 3 .Figure 170: Ultimate tensile strength of co-axial CuNbCuNb andco-cylindrical CuNbCu nanocomposite wires versus diameter.The improvement of the drawing conditions led us to applythe same procedure for the optimization of the extrusionconditions. In the framework of the NANOFILMAGproject, funded by the ANR and involving the PHYMAT,two CEA laboratories (DAPNIA, LTMEX), and an industrialpartner (Alstom/MSA), a program has been defined tooptimize the extrusion conditions. The aim is the preventionof fractures during the extrusion step of the ADB processand the scale-up of the size of the nanocomposite billets.A Ph.D student, J.B Dubois, is involved in the projectand shares his activity between LNCMI and PHYMAT.In addition, in-situ neutron diffraction (POLDI-PSI) and exsitulaboratory x-ray diffraction experiments (PHYMAT-Poitiers, ESRF) have been performed for different heattreatments to study the formation of the texture. Textureand microstructure in the copper after heat treatmentspresent radical differences depending on the size of the consideredcopper channels [to be published].F. Lecouturier, N. Ferreira, L. Bendichou, J.M. Lagarrigue, J.B. DuboisL. Thilly, P.O Renault (PHYMAT, Poitiers, France), H. van Swygenhoven, S. van Petegem (POLDI-Paul ScherrerInstitut, Switzerland), P. Olier (CEA-DEN-LTMEX, Saclay, France), C. Berriaud (CEA-IRFU-SACM, Saclay)120

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONMegagauss magnetic field generationThe year 2009 marks the first generation of a magneticfields in excess of 100 T at the Toulouse pulsed magnetfacility. The field has been obtained with a Megagauss generator(1 Megagauss = 100 Tesla) making use of capacitordrivensingle-turn coils to produce field pulses on a microsecondtimescale. The installation, originally developedat the Humboldt University in Berlin, between 1993 and1997, has been transferred to LNCMI-Toulouse in 2006.an improved cryostat permitting experiments at liquid-Hetemperature is underway.Prior to its recommissioning the generator has undergone amajor revision including the replacement of crucial componentsof the charge/discharge-circuit (60 kV charging supply,high-voltage switches) and the adaptation of remotecontroland security functions. In the second half of 2009the installation has been tested at moderate charging voltagesof up to 35 kV with 80 % of the nominal capacitance(16 out of 20 capacitors available). These tests have givenrise to the current 100 T field record. The field trace isshown in figure together with a simple test measurement ofthe Faraday rotation in CdS sample.This technique for generating magnetic field is destructive,in the sense that the coil explodes outwards and has to bereplaced after each pulse. However, the setup can be surroundedby an absorbing material (wood) which retains coilfragments, and notably prevents any fragments bouncingback. This means that the sample, and cryostat are not destroyedduring the pulse. Replacing the coil is a simpleoperation which can be performed quickly, and without removingthe sample or cryostat. This makes this techniquesfor generating magnetic fields practical from the point ofview of the user since many field shots are possible per day.Indeed, operation is simpler than a classical nitrogen cooledpulsed magnet since it is not necessary to wait for the coilto cool after each shot. The price to pay however, is theextremely short pulse, with only around 1µs available fordata acquisition at the maximum of the pulse.Starting from January 2010 the generator will be operated atfull capacitance and initially with moderate charging voltagesnot exceeding 45 kV. This will permit the generationof fields in excess of 150 T with little or no risk of insulationfailure. At this point the generator will be used forfirst scientific experiments making use of an optical setupfor transmission measurements in the mid-infrared rangethat has also been installed in 2009. The construction ofFigure 171: (a) Magnetic field generated with a 12×12 mm 2 single-turncoil (inner diameter × axial length) at a charging voltageof 35 kV. (b) Expanded view of the top of the magnetic field pulse.(c) Preliminary data of the Faraday rotation in a CdS sample todemonstrate the feasibility of the system for scientists.P.Y. Solane, F. Durantel, O. Portugall121

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009SEISM: A 60 GHz electron cyclotron resonance (ECR) ion source prototypeLinked to the EURISOL and EURO-ν projects (Designstudies for the second generation of exotic nuclei acceleratorand the future neutrino factory in Europe), the beta beamproject aims to use the β decay of radioactive ions to produceneutrinos beams. Within the Beta Beam work package(WP4), the ECR task is dedicated to the design of advancedmagnetic structures to be used in the ECR ion source technology.LPSC has proposed to develop a high frequency(60 GHz) pulsed ion source prototype for the beam preparation.A first ion source prototype was designed, basedon a cusp magnetic structure using the polyhelix coil technologyand was named SEISM for Sixty gigahertz Electroncyclotron resonance Ion Source using Megawatt magnets.Here we report progress in the construction of the prototypeand preparation for the first magnetic field measurementsscheduled in beginning 2010. This work has beencarried in the framework of an existing collaboration betweenLPSC and LNCMI.Simulations of the polyhelices have been carried out to finalizethe design. The temperature field in a coil dependingon the cooling water flow and on the geometry of the coil insulatorshas been investigated. The geometry and the numbersof insulators (glass fiber) to be positionned betweeneach turn of an helice has been specified in order to minimizethe pressure drop, maximize surface cooling and tosustain the 30 tons compression without any copper deformationthat could lead to short-circuit.The maximum electrical power needed is about 6 MW. Awater flow rate of ∼ 30 l/s in the two sets of coils is necessary.Thus, the average coils temperature varies from 80 to180 ◦ C while the peak temperature locally reaches 330 ◦ C.In these conditions, the maximum hoop stress in the coilsis 280 MPa, far below the copper alloy limit of elasticity(360 MPa). At full current, the two sets of coils repel eachother with a force of 300 kN. The CAD mechanical designof the magnetic structure, taking in account all these data,was performed at LPSC. Figure 172 represents a cut viewof the source. All mechanical parts will be ready for assemblingin December 2009.Figure 173: On-site implantation scheme for the magnetic fieldmeasurement at half-intensity. Hydraulic circuit is parallel to M5existing circuit, electrical cables are connected in series with M5magnet.Figure 172: (a) External CAD design of the ion source prototype,(b) internal view of the current and deionized water courses.M5 site was chosen as a temporary implantation site for themagnetic field validation at half-current. When suppliedwith 15 kA, each set of helices (injection set and extractionset) consumes only 0.75 MW of power from the currentsupply unit. Due to impedance tuning, the prototypeshould be electrically connected in series with an existing10 MW magnet, using flexible power cables. Water flowsof 24 l/s and 22 l/s at a pressure of 20 bars are respectivelyrequired at the injection and extraction. M5 hydraulic circuitwas modified in August 2009 in order to derive about50 l/s from the 150 l/s flow and a parallel hydraulic circuit isnow being installed (figure 173). The incoming water flowwill be controlled with tuneable valves and ultrasonic flowmeters. Hall probes will be mounted on step motor jacks inorder to establish a precise magnetic field map (1mm step).The test bench is planned to be upgraded with a high voltageplatform and a 28 GHz radiofrequency emitter to allowa first characterization of the plasma at the end of year 2010.These activities will be funded within the EURO-ν program(2008 - 2010).C. Trophime, F. Debray, J. Dumas, J. Matera, R. Pfister, N. VidalM. Marie-Jeanne, L. Latrasse, T. Lamy, T. Thuillier, C. Fourel, J. Giraud (LPSC, Grenoble, France)122

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONTowards developing a high T c superconducting magnetHigh critical temperature superconductors (HTS) open extremelyinteresting perspectives for high field applicationssuch as high field magnets for NMR, SMES or physicalinvestigations [J. Schwartz et al., IEEE trans. on Appl. Superc.,18,pp. 70, (2008)]. The demand is high for 25, 30 andeven 50 T magnets and is beyond the possibilities offeredby low critical temperature superconductors, for exampleNb 3 Sn. The superconducting solutions for very high fieldsmeet the requirements for sustainable development.Rutherford cable. These data show that the wires withstandthe cabling process and that they are magnetically isotropic.It will be possible to test the HTS magnets in liquid/gascooling or conduction cooling.Recently, the interest for Bi-2212 round wire has been reinforcedfor high field applications [Shen et al., Appl. Phys.Lett. 95, 152516, (2009)] These wires show very high performancein terms of critical currents at ultra high fields,at least on short samples. The main issue with Bi-2212 remainsthe heat treatment, which should be very preciselycontrolled. The melting temperature plays a crucial part andshould be within a window of one or two degrees to obtainthe required highest critical currents densities. Even morerecently, the second generation (2G) HTS conductors, theYBaCuO coated conductors, show also very exciting performancesin terms of current capacities under very highfields whereas their mechanical properties are excellent forthe IBAD route. The mechanical performance is of greatimportance for very high field magnets. Substantial advanceshave been achieved with the 2G HTS and they havenow reached a stage, in terms of lengths, where it is possibleto use them in real devices.Several laboratories in Grenoble (IN, LNCMI, CRETA andG2Elab) have begun works on HTS magnets. IN andG2Elab have built and successfully tested an importantHTS magnet (800 kJ) operating at 20 K in the context of aDGA project. The present works are carried out in the contextof a European Project ”EuCARD” and an ANR project”SUPER-SMES”. The purpose is to study the most suitableHTS materials and develop the technology for veryhigh field and high performance HTS magnets for SMES(storage) or high energy physics (magnets for accelerators)purposes. An identified issue with HTS magnets is theirprotection and studies have begun both from simulation andexperimental points of view. In the development program,the critical characteristics remain the fundamental data required.Advanced characterization tools have been developed,in particular to obtain the critical characteristic underhigh fields at variable temperatures and variable field orientations.After delicate adjustments, the system works perfectly.Figure 174 shows the sample holder and the newcryostat under construction to test HTS coils at variabletemperatures in two LNCMI high field magnets. Figure 175shows two PIT Bi-2212 short samples round wires criticalcharacteristics. These wires are extracted from a NexansFigure 174: Sample holder and variable temperature cryostat.Figure 175: Bi-2212 I c (B) performance for round wires.F. Debray, J. P. Domps, E. Mossang, S. DufresnesP. Brosse-Maron, J. P. Leggeri (Institute Neel, Grenoble), J. M. Rey (CEA, IRFU, Saclay), P. Tixador (INP, Grenoble)123

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009Status report of the 42+ Tesla hybrid magnet projectFor a given electrical power installation, the highest continuousmagnetic fields are obtained with hybrid magnets, i.e.the combination of a resistive inner coil with a superconductingouter one. The base-line for the coil sub-assembliesof the hybrid magnet under construction at LNCMI-G, isgiven in table together with magnetic field contributions.One of the particularities of this design is to use poly-helixcoil for the innermost part of the resistive insert. The fieldproduced by the poly-helix and Bitter coils are planned tobe further increased, typically up to 36 − 37 T, for the upgradephase of this project.Hybrid componentsField (baseline)14 series connected helix coils 24.5 T2 series connected Bitter coils 9 T1 superconducting pancake coil 8.5 TTable 2: Subparts of the 42+ Tesla hybrid magnet.The already existing infrastructure of the hybrid magnet hasfixed the maximum dimensions of the superconducting coil,which are listed in table 2 together with the other main parameters.A schematic view of the cryogenics circuits isshown in figure 176. A new helium liquefier producing upto 130 l/h is necessary for the hybrid magnet project as theestimated consumptions in equivalent liquid He at 4.5 Kwith and without magnet energisation are equal to 100 l/hand 65 l/h respectively.Extensive tests performed in collaboration with the CEA-Saclay and industrial partners have allowed the validationof the conceptual study as well as the preparation of theindustrialization processes.CharacteristicsValuesInner/outer radius550/913 mmHeight1400 mmInductance3 HNominal current (8.5 T)7100 AStored Energy76 MJOperating Temperature1.8 KTable 3: Main parameters of the superconducting coil.Figure 176:: Simplified cryogenic process flow diagramThe specification of the superconducting Rutherford Cableon Conduit Conductor (RCOCC) has been prepared and reviewed.The call for tender was issued and the contract forthe production of all unit lengths of the Nb-Ti/Cu Rutherfordcable (∼ 11 km) was signed at the end of 2009.W. Joss, R. Pfister, P. Pugnat, L. RonayetteC. Berriaud, A. Daël, P. Fazilleau, B. Hervieu, F.P. Juster, C. Mayri, J.M. Rifflet (CEA-Saclay, Gif-sur-Yvette, France)A. Bourquard, D. Bresson, (Alstom, Belfort, France)124

