2011 - Laboratoire National des Champs Magnétiques Intenses ...

LABORATOIRE NATIONAL DES CHAMPS MAGNÉTIQUES INTENSESANNUAL REPORT2011

Laboratoire National des Champs MagnétiquesIntensesGrenoble–ToulouseCentre National de la Recherche Scientifiquehttp://lncmi.cnrs.fr/Annual Report2011LNCMI is formally associated with the “Université Joseph Fourier”, Grenoble,the “Université Paul Sabatier”, Toulouse and the “Institut National des SciencesAppliquées”, Toulouse.Editors: Fabienne Duc and Duncan Maude

2011PrefaceDear Reader,You have before you the third 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 MagnétiquesIntenses (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 theFrench “Très Grands Equipements”. The laboratory is associated with the Université Joseph Fourier de Grenoble,the Université Paul Sabatier de Toulouse and the Institut National des Sciences Appliquées de Toulouse.Its activities are also supported by the EC FP7 “Large infrastructures” program, where it is the coordinator ofthe EuroMagNET2 Integrating Activity, a collaboration with our colleagues of the HFML (Nijmegen) and theHLD (Dresden). The same partners are performing a design study for the creation of a distributed EuropeanMagnetic Field Laboratory to serve the European high magnetic field user community even better.This report intends to provide a complete overview of the in-house and collaborative scientific and technicalactivities in 2011 of the LNCMI.The 24 MW power supply of the Grenoble site, dedicated to the generation of static fields, allows either tooperate simultaneously two resistive 12 MW magnets or to operate one 24 MW magnet. The highest fieldgenerated at the LNCMI-G is currently 35 T in a 34 mm bore diameter and we are working hard to increasethis value. This year, we have finally received the funding to construct a hybrid magnet, capable of generating43 T, that will become available in 2014.The 14 MJ capacitor bank generator of the Toulouse site, dedicated to the generation of pulsed fields, allows togenerate fields up to 80 T. The construction of a mobile extension of the bank with 6 MJ has started this year,opening the road to even higher fields. An installation for the generation of semi-destructive pulsed fields hasproduced fields up to 160 T and we hope to increase also this value even further.This report and the list of publications show the importance and the interest of results obtained in high magneticfields, either on the basis of in-house research or as a result of close collaborations with research groups frommany different countries.In addition, the LNCMI is proud to contribute to the training of young scientists by giving them the opportunityto prepare their 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 90people), our post docs and PhD students (almost 30) and the numerous visitors (over 200 this year) for theircontributions to the improvement of the installation and for the quality of their work.Geert RikkenDecember 2011v

2011CONTENTSFront CoveriCover PageiiiPrefacevList of FiguresxiThe LNCMI user facility 1CARBON ALLOTROPES 5Magnetic confinement and edge symmetry in graphene nanoribbons 7Carrier scattering from dynamical magneto-conductivity in quasi-neutral epitaxial graphene 8Carrier relaxation in epitaxial graphene photoexcited near the Dirac point 9Magnetotransport in graphene grown on silicon side of SiC wafers 10Quantum Hall effect in inhomogeneous few-layer graphene 11Quantum Hall effect in trilayer graphene 12The remarkably robust quantum Hall effect in bottom-gated epitaxial graphene 13Free electron kinetic energy terms and the SWM Hamiltonian 14Magneto-optical spectroscopy to probe the electron-hole asymmetry in graphite 15Magneto-optical spectroscopy in fields up to 160 T: exploring the band structure of graphite 16A de Haas-van Alphen investigation of the Fermi surface of graphite 17Cyclotron motion in the vicinity of a Lifshitz transition in graphite 18Raman scattering from electronic excitations in bulk graphite 19Magneto-phonon effect in bulk graphite 20Investigation of the Aharonov-Bohm effect in single antidot graphite structures 21The 1.4 eV Ni colour centre in synthetic diamond 22TWO-DIMENSIONAL ELECTRON GAS 23Direct resistive NMR measurement of the electronic spin polarization of fractional quantum Hall states 25Quantum-classical crossover and apparent metal-insulator transition in a weakly interacting 2D Fermi liquid 26Non-linear transport phenomena in electronic bilayers 27Polaronic effects in a single modulation doped GaAs quantum well 28Microwave-induced Hall resistance in a bilayer electron system 29Valley-spin phase diagram of the two-dimensional electron gas 30Temperature effect on spin gap of Landau levels in a bilayer 2DEG 31SEMICONDUCTORS AND NANOSTRUCTURES 33vii

2011Inter-shell electron-hole exchange interaction in CdTe/ZnTe quantum dots 35Probing the anisotropy of quantum-dots 36Magnetotransport studies of GaAs/AlGaAs core-shell nanowire 37Magnetically induced field effect in InAs nanowires 38Local and nonlocal magnetoresistance in two-dimensional topological insulator 39Quantum Hall effect in a quasi-three-dimensional HgTe film 40Ratchet photovoltage measurements in Si/SiGe heterostructures 41Hall Spectroscopy of the Dipolar Spin Waves of Nanomagnets 42Magnetoresistance in MOSFET saturation regime of operation 43Origin of the negative magnetoresistance in 2D polycrystalline SnO 2 thin films 44METALS, SUPERCONDUCTORS AND STRONGLY CORRELATED SYSTEMS 45Quantum oscillations the organic metal θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) 47dHvA investigation of the MAX phase compound Ti 3 SiC 2 48Magnetic-field-induced stripe order in underdoped YBa 2 Cu 3 O y 49Nuclear magnetic resonance study of (EDT-TTF-CONMe 2 ) 2 Br 50Fermi surface reconstruction across the 26 Tesla metamagnetic transition in CeRh 2 Si 2 51Field-induced Lifshitz transition in CeIrIn 5 52Direct observation of spin-dependent effective masses in CeCoIn 5 53Fermi-surface evolution in Yb-substituted CeCoIn 5 54Interplay of magnetism, Fermi surface reconstructions, and hidden-order in the heavy-fermion materialURu 2 Si 2 55Gap like structure in a nonsuperconducting layered oxycarbonate Bi 2+x Sr 4−x Cu 2 CO 3 O 8+δ single crystal 56Extended criticality in the electrical resistivity of La-doped Bi2201 57Fermi surface reconstruction by stripe order in cuprate superconductors 58Mapping the phase diagram of high-T c superconductors 59The upper critical magnetic field of (Ba 0.68 K 0.32 )Fe 2 As 2 and Ba(Fe 0.93 Co 0.07 ) 2 As 2 single crystals 60Fermi surface of 111 iron pnictide superconductors: LiFeP, LiFeAs 61MAGNETIC SYSTEMS 63NMR study of the “pinwheel” kagome compound Rb 2 Cu 3 SnF 12 65NMR study of the Bose-Einstein condensation in DTN 66Magnetic properties of Y 0.9 Gd 0.1 Fe 2 D 4.2 compound 67Magnetic properties of nanocrystalline Dy 3 Fe 5 O 12 garnet 68High-field magnetization study of ErCo 2 69High field-dependence of the surface spin magnetization in core-shell ferrite nanoparticles 70Magnetic field induced phase transition in a ferronematic liquid crystal 71viii

2011MOLECULAR MAGNETISM 73Multifunctional oxalate-based molecular magnets 75Π− stacked radicals question models 76Spectroscopic study of magnetic exchange between highly anisotropic spin centres 77Magnetic bistability of isolated giant-spin centers in a diamagnetic crystalline matrix 78APPLIED SUPERCONDUCTIVITY 79Tests of HTS model coils under very high field 81Transport J c measurements of YBCO coated conductors above 20 T 82Superconductivity of MgB 2 /Nb/Cu wires with amorphous carbon doping 83Superconductivity of Ca doped Bi-2212 single filament tapes 84High magnetic field HTS SuperSMES protection development 85Superconducting critical current measurements of MgB 2 based conductors 86MAGNETO-SCIENCE 87Vacuum Magnetic Birefringence (BMV) Experiment 89Electrodeposition of selenide semiconductors under magnetic field 90Hydrogen evolution in high magnetic field 91Morphology and phase composition of electrodeposited Co-Ni alloys under high magnetic field 92INSTRUMENTATION 93Towards the introduction of nuclear magnetic resonance as a method available in pulsed high-field magnets 95A flexible probe dedicated for Nuclear Magnetic Resonance experiments in pulsed fields 96Pulsed field setup for single crystal X-ray diffraction 97Sensor validation for polyhelix in-situ instrumentation 98Transient recording system for magnet sites 99Cryogenic development for pulsed magnetic fields 1003 He refrigerator development for pulsed and dc magnetic fields 101Dilution fridge for use in pulsed magnetic fields up to 60 T 102A new test station to measure the critical current of superconducting strands 103MAGNET DEVELOPMENT 105Pulsed high magnetic field coils 107Cu/Nb nanocomposite wires processed by severe plastic deformation for applications in high pulsed magnets108Pulsed energy supplies 109Helix development for DC high field magnets 110Copper/stainless steel polyhelix magnets 111First electron cyclotron resonance ion source using radially cooled polyhelices 112ix

2011Use of high field magnets for magnetic levitation 113Final design of the new Grenoble hybrid magnet 114Proposals for magnet time – Projects carried out in 2011 115PhD theses 126Habilitations à diriger la recherche 127List of Publications 2011 128List of patents 133Collaborating external laboratories 134Author index 137Index 138x

2011LIST OF FIGURES1 Number of weeks of availability of the resistive magnets at LNCMI-G . . . . . . . . . . . . . . . . . . . . . . . . . 22 Number of projects submitted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Summary of the use and power consumption of the resistive magnets in 2011 . . . . . . . . . . . . . . . . . . 34 Evolution of the number of magnetic field shots at LNCMI-T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Number of projects carried out by country of origin and by area of expertise . . . . . . . . . . . . . . . . . . . 46 Shubnikov de Haas oscillations for a 100 nm wide graphene nanoribbon . . . . . . . . . . . . . . . . . . . . . . . . 77 Electronic structure of a graphene nanoribbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Calculated magneto-electronic subbands versus B for graphene nanoribbons . . . . . . . . . . . . . . . . . . . 79 FIR magneto-transmission of highly doped epitaxial graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810 Differential magneto-transmission of epitaxial graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811 Pump-and-probe experiment in epitaxial graphene – energy and temperature dependence . . . . . . . . 912 Pump-and-probe experiment in epitaxial graphene -power dependence . . . . . . . . . . . . . . . . . . . . . . . . 913 Longitudinal and Hall resistance of epitaxial graphene from ITME Poland . . . . . . . . . . . . . . . . . . . . . 1014 Longitudinal and Hall resistance of epitaxial graphene from the University of Linköping . . . . . . . . . 1015 Longitudinal and Hall resistivities of few layer graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116 Hall resistivities of the two layer graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117 Quantum Hall effect in graphene, bilayer graphene and trilayer graphene . . . . . . . . . . . . . . . . . . . . . . 1218 ABA versus ABC stacking of trilayer graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219 Magnetic field dependence of our back gated epitaxial graphene sample . . . . . . . . . . . . . . . . . . . . . . . 1320 Calculated Landau level dispersion in graphite along k z . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1421 Magneto-transmission of graphite showing splitting of K and H-point transitions . . . . . . . . . . . . . . . 1522 Single-turn coil before and after a shot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1623 Single shot acquisition of both circular polarizations in the infrared transmission of graphite . . . . . 1624 Polarization resolved infrared transmission up to 140 T confirming the e-h asymmetry in graphite 1625 Angle dependence of the dHvA periods in natural graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1726 Cyclotron resonance in graphite in THz range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1827 Polarized Raman scattering spectra of bulk graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1928 Raman scattering on bulk graphite showing magneto-phonon effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 2029 Oscillatory part of magnetoresistance of graphite nano-structure with a single hole . . . . . . . . . . . . . . 2130 T-dependence of the amplitude of the magnetoresistance oscillations for a graphite nano-structure 2131 The emission spectra of Ni color center in diamond as a function of the magnetic field. . . . . . . . . . . 2232 The energy of the emission lines of Ni colour centre in diamond as a function of magnetic field . . . 2233 RDNMR showing Knight shift at ν = 5/2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534 Quantum-classical crossover and metal-insulator transition in a 2D Fermi liquid . . . . . . . . . . . . . . . . 2635 Zero differential resistance states in a wide quantum well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27xi