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONMeasuring the vacuum magnetic birefringencePreviously we reported measurements of birefringences assmall as 10 −16 with our experimental apparatus. During thelast year we have worked on the optimization of the signalto noise ratio.Our activity can be divided into five main themes.Vacuum system : in order to understand the ultimate pressurethat we are able to reach and the possibly systematiceffects that can occur, we have characterized the nature ofthe residual gases in our vessel using a Residual Gas Analyzerduring the entire pumping process. Because of theconductivity of the vacuum pipe passing through the magnetcryostat, the lower limit for the pressure that we canreach with our ionic pumps is of about 10 −8 mbar. We needa lower residual pressure. We are currently studying thefeasibility of deposing inside the vacuum pipe a substratetype “getter” which acts as a distributed pumping systemall along the inner surface of the pipe. Contact with theSAS getter company, the world experts of this technique,are under way. We are confident to reach a vacuum betterthan 10 −11 mbar which is enough for our experiment.Mirror intrinsic birefringence : static birefringence interferentialmirrors used in Fabry-Perot cavities is a limitingfactor of the signal to noise ratio. In reference [Bielsa etal., Applied Physics B 97, 457, (2009)], we reported newmeasurements that confirm that mirror phase retardation inducedby mirror birefringence decreases when mirror reflectivityincreases. Our study indicates that the origin ofthe intrinsic birefringence can be ascribed to the reflectinglayers close to the substrate. We believe that this is animportant step forward the realization of birefringence-freemirrors.Perot cavity (length 2.2 m) of finesse 198000 (figure 177).The linewidth of the cavity resonance (FWHM) is around350 Hz which is one of the smallest ever realized. TheLMA in Lyon is currently analysing the other mirrors toimprove their performances, which will allow us to furtherimprove our cavity finesse.Figure 177: The finesse of a Fabry Perot cavity is evaluatedthanks to the lifetime of the photon in the cavity. Here, the lifetimeτ is 463 µs. The corresponding finesse is F = πcτŁ = 198000.New coil : we have built a new coil in order to reach theproject goal of 25 T. It is based on our ”X” geometry, andwe have named it the XXLCoil. Its length is of about50 cm. We have tested it at the LNCMI in Toulouse andat the DHMFL in Dresden where we have reached 31 T(figure 178) (B 2 L ≥ 300 T 2 m). This result is a big stepforward the measurement of the vacuum magnetic birefringence,since we have fulfill the project requirements. In thefinal experiment, we will use three coils like this one.Dynamical response of the Fabry Perot cavity : our Fabry-Perot cavity stores photons for an average time which isbigger than 300 µ s. The magnetic pulse duration is a fewmilliseconds. When the storage time of photons approachesthe pulse duration, the effect of the cavity low-pass filtercannot anymore be neglected and the ellipticity signal is nolonger exactly proportional to the square of the magneticfield amplitude. We have observed such a behaviour, andwe have implemented the appropriate correction in the dataanalysis program.Very low loss cavity mirrors : we have tested four new mirrorsmade by LMA in Lyon. The mirrors currently used inour cavity have losses of the order of 20 ppm. LMA mirrorlosses are expected to have losses as low as a few ppm. Ourmeasurements confirm such expectations. We have measuredfor the best LMA mirror losses of about 5 ppm. As aconsequence, using such a mirror, we have realized a FabryFigure 178: Magnetic field obtained with a transverse coil(XXLCoil). The relevant parameter for the magnetic vacuum birefringencemeasurement is B 2 L=250 T 2 m. This factor has beenimproved by a factor ten in this new configuration.P. Berceau, F. Bielsa, J. Mauchain, R. BattestiA. Dupays, M. Fouché and C. Rizzo (Laboratoire Collisions Agrégats Réactivité, Toulouse, France)125

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009Special purpose NMR probe for spectroscopy of quadrupolar nuclei at 30 TAlthough Nuclear Magnetic Resonance (NMR) is a wellestablished technique, there is a continuous demand for improvedsensitivity and spectral resolution. This is the basisfor the implementation of NMR techniques in high magneticfields. State of the art NMR spectroscopy is based onsuperconducting magnets with field strengths up to 23.5 T.Higher dc magnetic fields may become possible with furtherdevelopment of superconducting technology, but theprogress is predicted to be incremental and expensive. Asan alternative, dc fields up to 34 T produced by resistivemagnets are already available, and there is considerable interestwithin the NMR community to implement generalpurpose and in particular high resolution NMR in thesemagnets. However, until recently, NMR in resistive magnetswas limited to low resolution spectroscopy due to thestrong field inhomogeneity and the limited stability of thehigh power installations. In order to overcome these intrinsicdrawbacks, the development of tailored and sensitivityoptimized NMR instrumentation is required.In this contribution we report on the commissioning of adedicated probe for NMR spectroscopy of quadrupolar nuclei(nuclear spin I > 1/2) at 30 T. In this case high magneticfield can overcome the problem of sensitivity andline broadening in NMR spectra of low-sensitivity nucleiwith strong quadrupole interactions [O. Pauvert et al., Inorg.Chem. 48, 8709 (2009)]. In order to fully benefit fromthe advantages of higher magnetic fields, the design of theNMR probe has to be optimized for this particular application.A basic origin of low sensitivity is a low gyromagneticratio γ of the examined nucleus (like 91 Zr or 25 Mg),that cannot be compensated as in the case of low naturalabundance, where isotope enrichment increases sensitivity.For a pulsed NMR experiment consisting of an excitation ofthe nuclear transition by a radio frequency (RF) field withstrength B 1 , followed by a detection period, a low value ofγ induces both a low signal (∝ γ 3 ) and a small spectral excitationwidth (∝ γB 1 ). Hence, the efficiency of an NMRexperiment involving low-γ nuclei is strongly reduced. Inresponse to these limiting constraints the design of the RFcircuit of the NMR probe has to ensure that strong RF fieldsB 1 can be applied and that electric losses are minimized.In figure 179 we present our realization of a room temperatureNMR probe meeting these constraints: Its bottomtuned RF circuit consists of the solenoid NMR RF excitationand detection coil of a given inductance L C that isconnected in series with a variable tuning capacitance C T .L C and C T form a tunable series resonance circuit matchedto the source impedance (50 Ω) by a variable matching inductanceL M that is connected in parallel. Since the probeis based on the concept of creating strong B 1 fields by theapplication of high RF power, the probe has to withstandhigh voltages of the order of several kV without electricbreakdown. Therefore the tuning capacitance is realized bya copper cylinder capacitor, that can be continuously filledwith an alumina tube. In addition all conducting parts arerounded to avoid sources of arcing. Low electric loss is ensuredby the usage of high conducting materials (copper,brass). The available tuning range of the probe (30 to 500MHz) covers most quadrupolar nuclei up to 30 T. The outerdiameter of the probe (28 mm) leaves space for future extensionslike passive shimming even in the 34 mm narrowbore M9 magnet of LNCMI-G.In addition, the probe is equipped with a second NMR circuitthat is used to stabilize the 24 MW magnet power supplyduring the experiment by an active, NMR based fieldstabilization (NMR spin-lock). For this purpose the 63 CuNMR signal of a CuCl reference sample is continuouslyrecorded and used for the generation of a control signal. Inorder to avoid crosstalks between the two RF circuits thespin-lock circuit is fully shielded by a metallic cap. Theactive field stabilization enables long time averaging of theNMR signal with reference drifts of less than 1 ppm.After adjustment and optimization the probe was proved tobe fully operational. It withstands pulsed RF power of morethan 1 kW with a B 1 efficiency of 18 G/ √ W for a solenoidNMR coil of 3 mm inner diameter and 5 mm length. At1 kW this corresponds to B 1 field strengths of 570 G. Subsequentlythe probe with the spin-lock option was usedto conduct 91 Zr NMR studies on a series of inorganic Zrcompounds at 30 T. The obtained results provide a systematicand quantitative determination of the relation betweenstructural parameters (bond lengths, bond angles, coordinationgeometry) and NMR parameters (chemical shift andquadrupole tensors) of Zr compounds.Figure 179: Schematic view of the RF part of NMR probe consistingof the main RF circuit (upper right) and the shielded NMRspin-lock part (lower left, shielding cut for clarity).S. Krämer, C. de Vallée, H. Stork, J. Spitznagel, M. Horvatić, C. BerthierF. Fayon, A. Rakhmatullin, O. Pauvert, C. Bessada, D. Massiot (CEMHTI-CNRS, Orléans, France)126

2009 MAGNET DEVELOPMENT AND INSTRUMENTATIONNew rotating sample holder for broad-band quasi-optical HF-EPRspectroscopyDuring the last year, we have developed and used a newrotating sample holder for single crystal orientation studies.The system was constructed for our broad-band quasiopticalhigh field electron paramagnetic resonance (HF-EPR) spectrometer. For the design, a construction similarto the one of the Fabry-Pérot resonator was used, with themain structure made of Torlon. A simplified scheme of therotating holder attached to a corrugated waveguide is depictedin figure 180 and a photograph of a part of the holderwith a sample is shown in figure 181.The rotating holder relies on a rotating piezoelectricnanopositioner ANRv50 (Attocube systems AG) with a resistiveencoder allowing for an absolute measurement of theangle. The calibrated encoder enables the direct measurementof the position in the range 0 ◦ −337 ◦ . Due to the lackof space in the cryostat, it is not possible to use directly thepiezoelectric element to rotate the sample holder. Thus, agear mechanism had to be introduced, with one gear drivenby the rotating piezoelectric device. The driven gear in turndrives the Teflon holder for the sample. The mechanicalgear is responsible for the main uncertainty in the rotationposition, which is however less than 1 ◦ . A direct use ofthe rotating piezoelectric nanopositioner should decreasethis uncertainty by one order of magnitude (this configurationwill be soon possible with the installation of a newmagnet with a 50 mm bore in the VTI ). A flat gold platedmirror is placed below the sample for the reflection of theMW. A smooth guide having 6 mm inner diameter is usedto propagate the MW from the corrugated taper to the top ofthe gear mechanism, a few millimetres apart from the sample(Fig. 180). A necessary modulation coil for continuouswave EPR is wound directly onto the Torlon housing of therotating sample holder. It has a maximum field amplitudeof 25 G at the sample (with the actual power supply).Figure 180: Scheme of the rotating holder attached to the corrugatedwaveguide. The sample (black point) is placed on a Teflonholder. The gears are driven by the rotating piezoelectric nanopositionerANRv50 (Attocube systems AG) with a resistive encoderallowing an absolute measurement of the angle. The modulationcoils are represented by the crossed area. One part of the holder(dark-gray) is fixed to the corrugated waveguide; the other part(light-gray) with the sample can be dismounted. A photograph ofthis part of the holder is in Fig. 181.Manipulation and loading of the sample are very easy andcomfortable. The holder has two parts. Whereas one partwith the modulation coil is fixed to the corrugated waveguide,the second part with the sample can be easily removedfrom the previous one (Fig. 181). The sample located onthe Teflon holder is accessible as well from the top thanfrom the sides. This construction allows safe manipulationwith the sample under the microscope and eliminates errorswhich can be introduced during the manipulation with thesample.Detailed information for the rotating holder as well asFabry-Pérot resonator can be found in [Neugebauer andBarra, Appl. Magn. Reson. 37, 833 (2009)].Figure 181: Photograph of a part of the rotating holder used fororientation studies of single crystals.P. Neugebauer, J. Florentin and A.-L. Barra127