201136 Magneto-transmission spectra of a GaAs QW up to 33.5 T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2837 Variation of δ 0 with the energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2838 Microwave induced Hall resistance in a wide GaAs quantum well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2939 Schematic illustrating our SOI transistor device. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3040 Topography map of tunnel spectra of a 2DEG tunnel diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3141 Magnetic field dependence of recombination of XX − transition in CdTe/ZnTe QD . . . . . . . . . . . . . . 3542 Polarisation dependence of the luminescence of a quantum dot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3643 Magnetic dependance of the luminescence of a quantum dot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3644 Magnetoresistance of a single radial core-shell nanowire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3745 Conductance versus backgate voltage for a 33 nm diameter InAs nanowire . . . . . . . . . . . . . . . . . . . . . 3846 Magneto-conductance measured on a 33 nm diameter InAs nanowire . . . . . . . . . . . . . . . . . . . . . . . . . . 3847 Local and nonlocal resistances in the HgTe topological insulator system . . . . . . . . . . . . . . . . . . . . . . . 3948 Quantum Hall effect in a quasi 3D HgTe film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4049 Rachet effect: Longitudinal photo-voltage at different temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . 4150 Rachet effect: Transverse photo-voltage at 1.4K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4151 AFM image of a dysprosium dot on the surface of a 2DEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4252 Microwave resonances in the photo-voltage of an array of magnetic dots . . . . . . . . . . . . . . . . . . . . . . . 4253 Extracted µ MR versus V d for different channel lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4354 µ MRsat vs µ MRlin at 270K from different channel lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4355 Extracted µ MRsat and v sats versus effective length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4356 Normalized magnetoconductance in a SnO 2 thin film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4457 Normalized magnetoconductance in a SnO 2 thin film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4458 Fermi surface of θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4759 Fourier analysis of dHvA in θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4760 Mass and Dingle plots from dHvA oscillations of θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) . . . . . . . . . . . . . . . . . . . . . 4761 Fourier analysis of the tunnel diode oscillator in-plane magnetoresistance . . . . . . . . . . . . . . . . . . . . . . 4762 SEM image of a Ti3SiC2 sample illustrating the step flow growth mode . . . . . . . . . . . . . . . . . . . . . . . 4863 Oscillatory dHvA signal of Ti 3 SiC 2 together with the Fourier transform . . . . . . . . . . . . . . . . . . . . . . . 4864 Splitting of the 63 Cu(2F) high frequency quadrupole satellite of YBa 2 Cu 3 O 6.5 . . . . . . . . . . . . . . . . . 4965 Temperature dependence of 13 C NMR spectra in EDTBr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5066 Phase diagram of EDTBr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5067 Metamagnetic transition in CeRh 2 Si 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5168 FFT spectra of the dHvA oscillations in CeRh 2 Si 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5169 Torque versus magnetic field in CeIrIn 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5270 FFT spectra of the dHvA oscillations in CeIrIn 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5271 dHvA oscillatory signal and FFT from 15 to 28 T in CeCoIn 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53xii

2011144 Example of a transient recorded on M9 outer Bitter magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99145 Newly installed liquid nitrogen cryostat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100146 Nitrogen cryostat for vacuum magnetic birefringence experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100147 Faraday cage for megagauss system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100148 Trigger noise reduction with Faraday cage on megagauss system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100149 3 He cryostat for pulsed magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101150 3 He cryostat for dc magnetic fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101151 Hold time of 3 He cryostat for dc magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101152 3 He/ 4 He dilution fridge for pulsed magnetic fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102153 Magnetoresistance of URu 2 Si 2 measured in a dilution fridge in pulsed magnetic fields . . . . . . . . . . . 102154 A new test station to measure the critical current of superconducting strands . . . . . . . . . . . . . . . . . . 103155 A new test station to measure the critical current of superconducting strands . . . . . . . . . . . . . . . . . . 103156 Magnetic field versus time for the 81 T pulse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107157 CAD view of the MAGFINS coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107158 A view of the 14 MJ, 24 kV, 65 kA generator in the basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109159 View of the new dedicated housing for polyhelix inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110160 The “mixed” insert to be tested in 2012 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110161 Yield strength versus conductivity for materials used in dc magnets and pulsed magnets . . . . . . . . . 111162 The pair of copper/stainless steel helices after electrical discharge machining . . . . . . . . . . . . . . . . . . . 111163 Magnetic field during the test of a copper/stainless steel pair of helices . . . . . . . . . . . . . . . . . . . . . . . . 111164 Cross section of the SEISM ECRIS prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112165 Axial magnetic induction measurements for the SEISM ECRIS prototype . . . . . . . . . . . . . . . . . . . . . . 112166 COLOSSECRIS superconducting bus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112167 Magnetic levitation: Isovalue of ɛ in a cylindrical grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113168 Detailed view of the levitation zone for liquid H 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113169 The outermost superconducting part of the hybrid magnet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114xv

2011 THE LNCMI USER FACILITYThe LNCMI user facilityIntroductionThe Laboratoire National des Champs Magnétiques Intenses is one of the large scale facilities of CNRS. Itworks as a research facility available both to its own researchers as well as to external users. The high fieldfacility is open to users from 27 countries of the European Union and from the associated states (Iceland, Israel,Liechtenstein, Norway, Switzerland and Turkey) under the same conditions as users of the CNRS. Researchersfrom other countries can also perform their experiments at the LNCMI, but they are required to cover themselvestheir travel and living expenses.High magnetic fields and instrumentation available to the usersThe LNCMI has resistive and superconducting magnets at the Grenoble site and pulsed magnets at the Toulousesite, achieving continuous magnetic fields up to 35 T and pulsed fields up to 80 T on which it is possible to installdifferent experimental set-ups.The technical and experimental environment also allows the user to combine very low temperatures and highpressures with very high fields. The laboratory offers a wide variety of instrumentation, allowing measurementsin high magnetic fields in the following areas:• EPR spectroscopy,• NMR spectroscopy,• Magnetization measurements (extraction technique, torque),• Magneto hydrodynamics,• Specific heat measurements,• Magneto-transport measurements,• Magneto-spectroscopy in the infra-red and visible area,• Characterization of superconductors for applications.Access to the facilityScientists who wish to obtain access to the resistive or pulsed magnets of the LNCMI must apply through theEuroMagNET web site (http://www.hfml.ru.nl/EuroMagNET2/). This application is drawn up according toa specific form and provides information demonstrating the relevance of the research work to be undertaken.The application is also used to ensure that the experimental conditions requested can be satisfied. Accessto the superconducting magnets of the LNCMI goes through an in-house procedure, which is open all year(http://ghmfl.grenoble.cnrs.fr/spip.php?rubrique126).Selection CommitteeAll applications are examined and ranked by the EuromagNET2 selection committee, twice a year, generally inthe course of December and at the end of June. The applications are ranked A, B or C. Projects rated A will beexecuted, those classified B are performed if the magnet time is available, and applications rated C are rejected.The ranking is made on the same basis for all applications, including those submitted by the local scientists.Accepted experiments are consequently programmed during the two semesters following this meeting. If theexperiment has not been performed in this period, one has to resubmit the proposal mentioning the reasonswhy it could not be performed (samples not ready, etc). The acceptance of an application means that the entirecost of the experiment (electricity, cryogenic fluids, etc) will be covered by the budget of the laboratory. In theframe of the EuroMagNET2 European contract, scientists from one of the countries belonging to the EuropeanUnion or to an associated state (see above) may obtain the reimbursement of their travel and living expenses(one person per project can be reimbursed).The Selection Committee is divided into six groups; each group with a different area of expertise (Semiconductors,Metals and Superconductors, Magnetism, Magnetic Resonance, Applied Superconductors, Soft Materials).In each area there are international experts from outside the EuromagNET2 consortium, chosen from within theEuropean Community, as well as scientists from the different high magnetic field laboratories of the Euromag-NET2 consortium, whose presence ensures that a project can be technically performed in the laboratory.1

THE LNCMI USER FACILITY 2011Local contactWhen the applicant is informed of the acceptance of his proposal, and of the number of pulses / shifts scheduledfor the experiment, he/she is at the same time informed of the name of his local contact in charge of facilitatinghis work at the LNCMI. The local contact is a research scientist or a engineer of the laboratory. If an applicantcannot identify a local contact, the laboratory management will appoint a scientist that the user can contact inorder to define with him the conditions for the organization of the experimental work.Programming the period of experimentThe schedule for utilization of the magnets is drawn up several weeks in advance and is made official at thebeginning of every month. Visitors are therefore strongly urged to get in touch with their local contact, toreserve as quickly as possible the most suitable period for carrying out their experiment at the LNCMI.Magnet time planningThe magnets operate on a schedule, displayed monthly, taking into account the availability of staff, magnetsand instrumentation. For the static field facility, the installation operates outside normal working hours withprofessional operators. This allows the magnets to be operated 20 hours per day during the week and 11 hoursper day during weekends and bank holidays. In order to reduce the electricity costs of the laboratory a newcontract which is effective since November 2011 has been signed with the electricity supplier. As a consequence,the resistive magnet site is shut down yearly for three months from December to February when the electricity ismost expensive. On the other hand, the access to these magnets is now possible during the summer months upfrom 2012. Due to the contract change the number of weeks where the 10 and 20 MW magnets were accessible wasreduced in 2011. The winter shutdown is used for maintenance, technical interventions, upgrading and testing.Notice that the new contract has no influence on the pulsed and superconducting activity. The evolution of thenumber of weeks the resistive magnets are available can be seen in figure 1.FIG. 1.Number of weeks of availability of the 10 and 20 MW resistive magnets at LNCMI-G.Projects submitted during the last yearsAn average of 182 projects have been submitted annually over the last three years, of which there were 75 projectsper year submitted for a 20 MW magnet, 31 projects per year submitted for a 10 MW magnet, 61 projects peryear submitted for a pulsed magnet and 16 external projects per year submitted for a superconducting magnet(see figure 2). The former increasing of the number of applications for 20 MW magnets has come to saturationover the last three years; these magnets are indeed totally occupied for all periods available. The success of thesemagnets is certainly linked both to the possibility of different bore diameters of the 20 MW magnets, and partlyto the increase of the field obtained (35 T in a 34 mm bore, 20 T in a 160 mm bore, 29 T and since November2011 30 T in a 50 mm bore).Projects carried out in 2011In 2011 the access to the Grenoble resistive magnets was possible during 37.5 weeks in which the laboratory hasprovided 2641 hours of magnet time to its users. More than 80% (2189 h which is more than 200 h more than in2

2011 THE LNCMI USER FACILITYFIG. 2. Number of projects submitted: 20 MW magnet refers refers to magnets using simultaneously the four 6 MWpower supplies of the facility whereas 10 MW magnets use only two power supplies. The projects on pulsed magnets usethe Toulouse facility and projects on superconducting magnets refers only to the external users.2010) of the magnet time was performed on the 20 MW magnets M8, M9 and M10 whereas the 10 MW magnetscovered almost 20% of the magnet time (see figure 3(a)). In October 2011 a new 10 MW magnet “M1” reaching23 T in a 50 mm bore has been put in operation replacing the old magnet “M6”. Another 10 MW magnet “M7”is about to be replaced by a more competitive one using the same helices insert of the 20 MW magnets. Thetotal power consumption of all resistive magnets in 2011 was about 15.7 GWh (see figure 3(b)).LNCMI-T, the pulsed field facility in Toulouse has provided 4202 shots in 2011 which corresponds to 400 shotsmore than in 2010. 40% of the shots used more than 80% of the available energy of pulsed field power supply.A summary of the evolution of the number of shots can be seen in figure 4.The left panel of figure 5 shows the country of origin of the projects that were performed on resistive, pulsedFIG. 3. Summary of the use (a) and power consumption (b) of the resistive magnets in 2011.3

THE LNCMI USER FACILITY 2011FIG. 4.Evolution of the number of magnetic field shots at pulsed facility over the last three years.and superconducting magnets in 2011. 40% of the projects came from France while the other 60% were dividedmainly between European countries. Some users also came from the USA, Canada, Russia, Brazil and Japan.Most scientists who have already used our facilities come back regularly, introducing and progressively formingscientists who become themselves, in turn, new users.The projects realized in 2011 using the Grenoble and the Toulouse facilities are thematically distributed asshown in the right panel of figure 5.FIG. 5.(left) Number of projects carried out by country of origin and (right) projects performed by area of expertise.ConclusionThe technical and scientific environment of the LNCMI, adapted to research in high magnetic field, is highlyappreciated by the international scientific community. A large majority of users shows great interest in thelaboratory and they come regularly in order to perform their experiments, thereby strengthening collaborationsbetween research groups. The number of projects submitted and realized at the LNCMI reached a significantnumber in recent years, demonstrating the international scope of the laboratory. The schedule of the Grenoble20 MW magnets is extremely tight and it is essential to optimize the use of these magnets.N. Bruyant, C. Warth-Martin, C. Vannier4

Carbon Allotropes

2011 CARBON ALLOTROPESMagnetic confinement and edge symmetry in graphene nanoribbonsThe two-probe transverse magnetoresistance of a100 nm wide graphene nanoribbons (GNR) exhibits,at first glance, the expected 1/B Shubnikov-de Haasperiodic oscillations at large magnetic field (figure 6).However, a strong departure from the 1/B periodicityis observed for large Landau index n, originating fromthe electronic confinement that starts to overcome themagnetic one.onto the lowest index Landau levels. For the narrowestGNR, a second maximum of resistance coincideswith the changeover of E F from the n = 1 Landaulevel (solid black line) to the second energy minimumat nonzero k (dashed black line).FIG. 7. Electronic and structure at 0 T (a) and 50 T (b)for a 70 nm wide armchair GNR.FIG. 6. SdH oscillations measured on a 100 nm wide GNR.In inset, the departure from the 1/B periodicity, fingerprintof the electronic confinement.Figure 7 shows the simulated band structures of anarmchair GNR (aGNR) around the charge neutralitypoint at B = 0 and 50 T. The armchair boundary conditionsentails the valley degeneracy lifting already atB = 0. As the magnetic field rises, magnetic and spatialconfinement starts competing, with a progressiveincrease of the bottom of the bands and the flatteningof their central region. The magnetic confinement inthe region of the bulk is so strong that these statesdo not feel the effect of the edges and they are valleydegenerateas in 2D graphene. However, when movingaway from the bulk, the effect of the edges becomesimportant again and the valley degeneracy is lost sincethe propagating states become more confined along thearmchair edges of the ribbon.We compare the oscillation of the Fermi energy (blue)and the magneto-subbands spectrum (black) with thelocations of the maxima of the experimental resistanceR(B), in red. Figure 8(a,b) depict the results obtainedon a 100 nm GNR at two different doping levels whilefigure 8(c) presents the analysis of a 70 nm wide GNR.The inflexion point of the E F (B) curves in between twosuccessive Landau levels well matches with the minimaof resistance. The widening of the resistance peaks atlarger V g and their shift to high fields are also a directconsequence of the stronger Fermi energy pinningFIG. 8. (a) Simulated magneto-electronic subbands versusB for the 100 nm-wide aGNR compared to the experimentalMR (red curve) at a selected E F in blue. (b) The same asin (a) for a lower doping level. (c) The same as in (a)for the 70 nm-wide aGNR, exhibiting the valley degeneracybreaking.This is an unambiguous signature of the valley degeneracylifting suggesting that the Landau pattern ispotentially dominated by an armchair contribution atthe edges, since the zigzag symmetry preserves the 2Dgraphene valley degeneracy [Ribeiro et al., Phys. Rev.Lett. 107, 086601 (2011)].R. Ribeiro, J-M Poumirol, W. Escoffier, M. Goiran, J-M Broto, B. RaquetA. Cresti (CEA-LETI, Grenoble, France), S. Roche (CI2N and ICREA, Barcelona, Spain)7

CARBON ALLOTROPES 2011Carrier scattering from dynamical magneto-conductivity inquasi-neutral epitaxial grapheneUnderstanding carrier scattering in graphene is one ofthe fundamental but still unclear issues in the physicsof electronic properties of this novel material. In thiswork, the energy-dependence of the electronic scatteringtime is probed by Landau level spectroscopyin quasi neutral multilayer epitaxial graphene. Toprobe this graphene system with a fixed Fermi energywe profit of the dynamical magneto-conductivitymethod and follow the energy dependence of the spectralbroadening Γ of inter Landau level transitions measuredin our infrared transmission experiments. Notably,the optical conductivity response of grapheneinvolves interband excitations across the Dirac pointand allows us to to determine the parameter Γ over abroad energy range.We have found that in the limit of strongly overlappingLLs, which corresponds to the quasi-classical limit ingraphene and in which the simple relation Γ = /τ(ɛ)is justified, this spectral broadening becomes Γ = α·|ɛ|,see figure 9. This linear dependence on energy is validat all magnetic fields and for all transition indexes(α = 0.026 in the sample investigated). Hence, wepresent a positive test of the 1/τ ∼ |ɛ| rule, which hasbeen originally predicted for graphene, but never observedin exfoliated specimens. This experimentallydetermined scattering rate 1/τ(ɛ) ∼ |ɛ| dependencerules out specific extrinsic scattering mechanisms, suchas resonant scatterers, ripples or charge impurities, inthe epitaxial graphene layers which are protected fromthe environment.Interestingly, besides the proper analysis of the data,which involved a direct comparison of theoretically calculatedand experimentally determined spectra, seeFigs. 9a and 9b, the same conclusion can be approximativelydrawn based on quasi-classical argument wheninspecting the differential spectra in Fig. 10. The numberof resolved transitions in these spectra is roughlymagnetic field independent, i.e., the onset of Landaulevel quantization remains always pinned to a certainlevel (n ∼ 10). The energy of this onset moves tohigher energies as |ɛ| ∼ √ B and the cyclotron frequencycorresponding to this onset thus necessarily followsa linear in energy dependence ω c = vF 2 eB/|ɛ| ∼|ɛ|. If we employ the classical condition for this onset,ω c τ ∼ 1, we immediately conclude 1/τ ∼ |ɛ|.For more information see [Orlita et al., Phys.Lett. 107, 216603 (2011)].Rev.FIG. 9. Part (a) and (b): Experimentally obtained transmissionspectrum of multilayer epitaxial graphene comparedto theoretically expected curves calculated for thebroadening parameter Γ = 3.9 and Γ = 0.026ω meV.FIG. 10. Differential magneto-transmission spectra T (B +∆B)/T (B − ∆B). The lowest two spectra have been takenfor ∆B = 0.05 T, the other with ∆B = 0.125 T. Experimentalcurves are accompanied by theoretical fits, assumingthe broadening parameter Γ = 0.026ω. Transitions arealways well-resolved up to the Landau level index n ∼ 10.M. Orlita, C. Faugeras, G. Martinez, M. PotemskiR. Grill (Charles University, Prague, Czech Republic), A. Wysmo̷lek (Warsaw University, Poland), W.Strupiński (ITME, Warsaw, Poland), C. Berger, W. A. de Heer (GeorgiaTech, Atlanta, USA)8

2011 CARBON ALLOTROPESCarrier relaxation in epitaxial graphene photoexcited near the DiracpointCarrier dynamics in graphene has been studied viapump-and-probe technique using a tunable source ofpulsed infrared radiation - the free electron laser. Incontrast to previous studies, mostly single- or twocolorpump-and-probe experiments in visible or nearinfraredrange, probing the carrier relaxation at relativelyhigh energies, here we address dynamics of masslessDirac fermions in a close vicinity of the Dirac point.FIG. 11. Part (a): Normalized pump-induced transmissionfor different photon energies E (a) measured at 10 K. Part(b): Illustration of the optical pumping in comparison withrelaxation via optical phonons, arrows in colors correspondto the part (a). Parts (c) and (d): Temperature dependenceof the pump- induced transmission for E = 245 meV andE = 72 meV. The symbols are experimental data, the linesare fits taking into account the laser pulse duration and abi-exponential decay.The study has been performed on quasi-neutral sheetsof multi-layer epitaxial graphene specimens preparedby thermal decomposition of carbon-terminated surfaceof 4H-SiC. The experimental data, i.e., the relativechange of the induced transmission as a functionof photon energy and temperature is plotted in figure11(a,c and d). Let us now summarize the mainexperimental findings.Our data exhibit an overall bi-exponential decay withthe fast and slow components in the ps and sub-nsranges, respectively. The faster component is characterizedby a decay time that decreases monotonicallywith the photon energy. For instance, at temperatureof 10 K, it falls from τ = 25±2 ps at the photon energyof 30 meV down to τ = 5.2±0.2 ps at the excitation energyof 245 meV. In addition, the decay times decreasewith rising temperature, especially for lower photon energies.Our experiments have been complemented bymicroscopic modeling which revealed the interactionwith optical phonons as a dominant relaxation mechanismof photo-excited carriers. Perhaps surprisingly,this conclusion remains valid also in the case of excitationat energies significantly lower than the opticalphonon energy in graphene (ω ≪ 200 meV).Intriguing results have been obtained in experimentsat excitation energies close to the absorption edgedue to interband transitions (around 2E F ). As documentedin figure 12, the sign of pump-and-probe signalis inverted, i.e., the induced transmission (bleaching)is switched into the pump-induced absorption, whenphoton drops below 2E F , estimated in our case as|E F | ≈ 13 meV. The induced absorption is explainedby heating the system induced by the pump pulse,which spreads originally sharp thermal distribution.This enables absorption of photons with energies below2E F and simultaneously reduces absorption strengthabove 2E F .For more information see [S. Winnerl et al., Phys. Rev.Lett. 107, 237401 (2011)].FIG. 12. Experimental pump-induced transmission changefor three different fluences for the photon energy 30 meV(a) and 20 meV (b).M. Orlita, P. Plochocka, M. PotemskiS. Winnerl, H. Schneider, M. Helm (Helmholtz-Zentrum Dresden-Rossendorf, Germany), P. Kossacki(Warsaw University, Poland), T. Winzer, E. Malic, A. Knorr (Technische Universität Berlin, Germany),M. Sprinkle, C. Berger, W. A. de Heer (GeorgiaTech, Atlanta, USA)9

CARBON ALLOTROPES 2011Magnetotransport in graphene grown on silicon side of SiC wafersGraphene is one of the leading candidates for replacingsilicon as a dominant material in future electronics.While silicon electronics reaches presently fundamentallimits, graphene with its remarkable properties couldprove useful in many future applications.FIG. 13. Longitudinal (red curve) and Hall (blue curve)magnetoresistances measured on sample prepared in ITMEWarsaw, Poland.Several methods are used for preparation of graphenesheets.The simplest one is exfoliation from natural graphiteresulting in very tiny flakes of graphene, which is notreally suitable for large scale applications. With theaim to increase the size of graphene grains the chemicalvapor deposition (CVD) of carbon atoms onto varioussubstrates, e.g. the copper foils, is widely used.The source of the carbon atoms is dissociation of CH 4at high temperatures. The most promising method ofobtaining macroscopic sheets of graphene is to heat siliconcarbide (SiC) to high temperatures to reduce itssurface layer to graphene. This process produces epitaxialgraphene with dimensions dependent upon thesize of the SiC wafer.Samples under study have been prepared on wafersfrom two different sources, University of Linköping,Sweden and ITME Warsaw Poland. Devices were fabricatedin the Institute of Physics, v.v.i., Prague, bystandard photolithographic techniques. Hall bars witha 100 micrometers wide conducting channel, two currentcontacts and six contacts for measurement of thelongitudinal magnetoresistance and the Hall voltageswere formed by oxygen plasma etching. The separationof potential leads was 300 micrometers. Ohmiccontacts were deposited by the electron beam assisteddeposition and lift-off of TiAu. The quality of thesamples has been verified by the Raman spectroscopywhich confirm the existence of few layer of graphene.The low temperature magnetotransport measurementswere conducted at 4.2 K in 20 T resistive magnet (figures13 and 14). Transverse magnetoresistance andHall resistance has been measured. The Hall voltagehas been corrected for misalignment of Hall potentialcontacts. Both resistances exhibit two periods of oscillationsindicating two different parallel conductingchannels with a high concentration of carriers. Asthe oscillations disappear on magnetoresistance curvesmeasured in magnetic fields parallel with the sampleplane, we can conclude that both channels are formedby two-dimensional systems of carriers. The measuredHall effect implies that one of the channels correspondsto Dirac electrons with concentrations N in the interval9 − 10 × 10 11 cm −2 . The position of Hall plateausshows the slightly lower resistance that predicted theoretically,probably due to existence of the second parallelchannel. The mobility of carriers in the first channelat He temperature was in the range 8 × 10 3 − 16 × 10 3cm 2 /Vsec. The second channel has a higher concen-FIG. 14. Longitudinal (red curve) and Hall (blue curve)magnetoresistances measured on sample prepared in Universityof Linköping, Sweden.tration of carriers N ≈ 5.3 − 5.8 × 10 12 cm −2 and thecarrier mobility in the range 1×10 3 −2×10 3 cm 2 /Vsec.The existence of such channel on Si side of SiC was notreported before. We should remark, that this channelwas found on samples from two different sources employingslightly different methods of preparation. Onlythe lithographic procedure to make both groups ofsamples was identical. The origin of the second channelis unclear at present and will be subject to furtherinvestigation.M. Orlita, D. K. Maude,N. A. Goncharuk, V. Jurka, P. Vašek, P. Svoboda, L. Smrčka (Institute of Physics ASCR, v.v.i.,Prague,Czech Rep.), W. Strupinski (ITME Warsaw, Poland), R. Yakimova, Jawad-ul-Hassan (Linköping University,Sweden)10