MAGNET DEVELOPMENT AND INSTRUMENTATION 2009A new magnetometer for use in dc magnetic fields in excess of 28 T.Since many years we have developed intense collaborationwith French and foreign groups in the frame of the studyof the magnetic properties of solids under strong magneticfield and that in the 1.5 − 400 K temperature range. In2009 these exchanges with different groups (France, USA,Poland, Turkey, Romania, Canada.) have been then maintained.A new challenge originates from the capability ofproduction of continuous magnetic field at the 35 T level(M9 magnet). Such a field value leads to an important reductionof the inner bore of the magnet that does not exceed34 mm. The construction of a new home-made magnetometerwas under taken in the second half of the year2008. In March 2009, this new magnetometer has been successfullytested on the M9 magnet with magnetic fields upto 35 T in the 1.5 − 350 K temperature range. It is worthnoting that the two main challenges were (i) one uniquesample holder which can be used either in a 50 mm boreor in a 34 mm bore in Grenoble or in the Vibrating SampleMagnetometer in the NHMFL (Tallahassee USA). Thischoice allows an accurate comparisons of the magnetization(M) values delivered by the different magnetometers underidentical experimental conditions; (ii) Precise absolute calibrationof the magnetometer was performed using differentsingle crystals.Finally, the set sup offers the advantage of a good sensitivityin the order of 2 × 10 −3 emu; the relative accuracy isestimated to be 0.1% when M is of the order of one emu,while the reproducibility on the field and on temperatureare estimated to be 0.01 T and 0.2 K, respectively. Thesample cavity is a cylinder whose axis is parallel to the appliedmagnetic field; its volume is of about 125 mm 3 inthe 4.2 − 400 K temperature range. In the 1.5 − 4.2 K domain(pumped helium bath) the available diameter of thesample increases up to 12 mm. The sample may be eitherpowder or oriented powder or single crystal. Isothermal orisofield magnetization curves may be obtained in positiveand (or) negative field. The quality of the measurementsperformed using this new home made magnetometer is illustratedby the figures 182 and 183. From each isothermalM T (H) obtained for the deuteride Y 0.7 Er 0.3 Fe 2 (D) 4.2the temperature variation of the differential magnetic susceptibility(dM/dH)T versus the applied field was deduced(figure 182). The critical field of the transition is then definedas the field corresponding to the maximum.In November 2009, in collaboration with the Academy ofSciences of Prague magnetic measurements under pressure(P up to 11 kbar) were performed. Although the new M1magnet failed in relatively low field (100 kOe) the figure183 shows that measurements of excellent quality canbe performed. In the following month further experimentsare expected with H up to 330 kOe in the 1.6 − 4.2 K temperaturerange.Figure 182: Differential magnetic susceptibility versus appliedfield (up to 35 T) for the Y 0.7 Er 0.3 Fe 2 (D) 4.2 deuteride.Figure 183: Isothermal magnetization curves for the Ce 2 Fe 17alloy at 4.2 K under different pressures.M. GuillotA. Zdenek, K. Jiri, S. Yuriy, M. Martin, (Institute of Physics, The Academy of Sciences of the Czech Republic, Prague)C.V. Colin, O. Isnard (Institut Néel, CNRS and Université Joseph Fourier)128

2009 PROPOSALSProposals for Magnet Time – Projects Carried Out in 2009APPLIED SUPERCONDUCTORSInvestigation of the magnetic flux penetration and concentration in bulk high-Tc Superconductors shaped with an array ofholes under pulsed magnetization conditionsChaud - CNRS/CRETA, Grenoble, FranceDr. HaanappelTransport critical current density Jc in carbon-doped multifilamentary PIT-MgB 2 wires in high magnetic fieldSulpice A. - CNRS, Institut Néel, Grenoble, FranceDr. Wang, Dr. MossangTransport properties of MgB 2 superconducting wires with improved behaviour in high magnetic fieldsMalagoli A. - CNR-INFM, LAMIA, Genova, ItalyDr. Fanciulli, Dr. Vignolo, Dr. RomanoCritical currents of HTS wires under high magnetic flux densities and at variable temperatures for high field magnetsTixador P. -CNRS, Institut Néel, Grenoble, FranceJ.P. Domps, Dr. Porcar, Dr.J.M. ReyTransport critical current density in carbon-doped MgB 2 wiresSulpice A. - CNRS, Institut Néel, Grenoble, FranceDr. Wang, Dr. MossangTransport properties of MgB 2Malagoli A. - CNR-INFM, LAMIA, Genova, ItalyDr. Vignolo, Dr. RomanoMAGNETISMHigh field magnetotransport in GeMn magnetic semiconductorsGerber - Tel Aviv University, IsraelDr. RaquetHigh field magnetization experiments on single crystalline (5-MAP) 2 CuBr 4Suellow - TU Braunschweig, Institute for Physics of Condensed Matter, GermanyBallonHigh field anisotropy of hematite and goethiteDekkers - Department of Earth Sciences, Utrecht University, The NetherlandsF. DurantelHigh field magnetisation studies of the low-dimensional zig-zag spin chains in Na 2 Cu 5 Si 4 O 14Klingeler - Insitute for Solid State Research @ IFW Dresden, GermanyDr. Broto129

PROPOSALS 2009High field torque measurements on a molecular dysprosium triangleSessoli R. - Universita degli Studi de Firenze, Dipartimento di Chimica, Firenze, ItalyDr. LuzonSpin-Jahn-Teller effect in antiferromagnetic molecular whells: A systematic study of the field-orientation dependenceWaldmann O. - Physikalisches Institut, Universität Freiburg, GermanyDr. LortzeMagnetic properties of new transition-metal thiospinels: study of the inter-network interactions in Mn 1−x Cd x Cr 2 S 4(0< x

2009 PROPOSALSSpin-Jahn-Teller effect in antiferromagnetic wheels: Its effect on the dielectric constantWaldmann O. - Physikalisches Institut Universität Freiburg, GermanyDr. LotzeQuantum oscillations of the total spin induced by Dzyaloshinski-Moriya interactions in the frustrated Cr 8 Ni AF ringAffronte M. -Instituto Nazionale di Fisica della Material, S3 National Research Centre, Modena, ItalyDr. CarrettaSaturation magnetization of Fe 7 S 8 monoclinic 4C pyrhotite crystalsFillion G. - CNRS, Institut Louis Néel, Grenoble, FranceGaMnAs valence band spectroscopy: energy-and momentum-resolved tunneling anisotropic magneto-resistanceDr. Giraud - CNRS, LPN, Marcoussis, FranceDr. FainiTorque and magnetization studies on the high field phases of the 2D frustrated spin system SrCu 2 (BO 3 ) 2Takigawa M. - University of Tokyo, Institute for Solid State Physics, Kahiwa-shi, Chiba, JapanDr. LevyStructural transitions in liquid crystal with negative and extremely low diamagnetic anisotropy doped with magneticparticlesKopcansky P. - Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, SlovakiaDr. Koneracka, Dr. Timko, Dr. Koneracka, Dr. TomasovicovaSpin-Jahn-Teller effect in antiferromagnetic wheels: a systematic study of the field-orientation dependenceWaldmann O. - Physikalisches Institut Universität Freiburg, GermanyDr. LotzeNMR investigation of high field phases of Volborthite with a distorted Kagome latticeTakigawa M. - University of Tokyo, Institute for Solid State Physics, Kahiwa-shi, Chiba, JapanDr. YoshidaGiant coercive fields in LuFe 2 O 4 at low temperaturesde Groot J. - Institut für Festkörperforschung, Forschungszentrum Jülich, GermanyDr. AngstMAGNETIC RESONANCEMicrowave absorption in high pulsed magnetic field and crystal field problem in tetragonal compounds Ba 2 Cu 3 O x (x =6.0, 6.3)Kazei - Moscow State University, RussiaDr. GoiranMicrowave absorption peculiarities at the spontaneous and magnetic field induced phase transitions in the Jahn-Tellercompound DyVO 4Kazei - Moscow State University, RussiaDr. GoiranParamagnetic relaxation enhancement in NMR at very high magnetic field131

PROPOSALS 2009Kowalewski J. - Physical Chemistry, Arrhenius Laboratory, Stockholm University, SwedenDr. Fries91 Zr solid-state NMR spectroscopy at very high magnetic fieldFayon F. - CEMHTI-CNRS, Orléans, FranceDr. Rakhmatullin Milan, O. Pauvert, Dr. Bessada, Dr. MassiotNMR investigation of the Bose-Einstein condensation in the Han purple compound BaCuSi 2 O 6Stern R. - National Institute of Chemical Physics, NICPB, Tallinn, EstoniaDr. Kimura, F. AimoHigh field NMR study of the ortho-VIII YBaCuO 6,7Julien M.H. - UJF, Laboratoire de Spectrométrie Physique, Saint Martin d’Hères, FranceDr. BonnMETALS AND SUPERCONDUCTORSHigh Field Hall Effect in YBCODoiron-Leyraud - Universite Sherbrooke, CanadaDr. ProustQuantum Oscillations in high purity, underdoped cuprate superconductorsBonn - University of British Columbia, CanadaDr. ProustTDO experiments on heavy-fermion systems in pulsed magnetic fieldsKnafo - LNCMI-T, FranceF. DurantelPulsed field transport measurements on FeSeTe single crystalsBraithwaite - CEA, FranceDr. KnafoFermi surface studies and upper critical field anisotropy in iron chalcogenitesColdea - Bristol University, UKDr. ProustPoint contact spectroscopy of high Tc superconductors in pulsed magnetic fieldsRikken - LNCMI Toulouse, FranceDr. GalibertConductivity tensor of Bi 1−x Sb x beyond the quantum limitBehnia - Ecole Supérieure de Physique et de Chimie Industrielles, Paris, FranceDr. ProustQuantum interference and closed orbits in the quasi-two-dimensional organicconductorß”-(BEDT-TTF) 4 H 3 O[Fe(C 2 O 4 ) 3 ] dichlorobenzeneLaukhin - Institut de Ciencia de Materials de Barcelona (CSIC), Bellaterra, SpainDr. Audouard132

2009 PROPOSALSQuantum Oscillations in high purity, underdoped cuprate superconductorsBonn - University of British Columbia, Vancouver, CanadaDr. ProustdHvA experiment in high Tc superconductors in extreme conditions of temperature and magnetic fieldVignolles - LNCMP, Toulouse, FranceDr. VignollesQuantum oscillations and anomalous transport in overdoped cupratesHussey - University of Bristol, UKDr. ProustQuantum oscillations and Hall effect measurements in the cupratesTaillefer - Université de Sherbrooke, CanadaDr. ProustHigh-field magnetization of the heavy-fermion system Ce 1−x La x Ru 2 Si 2Knafo - LNCMP, Toulouse, FranceDr. KnafoLandau States in the electronic spectrum of MWCNTs under 60 TRaquet - LNCMP, Toulouse, FranceDr. RaquetSuppression of superconducting fluctuations by high magnetic fields in high-Tc cuprates and iron pnictidesRullier-Albenque - SPEC- IRAMIS - CEA, Saclay, FranceDr. ProustFermi surface properties under high magnetic fields in CeRh 2 Si 2 and URu 2 Si 2Aoki - INAC/SPSMS, CEA-Grenoble, FranceDr. KnafoExploring the Fermi Surface on Iron Based Pcnitite and Chalcogenite SuperconductorsColdea - Bristol University, UKDr. ProustDe Haas-van Alphen oscillations in (BEDT-TTF) 8 [Hg 4 Cl 1 2(C 6 H 5 Br)] 2Lyubovskii - Institute of Problems of Chemical Physics, RAS, RussiaDr. AudouardTests up to 2 GPa of a pressure cell designed for isothermal transport measurements at liquid helium temperatures in 55T pulsed magnetic fields. Application to the organic metal (BEDT-TTF) 8 Hg 4 Cl 1 2(C 6 H 5 Br)Audouard - LNCMI-T, FranceDr. AudouardMagneto-transport measurement in the heavy Fermion CeCoIn5Dr. Vignolles - LNCMI-T, FranceVignolles133

PROPOSALS 2009High field ultrasound measurements on the magnetoelectric compound CuFeO 2Quirion - Dep. of Physics, Memorial University, St. John’s, CanadaDr. ProustStudies of Shapiro step response in the interchain transport in NbSe 3 in high magnetic fieldLatyshev Y. - Russian Academy of Sciences, Institute of radioengineering and electronics, Moscow, RussiaDr. Monceau, A.P. OrlovStudies of Aharonov-Bohm effect on MW nanotubes and few layers graphite with columnar defectsLatyshev Y. - Russian Academy of Sciences, Institute of radioengineering and electronics, Moscow, RussiaDr. Monceau, A.P. OrlovInvestigation in the possible Fulde-Ferrell-Larkin-Ovchinnikov state in the magnetic-field-induced organicsuperconductor Lambda-(BETS) 2 FeCl 4 by means of magnetic torque experimentsLortz R. - Physics Dept. Hong Kong University of Science and Technology, Hong KongDr. Kobayashi, Dr. Sheikin, Dr. de MuerAngular-dependent Nernst effect in bismuth beyond the quantum limitBehnia K. - ESPCI, Laboratoire de Physique Quantique, Paris, FranceDr. ZhuMapping the fermi surface of ternary pnictidesBartkowiak M. - HLD, Dresden, GermanyDr. GoodrichAngular-dependent Nernst effect in bismuthBehnia K. - ESPCI, Laboratoire de Physique Quantique, Paris, FranceDr. FauquéNormal state magnetotransport anisotropy of the electron-doped high Tc uperconductor Nd 2−x Ce x CuO 4Kartsovnik M. - Walther-Meissner-Institut, Garching, GermanyDr. HelmHigh field de Haas-van Alphen effect measurements in non-centrosymmetric CeCoGe 3Sheikin I. - CNRS, LNCMI, Grenoble, FranceDr. SettaiInterplay of antiferromagnetism and superconductivity and enormous upper critical field in CeIrSi 3 near itspressure-induced quantum critical pointSettai R. - Osaka University, Graduate School of Science, Osaka, JapanDr. Fittipaldi, Dr. SheikinMulti-band conduction in the newly discovered quaternary oxypnictidesFerdeghini C. - CNR - INFM/LAMIA, Dipartimento di Fisica, Genova, ItalyDr. Fanciulli, Dr. TropeanoQuantum oscillation study of the hidden order state by using high quality single crystal of URu 2 Si 2Aoki D. - INAC/SPSMS, CEA Grenoble, Grenoble, FranceDr. Sheikin134