2011 CARBON ALLOTROPESQuantum Hall effect in inhomogeneous few-layer grapheneWe show the magnetotransport experiments with bilayer(2LG), trilayer (3LG) and four layered graphene(4LG) which we have performed. These measurementsreveal the difference between graphene as we increasethe number of layers. All the samples have been fabricatedby means of mechanical exfoliation of naturalgraphite.For the 3LG sample, we obtain the Hall resistivity usingρ xy = V 1−3 /I, and the longitudinal resistivities areobtained using ρ t xx = V1−2 WI L and ρb xx = V3−4 WI L. In figure15(a-b) we show the Hall and longitudinal resistivitiesas a function of the magnetic field and measuredFor the 2LG sample, we define the longitudinal resistanceas R xx = (V 1−2 /I) (see inset in figure 15(b))and the Hall resistance is defined as: R xy = (V 1−3 /I).The Dirac point of this sample is at 25 V. We havemeasured both resistivities at different gates voltagesobserving the ν = ±4, ±8, ±12, ±16 ± 20 plateaus (figure16), which is different than the series observed inmonolayer graphene.FIG. 16. Top panel: Hall resistivities of the 2LG at differentback gate voltages measured using the magnetic fieldas driving parameter. Plateaus ν = ±4, ±8, ±12, ±16 areclearly observed. Bottom panel: Longitudinal resistivitiesof the 2LG at different back gate voltages in function of themagnetic field.FIG. 15. (a) longitudinal resistivities and (b) Hall resistivities,versus magnetic field at different temperatures.Plateaus ν = ±6 are clearly observed. The inset showsan image of the 3LG flake and the contact labelling usedfor both the 2LG and 3LG samples. (c) Hall resistivitiesof the 4LG at different back gate voltages as a function ofthe magnetic field. At a gate voltage close to the Diracpoint we observe that the Hall resistivity is not a straightline, which arises from the fact that at this point carriersare both electron-like and hole-like. The inset displays theDirac point at 140 Vat different temperatures. The longitudinal resistivitiesare not symmetric with the magnetic field becauseof the inhomogeneities present on the sample, as shownin [Cobaleda et al., Phys. E. 44 (2011) 530-533]Our 4LG sample was inhomogeneous and the Diracpoint was at V g = 140 V. We measured the Hall resistivityas a function of the magnetic field at differentgate voltages (see figure 15(c)). Due to the inhomogeneityof the sample and its low mobility a resolvedplateau is not observed. Nevertheless, these arepromising data which encourage us to process bettersamples and look for resolved plateaus in 4LG.D. K. Maude, B. A. PiotC.Cobaleda, E. Díez (University of Salamanca, Spain), F. Rossella, V. Bellani (University of Pavia, Italy)11

CARBON ALLOTROPES 2011Quantum Hall effect in trilayer grapheneThe integer quantum Hall effect (IQHE) in grapheneexhibits conductance plateaus at σ xy = (n + 1 2 )g.e2 /hwhere g is the Landau level degeneracy due to spin andvalley degrees of freedom. This special Hall conductancequantization arises from the Dirac-like nature ofthe electronic excitations with Berrys phase π. Later,it was shown that IQHE in bilayer graphene shows anotherform of IQHE due to the 8-fold degeneracy of thezeroth Landau level and 2π Berrys phase. This discoverywas very important since it has been demonstratedthat two layers of graphene stacked upon each other donot behave as two independent graphene-like systemsbut gives rise to a novel system with specific electronicproperties. Since then, it has been anticipated that themutual interaction between the graphene layers wouldyield several new forms of IQHE, depending on the totalnumber of layers involved and their relative stackingconfiguration. This particularly holds for trilayergraphene, where the 12-fold degenerate n = 0 Landaulevel was theoretically anticipated long ago. However,trilayer graphene displays a much lower mobility thanits graphene or bi-layer graphene counterparts whichsignificantly delayed its experimental discovery, unlessvery intense magnetic field is used to unravel the IQHEfeatures as shown below.model] and for both Bernal (ABA) and rhombohedral(ABC) stacking order of the three layers. As shown infigure 18, the extracted theoretical Hall resistance isfavorably compared to the experimental data only forone of the two scenarii, allowing an unambiguous determinationof the ABC stacking order between layers[Kumar et al., Phys. Rev. Lett. 107, 126806 (2011)].The plateau sequences for ABC and ABA trilayersarise due to the significant differences in their respectiveLandau level structures (figure 18). The ABA trilayerlacks inversion symmetry and therefore the valleysK and K’ are quite distinct from each other, especiallyin the presence of an electric field across thegraphene layers due to the gate voltage induced chargeredistributions. On the other hand, the ABC-stackedtrilayer is inversion symmetric and its Landau levelband structure is much less affected by electrostaticeffects. For the latter, all the Landau levels evolveroughly by bunches of four (when disorder effects aretaken into account) which leads to quantum Hall stepsof ∆R xy = h/4.e 2 at high enough magnetic field, asexperimentally observed.FIG. 17. IQHE in tri-, bi-, and monolayer graphene forequivalent carrier density n = 3.4 × 10 12 cm −2 and similarmobility µ ∼ 1200 cm 2 .V −1 .s −1 . Inset: Raman spectrumof trilayer graphene (red curve) and monolayer graphene(black curve) measured on the same substrate.Using the standard micro-mechanical exfoliation ofgraphite, a tri-layer graphene sample has been identifiedon Si/SiO 2 substrate by optical microscopy andRaman spectroscopy. As shown in figure 17, theHall resistance evolution is similar to the one ofthe graphene monolayer, except for the absence of aplateau at a filling factor of ν = 2. The Hall resistancehas been computed theoretically using a self-consistentHartree calculations of the Landau levels [based onthe Slonczewski-Weiss-McClure (SWM) tight-bindingFIG. 18. Top: ABA and ABC-stacked trilayer graphene.Middle: Theoretical Landau level structure from valleys K(solid) and K’ (dashed). Bottom: Quantized Hall resistancefor theory (black lines) and experiment (red lines).J.M. Poumirol, A. Kumar, W. Escoffier, B. Raquet, M. Goiran and C. FaugerasD. P. Arovas and M. M. Fogler (University of California, San Diego, USA), F. Guinea (ICMM, Madrid,Spain), S. Roche (CI2N and ICREA, Barcelona, Spain)12

2011 CARBON ALLOTROPESThe remarkably robust quantum Hall effect in bottom-gatedepitaxial grapheneGraphene grown on SiC (epitaxial graphene) is promisingin terms of applications. In order to control thecarrier density, we have tested bottom-gated devices(figure 19(a), in which the gate is created at 200nm by implantation of nitrogen atoms under the carbonface of a 6H-SiC substrate. The implantation ofnitrogen donors was performed with an accelerationenergy of 160 keV and a dose of 5 × 10 13 cm −2 in aEATON 4200 NV implanter. Graphene monolayer ribbonswith typical widths of 5 − 10µm and lengths upto 300 µm have been obtained by graphitization andthey have been contacted using e-beam lithography byCr/Au deposition of invasive contacts (figure 19(b)).We found that the gate works in a limited temperaturerange (30−100 K). At high temperatures, currentleaks through the gate dominate the transport. At lowT , the gate is frozen, but it is still possible to modulatethe graphene carrier concentration by tuning the gatevoltage during the cool-down.Figure 19(c) shows the longitudinal and transverse resistivitiesof the device in a magnetic field B = 2 T,T = 46 K. The Dirac point is visible at V g ≈ 1 V.We checked that the leakage current, measured betweenthe gate and the drain terminals of the device,is much lower that the source-drain current in thisregime. While the electron concentration can be modulatedup to 3× 10 12 cm −2 , the hole concentration remainspinned to ≈ 5× 10 12 cm −2 . We speculate thatthe pinning originates from charge transfer betweengraphene and interface states located below the energyof the Dirac point. Figure 19(d) shows the conductivityas a function of the carrier concentration n s = B/eρ xy(B = 2 T). The conductivity is roughly linear as afunction of n s , indicating that scattering processes aredominated either by long range Coulomb scatterers,or by short-range neutral resonant-like scatterers. Thelast ones are most likely, as they is no obvious sourceof Coulomb scatterer in SiC (by contrast to SiO 2 ). Atlow temperature, a very large plateau develops in thequantum Hall regime when the electron concentrationis tuned at ≈ 7× 10 11 cm −2 (figure 19(e-h)). The widthof the plateau exceeds 20 T. The exceptional width ofthe plateau is not driven by charge transfer betweengraphene and interface states, as it is the case for epitaxialgraphene on the Si face. Here, the electron concentrationin graphene is constant as a function of magneticfield. Indeed, the critical magnetic field for thecurrent breakdown is observed at B c ≈ 14T. Assumingthat at this field corresponds a filling factor 2, we retrievethe electron concentration which is given by theslope of the classical Hall effect at low field. The breakdowncurrent is quite weak ≈ 1µA , as the distancebetween the Hall electrodes does not exceed 2µm.To conclude, these experiments mainly demonstratethat i) implantation allows to tune the carrier densityof graphene; ii) interface states probably play aimportant role for epitaxial graphene on the carbonface; iii) quantum Hall effect is extremely robust onthis material.FIG. 19. (a) Sketch of the graphene device with implantedSiC substrate. (b) top view of one of the devices (c) Resistanceas a function of the gate voltage V g, showing theDirac peak close to V g = 0 V; the Hall resistance is also indicated(blue), revealing the change of sign of the carriers.(d) the conductivity is roughly linear as a function of theHall concentration. (e,f) Current dependence of the longitudinaland transverse resistivities, in magnetic fields up to28 T. The current breakdown of the plateau is maximal atB ≈ 14 T, the value which corresponds to filling factor 2from the slope of the classical Hall effect at low field. (g,h)Temperature dependence of the resistivities.D. K. Maude, B. A. PiotB. Jabakhanji, N. Camara, B. Jouault (L2C, Montpellier,France)13

CARBON ALLOTROPES 2011Free electron kinetic energy terms and the SWM HamiltonianThe band structure of graphite in a magnetic field is accuratelydescribed by the Slonczewski, Weiss and Mc-Clure (SWM) Hamiltonian with its seven tight bindingparameters γ 0 , .., γ 5 , ∆. The SWM Hamiltonian hasinfinite order but can be truncated and numericallydiagonalized to find the eigen values (Nakao, J. Phys.Soc. Japan 40, 761 (1976)). At the H-point the effectof γ 3 vanishes and the SWM Hamiltonian can beanalytically solved to give a Landau level energy spectrumwhich depends only on γ 0 . This is the origin of awidespread misconception in the literature that thereis no electron-hole asymmetry at the H-point.Unfortunately, when writing the expression for Landaulevel energy spectrum at the H-point Nakao neglectedfor simplicity the small free electron kinetic energyterms 2 k 2 /2m, where k is the in plane wave vectorand m is the free electron mass. The full expressionsfor Landau level energies at the H-point are,E n 3± = ∆ ± √ (∆ + 2 s/2m) 2 + 3nsγ 2 0 a2 02+ n2 s2m ,Landau levels (LL0 and LL-1) whose energy remainsclose to zero. The bilayer model correctly predictsthe energy of LL-1 using n = 0. LL0 is missing butcan be reproduced accurately between 0 − 150 T usingn = 0 if the free electron term is replaced by(n + 3/2) 2 s/2m − 16( 2 s/2m) 2 .The importance of the free electron kinetic energyterms is demonstrated by numerically diagonalizingthe SWM matrix for a magnetic field B = 0.3 T. Thecalculated Landau level dispersion along k z is shownin figure 20(a) including the free electron terms. Thesymbols are taken from the paper of Nakao at the samemagnetic field and clearly there is perfect agreementbetween the two calculations. On the other hand, thecalculations in Fig.20(b) which neglect the free electronterms are significantly different. Notably, the electroncyclotron energy is underestimated, while the hole cyclotronenergy is overestimated. Thus, the free electronterms have to be included in the SWM Hamiltonian ifthe correct energy spectrum is to be obtained.E n 1,2 = ∆ ± √ (∆ − 2 s/2m) 2 + 3(n + 1)sγ 2 0 a2 02+ (n + 1)2 s2m ,where n = 0, 1, 2, ... is the orbital quantum number,s = 2eB/ and a 0 = 0.246 nm. The Zeeman termhas been omitted since it simply shifts the energies by±gµ B B/2 and can easily be added if required. At theH-point the electron hole asymmetry is provided bythe free electron term n 2 s/2m. Note that 2 s/2m ≪sγ0a 2 2 0 so that to a very good approximation E3 n+1±=E1,2 n i.e. the Landau ladders remain doubly degenerateat the H-point. Note that the free electron term hasthe expected phase (0) for massless Dirac fermions.The bi-layer expression can be modified phenomenologicallyto include the free electron term for massivefermions with a phase of (1/2) at the K-pointE n 3± = ± 1 √2[(λγ1 ) 2 + (2n + 1)ε 2− √ (λγ 1 ) 4 + 2(2n + 1)ε 2 (λγ 1 ) 2 + ε 4 ] 1/2+ (n + 1 2 )2 s,2m√where n = 0, 1, 2..., λ = 2, ε = v f 2eB is thecharacteristic magnetic energy, v f = √ 3ea 0 γ 0 /2 isthe Fermi velocity. The full SWM model has quantumnumbers −1, 0, 1, 2, ... and there are two specialFIG. 20. (a) Calculated Landau level dispersion (solidlines) along k z using the SWM parameters of Nakao andincluding the free electron terms. For comparison the calculatedvalues of Nakao (symbols) are shown. (b) CalculatedLandau level dispersion along k z neglecting the freeelectron terms.Finally, we note that the Landau level energy spectrumof graphene can be derived from the SWM Hamiltoniansimply by setting all the inter-layer coupling parametersγ 1 , .., γ 5 = 0. This implies that the electron-holeasymmetry observed in the cyclotron resonance of exfoliatedgraphene (Deacon et al., Phys. Rev. B 76,081406 (2007)) also originates from the neglected freeelectron kinetic energy terms.P. Plochocka, D. K. Maude, J. M. Schneider, B. A. Piot, P. Y. Solane, O. Portugall and G. L. J. A.RikkenR. J. Nicholas (Clarendon Laboratory, Oxford University)14

2011 CARBON ALLOTROPESMagneto-optical spectroscopy to probe the electron-hole asymmetryin graphiteMagneto-transmission has been used to probe theelectron-hole asymmetry in graphite extending our previousmeasurements (Ubrig et al. Physical Review B83, 073401 (2011)) to lower energies, lower temperatures,and higher magnetic fields. Suitably thin sampleswere fabricated by exfoliating natural graphite andthe measurements performed in pulsed fields ≤ 60 T.A tungsten halogen lamp provides a broad spectrumin the visible and near infra-red range and the absorptionis measured in the Faraday configuration with thec-axis of the graphite sample parallel to magnetic field.A nitrogen cooled InGaAs photodiode array, or an extendedInGaAs detector analyzed the transmitted lightdispersed by a spectrometer. The use of two detectorsallows us to cover a wide energy range 0.6 − 1.1 eV.parameters; the size of the splitting is simply givenby 2 s/2m ≃ 0.23 meV/T. In addition, the observedsplitting of the “effective” E 3− → E 3+ transitions with“apparent” selection rules ∆n = ±2 (dashed red lines)is twice as large in agreement with the predictions forelectron hole asymmetry.For the K-point transitions, a comparison of the SWMsplitting ≃ 23 meV at B = 60 T with s/2m ≃ 14 meVsuggests that γ 4 and γ 5 are responsible for approximately40% of the splitting. The relative importanceof the contribution of the free electron kinetic energyterms to the electron-hole asymmetry means that anydata analysis which neglects them would lead to a significantover estimation of size of γ 4 or γ 5 .The differential absorption spectra, shown in Fig.21(a),contain a large number of lines reflecting the numerousK and H point transitions which cross in this energyregion. Nevertheless, a clear splitting of the H-pointand the K-point transitions is observed (arrows). Theenergy of the observed transitions are plotted as a functionof magnetic field in Fig.21(b). Before discussingthese results it is useful to consider the possible transitionsat the H-point. Dipole allowed transitions havea change in the orbital quantum number of ±1. Dueto the doubly degenerate Landau level spectrum at theH-point with E3 n+1±= E1,2 n there are a large number ofallowed transitions between the valence band (E 3− orE 2 ) and the conduction band (E 3+ or E 1 ). However,the understanding of the problem is greatly facilitatedby fact that all transitions involving bands E 2 or E 1are degenerate with E 3− → E 3+ transitions with “apparent”selection rules ∆n = 0 and ∆n = ±2. Theelectron hole asymmetry splits both the ∆n = ±1 andthe ∆n = ±2 transitions, while the ∆n = 0 transitionsremain unaffected. The splitting of the ∆n = ±2 transitionsshould be δE = 2 s/m i.e. twice the size of thesplitting of the ∆n = ±1 transitions.The energy of the observed H and K-point transitionsare plotted as a function of magnetic field in Fig.21(b).As seen in the raw data, a splitting of the H-point andthe K-point transitions is observed. The calculatedSWM transitions energies, using the accepted valuesfor the SWM tight binding parameters γ 0 , .., γ 5 , ∆ areindicated by the solid and broken lines. The energy ofthe H-point transitions depends only on γ 0 = 3.15 eVand the calculated splitting is independent of all otherSWM parameters and vanishes only if the free electronterms are not included in the Hamiltonian. The→ E n+1(n)3+observed splitting of the H-point E n(n+1)3−transitions (blue solid lines) is beautifully reproducedby the calculations. We stress that there are no fittingFIG. 21. (a) Differential magneto-transmission spectra ofnatural graphite measured at magnetic fields in the range55 − 59 T at T ≃ 1.8 K. (b) Magnetic field dependence ofthe observed optical transitions in natural graphite. Thecalculated SWM energies of the transitions are shown aslines: H-point ∆n = ±1 (thin blue lines), “effective” H-point ∆n = ±2 (dashed red lines), ∆n = 0 (dotted greenlines) and K point ∆n = ±1 (thick black lines).P. Plochocka, P. Y. Solane, J. M. Schneider, B. A. Piot, D. K. Maude, O. Portugall and G. L. J. A.RikkenR. J. Nicholas (Clarendon Laboratory, Oxford University)15

CARBON ALLOTROPES 2011Magneto-optical spectroscopy in fields up to 160 T: exploring theband structure of graphiteAfter substantial improvements of the experimentalinfrastructure for magneto-optical spectroscopy theMegagauss generator has finally gone into regular operation.The installation is routinely used at 40 % ofits nominal energy which permits generation of 160 Tin 8 mm diameter (see figure 22).(b) −→←− (a)←− (d)←− (c)FIG. 22. Single turn coil before and after a shot. On the leftthe coil (a) with the current-feed flanges (b) and a plastictube (c) that contains the sample holder and cryostat areshown. The inside of the coil and feed flanges is insulatedwith a strip of polyimid film (d). On the right the ensembleis shown after the shot. The feed-flanges are bent open andthe insulation strip has popped up.Experiments were primarily performed on epitaxialgraphene and exfoliated layers of natural graphite.Figure 23 shows an example of original data obtainedwith graphite during a single shot at comparably lowmagnetic field. As the field oscillates repeatedly therecording demonstrates both the remarkable reproducibilityof the measurement and the characteristicdifference between left- and right-hand circular polarizations.Figure 24 shows the magnetic field dependence of thegraphite data displayed in Fig. 23 together with measurementsup to 140 T. The quantum-oscillatory likebehaviour stems from transitions between the E 3+ andE 3− bands at the K-point of the Brillouin-zone. Aspositive magnetic fields correspond to σ − -polarizationthe respective resonances refer to the ∆ n = −1 transitionsof holes. Resonances at negative magnetic fieldsare subject to σ + -polarization and hence correspondto the ∆ n = +1 transitions of electrons.The data shown in Fig. 24 exhibits characteristic deviationsbetween the resonance positions for σ + and σ − -polarizations. Up to 50 T these deviations are in goodagreement with the e-h asymmetry caused by the freeelectronkinetic-energy terms in the standard Hamiltonianof graphite. The situation is more complex forthe resonances observed around 100 T as the respective0 → 1 and 1 → 0 transitions should not show anye-h asymmetry. The small observed splitting of 0 → 1and 1 → 0 transitions at around 100 T in figure 24 isprobably due to the exchange enhanced spin gap of the0 Landau level, which can give rise to e-h asymmetrywhen filling arguments are taken into account.FIG. 23. Single shot acquisition of both circular polarizationsin the infrared transmission of graphite. The opticaltrace exhibits perfect correspondence between partsrecorded during the up- and down-sweeps of each field oscillationas well as during the first and third half-waves.By comparison the transmission changes characteristicallyon the second, positive half-wave as the sense of the circularpolarization is effectively reversed. Note that dataobtained during the third half-wave misses one resonancedue to the diminished field yet resolves all other resonancesmuch better. The measurement was performed at 300 Kwith a CO-laser producing radiation at 229 meV.FIG. 24. Polarization resolved infrared transmission up to140 T confirming the e-h asymmetry in graphite. Differentcoil sizes (10, 12 and 15 mm) and charging voltages (30 and40 kV) were used in order to optimize both peak fields andthe resolution of data at lower fields. Deviations betweenthe resonance positions for σ + and σ − -polarization shownin the upper half of the figure were thus confirmed to bereproducible. In the lower half their quantitative agreementwith calculated transition energies is shown.P. Y. Solane, P. Plochocka, D. K. Maude, O. Portugall and G. L. J. A. RikkenR. J. Nicholas (Clarendon Laboratory, Oxford University)16

2011 CARBON ALLOTROPESA de Haas-van Alphen investigation of the Fermi surface of graphiteThe Fermi surface of graphite has electron and holemajority carrier pockets with maximal extremal crosssections at k z = 0 (electrons) k z ≈ 0.35 (holes). Wehave investigated these using dHvA measurements. Amm-size piece of natural graphite was mounted on aCuBe cantilever which forms the mobile plate of a capacitivetorque meter. In figure 25 we plot the amplitudeof the Fourier transform of the oscillatory torqueτ osc (1/B) as a function of the period of the oscillations)and the tilt angle θ. Well pronounced quantumoscillations are observed even at θ = 90 ◦ demonstratingunequivocally that the Fermi surface of graphiteis 3D and closed. For angles θ < 60 ◦ , the dHvA periodfor both electrons and holes follow a cos(θ) dependence.The non cylindrical nature of the Fermi surfaceis clearly revealed for θ ≥ 60 ◦ where deviations fromthe cos(θ) behavior, are observed.We approximate the Fermi surface of graphite usinghighly elongated ellipsoids. The fundamental frequenciesB F = A/2πe are directly proportional to the extremalcross sectional areas A of the Fermi surface. Forthe maximal extremal orbits the area is given by theintersection of a plane with the ellipse,√B F ∝ A = πab/ sin 2 θ + (a 2 /b 2 ) cos 2 θ, (1)where a and b are the semi-major and semi-minor axesof the ellipse and θ is the angle between the magneticfield and the c-axis of the graphite crystal. Forelongated ellipsoids (a/b ≥ 5) this follows very closelyπb 2 / cos θ for θ < 60 ◦ i.e. follows closely the behaviorof a 2D cylindrical Fermi surface.For our data, matters are further complicated by theobserved spin splitting for θ > 70 ◦ . The oscillatoryterm can be written as,() ( )|E f |B ↑↓τ osc ∝ cos ( e)F+ φ ≡ cosm∓ g ∗ µ ∗ B B B + φ ,(2)where E f is the Fermi energy, m ∗ is the angle dependenteffective mass, g ∗ is the Landé g-factor, µ B theBohr magnetron and φ is a phase factor. From Equation2 the frequency in the absence of spin splitting isB F = |E f |m ∗ /e. Thus, we have a simple relation betweenthe frequency (period) of oscillations calculatedfor the ellipsoid and the expected frequencies when spinsplitting is included,1/B ↑↓F = 1/B F ∓ g ∗ µ B /2|E f |so that the expected splitting of the period is simply±g ∗ µ B /2|E f | independent of the angle θ. The crosssectional area of the ellipsoids are obtained by fittingto the majority electron and hole frequencies at θ = 0 ◦and at high tilt angles θ > 70 ◦ . The parameters usedare summarized in table I. The results of such a fit areplotted in figure 25 as thick solid and dot-dash linesfor the majority hole and electron pockets. The simplemodel fits the experimental data remarkably well,reproducing the observed angular dependence and theelectron and hole spin splitting.FIG. 25. (a-b) Color plot : Amplitude of the Fouriertransform of τ osc(1/B) as a function of the period of theoscillations and the tilt angle (θ) at T ≃ 400 mK. Thecalculated dHvA periods for ellipsoidal electron and holeFermi surfaces are also shown for hole (solid lines), holeharmonic (thin solid lines), and electrons (dot-dash).B f⊥ (T) A ⊥ A ‖ A ‖ /A ⊥Maj. hole 4.7 4.49 40.3 9.0Maj. elec. 6.45 6.15 43.1 7.0TABLE I. Frequencies and areas of the extremal orbits (inunits of 10 12 cm −2 ) for the Fermi surface of graphite.J. M. Schneider, B. A. Piot, I. Sheikin, and D. K. Maude17