2009 PROPOSALSThermoelectric tensor of URu 2 Si 2 up to 28 T in a dc fieldMatsuda T. - CEA-Grenoble, Grenoble, FranceDr. SheikinHigh field de Haas-van Alphen effect measurements in non-centrosymmetric CeIrSi 3Settai R. - Osaka University, Graduate School of Science, Osaka, JapanDr. SheikinEnormous upper critical field in CeCoGe3 near its pressure-induced quantum critical pointSettai R. - Osaka University, Graduate School of Science, Osaka, JapanDr. SheikinHigh field de Haas-van Alphen effect measurements in non-centrosymmetric CePt 3 SiSettai R. - Osaka University, Graduate School of Science, Osaka, JapanDr. SheikinHigh field resistivity measurements on single crystalline UPt 2 Si 2Schulze Grachtrup D. - Institute für Physik der Kondensierten Materiel, TU Braunschweig, Braunschweig, GermanyNernst effect in the high-Tc superconductor YBCOTaillefer L. - Université de Sherbrooke, Sherbrooke, QC, CanadaDr. Behnia, Dr. Liangde Haas-van Alphen effect measurements across the 28 T metamagnetic transition in CeIrIn 5 : in search for field-inducedmodification of the Fermi-surfaceSheikin I. - CNRS, LNCMI, Grenoble, FranceDr. AokiInvestigation of the uppper critical field of superconducting carbon nanotubes fabricated in the channels of AlPO4-5zeolite crystalsLortz R. - Hong Kong University of Science and Technology, Hong Kong, ChinaDr. WangAnisotropy of the in-plane normal state magneto-resistance of Sr 1−x La x CuO 2 epitaxial thin filmsRaffy H. - Université Paris Sud, Laboratoire de Physique des Solides, Orsay, FranceDr. JovanovicTransport measurements of Hc 2 and its anisotropy in FeSe 1−x Te x single crystals in static magnetic fieldBraithwaite D. - INAC/SPSMS/IMAPEC, CEA Grenoble, FranceMagnetotransport in thin single crystals of graphite and graphene containing nanoholesLatyshev Y. - Russian Academy of Sciences, Institute of radioengineering and electronics, Moscow, RussiaDr. Monceau, A.P. OrlovMagnetostriction measurements across the 26 T metamagnetic transition in CeRhSi 2Sheikin I. - CNRS, LNCMI, Grenoble, FranceDr. de Muer, Dr. Rodiere, Dr. Haen, Dr. Lejay135

PROPOSALS 2009Angle resolved high field magnetic torque measurements on Sr 4 Ru 3 O 1 0 single crystals and Sr 4 Ru 3 O 1 0/Sr 3 Ru 2 O 7 eutecticcrystalsZola D., Department of Physics, University of Salerno, ItalyDr. Tedesco, Dr. Polichetti, Dr. FittipaldiSEMICONDUCTORSMagnetotransport in silicon (110) MOSFETGold - CEMES-CNRS, FranceDr. GoiranPhotoluminescence efficiency and energy in the high mobility 2DEG. in the extreme quantum limit.Bellani - Dep. of Physics, ItalyEscoffierElectronic properties of graphene nano-ribbons in high magnetic fieldEscoffier - LNCMP, Toulouse, FranceDr. EscoffierMono and bilayer graphene in very high magnetic fieldsShukla - Institut de Minéralogie et de Physique des Milieux condensés (IMPMC), Paris, FranceDr. EscoffierAnomalous Quantum Hall effect in graphene and bi-layer grapheneEscoffier - LNCMP, Toulouse, FranceDr. EscoffierHigh field spectroscopy of epitaxial graphene layersPlochocka - Grenoble High Magnetic Field Laboratory, FranceN. UbrigInvestigation of the dynamic alignement of DNA-wrapped single walled carbon nanotubes in liquid SuspensionKono - Department of Electrical and Computer Engineering, Rice University, Houston, USAN. UbrigLandau Level spectroscopy of graphene monolayerUbrig - LNCMP, Toulouse, FranceN. UbrigElectron effective mass in InN under pressureLeotin - LNCMP, Toulouse, FranceDr. BrotoHigh magnetic field magnetoabsorption under high pressure in InSeSegura - Departamento de Física Aplicada Ed. Investigación, Valencia, SpainDr. BrotoMagneto-transport studies of magnetic impurities doped graphiteKumar - Neel Institute, CNRS/UJF, Grenoble, France136

2009 PROPOSALSDr. EscoffierCyclotron resonance experiments in novel semiconductor dilute nitride alloysPatane - University of Nottingham, UKDr. GoiranTwo-dimensional electron gas mobility and density in AlInN/AIN/GaN heterostructuresGrandjean - EPFL, Lausanne, SwitzerlandDr. GoiranHigh field magneto-transport in FeSi thin filmsMarrows C.H. - School of Physics and Astronomy, University of Leeds, Leeds, U.K.Dr. PorterPolarized magneto-Raman scattering of optical phonon in epitaxial grapheneFaugeras C. - CNRS, LNCMI, Grenoble, FranceDr. Potemski, Dr. P. KossackiMagneto-optical investigation of the main absorption line of epitaxial graphene in the extreme quantum limitMartinez G. - CNRS, LNCMI, Grenoble, FranceDr. De Heer, Dr. Orlita, Dr. Potemski, Dr. BergerPolaronic effects in single doped GaAs based quantum wellMartinez G. - CNRS, LNCMI, Grenoble, FranceDr. Friedland, Dr. Orlita, Dr. FaugerasTemperature dependence of a quantum Hall effect in GrapheneDiez E. - Universidad de Salamanca, Facultad de Ciencias Fisicas, Salamanca, SpainDr. AmadoCurie temperature of diluted ferromagnetic semiconductorsVasek P. - Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech RepublicDr. Smrcka, Dr. SvobodaPolarized magneto-optical investigation of the main absorption line of epitaxial graphene in the extreme quantum limitMartinez G. - CNRS, LNCMI, Grenoble, FranceDr. Potemski, Dr. OrlitaMagneto-optical studies of combined exciton- cyclotron resonance in two dimensional hole gasBryja L. - Institute of Physics, Wroclaw Technical University, Wroclaw, PolandDr. JadczakEffects of magnetic fields on excitonic features in optical spectra of CuInSe 2 single crystalsYakushev M. - University of Strathclyde, Department of Physics, Glasgow, U.K.Dr. MartinFractional quantum Hall effect in CdTe/CdMgTe quantum wellsKunc J. - Charles University, The Institute of Physics, Prague, Czech Republic137

PROPOSALS 2009Dr. Potemski, Dr. PiotHigh field carrier transport studies in Mn doped II-IV-V2 semiconductorsKilanski L. - Institute of Physics, PAS, Warsaw, PolandDr.DobrowolskiMagneto-optical properties of semiconductor quantum wells subject to in-plane electric fieldTuçek J. - Institute of Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech RepublicDr. Kunc, Dr. OrlitaSystematic study of the temperature dependence of the quantum Hall effect in graphene at high magnetic fieldDiez Fernandez E. - Universidad de Salamanca, Facultad de Ciencias Fisicas, Salamanca, SpainDr. Rossella, Dr. Amado MonteroNIR magneto-spectroscopy of exfoliated graphene membraneFaugeras C. - CNRS, LNCMI, Grenoble, FranceDr. PotemskiResonant magneto-polaron effect of a free 2D electron gas in CdTe QW with an in plane magnetic fieldFaugeras C. - CNRS, LNCMI, Grenoble, FranceDr. PotemskiInfrared spectroscopy of graphene membranesOrlita M. - CNRS, LNCMI, Grenoble, FranceDr. Potemski, Dr. FaugerasSearch for free holes in InN:MgDmowski L.H. - Institute of High Pressure, Polish Academy of Sciences, Warsaw, PolandDr. Baj, Dr. KonczewiczElectricaly detected spin resonance in silicon MOSFETS and magnetically modulated Si MOSFETSSaraiva P. - Department of Physics, University of Bath, U.K.Dr. NogaretFractional quantum Hall effect in trilayer system in a tilted fieldGusev G. - Instituto de Fisica, Universidade de Sao Paulo, BrasilStudy of coupling in a self-assembled QD systemKazimierczuk T. - Institute of Experimental Physics, University of Warsaw, PolandDr. Potemski, Dr. Goryca, Dr. Kossacki, Dr. PlochockaAbsorption of the graphite in the magnetic fieldPlochocka P. - CNRS, LNCMI, Grenoble, FranceDr. Potemski, Dr. Orlita, Dr. KossackiStudy of high excitonic states in quantum dots with single magnetic atomsGoryca M. - Institute of Experimental Physics, University of Warsaw, PolandDr. Potemski, Dr. Plochocka, Dr. Kossacki, Dr. Kazimierczuk138

2009 PROPOSALSHigh field magnetotransport in ferromagnetic semiconductor CdMnGeAs 2Kilanski L. - Institute of Physics, Polish academy of Science, Warsaw, PolandInvestigation of the energy fine structure of localized emission from GaN(In)As layers in high magnetic fieldKudrawiec R. - Institute of Physics, Wroclaw Institute of Technology, PolandDr. PotemskiMagnetotransport in graphiteVasek P. - Institute of Physics, ASCR, Prague, Czech RepublicDr. Smrcka, Dr. SvobodaAbsorption of the 2DEG system in integer Quantum Hall regimePlochocka P. - CNRS, LNCMI, Grenoble, FranceDr. Potemski, Dr. MaudeSOFT MATTER AND OTHERSInterplay between photoluminescence and magnetism in MnF 2 , KMnF 3 and RbMnF 3 at high pressure and high magneticfieldRodriguez - Dpt. Ciencias de la Tierra y Fisica de la Materia Condensada, Facultad de Ciencias, Universidad deCantabria, SpainDr. BrotoSpectroscopic study of Yb 3+ -doped oxides under hydrostatic pressure in magnetic fieldValiente - Universidad de Cantabria, Santander, SpainDr. BrotoMHD effect on electrodeposition of nickel alloys: catalysts for hydrogen evolutionZabinski P. - AGH University of Sciences and Technology, Krakow, PolandDr. AnnaMagneto electro chemistry: Influence of the magnetic forces on the mass transferBaaziz D. - GAMAS GDRE, EPM/SIMAP, CNRS, D.U., Saint Martin d’Hères, FranceDr. AlemanyMagnetic dependence of the ferroelectric soft mode frequency in the tensile strained EuTiO 3 thin filmKamba S. - ASCR, Institute of Physics, Prague, Czech RepublicDr. GoianCalcium hydroxyapatite electrodepositionChopart J.P. - Laboratoire DTI, UFR Sciences Naturelles, Reims, FranceDr. Daltin, Dr. BaudartAlfven waves in liquid sodium: non-linear interactions and turbulenceAlboussiere T. - CNRS, Laboratoire LGIT, St Martin d’Hères, FranceD. Schmitt, P. Cardin, F. Plunian, H.C. Nataf, J.P. Masson, P. La RizzaEffect of a magnetic field on the dendrite morphology during crystal growth of aluminium and Zinc-based alloys.Analysis of the various effects such as thermo-effects, diffusion, modification of the structures139

PROPOSALS 2009Fautrelle Y. - INPG/EPM, ENSHMG, St Martin d’Hères, FranceDr. Saadi, Dr. Li140

2009 THESESPhD Theses 20091. Nanot SébastienStructure de bandes et transport électronique dans les nanotubes de carbone sous champ magnétique intense.Doctorat de l’Université de Toulouse, délivré par l’Université de Toulouse III Paul Sabatier Thèse soutenue le vendredi30 octobre 20092. Millot MariusSpectroscopies sous haute pression et champ magnétique intense.Doctorat de l’Université de Toulouse, délivré par l’Université de Toulouse III Paul Sabatier Thèse soutenue le 13Novembre 20093. Jaudet CyrilEffet de Haas-van Alphen dans les supraconducteurs haute température critique.Doctorat de l’Université de Toulouse, délivré par l’Université de Toulouse III Paul Sabatier Thèse soutenue le 17Novembre 2009141

PUBLICATIONS 2009List of Publications 2009[1] F. Aimo, S. Kraemer, M. Klanjsek, M. Horvatic, C. Berthier and H. Kikuchi. Spin configuration in the 1/3 magnetization plateauof azurite determined by NMR. Physical Review Letters 102, 127205 (2009).[2] L. E. G. Armas, G. M. Gusev, T. E. Lamas, A. K. Bakarov and J. C. Portal. Quantum Hall ferromagnet in a double well withvanishing g-factor. International Journal of Modern Physics B 23, 2933 (2009). 18th International Conference on High MagneticFields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[3] A. Audouard, C. Jaudet, D. Vignolles, R. X. Liang, D. A. Bonn, W. N. Hardy, L. Taillefer and C. Proust. Multiple quantumoscillations in the de Haas-van Alphen spectra of the underdoped high-temperature superconductor YBa 2 Cu 3 O 6.5 . PhysicalReview Letters 103, 157003 (2009).[4] S. Awirothananon, S. Raymond, S. Studenikin, M. Vachon, W. Render, A. Sachrajda, X. Wu, A. Babinski, M. Potemski, S. Fafard,S. J. Cheng, M. Korkusinski and P. Hawrylak. Single-exciton energy shell structure in InAs/GaAs quantum dots. Physical ReviewB 78, 235313 (2008).[5] A. Babinski, A. Golnik, J. Borysiuk, S. Kret, P. Kossacki, J. A. Gaj, S. Raymond, M. Potemski and Z. R. Wasilewski. Threedimensionallocalization of excitons in the InAs/GaAs wetting layer - magnetospectroscopic study. Physica Status Solidi B-BasicSolid State Physics 246, 850 (2009). 5th International Conference on Semiconductor Quantum Dots, Gyeongju, SOUTH KOREA,MAY 11-16, 2008.[6] M. G. Banks, R. K. Kremer, C. Hoch, A. Simon, B. Ouladdiaf, J. M. Broto, H. Rakoto, C. Lee and M. H. Whangbo. Magneticordering in the frustrated heisenberg chain system cupric chloride CuCl 2 . Physical Review B 80, 024404 (2009).[7] F. Bert, A. Olariu, A. Zorko, P. Mendels, J. C. Trombe, F. Duc, M. A. de Vries, A. Harrison, A. D. Hillier, J. Lord, A. Amatoand C. Baines. Frustrated magnetism in the quantum Kagome Herbertsmithite ZnCu 3 (OH) 6 Cl 2 antiferromagnet. Journal ofPhysics: Conference Series 145, 012004 (2009).[8] F. Bielsa, A. Dupays, M. Fouche, R. Battesti, C. Robilliard and C. Rizzo. Birefringence of interferential mirrors at normalincidence. Applied Physics B-Lasers and Optics 97, 457 (2009).[9] L. Bogani, C. Danieli, E. Biavardi, N. Bendiab, A.-L. Barra, E. Dalcanale, W. Wernsdorfer and A. Cornia. Single-molecule-magnetcarbon-nanotube hybrids. Angewandte Chemie-International Edition 48, 746 (2009).[10] N. Bréfuel, H. Watanabe, L. Toupet, J. Come, N. Matsumoto, E. Collet, K. Tanaka and J.-P. Tuchagues. Concerted spin crossoverand symmetry breaking yield three thermally and one light-induced crystallographic phases of a molecular material. AngewandteChemie International Edition 48,49, 9304 (2009).[11] L. Bryja, A. Wojs, J. Misiewicz, P. Plochocka, M. Potemski, D. Reuter and A. Wieck. Photoluminescence studies of positivelycharged excitons in asymmetric GaAs/Ga 1−x Al x As quantum wells with a two-dimensional hole gas. International Journal ofModern Physics B 23, 2718 (2009). 18th International Conference on High Magnetic Fields in Semiconductor Physics andNanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[12] M. V. Budantsev, A. G. Pogosov, A. K. Bakarov, A. I. Toropov and J. C. Portal. Effect of an in-plane magnetic field on magnetoresistancehysteresis of the two-dimensional electron gas in the integer quantum Hall effect regime. JETP Letters 89, 92(2009).[13] M. Casse, F. Rochette, L. Thevenod, N. Bhouri, F. Andrieu, G. Reimbold, F. Boulanger, M. Mouis, G. Ghibaudo and D. K. Maude.A comprehensive study of magnetoresistance mobility in short channel transistors: Application to strained and unstrained siliconon-insulatorfield-effect transistors. Journal of Applied Physics 105, 084503 (2009).[14] X. Chaud, J. Noudem, T. Prikhna, Y. Savchuk, E. Haanappel, P. Diko and C. P. Zhang. Flux mapping at 77 k and localmeasurement at lower temperature of thin-wall YBaCuO single-domain samples oxygenated under high pressure. Physica C-Superconductivity And Its Applications 469, 1200 (2009).[15] R. A. Cooper, Y. Wang, B. Vignolle, O. J. Lipscombe, S. M. Hayden, Y. Tanabe, T. Adachi, Y. Koike, M. Nohara, H. Takagi,C. Proust and N. E. Hussey. Anomalous criticality in the electrical resistivity of La 2−x Sr x CuO 4 . Science 323, 603 (2009).[16] C. Danieli, A. Cornia, C. Cecchelli, R. Sessoli, A.-L. Barra, G. Ponterini and B. Zanfrognini. A novel class of tetrairon(III) singlemoleculemagnets with graphene-binding groups. Polyhedron 28, 2029 (2009). 11th International Conference on Molecule-BasedMagnets (ICMM 2008), Florence, ITALY, SEP 21-24, 2008.[17] C. R. Dean, B. A. Piot, G. Gervais, L. N. Pfeiffer and K. W. West. Current-induced nuclear-spin activation in a two-dimensionalelectron gas. Physical Review B 80, 153301 (2009).[18] E. del Corro, J. Gonzalez, M. Taravillo, W. Escoffier and V. G. Baonza. Graphite under non-hydrostatic conditions. High PressureResearch 28, 583 (2008).[19] W. Desrat, S. Kamara, F. Terki, S. Charar, J. Sadowski and D. K. Maude. Antisymmetric magnetoresistance anomalies andmagnetic domain structure in GaMnAs/InGaAs layers. Semiconductor Science and Technology 24, 065011 (2009).[20] C. v. Dewitz, F. Hatami, M. Millot, J. M. Broto, J. Leotin and W. T. Masselink. Evidence of type-I direct recombination in InP/GaPquantum dots via magnetoluminescence. Applied Physics Letters 95, 151105 (2009).142

2009 PUBLICATIONS[21] O. Drachenko, D. V. Kozlov, V. Y. Aleshkin, V. I. Gavrilenko, K. V. Maremyanin, A. V. Ikonnikov, B. N. Zvonkov, M. Goiran,J. Leotin, G. Fasching, S. Winnerl, H. Schneider, J. Wosnitza and M. Helm. High-field splitting of the cyclotron resonanceabsorption in strained p-InGaAs/GaAs quantum wells. Physical Review B 79, 073301 (2009).[22] C. A. Duarte, G. M. Gusev, T. E. Lamas, A. K. Bakarov and J. C. Portal. Valley splitting and g-factor in AlAs quantum wells. InternationalJournal of Modern Physics B 23, 2948 (2009). 18th International Conference on High Magnetic Fields in SemiconductorPhysics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[23] C. Duboc and M.-N. Collomb. Détermination des proprités électroniques de complexes du manganèse. spectroscopie derésonance paramagnétique électronique à haut champ (RPE-HF) et calculs théoriques : une combinaison gagnante. ActualitéChimique 326, 19 (2009).[24] C. Duboc and M.-N. Collomb. Multifrequency high field EPR investigation of a mononuclear manganese(IV) complex. Chem.Comm. 2717 (2009).[25] C. Faugeras, M. Amado, P. Kossacki, M. Orlita, M. Sprinkle, C. Berger, W. A. de Heer and M. Potemski. Tuning the electronphononcoupling in multilayer graphene with magnetic fields. Physical Review Letters 103, 186803 (2009).[26] C. Faugeras, M. Orlita, S. Deutchlander, G. Martinez, P. Y. Yu, A. Riedel, R. Hey and K. J. Friedland. Measurement of the infraredtransmission through a single doped GaAs quantum well in an external magnetic field: Evidence for polaron effects. PhysicalReview B 80, 073303 (2009).[27] B. Fauqué, B. Vignolle, C. Proust, J. Issi and K. Behnia. Electronic instability in bismuth far beyond the quantum limit. NewJournal of Physics 11, 113012 (2009).[28] B. Fauqué, H. Yang, I. Sheikin, L. Balicas, J.-P. Issi and K. Behnia. Hall plateaus at magic angles in bismuth beyond the quantumlimit. Physical Review B 79, 245124 (2009).[29] O. M. Fedorych, S. A. Studenikin, S. Moreau, M. Potemski, T. Saku and Y. Hirayama. Microwave magnetoplasmon absorptionby a 2DEG stripe. International Journal of Modern Physics B 23, 2698 (2009). 18th International Conference on High MagneticFields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[30] D. Fishman, C. Faugeras, M. Potemski, A. Revcolevschi and P. H. M. van Loosdrecht. Magneto-optical readout of dark excitondistribution in cuprous oxide. Physical Review B 80, 045208 (2009).[31] M. Fittipaldi, L. Sorace, A.-L. Barra, C. Sangregorio, R. Sessoli and D. Gatteschi. Molecular nanomagnets and magnetic nanoparticles:the EMR contribution to a common approach. Physical Chemistry Chemical Physics 11, 6555 (2009).[32] S. Frantz, M. Sieger, I. Hartenbach, F. Lissner, T. Schleid, J. Fiedler, C. Duboc and W. Kaim. Structure, electrochemistry, spectroscopy,and magnetic resonance, including high-field EPR, of ((µ-abpy)[Re(CO) 3 X] 2 ) 0 , - where abpy = 2,2 ′ -azobispyridineand X = F, Cl, Br, I. J. Organomet. Chem. 694, 1122 (2009).[33] K. J. Friedland, A. Siddiki, R. Hey, H. Kostial, A. Riedel and D. K. Maude. Quantum hall effect in a high-mobility two-dimensionalelectron gas on the surface of a cylinder. Physical Review B 79, 125320 (2009).[34] A. Goiran, J. M. Poumirol, M. P. Semtsiv, W. T. Masselink, D. Smirnov, V. V. Rylkov and J. Leotin. Magnetospectroscopy of AIPquantum wells. Transport and Optical Properties of Nanomaterials 1147, 3 (2009).[35] M. Goiran, M. P. Semtsiv, W. T. Masselink and J. Leotin. AlP/GaP quantum wells for implementing intersubband devices in the30-60 µm wavelength region. 2008 33rd International Conference On Infrared, Millimeter And Terahertz Waves, Vols 1 And 2350 (2008).[36] J. Gonzalez, C. Power, E. Belandria, J. Jorge, F. Gonzalez-Jimenez, M. Millot, S. Nanot, J. M. Broto and E. Flahaut. Pressuredependence of Raman modes in double wall carbon nanotubes filled with -Fe. High Pressure Research 28, 577 (2008).[37] L. Gregoli, C. Danieli, A.-L. Barra, P. Neugebauer, G. Pellegrino, G. Poneti, R. Sessoli and A. Cornia. Magnetostructural correlationsin tetrairon(III) single-molecule magnets. Chemistry-A European Journal 15, 6456 (2009).[38] G. M. Gusev, S. Wiedmann, O. E. Raichev, A. K. Bakarov and J. C. Portal. Emergent and reentrant fractional quantum hall effectin trilayer systems in a tilted magnetic field. Physical Review B (Condensed Matter and Materials Physics) 80, 161302 (2009).[39] K. Haas, T. Kazimierczuk, P. Wojnar, A. Golnik, J. Gaj and P. Kossacki. Control of local electric fields influencing the photoluminescenceof an individual CdTe/ZnTe quantum dot. Acta Physica Polonica A 116, 896 (2009).[40] J. Jadczak, L. Bryja, P. Plochocka, A. Wojs, J. Misiewicz, D. Maude and M. Potemski. Combined exciton-cyclotron resonance inphotoluminescence of a two-dimensional hole gas. Acta Physica Polonica A 116, 852 (2009).[41] S. Jandl, A. A. Mukhin, V. Y. Ivanov, A. Balbashov and M. Orlita. Nd 3+ crystal-field study of weakly doped Nd 1−x Ca x MnO 3 .Journal of Magnetism and Magnetic Materials 321, 3607 (2009).[42] C. Jaudet, J. Levallois, A. Audouard, D. Vignolles, B. Vignolle, R. Liang, D. A. Bonn, W. N. Hardy, N. E. Hussey, L. Tailleferand C. Proust. Quantum oscillations in underdoped YBa 2 Cu 3 O 6.5 . Physica B-Condensed Matter 404, 354 (2009).[43] B. Jouault, M. Gryglas, M. Baj, A. Cavanna, U. Gennser, G. Faini and D. K. Maude. Spin filtering through a single impurity in aGaAs/AlAs/GaAs resonant tunneling device. Physical Review B 79, 041307 (2009).[44] M.-H. Julien. Enhanced low-energy spin dynamics with diffusive character in the iron-based superconductor (La 0.87 Ca 0.13 )FePO:Analogy with high T-c cuprates. Journal of the Physical Society of Japan 77, 125002 (2008).[45] M. H. Julien, H. Mayaffre, M. Horvatic, C. Berthier, X. D. Zhang, W. Wu, G. F. Chen, N. L. Wang and J. L. Luo. Homogeneousvs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co- and K-doped iron pnictides. EuroPhysics Letters 87, 37001 (2009).[46] A. A. Kapustin, A. A. Shashkin, V. T. Dolgopolov, M. Goiran, H. Rakoto and Z. D. Kvon. Spin susceptibility and polarizationfield in a dilute two-dimensional electron system in (111) silicon. Physical Review B 79, 205314 (2009).[47] Z. A. Kazei, V. V. Snegirev, N. P. Danilova, M. Goiran, L. P. Kozeeva and M. Y. Kameneva. Microwave absorption spectra andthe problem of the crystal field in tetragonal compounds HoBa 2 Cu 3 O x (x=6.0, 6.3). JETP Letters 88, 725 (2008).143