CARBON ALLOTROPES 2011Cyclotron motion in the vicinity of a Lifshitz transition in graphiteA Lifshitz transition (also known as an electronictopological transition) is a change in the Fermi surfacetopology occurring upon a continuous change ofsome external parameter, such as pressure, magneticfield or, most naturally, doping. This transition doesnot involve a symmetry breaking, unlike conventionalphase transitions of the Landau type, but still leadsto observable singularities in thermodynamics, electrontransport, sound propagation, and magnetic responseof metals. Saddle points in the electronic dispersion,often apparent in complex metals, have only recentlybeen visualized with the spectroscopy method of angleresolvedphotoemission. Here we show how the proximityto a Lifshitz transition manifests itself in cyclotronresonance absorption experiments on graphite (in thevicinity of the K point), a model system with saddlepoints due to the trigonal warping of electronic bands.Cyclotron resonance absorption was measured usingthe setup routinely used for high-frequency electronparamagnetic resonance experiments. A flake of naturalgraphite (50 µm thick, area 1 mm 2 ) was placed ina Fabry-Perot cavity mounted inside a superconductingcoil. The magnetic field was applied perpendicularto the graphene layers. Linearly polarized microwaveradiation from a Gunn diode tripled to frequency of283.2 GHz (1.171 meV) was delivered to the sample viaquasi-optics waveguides. The field-modulation techniquewas applied to enhance the detection sensitivity.A fully classical approach is employed to describe ourexperimental data, which are shown to reflect the proximityof the Lifshitz transitions in graphite. In thissystem, the Fermi level at the K point lies only 6 meVabove the upper of two separatrix lines, see figure 26(ab).This proximity manifests itself in a high number ofcyclotron resonance harmonics and also in a decrease ofthe cyclotron resonance frequency in comparison to expectationsof the parabolic-band limit. Additional evidencefor the vicinity of the Lifshitz transition is providedby a thorough analysis of the lineshapes whichinvolve integration over states away from the K point(k z = 0).In summary, we have introduced CR experiments as anew tool to study Lifshitz transitions. We have shownhow the proximity to the Lifshitz transition manifestsitself in the CR spectrum of a model system, bulkgraphite. The similarity between the band structureof graphite near the K point and that of a bilayergraphene logically suggests to probe Lifshitz transitionin the latter system by CR methods, and to shed lighton the currently debated issue of spontaneous symmetrybreaking in a bilayer graphene [Mayorov et al.,Science 333, 860 (2011)].For more information see [Orlita et al., Phys.Lett. 108, 017602 (2012)].Rev.Our key observations, see figure 26(c), are; i) the appearanceof a large number (up to 20) of cyclotron resonanceharmonics, ii) an enhanced strength of 3k + 1harmonics as compared to the strength of the 3k − 1series at B > 0 (and vice versa at B < 0) and finally,iii) a very characteristic, asymmetric broadening of theobserved resonances, enhanced on the low frequency(high-field) sides of the absorption peaks.FIG. 26. Parts (a) and (b): Electronic band structure atthe K point of bulk graphite. The Fermi level is located6 meV above the upper separatrix, which separates regionswith different topology of the Fermi level. Part (c): Lowfieldmagneto-absorption spectrum in bulk graphite plottedas a function of ω/ω c, where ω is the applied microwavefrequency and ω c = eB/m with m = 0.056m 0.M. Orlita, P. Neugebauer, C. Faugeras, A.-L. Barra, M. PotemskiF. M. D. Pellegrino (Universita di Catania, Italy) , D. M. Basko (LPMMC UJF/CNRS, Grenoble, France)18

2011 CARBON ALLOTROPESRaman scattering from electronic excitations in bulk graphiteElementary excitations of graphene-based systemshave been mostly studied by transmission/refletivitytechniques, in the infrared range of energy. These techniqueshowever suffer from the lack of polarization possibilitiesin the long wavelength range of energy, andalso from the difficulty of focusing infrared radiationsat the micrometer scale. Because Raman scatteringexperiments are performed in the visible range of energy,they overcome these two main limitations andoffer a complementary method to study low energy excitations.scattering response of bulk graphite using a designedand built in-house micro-Raman scattering set-up atlow temperature and in high magnetic fields. The sampleis mounted on piezzo stages that allow to move thesample under the laser spot with a sub-micrometer resolutionand to spatially map the surface of the sample.Band pass filters are used to reject the scattered laserand provide a cut-off energy of ∼ 350 cm −1 . To studythe electronic properties of bulk graphite, we selecta spatial location outside of the decoupled grapheneflakes that appear naturally on the surface of this material.Controlling the circular polarization of the excitationlaser and of the scattered light allows to select the symmetryof the electronic excitations by controlling theangular momentum transferred to the system. Thisis shown in figure 27 which show, for different valuesof the magnetic field, electronic excitations observedin the co- and crossed circular polarization configurations.These two configurations select respectivelyelectronic excitations with ∆|n| = 0 and ∆|n| = ±2 or∆|n| = ±1 (allowed thanks to the trigonal warping),where n is the Landau level index.FIG. 27. a) Co-circularly polarized Raman scatteringspectra of bulk graphite (Raw spectra) b) Crossed circularlypolarized Raman scattering spectra. Measurementperformed at T = 4.2 K.We have recently demonstrated the observation ofelectronic excitations in the Raman scattering responseof graphene flakes on the surface of bulkgraphite [Faugeras et al., Phys. Rev. Lett. 107,036807 (2011)] through low temperature and polarizationresolved measurements. Similar experiments performedon the surface of bulk graphite reveal a veryrich electronic excitation spectrum, which provides aunique insight into the 3D band structure of this materialand a hope of determining more precise valuesfor the seven SWM tight binding parameters.We have measured the circularly polarized RamanThe Raman scattering spectrum of bulk graphite showsa very rich evolution with increasing magnetic fields.figure 27(a), shows that low energy electronic excitationscontribute the B = 0 Raman scattering responseof bulk graphite. This response has a metallic character,is flat, but can be identified by applying a magneticfield. It then transforms into discrete features due toLandau quantization. The shape of these features canbe modelled by calculating the joint density of statesof ∆|n| = 0 electronic excitations, which are relevantin this polarization configuration. The results of sucha calculation are presented in figure 27 as solid anddashed lines.In the crossed circular polarization configuration,mostly ∆|n| = ±2 excitations are observed. This situationis different that the one observed in graphenebecause of the different origin of the trigonal warping inthese two distinct material systems. By selecting either∆|n| = +2 or ∆|n| = −2 excitations, it is then possibleto reveal and to quantify the strong electron-holeasymmetry of K-point massive electrons in this material.Interesting is also the observation of an excitationinvolving states at the Fermi level, with a significantlydifferent line shape. These results can be describedwithin the Slonczewski-Weiss-McClure model for theband structure of graphite, which involves seven tightbinding parameters that can be deduced from thesepolarization resolved experiments.P. Kossacki, C. Faugeras, M. Kühne, M. Orlita, A. A. L. Nicolet, M. PotemskiD. M. Basko (LPMMC-UJF-CNRS, Grenoble, France)Y. I. Latyshev (Kotelnikov Institute of Radio-Engineering and Electronics RAS, Moscow, Russia)19

CARBON ALLOTROPES 2011Magneto-phonon effect in bulk graphiteThe electron-phonon interaction in sp 2 carbon allotropesis of particular interest because it can be tunedby modifying the low energy electronic excitation spectrumof these materials. In the case of graphene, thishas been achieved by tuning the position of the Fermienergy with a back gate [Yan et al., Phys. Rev. Lett.98, 166802 (2007)] or by applying a strong magneticfield perpendicular to the graphene crystal [Faugeraset al., Phys. Rev. Lett. 103, 186803 (2009)]. Thecase of bulk graphite, graphene sheets in a Bernal typeof stacking is of particular interest because of the 3Dnature of this material and of the associated dispersionalong k z , the wave vector measure in units ofthe inverse inter-layer spacing. Under an applied magneticfield, electronic states in this material belong toLandau bands with a dispersion along k z and electronicexcitations are characterized by a large width(FWHM∼ 50 cm −1 ) due to the contribution of statesat different values of k z . As a result, the electronphononinteraction is also spread on such a wide rangeof energy.We have measured the low temperature polarizationresolved Raman scattering response of optical phonons(E 2g phonons) of bulk graphite in magnetic field.Fig. 28(a) shows typical spectra of the phonon featurewhich is strongly modified by the magnetic field, showingpronounced oscillations of its energy position andof its Full Width at Half Maximum (FWHM). Theevolution of these two these parameters is shown inFig. 28(b) and c) as a function of the magnetic fieldtogether with the results of the theoretical modellingof this effect, taking the full 3D band structure intoaccount. It appears that the observed oscillations aremainly due to K-point massive electrons and the differentbehavior observed in the two different crossed circularpolarization configuration reveals the contributionfrom massless holes at the H point of the graphiteBrillouin zone where the zeroth Landau level is fullyunoccupied and only excitations from lower Landaulevels to this level exist. The contribution of these Hpoint carriers can also be seen at B = 5 T where astep like behavior is observed in both the phonon energyand FWHM as function of the magnetic field.The electronic dispersion along k z appears in two differentforms. First, the oscillations of the FWHM arestrongly asymetric, with pronounced tail at values ofthe magnetic field lower than the resonant position.This reveals the coupling of a quasi continuum of electronicexcitations from k z = 0 (K point) to highervalues of k z . Then, the amplitude of these oscillationsis strongly reduced with respect to the ones observedin graphene and this is a fingerprint of the Landauband structure of electronic states in bulk graphite, incontrast to discrete Landau levels in graphene where afully resonant coupling can be achieved.These experiments show that (i) if the Fermi energy ofbulk graphite cannot be varied by gate effects, a strongmagnetic field can be used as a tool to investigate theFIG. 28. a) Low temperature Raman scattering spectrameasured at different values of the magnetic field in theσ − /σ + configuration. b) and c) Evolution of the Ramanshift and of the FWHM of the G band feature respectivelyas a function of the magnetic field. Solid lines are theoreticalcalculations of this effectelectron-phonon coupling in this material and (ii) themagneto-phonon effect also exists in bulk graphite, itinvolves mainly massive electrons and it can be usedto investigate details of the graphite band structuresuch as the electron-hole asymmetry which shifts theresonant magnetic field for the electron-phonon interactionin the two distinct crossed-circular polarizationconfigurations.P. Kossacki, C. Faugeras, M. Kühne, M. Orlita, A. A. L. Nicolet, M. PotemskiD. M. Basko (LPMMC-UJF-CNRS, Grenoble, France)Y. I. Latyshev (Kotelnikov Institute of Radio-Engineering and Electronics RAS, Moscow, Russia)20

2011 CARBON ALLOTROPESInvestigation of the Aharonov-Bohm effect in single antidot graphitestructuresThe first experiments on thin natural graphite samplesirradiated with heavy ions showed field periodicoscillations of the magnetoresistance with a period of7.5 T roughly corresponding to the flux quantum hc/eper an area of columnar defect (Latyshev et al., JETPLett. 90, 480 (2009)). That pointed to the dominantcontribution of Dirac fermions to the Aharonov-Bohm(A-B) effect as that has been demonstrated on mesoscopicgraphene rings (Russo et al., Phys. Rev. B 77,085413 (2008)).However, since the structure of columnar defects isstill not well defined it was important to reproducethis effect on real nano-hole with well defined geometricalsize. The structure of this type containingsingle nano-hole with a diameter of 35 nm hasbeen recently fabricated with the use of focused ionbeam (FIB) technique. The magnetoresistance of thisstructure has been measured at fields up to 22 T. Atfields below 8 T the structure demonstrates typical forgraphite Shubnikov-de Haas oscillations of magnetoresistancewith inverse field periodicity of 0.2 T −1 . NormallyShubnikov-de Haas oscillations are terminatedin graphite at fields above 8.5 T and field dependenceof the magnetoresistance becomes smooth.The main series of the peaks has a periodicity of 3.3 Tcorresponding to the flux quantum to the hole area.That implies that the orbits of carriers contributing tothe A-B oscillations are located very near to the edgeof the hole. That is possible if one can accept the existenceof the edge states around the hole. These statescan localize the carriers near the edge and they cancontribute coherently to the A-B interference effect. Atheory of the edge states of Tamm-type in graphenehas been recently developed (Volkov et al., J. of Phys.:Conf. Series 193, 012113 (2009)). It was shown thatFIG. 30. Temperature dependence of the amplitude of themagnetoresistance oscillations at H = 17.8 T for a graphitenano-structure with a single hole.the carriers localized at the edge states have a linearspectrum E(k) but with a slope dE/dk much less thanFermi velocity.FIG. 29. Oscillatory part of magnetoresistance of graphitenano-structure with a single hole at T = 1.5 − 45 K.In contrast, in the structure with a single nano-holewe clearly observe field periodic oscillations in the fieldrange of 8 − 22 T. Figure 29 shows oscillating part ofmagnetoresistance in this range at various temperatures.The oscillations are observed up to about 50 K.To prove this prediction we studied the temperaturedependence of the amplitude of oscillations A(T ). Figure30 shows that A(T ) follows an exponential dependenceA(T ) ∝ exp(−T/E 0 ) with E 0 ≃ 20 K. The valueof E 0 corresponds to the energy of the edge states fora given angular velocity ω 0 , E0 = ω 0 . Taking into accountthat linear velocity v = ω 0 r, where r is the radiusof the hole one obtains v = 10 7 cm/c, ten times lessthan v F . In a summary, our observations of the A-B oscillationson a single hole graphite nano-structure andthe analysis of their temperature dependence stronglysupport the existence of the edge states around thenano-hole in thin graphite or graphene samples.B. A. PiotYu.I. Latyshev, A.P. Orlov (Kotelnikov Institute of Radio-Engineering and Electronics RAS, Moscow, Russia),P. Monceau (Institute Néel, CNRS, Grenoble, France)21

CARBON ALLOTROPES 2011The 1.4 eV Ni colour centre in synthetic diamondThe nickel colour centre in diamond has attracted greatinterest since it behaves as a bright and almost perfectsingle photon source which works at room temperature.However, its exact crystallographic site and electronicproperties are still under debate. In order to try toelucidate the microscopic structure of Ni in diamond,we have performed a magneto-optical study. The Nicolour centre in synthetic diamond is characterized bytwo zero-phonon lines at 1.401 and 1.404 eV. Photoluminescencehas been measured at T = 4.2 K in pulsedmagnetic fields up to 56 T using a Ti:Sapphire laserat 720 nm for the excitation. The magnetic field wasapplied for different orientations of the diamond crystal.A typical compilation of the emission spectra ispresented in figure 31 where the two zero phonon linessplit into 2 × 2 lines due to the Zeeman effect with themagnetic field applied in the < 111 > direction.Nickel atom has spin 1/2 and trigonal symmetry andit is incorporated as interstitial Ni + .The data taken for orientation < 111 >‖ B are ina very good agreement with the theoretical prediction,showing the correct magnetic field dependenceand level crossing. However, for the second orientation(< 011 >‖ B the data are not well described by themicroscopic model. Indeed, for the < 001 >‖ B theemission lines seems to merge rather than to cross, indisagreement with the model. Further detailed calculationsare underway to elucidate this point.FIG. 31. The photoluminescence spectra at T = 4.2 K ofNi color center in synthetic diamond as a function of B ‖. The two zero phonon lines split in the magneticfield due to the Zeeman effect.In order to facilitate the comparison of the observedZeeman splitting with existing theory the position ofthe emission lines as a function of the magnetic fieldare presented in figure 32 with data taken for two differentorientations of the crystal: < 111 >‖ B and< 011 >‖ B. The results of the calculations based on[Nazare et al., Phys. Rev. B 43, 14196 (1999)] areindicated by the solid lines. The theoretical model assumesthat, after incorporation to the diamond lattice,FIG. 32. The energy of the emission lines as a functionof B ‖< 111 > and B ‖< 011 >. The solid lines arethe calculated behaviour for interstitial, s = 1/2, Ni withtrigonal symmetry.In summary, the zero phonon lines show a Zeemansplitting, as predicted by theory, but also, and for thefirst time, a clear interaction between the Zeeman splitlevels, potentially giving new insight as to the electronicstructure of this Ni center. A detailed analysisis underway to extract precise values for the g-factorand to understand the polarization selection-rules.P. Y. Solane, P. Plochocka, O. Portugall, G. RikkenE. Gheeraert, E. Bustarret (Institut Néel, CNRS and Université Joseph Fourier, Grenoble, France), H.Kanda (National Institute for Materials Science, Tsukuba, Japan)22

Two-Dimensional Electron Gas

2011 TWO-DIMENSIONAL ELECTRON GASDirect resistive NMR measurement of the electronic spinpolarization of fractional quantum Hall statesWhen a strong magnetic field is applied perpendicularlyto a sheet confining electrons to two dimensions,quantum Hall states emerge at rational fillings of theLandau levels as a result of electron-electron interactions.Among these so-called fractional quantum Hallstates, the quantization of the Hall resistance at thefilling factor ν=5/2 has recently raised considerable interests.Indeed, the wave function which is most-likelyexpected to describe this unique “even-denominator”state, the so-called Moore-Read wave function, exhibitspeculiar non-abelian anyonic quantum statistics.Recent experiments, such as the observation of fractionallycharged e/4 quasi-particles at ν=5/2, havebrought further support in favor of the Moore-Readdescription. However, the spin polarization of electronsin this state, which should be complete, is stillactively debated.The fragility of the ν=5/2 state (which activation gapis only ∼ 300 mK in the highest quality samples), aswell as the small number of spin in a 2D electron gas(2DEG), impose a highly sensitive measurement performedat very low temperature. Resistively-detectedNuclear Magnetic Resonance (RDNMR), which probesthe coupling between electron and nuclear spins via thehyperfine interaction, is a good experimental tool tosatisfy theses constraints. This technique relies on thedependence of the longitudinal resistance R xx of the2DEG on the Zeeman energy, which can be written asE Z = ± 1 2 gµ B (B + B N ) if one includes the hyperfinecoupling in a mean-field approach where B N is the localmagnetic field created by the nuclei. By applyinga radio frequency (RF) pulse at a nuclei resonant frequency,it is possible to depolarize the nuclear spinswhich consequently modifies E Z and, in turn, R xx .An important obstacle is nevertheless that, at ν=5/2exactly, there is no significant dependance of the longitudinalresistance on the Zeeman energy. A recentexperimental work [Tiemann et al., APS March meeting.(2011)] has overcome this problem by performing“pulsed-RDNMR”, in which the system is excitedat the filling factor of interest (here ν=5/2) and thenrapidly switched to another filling factor (the detectionpoint) where a change in the Zeeman energy significantlyaffects the resistance. The resistance changeinduced at the detection point by the nuclear depolarizationis still visible, because it generally takes muchmore time for the nuclei to relax than it does to moveto the detection point by applying a gate voltage tochange the electron density. In a similar approach, wehave recently achieved a detection of a RDNMR signalat ν=5/2. In figure 33, we show a pulsed-RDNMRsignal at ν=5/2, in which the detection point is chosento be close to ν=2/3.The resistance dip, corresponding to the NMR of the75 As nuclei, forms at ∼ 7.6 kHz lower than the resonanceminimum obtained when exciting the systemat filling factor ν=2, where the electronic polarizationis known to be null. The so-called Knight shift (K S )between both response results from the polarized electronscreating an additional local magnetic field seenby the nuclei, and is thus directly related to the electronspin polarization. In our experiment, its valuewould be consistent with the expected full spin polarizationof the ν=5/2 state. However, attention shouldbe paid to the fact that the actual electron temperatureduring the RF pulse can be much higher thanthe bath temperature. In figure 33, the bath temperaturestabilizes to 39 mK, while during the +17dBmRF pulse the electronic temperature is estimated to beclose to 1K, a temperature at which the ν=5/2 state isdestroyed. As a result, only the single particle physicsat ν=5/2 is likely to be probed. We are currently tryingto optimize the pulse sequence and power to reducethe electronic temperature during the RF pulse.FIG. 33. Pulsed-RDNMR of the 75 As nuclei at filling factorν=5/2 (blue circles) and filling factor ν=2 (green line). Thesystem is excited at the filling factor of interest with a short(100 ms) RF pulse, and then switched to the detectionpoint at 2/3 for 100 s before moving to the next frequency.∆R/R xx is the relative change of resistance at the detectionpoint before and after the pulse for a given frequency.B. A. Piot, P. Plochocka, D. K. MaudeM. Stern, Y. Vardi, V. Umansky, I. Bar-Joseph (Weizmann Institute of Science, Rehovot, Israel)25

TWO-DIMENSIONAL ELECTRON GAS 2011Quantum-classical crossover and apparent metal-insulator transitionin a weakly interacting 2D Fermi liquidIn a previous study [Zhou et al., Phys. Rev. Lett.104, 216801 (2010)], a parallel field induced colossalmagnetoresistance (CMR) was observed in an ultraclean2D electron gas sample and at low temperaturelimit. The CMR has been interpreted as the result ofmagneto-orbital coupling between the magnetic fieldand the ẑ-degree of freedom of quasiparticle motion. Inthis 40 nm quantum well sample, at approximately 9 Twhen the magnet field length is equal to the quasiparticleconfinement width, the magneto-orbital couplingeffect becomes so significant that it leads to mixingof 2D subbands. This gives rise to a quasi-3D phaseabove 9 T, which has no conventional analogy. To furtherinvestigate the nature of this novel phase, in thiswork we investigate its temperature dependence up toLN 2 temperature, with the measurement current beingapplied parallel to the magnetic field.The experiment was performed in the pulsed field facilityin Toulouse. The sample chosen in this studyis a 2D GaAs/AlGaAs 40 nm quantum well system,with a high charge carrier density and an ultra highmobility. Using an in situ rotator, its 2D plane wascarefully aligned to be parallel to the applied magneticfield. A DC measurement current of 100 nA was suppliedalong the edge of the sample, which was alignedparallel to the applied field as well. Within field pulsesup to 60 T, the longitudinal resistance R xx as a functionof field was measured at fixed temperatures usingstandard four points measurement technique. Possiblesystematic errors such as mixing of Hall resistanceand induced voltage are extracted and removed by reversingthe measurement currents and magnet polarity.From the field sweep data we were able to constructthe temperature dependence of magnetoresistance atseveral fields.The main result of this work is an apparent parallelfield induced metal-insulator transition phenomenologycharacterized by a change of sign in ∂ρ xx /∂T . Aspresented in figure 34, the magnetoresistance below the2D to quasi-3D transition (around 10.5 T) shows typical“metallic” behaviour, i.e., a positive temperaturedependence. Furthermore, the linear temperature dependencebelow 40 K and the upturn above 60 K areconsistent with established theoretical descriptions ofelectron-phonon scattering in a 2D Fermi liquid system,and remain qualitatively unchanged in the presenceof magnetic fields below ∼10.5 T. However, an∂ρ xx∂T“insulator-like” behaviour (i.e. < 0) is observedbelow ∼10 K when the field is increased above ∼10.5 T,coinciding with the 2D to quasi-3D crossover transition.The negative temperature dependence slope below∼10 K becomes stronger with an increasing field,while the positive temperature dependence at highertemperature (above ∼25 K) remain qualitatively similarto that in the zero field.Even though the negative temperature dependence resemblesthat of a conventional insulator, in our studythe metallicity parameter k F λ remains large (λ beingthe mean free path). This suggests that the negativetemperature dependence might not correspond to atrue insulating phase. Furthermore, the “transition”occurs in a narrow field window instead of a singlecritical field, suggesting that the sign change does notcorrespond to a true phase transition. Instead, thephenomenology is fully consistent with the quantumclassicalcrossover arguments. That is, with the Fermitemperature being suppressed by the magnetic field,our sample evolves from a 2D quantum (i.e. T F (B) ≫T ) metal to a quasi-3D classical (i.e. T F (B) ≪ T )Fermi gas, the resistance of which has a characteristicnegative temperature dependence ρ xx ∝ T F /T . Thisquantum-classical crossover effect signals an emergingfield for further theoretical understanding and experimentalexploration.FIG. 34. Temperature dependence of the magnetoresistanceat several fields. Below the 2D to quasi-3D crossovertransition (around 10.5 T), the resistance increases monontonicallywith increasing temperature. By contrast, abovethe transition, there is an emerging negative temperatureslope of resistance below ∼10 K with an increase field, resemblingthat of an insulator. Nevertheless, even with astrong negative temperature slope, the resistance is stillmuch smaller than the resistance quantum ≃ h/e 2 (25 kΩ),suggesting that electrons are still mobile.C. ProustXiaoqing Zhou, B. Schmidt, G. Gervais (McGill University, Montreal, Canada), L. N. Pfeiffer, K. W.West (Princeton University, Princeton, USA), S. Das Sarma (University of Maryland, College Park, USA)26