PUBLICATIONS 2009[48] T. Kazimierczuk, A. Golnik, M. Goryca, P. Wojnar, J. Gaj and P. Kossacki. Anisotropic exchange interaction between p-shellelectron and s-shell hole in CdTe/ZnTe quantum dots. Acta Physica Polonica A 116, 882 (2009).[49] W. Knafo, C. Meingast, A. V. Boris, P. Popovich, N. N. Kovaleva, P. Yordanov, A. Maljuk, R. K. Kremer, H. V. Lohneysen andB. Keimer. Ferromagnetism and lattice distortions in the perovskite YTiO 3 . Physical Review B 79, 054431 (2009).[50] W. Knafo, C. Meingast, S. Sakarya, N. H. van Dijk, Y. Huang, H. Rakoto, J. M. Broto and H. V. Lohneysen. Critical scalingof the magnetization and magnetostriction in the weak itinerant ferromagnet UIr. Journal of the Physical Society of Japan 78,043707 (2009).[51] W. Knafo, S. Raymond, P. Lejay and J. Flouquet. Antiferromagnetic criticality at a heavy-fermion quantum phase transition.Nature Physics 5, 753 (2009).[52] J. Kobak, M. Goryca, P. Kossacki, A. Golnik, G. Karczewski, T. Wojtowicz and J. Gaj. Magnetization dynamics of a (Cd, Mn)Tequantum well in pulsed magnetic field. Acta Physica Polonica A 116, 907 (2009).[53] Y. Kopelevich, B. Raquet, M. Goiran, W. Escoffier, R. R. da Silva, J. C. M. Pantoja, I. A. Luk’yanchuk, A. Sinchenko andP. Monceau. Searching for the fractional quantum Hall effect in graphite. Physical Review Letters 103, 116802 (2009).[54] M. Koperski, T. Kazimierczuk, M. Goryca, P. Wojnar, A. Golnik, P. Kossacki and J. Gaj. Numerical rate equation approach topicosecond charge state dynamics in CdTe/ZnTe quantum dots. Acta Physica Polonica A 116, 893 (2009).[55] G. Koutroulakis, J. M. D. Stewart, V. F. Mitrovic, M. Horvatic, C. Berthier, G. Lapertot and J. Flouquet. Field evolution ofcoexisting superconducting and magnetic orders in CeCoIn 5 . Phys. Rev. Lett. , in press (arXiv:0912.3548) (2010).[56] Y. I. Latyshev, A. P. Orlov, A. Y. Latyshev, A. M. Smolovich, P. Monceau and D. Vignolles. Recent experiments on interlayertunneling spectroscopy and transverse electric field effect in NbSe 3 . Physica B-Condensed Matter 404, 399 (2009).[57] J. Levallois, K. Behnia, J. Flouquet, P. Lejay and C. Proust. On the destruction of the hidden order in URu 2 Si 2 by a strongmagnetic field. Euro Physics Letters 85, 27003 (2009).[58] F. Levy, I. Sheikin, C. Berthier, M. Horvatic and M. Takigawa. Reply to the Comment by S. E. Sebastian and N. Harrison. EuroPhysics Letters 85, 67008 (2009).[59] F. Levy, I. Sheikin, B. Grenier, C. Marcenat and A. Huxley. Coexistence and interplay of superconductivity and ferromagnetismin URhGe. Journal of Physics-Condensed Matter 21, 164211 (2009). 25th International Conference on Low Temperature Physics(LT25), Amsterdam, NETHERLANDS, AUG 06-13, 2008.[60] O. Lipscombe, B. Vignolle, T. Perring, C. Frost and S. Hayden. Emergence of coherent magnetic excitations in the high temperatureunderdoped La 2 − xSr x CuO 4 superconductor at low temperatures. Physical Review Letters 102, 167002 (2009).[61] G. P. Lousberg, J. Gagnard, E. Haanappel, X. Chaud, M. Ausloos, B. Vanderheyden and P. Vanderbemden. Pulsed field magnetizationof drilled high temperature superconductors: flux front propagation in the volume and on the surface. SuperconductorScience and Technology 22, 125026 ((2009)).[62] T. T. A. Lummen, C. Strohm, H. Rakoto, A. A. Nugroho and P. H. M. van Loosdrecht. High-field recovery of the undistortedtriangular lattice in the frustrated metamagnet CuFeO 2 . Physical Review B 80, 012406 (2009).[63] A. Malagoli, V. Braccini, M. Tropeano, M. Vignolo, C. Bernini, C. Fanciulli, G. Romano, M. Putti, C. Ferdeghini, E. Mossang,A. Polyanskii and D. C. Larbalestier. Effect of grain refinement on enhancing critical current density and upper critical field inundoped MgB 2 ex situ tapes. Journal of Applied Physics 104, 103908 (2008).[64] M. Matsuda, K. Ohoyama, S. Yoshii, H. Nojiri, P. Frings, F. D. andB. Vignolle, G. L. J. A. Rikken, L.-P. Regnault, S.-H. Lee,H. Ueda, and Y. Ueda. Universal magnetic structure of the half-magnetization phase in Cr-based spinels. Physical Review Letters(2009). Accepted for publications.[65] C. Meingast, Q. Zhang, T. Wolf, F. Hardy, K. Grube, W. Knafo, P. Adelmann, P. Schweiss and H. von Lohneysen. Resistivity ofMn 1−x Fe x Si Single Crystals: Evidence for Quantum Critical Behavior. Properties and Applications of Thermoelectric Materials261 (2009).[66] M. Millot, S. George, J. M. Broto, B. Couzinet, J. C. Chervin, A. Polian, C. Power and J. Gonzalez. New diamond anvil cell foroptical and transport measurements under high magnetic fields up to 60T. High Pressure Research 28, 627 (2008).[67] M. Millot, S. Gilliland, J. M. Broto, J. Gonzalez, J. Leotin, A. Chevy and A. Segura. High pressure and high magnetic fieldbehaviour of free and donor-bound-exciton photoluminescence in InSe. Physica Status Solidi B-Basic Solid State Physics 246,532 (2009).[68] M. Nannini, H. Cloez, S. Girard, C. Roux, J. P. Serries, M. Tena, L. Zani and E. Mossang. Characterization of industrial NbTistrands at variable field for JT-60SA toroidal field coils. Fusion Engineering and Design 84, 1404 (2009). 25th Symposium onFusion Technology, Rostock, GERMANY, SEP 15-19, 2008.[69] S. Nanot, R. Avriller, W. Escoffier, J.-M. Broto, S. Roche and B. Raquet. Propagative landau states and fermi level pinning incarbon nanotubes. Physical Review Letters 103, 256801 (2009).[70] S. Nanot, W. Escoffier, B. Lassagne, J. M. Broto and B. Raquet. Exploring the electronic band structure of individual carbonnanotubes under 60 T. Comptes Rendus Physique 10, 268 (2009).[71] P. Neugebauer and A. Barra. New cavity design for broad-band quasi-optical HF-EPR spectroscopy. Appl. Magn. Reson. 37, 833(2009).[72] P. Neugebauer, M. Orlita, C. Faugeras, A. L. Barra and M. Potemski. How perfect can graphene be? Physical Review Letters103, 136403 (2009).[73] A. Nish, R. J. Nicholas, C. Faugeras, Z. Bao and M. Potemski. High-field magnetooptical behavior of polymer-embedded singlewalledcarbon nanotubes. Physical Review B 78, 245413 (2008).[74] A. Nogaret, J.-C. Portal, H. E. Beere, D. A. Ritchie and C. Phillips. Quantum interference of magnetic edge channels activatedby intersubband optical transitions in magnetically confined quantum wires. Journal of Physics-Condensed Matter 21, 025303(2009).144

2009 PUBLICATIONS[75] S. Nowak, T. Jakubczyk, M. Goryca, P. Ciosmak, A. Golnik, P. Kossacki, P. Wojnar and J. Gaj. Emission of self assembledCdTe/ZnTe quantum dot samples with different cap thickness. Acta Physica Polonica A 116, 890 (2009).[76] Y. Oener and M. Guillot. Magnetic disorder in Ti doped ErCo 2 : High-magnetic-field study. Journal of Applied Physics 105,07E120 (2009). 53rd Annual Conference on Magnetism and Magnetic Materials, Austin, TX, NOV 11-14, 2008.[77] M. Orlita, C. Faugeras, G. Martinez, D. K. Maude, J. M. Schneider, M. Sprinkle, C. Berger, W. A. de Heer and M. Potemski.Magneto-transmission of multi-layer epitaxial graphene and bulk graphite: A comparison. Solid State Communications 149,1128 (2009). Graphene Week 2008 International Conference, Trieste, ITALY, 2008.[78] M. Orlita, C. Faugeras, P. Plochocka, P. Neugebauer, G. Martinez, D. K. Maude, A. L. Barra, M. Sprinkle, C. Berger, W. A.de Heer and M. Potemski. Approaching the Dirac Point in High-Mobility Multilayer Epitaxial Graphene. Physical Review Letters101, 267601 (2008).[79] M. Orlita, C. Faugeras, J. M. Schneider, G. Martinez, D. K. Maude and M. Potemski. Graphite from the viewpoint of Landaulevel spectroscopy: An effective graphene bilayer and monolayer. Physical Review Letters 102, 166401 (2009).[80] I. Pallecchi, C. Fanciulli, M. Tropeano, A. Palenzona, M. Ferretti, A. Malagoli, A. Martinelli, I. Sheikin, M. Putti and C. Ferdeghini.Upper critical field and fluctuation conductivity in the critical regime of doped SmFeAsO. Physical Review B 79, 104515(2009).[81] J. Papierska, M. Goryca, P. Wojnar and P. Kossacki. Temperature of a single Mn atom in a CdTe quantum dot. Acta PhysicaPolonica A 116, 899 (2009).[82] A. Patane, W. H. M. Feu, O. Makarovsky, O. Drachenko, L. Eaves, A. Krier, Q. D. Zhuang, M. Helm, M. Goiran and G. Hill.Effect of low nitrogen concentrations on the electronic properties of InAs 1−x N x . Physical Review B 80, 115207 (2009).[83] O. Pauvert, F. Fayon, A. Rakhmatullin, S. Kraemer, M. Horvatic, D. Avignant, C. Berthier, M. Deschamps, D. Massiot andC. Bessada. Zr-91 Nuclear Magnetic Resonance Spectroscopy of Solid Zirconium Halides at High Magnetic Field. InorganicChemistry 48, 8709 (2009).[84] M. L. Peres, V. A. Chitta, N. F. Oliveira, Jr., D. K. Maude, P. H. O. Rappl, A. Y. Ueta and E. Abramof. Antilocalization of holecarriers in Pb 1−x Eu x Te alloys in the metallic regime. Physical Review B 79, 085309 (2009).[85] A. P. Petrović, Y. Fasano, R. Lortz, C. Senatore, A. Demuer, A. B. Antunes, A. Paré, D. Salloum, P. Gougeon, M. Potel andO. Fischer. Real-space vortex glass imaging and the vortex phase diagram of SnMo 6 S 8 . Physical Review Letters 103, 257001(2009).[86] B. A. Piot, C. R. Dean, G. Gervais, Z. Jiang, L. W. Engel, L. N. Pfeiffer and K. W. West. Distortion of the 2D Wigner crystalinto a ’quasi-3D’ insulator. International Journal of Modern Physics B 23, 2713 (2009). 18th International Conference on HighMagnetic Fields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[87] B. A. Piot, D. K. Maude, U. Gennser, A. Cavanna and D. Mailly. Interplay among spin, orbital effects, and localization in a GaAstwo-dimensional electron gas in a strong in-plane magnetic field. Physical Review B 80, 115337 (2009).[88] P. Plochocka, P. Kossacki, A. Golnik, T. Kazimierczuk, C. Berger, W. A. de Heer and M. Potemski. Slowing hot-carrier relaxationin graphene using a magnetic field. Physical Review B (Condensed Matter and Materials Physics) 80, 245415 (2009).[89] P. Plochocka, J. M. Schneider, D. K. Maude, M. Potemski, M. Rappaport, V. Umansky, I. Bar-Joseph, J. G. Groshaus, Y. Gallaisand A. Pinczuk. Optical absorption to probe the quantum Hall ferromagnet at filling factor ν = 1. Physical Review Letters 102,126806 (2009).[90] M. Potemski. Landau level spectroscopy of dirac-like fermions in multilayer graphene. International Journal of Modern PhysicsB 23, 2665 (2009). 18th International Conference on High Magnetic Fields in Semiconductor Physics and Nanotechnology, SaoPedro, BRAZIL, AUG 03-08, 2008.[91] V. Preisler, T. Grange, R. Ferreira, L. A. de Vaulchier, Y. Guldner, F. J. Teran, M. Potemski and A. Lemaitre. Investigationof interband optical transitions by near-resonant magneto-photoluminescence in InAs/GaAs quantum dots. European PhysicalJournal B 67, 51 (2009).[92] G. Pristas, M. Reiffers, E. Bauer, A. G. M. Jansen and D. K. Maude. Suppression of asymmetric differential resistance in thenon-Fermi-liquid system YbCu 5−x Al x (x=1.3-1.75) in high magnetic fields. Physical Review B 78, 235108 (2008).[93] C. Proust and D. Poilblanc. Des champs magnétiques intenses pour sonder les supraconducteurs. Images de la Physique 87–92(2008).[94] G. Quirion, M. L. Plumer, O. A. Petrenko, G. Balakrishnan and C. Proust. Magnetic phase diagram of magnetoelectric CuFeO 2in high magnetic fields. Physical Review B 80, 064420 (2009).[95] C. Robilliard, B. Pinto Da Souza, F. Bielsa, J. Mauchain, M. Nardone, G. Bailly, M. Fouche, R. Battesti and C. Rizzo. The BMVproject: Search for photon oscillations into massive particles. Canadian Journal of Physics 87, 735 (2009).[96] V. V. Rylkov, B. A. Aronzon, A. S. Lagutin, V. V. Podol’skii, V. P. Lesnikov, M. Goiran, J. Galibert, B. Raquet and J. Leotin.Transport features in laser-plasma-deposited InMnAs layers in strong magnetic fields. Journal of Experimental and TheoreticalPhysics 108, 149 (2009).[97] J. M. Schneider, M. Orlita, M. Potemski and D. K. Maude. Consistent Interpretation of the Low-Temperature Magnetotransport inGraphite Using the Slonczewski-Weiss-McClure 3D Band-Structure Calculations. Physical Review Letters 102, 166403 (2009).[98] J. Shaver, A. N. G. Parra-Vasquez, S. Hansel, O. Portugall, C. H. Mielke, M. von Ortenberg, R. H. Hauge, M. Pasquali andJ. Kono. Alignment dynamics of single-walled carbon nanotubes in pulsed ultrahigh magnetic fields. Acs Nano 3, 131 (2009).[99] J. Shaver, A. Srivastava, J. Kono, S. A. Crooker, H. Htoon, V. I. Klimov, J. A. Fagan, E. K. Hobbie, N. Ubrig, O. Portugall,V. Perebeinos and P. H. Avouris. High field magneto-optical spectroscopy of highly aligned individual and ensemble single-walledcarbon nanotubes. International Journal of Modern Physics B 23, 2667 (2009).[100] N. Silva, A. Millán, F. Palacio, E. Kampert, U. Zeitler, H. Rakoto and V. Amaral. Temperature dependence of antiferromagneticsusceptibility in ferritin. Physical Review B 79, 104405 (2009).145

PUBLICATIONS 2009[101] R. Stoyanova, A. L. Barra, E. Zhecheva, R. Alcantara, G. Ortiz and J.-L. Tirado. Local Coordination of Fe 3+ in LayeredLiCo 1−y Al y O 2 Oxides Determined by High-Frequency Electron Paramagnetic Resonance Spectroscopy. Inorganic Chemistry48, 4798 (2009).[102] P. Strobel, H. Muguerra, S. Hebert, E. Pachoud, C. Colin and M.-H. Julien. Effect of ruthenium substitution in layered sodiumcobaltate Na x CoO 2 : Synthesis, structural and physical properties. Journal of Solid State Chemistry 182, 1872 (2009).[103] L. Thilly, S. Van Petegem, P. O. Renault, F. Lecouturier, V. Vidal, B. Schmitt and H. Van Swygenhoven. A new criterion for elastoplastictransition in nanomaterials: Application to size and composite effects on Cu-Nb nanocomposite wires. Acta Materialia57, 3157 (2009).[104] C. A. Thuesen, A.-L. Barra and J. Glerup. Single crystal electron paramagnetic resonance spectra of CS 2 MnF 6 and K 2 MnF 6diluted in the isomorphous germanium salts. Inorganic Chemistry 48, 3198 (2009).[105] T. D. Tzima, G. Sioros, C. Duboc, D. K. Demertzi, V. S. Melissas and Y. Sanakis. Multifrequency electron paramagnetic resonanceand theoretical studies of a mn(II) (S = 5/2) complex. the role of geometrical elements on the zero field splitting parameters.Polyhedron 28, 3257 (2009).[106] M. Vachon, S. Raymond, A. Babinski, J. Lapointe, Z. Wasilewski and M. Potemski. Energy shell structure of a single InAs/GaAsquantum dot with a spin-orbit interaction. Physical Review B 79, 165427 (2009).[107] L. Van Khoi, A. Avdonin, R. Szymczak, R. R. Galazka and M. Potemski. Electroluminescence and positive magnetoresistancenear the curie-weiss temperature in the Zn 1−x Mn x Te light emitting devices. Journal of Applied Physics 106, 036102 (2009).[108] L. Van Khoi, A. Avdonin, R. Szymczak, R. R. Galazka and M. Potemski. Magnetoresistance and electroluminscence near thecurie-weiss temperature in the Zn 1−x Mn x Te light emitting devices. Acta Physica Polonica A 116, 941 (2009).[109] V. Vidal, L. Thilly, S. Van Petegem, U. Stuhr, F. Lecouturier, P. O. Renault and H. Van Swygenhoven. Plasticity of nanostructuredCu-Nb-based wires: Strengthening mechanisms revealed by in situ deformation under neutrons. Scripta Materialia 60, 171 (2009).[110] D. Vignolles, A. Audouard, V. N. Laukhin, E. Canadell, T. Prokhorova and E. B. Yagubskii. Indications for the coexistence ofclosed orbit and quantum interferometer with the same cross section in the organic metal β”-(ET) 4 (H 3 O)[Fe(C 2 O 4 ) 3 ]·C 6 H 4 Cl 2 :Persistence of Shubnikov-de Haas oscillations above 30 K. European Physical Journal B 71, 203 (2009).[111] D. Vignolles, A. Audouard, V. N. Laukhin, M. Nardone, E. Canadell, N. G. Spitsina and E. B. Yagubskii. Magnetic oscillationsamplitude of a dirty quasi two-dimensional organic metal. Synthetic Metals 158, 973 (2008).[112] D. Vignolles, A. Audouard, R. B. Lyubovskii, M. Nardone, E. Canadell, E. I. Zhilyaeva and R. N. Lyubovskaya. ShubnikovdeHaas oscillations spectrum of the strongly correlated quasi-2D organic metal (ET) (8) [Hg 4 Cl 12 (C 6 H 5 Br) (2) ] under pressure.European Physical Journal B 66, 489 (2008).[113] G. Wagnières and G. Rikken. Chirality and magnetism: Free electron on an infinite helix,ncd,mcd,and magnetochiral dichroism.Chem. Phys. Lett. 481, 166 (2009).[114] O. Waldmann, T. C. Stamatatos, G. Christou, H. U. Guedel, I. Sheikin and H. Mutka. Quantum Phase Interference and Neel-Vector Tunneling in Antiferromagnetic Molecular Wheels. Physical Review Letters 102, 157202 (2009).[115] D. Wang, Y. Ma, Z. Gao, X. Zhang, L. Wang, E. Mossang, G. Nishijima, S. Awaji and K. Watanabe. Enhancement of the High-Field J (c) properties of MgB 2 /Fe Tapes by Acetone Doping. Journal of Superconductivity and Novel Magnetism 22, 671 (2009).[116] D. Wang, X. Zhang, Z. Gao, L. Wang, Y. Ma, S. Awaji, G. Nishijima, K. Watanabe and E. Mossang. Effect of processingtemperature on the superconducting properties of acetone doped MgB 2 tapes. Physica C-Superconductivity and its Applications469, 23 (2009).[117] H. Weihe, S. Piligkos, A. L. Barra, I. Laursen and O. Johnsen. EPR of Mn 2+ impurities in calcite: a detailed study pertinent tomarble provenance determination. Archaeometry 51, 43 (2009).[118] S. Wiedmann, G. M. Gusev, O. E. Raichev, A. K. Bakarov and J. C. Portal. High-order fractional microwave-induced resistanceoscillations in two-dimensional systems. Physical Review B 80, 035317 (2009).[119] S. Wiedmann, G. M. Gusev, O. E. Raichev, T. E. Lamas, A. K. Bakarov and J. C. Portal. Magnetoresistance oscillations in doublequantum wells under microwave irradiation. International Journal of Modern Physics B 23, 2943 (2009). 18th InternationalConference on High Magnetic Fields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[120] S. Wiedmann, N. C. Mamani, G. M. Gusev, O. E. Raichev, A. K. Bakarov and J. C. Portal. Magnetoresistance oscillations inmultilayer systems: Triple quantum wells. Physical Review B (Condensed Matter and Materials Physics) 80, 245306 (2009).[121] A. Wojs, L. Bryja and M. Potemski. Effects of ionized impurities on binding and recombination of positive and negative quasitwo-dimensionalmagneto-trions. International Journal of Modern Physics B 23, 2964 (2009). 18th International Conference onHigh Magnetic Fields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[122] A. Wysmolek, M. Kaminska, A. Twardowski, M. Potemski, M. Bockowski and I. Grzegory. Magneto-luminescence of gadoliniumdoped gallium nitride. International Journal of Modern Physics B 23, 2994 (2009). 18th International Conference on HighMagnetic Fields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[123] A. Wysmolek, R. Stepniewski, K. Wardak, J. Baranowski, M. Potemski, E. Tymicki and K. Grasza. 1.4 eV - luminescence bandin 6h-SiC: symmetry of the associated defect. International Journal of Modern Physics B 23, 3019 (2009). 18th InternationalConference on High Magnetic Fields in Semiconductor Physics and Nanotechnology, Sao Pedro, BRAZIL, AUG 03-08, 2008.[124] S. Yoshii, K. Ohoyama, K. Kurosawa, H. Nojiri, M. Matsuda, P. Frings, F. Duc, B. Vignolle, G. L. J. A. Rikken, L. P. Regnault,S. Michimura and F. Iga. Neutron diffraction study on the multiple magnetization plateaus in TbB 4 under pulsed high magneticfield. Physical Review Letters 103, 077203 (2009).[125] X. Zhang, Y. Ma, D. Wang, Z. Gao, L. Wang, Y. Qi, S. Awaji, K. Watanabe and E. Mossang. Phthalocyanine doping to improvecritical current densities in MgB 2 tapes. Superconductor Science and Technology 22, 045019 (2009).[126] A. Zorko, S. Nellutla, J. van Tol, L. C. Brunel, F. Bert, F. Duc, J.-C. Trombe and P. Mendels. Electron spin resonance investigationof the spin-1/2 kagomé antiferromagnet ZnCu 3 (OH) 6 Cl 2 . Journal of Physics : Conference Series 145, 012014 (2009).146

Contributors of the LNCMI to the Present ReportAMANDO M. : 16, 17ANTUNES A.B. : 55, 66, 80, 81, 86, 91, 92, 93, 94AUDOUARD A. : 68, 69, 70AUTERNAUD C. : 115, 117BARRA A.L. : 15, 98, 127BATTESTI R. : 125BÉARD J. : 119BENDICHOU L. : 120BERCEAU P. : 125BERTHIER C. : 60, 62, 82, 83, 126BIELSA F. : 125BILLETTE J. : 119BREFUEL N. : 97BROTO J.M. : 7, 8, 9, 19, 43, 46, 52, 77DAUZHENKA T.A. : 6, 48DEBRAY F. : 104, 107, 108, 109, 110, 111, 115, 116, 117,122, 123DE VALLÉE C. : 126DOMPS J.P.: 104, 123DUBOIS J.B. : 120DUC F. : 70, 84, 85DUFRESNES S. : 104, 123DUMAS J. : 115, 116, 117, 122DURANTEL F. : 121ESCOFFIER W. : 7, 8, 9, 13, 19FAUGERAS C. : 12, 15, 16, 17, 18, 23, 24, 39FERREIRA N. : 120FLORENTIN J. : 127FRINGS P. : 84, 118, 119GALIBERT J. : 6, 48GEORGE S. : 5, 43, 46, 52, 77GIQUEL F. : 119GOIRAN M. : 9, 19, 45GORYCA M. : 51GRIFFE B. : 118GUILLOT M. : 87, 88, 89, 90, 128HORVATIC M. : 60, 62, 82, 83, 126JAUDET C. : 61JOSS W. : 72,73, 124KAMKE M. : 115, 117KLEIN Y. : 61KNAFO W. : 59, 61KOSSACKI P. : 10, 16, 17, 22, 51KOWALIK K. : 29KRÄMER S. : 83, 126KUNC J. : 29, 50KUMAR A. : 13LABBE-LAVIGNE S. : 116LAGARRIGUE J.-M. : 119, 120LECOUTURIER F. : 120LEOTIN J. : 52, 45MALONE L. : 66MARTINEZ G. : 23, 24, 39MATERA J. : 115, 122MAUCHAIN J. : 119, 125MAUDE D.K. : 11, 12, 14, 20, 21, 23, 24, 27, 28, 29, 30,44, 47, 49, 50, 57, 79MILLOT M. : 43, 45, 46, 52, 77MOLLARD C. : 115MOSSANG E. : 101, 102, 103, 104, 123NANOT S. : 7, 8NARDONE M. : 70NEUGEBAUER P. : 15, 98, 127NICOLIN J.-P. : 118ORLITA M. : 15, 17, 18, 20, 21, 22, 23, 24, 39, 50, 78PARDO B. : 115, 117PIOT, B.A. : 12, 28, 30, 57PFISTER R. : 115, 116, 117, 122, 124PLOCHOCKA P. : 10, 22, 27, 29, 51PONTON D. : 115PORTAL J.C. : 31 32, 33, 34, 35, 36, 37, 38, 40PORTUGALL O. : 5, 22, 121POTEMSKI M. : 10, 12, 15, 16, 17, 18, 20, 21, 23, 24,27, 29, 51POUMIROL J.M. : 9, 13, 19, 45PROUST C. : 55, 56, 61, 67, 65PUGNAT P. : 124RAQUET B. : 7, 8, 9, 19RIBEIRO R. : 13RIKKEN G.L.J.A. : 22, 84RONAYETTE L. : 124SCHIAVO T. : 118SCHNEIDER J.M. : 20, 21, 23, 24SHEIKIN I. : 55, 58, 59, 63, 64, 71, 81, 83SOLANE P.Y. : 121SPITZNAGEL J. : 115, 126STORK H. : 126TERAN F.J. : 29TROPHIME C. : 115, 116, 117, 122UBRIG N. : 5, 22VERNEY E. : 115, 117VEYS S. : 115VIDAL N. : 115, 116, 117, 122VIGNOLLE B. : 56, 61, 65, 67, 84VIGNOLLES D. : 61, 65, 69, 70WIEDMANN S. : 31, 32, 33, 34, 35, 36, 37, 38, 40

Collaborating External LaboratoriesAlgeriaFluids Mechanics Laboratory, University of USTHB, Algiers : 108BelarusBelarus State University, Dept Phys. SC & Nanoelectronics, BY-Minsk : 48Belarus State Univ. Research Institute of Physicochemical Problems, BY-Minsk: 48BrazilEPUSP- Escola Politécnica da Universidade de São Paulo, São Paulo : 49Federal University of São Carlos : 50Instituto de Física da Universidade de São Paulo : 31, 32, 33, 34, 35, 36, 37CanadaInstitute of Microstructural Sciences, NRC, Ottawa : 28, 39University of British Columbia, Vancouver : 55University of Sherbrooke : 55, 78ChileDepartamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago : 92Instituto de Ciencias Básicas, Universidad Católica del Maule, Talca : 92Facultad de Ciencias Químicas, Universidad de Concepción, Concepción : 94ChinaBeijing National Laboratory for Condensed Matter Physics : 60China Northwest Institute for Non-Ferrous Metal Research, Xi’an: 102Institute of Physics, Chinese Academy of Science, Beijing : 60Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing : 103University of Shanghai, Shanghai : 111Czech RepublicAcademy of Science, Prague : 21, 128FranceAlstom, Belfort : 124CEA-DEN-LTMEX, Saclay : 120CEA, DRFMC-SPSMS-MDN, Grenoble : 84CEA, IRFU, Saclay : 123CEA-IRFU-SACM, Saclay : 120CEA, Saclay : 124CEMHTI-CNRS, Orléans : 126Commissariat à l’Energie Atomique, CEA-Grenoble : 59, 61, 63Commissariat à l’Energie Atomique, INAC, SP2M, Grenoble : 7, 9CMTR, ICMPE, CNRS, Thiais : 90CRETA, CNRS, Grenoble : 107DSM-DAPNIA-SACM, CEA Saclay : 104EPM-SIMAP Laboratory, Grenoble : 108ESPCI, Paris : 55, 66, 67European Synchrotron Radiation Facility, Grenoble : 84, 116GES-UM2, Montpellier : 28, 44IMEP, Grenoble : 49INAC, SPSMS, CEA Grenoble : 62

Institut Jean Lamour, Nancy : 68Institut Laue Langevin, Grenoble : 116Institut Néel, Grenoble : 88, 89, 93, 102, 104, 123, 128INP, Grenoble : 123Laboratoire Collisions Agrégats Réactivité, Toulouse : 125Laboratoire de Photonique et de Nanostructures, Marcoussis : 30Laboratoire de Spectrométrie Physique, Université J. Fourier, Grenoble : 60LEGI CNRS, INP, UJF, Grenoble : 117LGIT/CNRS/OSUG/UJF, Grenoble : 109LPEC, Universit du Maine, Le Mans : 87LPSC, CNRS, Grenoble : 116, 122PHYMAT, Poitiers : 120SERAS, CNRS, Grenoble : 115, 117Sciences Chimiques de Rennes, Université de Rennes 1, Rennes : 91, 92, 93, 94SIMAP, EPM, CNRS, Grenoble : 111Université Joseph Fourier, Grenoble : 88, 89, 128Université de Nice, Nice : 18University of Paris XII, Thiais: 90University of Rennes : 97University of Toulouse : 97GermanyApplied Physics Department, University of Erlangen-Nürnberg : 11Forschungszentrum Jülich GmbH, Jülich : 86Humboldt-Universitt, Berlin : 52Physikalisches Institut, Universität Karlsruhe : 82TU Braunschweig, Institute for Physics of Condensed Matter, Braunschweig : 64University of Karlsruhe : 81University of Stuttgart : 81Walther-Meissner-Institut, Garching : 58Hong KongThe Hong Kong University of Science and Technology, Kowloon : 71IsraelThe Institute of Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan : 72, 73The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem : 72, 73Weizmann Institute, Rehovot : 27ItalyCNR-INFM LAMIA, Genova : 101Columbus Superconductors S.p.A., Genova : 101Universitá degli studi di Pavia : 14University of Florence : 81, 98University of Modena : 98JapanInstitute for Materials Research, Tohoku University, Sendai : 84, 103ISSP, University of Tokyo : 83JAEA, Tokai : 84Nihon University, Tokyo : 71University of Kumamoto : 97University of Kyoto : 83, 65, 97University of Osaka, Osaka : 71, 65University of Tokyo, Kashiwa : 55

LithuaniaSC Institute LI-Vilnius : 6PolandAGH University of Science and Technology, Krakow : 110Institute of Experimental Physics, Warsaw University : 10, 44, 51Institute of High Pressure Physics, Warsaw : 44Institute of Physics Polish Academy of Science, Warsaw : 29, 51, 79, 80RussiaInstitute of General and Inorganic Chemistry, Russian Academy of Science, Moscow : 79Institute of Microelectronics Technology of RAS, Chernogolovka, Moscow district : 40Institute of Semiconductor Physics, Novosibirsk : 31, 32, 33, 34, 35, 36, 37, 38IPCP, Chernogolovka, Russian Federation : 69, 70General Physics Institute of the Russian Academy of Sciences, Moscow : 78Moscow Power Engineering Institute, Moscow : 78P.N. Lebedev Physical Institute, Russian Academy of Science, Moscow : 57SpainCT-ISOM, Universidad Politécnica de Madrid : 14DCITIMAC, Santander : 43, 52, 77Electrocerámicas, Instituto de Cerámica y Vidrio, CSIC, Madrid : 91, 93ESI, Bilbao : 77ICMAB, Barcelona : 69, 70ICMUV, Valencia : 46Laboratorio de bajas temperaturas, Universidad de Salamanca : 14SwitzerlandEMPA, Dübendorf : 77EPFL, Lausanne : 83POLDI-Paul Scherrer Institut : 120EPFL, Switzerland : 8The NetherlandsHMFL, Nijmegen : 65MESA+, Enschede : 77UkraineInstitute of Materials Science Problems, Ukrainian Academy of Sciences, Chernovtsy : 80Institute of Semiconductor Physics, NAS of Ukraine, Kiev : 31, 32, 33, 34, 35, 36, 37United KingdomSchool of Physics and Astronomy, University of Leeds : 47School of Physics and Astronomy, University of Nottingham : 50University of Bristol, Bristol : 56, 65University of Edinburgh, Edinburgh : 85University of Manchester, Manchester : 18USABrown University, Providence : 62Center for Microwave Magnetic Materials and Integrated Circuits, Northeastern University, Boston : 87Columbia University, New York : 27Georgia Tech., Atlanta : 10, 12, 16, 17, 24Garcia center Polymers at Engineered Interfaces, Stony Brook Univ. NY : 6

Lawrence Berkeley National Laboratory, Berkeley : 45Physics department and Chemistry Laboratory, Stanford : 9Rice University : 5RoseStreet Labs Energy, Phoenix : 45

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