2011 SEMICONDUCTORS AND NANOSTRUCTURESNon-linear transport phenomena in electronic bilayersNonlinear transport phenomena in very high mobilitytwo-dimensional electron systems have become a generaltopic in physics, in particular after the observationof “zero resistance state” (ZRS). In the regime of ZRS,samples exhibit vanishing resistance under microwaveirradiation and in the presence of a weak magneticfield. In general, non-linear transport phenomena canbe divided into two classes; microwave induced magnetoresistanceoscillations (MIROs) and Hall inducedmagnetorsistance oscillations (HIROs). It is worthnoting that MIROs evolve into ZRS for high electronmobility and elevated microwave power, while HIROsevolve into zero differential resistance states (ZdRS) athigh DC field. The two seemingly different phenomenaare explained within the same model assuming formationof current domains. We demonstrate that the phenomenonof ZdRS and HIROs is not limited to singlesubband systems. In contrast to recent experimentalobservations in highquality systems with one occupiedsubband, where ZdRS appear at nearly discrete valuesof B [Bykov et al., Phys. Rev. Lett. 99, 116801(2007)] or in a wide region of magnetic fields [Hatke etal, Phys. Rev. B 82, 041304(R) (2010)], we show thatZdRS in bilayer electron systems are periodic in 1/Band are separated by maxima in differential resistancer xx .A theoretical calculation of the nonlinear magnetoresistancegives a good agreement with experiment concerningthe shape and periodicity of the oscillation picture.The comparison of theory and experiment indicatesa significant role of electron scattering by theshort-range random potential in our samples. Furthermore,we show that HIROs in the region of low magneticfields coexist with zero differential resistance athigher fields.To summarize, we show that ZdRS are developed fromOur samples are high mobility wide quantum wellswith two occupied subbands which also exhibit ZRSunder microwave excitation (see sample details and observationof ZRS in [Wiedmann et al, Phys. Rev. Lett.105, 026804, (2010)].In figure 35(a) we plot differential resistance r xx asa function of B first without DC excitation andthen for I DC = 20µA at 50 mK where minima ofmagneto-intersubband (MIS) oscillations evolve intoZdRS. Since ZdRS exhibit 1/B periodicity, we also illustratethis phenomenon by sweeping I DC at a constantmagnetic field, see figure 35(b), for several chosenmagnetic fields (1)-(4). For a characteristic thresholdcurrent I th , ZdRS are fully developed. If we integratethe data at B = 0.254 T, we find a typicalvoltage-current characteristic with saturation of V xxcorresponding to the onset of ZdRS. An increase inDC excitation leads to the formation of HIROs at lowmagnetic fields. In electronic bilayers, HIROs evolvefrom MIS oscillations which are enhanced damped orinverted.FIG. 35. (a) Magnetoresistance and differential resistancer xx for I DC = 20µA as a function of B at 50 mK. PeriodicZdRS are observed; see (1) and (4). (b) r xx as a function ofI DC for several chosen magnetic fields; for a characteristicI th , MIS minima evolve into ZdRS. (c) I − V plot for B =0.254 T.intersubband oscillations and occur as a result of currentinstability under negative differential resistance[Gusev et al., Phys. Rev. B 83, 041306, (2011)]. Withthe present observation, we give evidence that the domainmodel [Andreev et al., Phys. Rev. Lett. 91,056803 (2003).] very likely applies to multisubbandsystems.S. Wiedmann, J. C. PortalG. M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), O. E. Raichev (Instituteof Semiconductor Physics, NAS of Ukraine, Kiev, Ukraine), A. K. Bakarov (Institute of SemiconductorPhysics, Novosibirsk, Russia)27

TWO-DIMENSIONAL ELECTRON GAS 2011Polaronic effects in a single modulation doped GaAs quantum wellAbsolute magneto-transmission experiments in the farinfra-red,as a function of the magnetic field B upto 33.5 T, have been performed on a single modulationdoped GaAs Quantum Well (QW) with awidth dw = 13 nm. The QW is sandwiched betweentwo GaAs/AlAs superlattices, the whole epilayer beinglifted-off from the GaAs substrate and depositedon a wedged Si substrate. The carrier concentrationN s = 3.8 × 10 11 cm −2 with mobilities exceeding10 6 cm 2 /V/sec at low temperatures. Due to the absenceof the GaAs substrate, the magneto-transmissionof the sample, mainly governed by the cyclotron (CR)absorption line, can be followed continuously over thewhole range of energies as shown in figure 36. Thein which the x-axis is assumed to lie in the plane ofthe QW, ε L is the contribution of the GaAs lattice,ω NP is the contribution to the CR frequency of thenon-parabolicity (NP) effects, η is the field independentcontribution of the background defects to the linewidth while Σ(ω) represents the self energy correctiondue to interactions: Im(Σ(ω)) and Re(Σ(ω)) are relatedeach other by Kramers-Krönig (KK) transformation.The effective mass m ∗ entering the plasmafrequency ω p is calculated with the same NP model.We note that the lineshape and intensity of the CRtransitions at low and high fields are practically thesame. Hence we can assume that there is no loss ofcarrier density during the magnetic field runs. In thefitting process, we first neglect the frequency dependenceof Σ and are left with two independent fittingparameters ω c = ω NP − Re(Σ) and δ 0 = η + Im(Σ)for each magnetic field. δ 0 and ω c /B are plotted infigure 37 as a function of the energy. Injecting intoFIG. 36. Evolution of the magneto-transmission spectra asa function of the energy for different values of the magneticfield between 19 and 33.5 T.spectra show different “anomalies” around the energiesof the transverse optical (TO) phonons of GaAs andAlAs and a strong damping of the CR line for energiesabove that of the longitudinal optical (LO) phonon ofGaAs. This latter effect is the manifestation of the polaroniccoupling due to the Fröhlich interaction of theCR line with the GaAs LO-phonon. We analyze thespectra with a multi-layer dielectric model includingthe dielectric function ε of the doped QW in additionto the appropriate dielectric functions of undoped barrierlayers. For the doped QW, the diagonal part ε xxis written as,ε xx = ε L −ω[ω − (ω NP − Re(Σ(ω))) + ı(η + Im(Σ(ω)))]ω 2 pFIG. 37. Top panel: Variation of δ 0 with the energy; theopen squares are values obtained from the fit of transmissiondata and the dotted and dashed lines represent thedifferent simulated contributions to δ 0. The full line is thetotal contribution. Bottom panel: variation of ω c/B withthe energy; the open squares are values obtained from thefit of transmission data and the full line, the KK transformof the total contribution to δ 0. The dashed line is thenon-parabolicity contribution to ω c/B.the multi-layer dielectric model the two contributionsdisplayed as continuous lines in figure 37 allows to reproduceall features observed experimentally includingthe “anomalies” mentioned at the TO energies whichappear to be pure dielectric artefact. The interactionwith the CR line occurs at two different levels; thefirst one corresponds to the Fröhlich interaction withthe GaAs LO-phonon and the second one, appearingat the energy E XP , to the interaction with interfacephonons (dashed and dotted lines respectively in theupper panel of figure 37). The LO-Fröhlich interactionhas been simulated with the model of Peeters and Devreese(PD)[ Phys. Rev. B 28, 6051 (1983)] whereasthat of the interface phonon with the model of Moriand Ando [Phys. Rev. B 40, 6175 (1989)].With the introduction of the PD model, it is possible toquantify the Fröhlich interaction and it is important toverify how this interaction disappears when increasingthe carrier density N s . This work is now in progress.C. Faugeras, M. Orlita, G. MartinezA. Riedel, R. Rey, K. J. Friedland (Paul Drude Institute, Berlin, Germany)28

2011 TWO-DIMENSIONAL ELECTRON GASMicrowave-induced Hall resistance in a bilayer electron systemIn the last year we have shown that microwave-induced(MW) zero-resistance states (ZRS) occur in a twodimensionalelectron system with two populated 2Dsubbands and are not limited to high-quality singlesubband systems [Wiedmann et al., Phys. Rev. Lett.105, 026804 (2010)].High mobility (µ = 1.9 × 10 6 cm 2 /V s) GaAs widequantum wells with a well width of 45 nm and highelectron density have been studied. Due to charge redistribution,we find a bilayer configuration with twooccupied 2D subbands. Samples in Hall-bar geometryare exposed to microwave (MW) irradiation using thedouble-modulation technique. The MWs that are absorbedby the sample are amplitude modulated withan additional frequency of 333 Hz. This enables us todirectly probe ∆R. We study the symmetry of ∆R xxand ∆R xy under field reversal and find a deviationfrom odd symmetry under magnetic-field reversal inthe microwave-induced Hall resistance ∆R xy , whereasthe dissipative resistance ∆R xx obeys even symmetry.field vector E ω ), demonstrate that whereas ∆R xx doesnot change sign under field reversal, ∆R xy does (fordetails see Ref. [Wiedmann et al., Phys. Rev. B 83,195317 (2011)]).To summarize, our studies of ∆R xy as a function ofthe microwave electric field and polarization exhibit astrong and nontrivial power and polarization dependence.Indeed, since MW-excited electron systems arefar from thermodynamic equilibrium conditions, it ispossible that the symmetry of the resistivity tensoris essentially broken under MW irradiation. Furtherexperimental investigations of MW-induced Hall re-In figure 38, we illustrate the power dependence ofphotoresistance for 143 GHz at 1.4 K. For ∆R xx infigure 38(a), we see that the amplitude decreases withdecreasing MW power (attenuation from 0 to -5 dB)while preserving even symmetry under field reversal.However, the main peak of ∆R xy at ±0.4 T, see figure38(b), changes its sign with decreasing MW powerfrom 0 to -7.5 dB (in contrast to the peak at -0.4 Twhose sign remains unchanged), so we find a transitionfrom odd to even symmetry approximately at -5 dB.The Hall resistance (amplitude of the MIS peak) at±0.4 T is plotted as a function of MW power in figure38(c). In contrast to this change in symmetry at±0.4 T with decreasing MW power, we notice that allother features in ∆R xy , e.g., at ±0.2 and ±0.27 T, donot exhibit an apparent change in symmetry.The inversion of one particular feature in Hall resistanceleading to breaking of odd symmetry under fieldreversal has been observed at high frequencies, powerdependentmeasurements of ∆R xy for f

TWO-DIMENSIONAL ELECTRON GAS 2011Valley-spin phase diagram of the two-dimensional electron gasWhat determines whether a 2-dimensional electronsystem (2DES) is a metal or an insulator remains aquestion of continued debate [Spivak et al, Rev. Mod.Phys. 82, 1743 (2010)] owing to the importance ofthe 2DES in modern solid-state physics, the rich phenomenologyand theoretical treatments, and the fundamentalnature of underlying concepts. A key conceptis the role played by degeneracy. Degeneracy is providedby spin and valley degrees of freedom, the latterbeing an additional freedom due to there being morethan one conduction band minimum (valley) at thesame energy in materials such as silicon, aluminiumarsenide and graphene. The crucial importance of thespin degree of freedom has been experimentally establishedby a number of authors working with differentmaterials. Lifting the spin-degeneracy by applicationof an in-plane magnetic field firstly increases the resistivityand secondly, tends to drive a metallic system tobecome an insulating one, although the detailed phenomenologyis more complicated and remains to befully understood.Since interactions between electrons are mediated bythe Coulomb interaction, it might be expected thatthe role played by the valley degree of freedom is, inessence, equivalent to that of the spin degree of freedom.This has been demonstrated in recent experimentsin AlAs quantum wells [Gunawan et al., NaturePhysics, 3, 388 (2007)] in which valley polarization isinduced and tuned by applying strain. These experimentsshowed that for certain fixed electron densities,both spin and valleys need to be polarized before insulatingbehaviour is observed. However, these experimentswere limited to a narrow selection of electrondensity, which is known to be critical in determiningthe nature of the 2DES and there remain many uncertaintiesas to how this relates to the complex phenomenologyobserved in other systems. This calls forresistivity and its temperature dependence to be investigatedin conjunction with spin and valley polarisationand density in order to obtain a complete picture.We have recently demonstrated that the valley polarizationcan be electrically tuned in situ, in additionto, and independently from the electron densityin certain silicon-on-insulator (SOI) quantum well devices[Takashina et al., Phys. Rev. Lett., 96, 236801(2006)]. In these experiments here, we use these devices(see figure 39) to establish the behaviour of thetemperature dependent resistivity on valley and spinpolarization and electron density.Preliminary measurements at relatively low magneticfield have revealed rich behaviour with valley polarization,evolving qualitatively with density. Furthermore,in-plane magnetic field measurements stronglysuggest that the complete picture involves physics inwhich spin and valley play equivalent roles, as well asphysics requiring both degrees of freedom [Takashinaet al., Phys. Rev. Lett., 106, 196403 (2011)].Through the use of a DC magnetic field of up to 28 Tat the Grenoble High Magnetic Field facility, we areable to freeze the spin degree of freedom up to a highelectron density. This enables us to observe saturationof behaviour at full spin polarization, under differentvalley-polarization conditions over a wide densityrange. In turn, this allows us to obtain unique insightsinto the roles played by spin in determining transportproperties, with and without the valley degree of freedom.FIG. 39. (i) A cross-section schematic of our device. The2-dimensional electrons reside in the silicon quantum well.(ii) An illustration of the confinement potential. The electricfield in the binding oxide layers are controlled by thefront gate voltage and a voltage applied to the substratewhich forms the back gate. (iii) Bulk silicon has six-foldvalley degeneracy but with confinement along a principleaxis, such as in our (001) quantum well, there is only a2-fold valley degeneracy. (iv) The degeneracy of these valleyscan be lifted by controlling the out-of-plane electricfield (quantified by δn) in turn controlled by the front andback gates. (v) The density of states is altered by the valleysplitting. Left: no valley splitting, right: with valleysplitting ∆ V .B. A. Piot, D. K. MaudeTakashina (University of Bath, Bath, UK), V. Renard (Institut Néel, Grenoble, France), Y. Niida, Y.Hirayama (Tohoku University, Sendai, Japan), A. Fujiwara (NTT Basic Research Laboratories, NTT Corporation,Atsugi, Japan)30

2011 TWO-DIMENSIONAL ELECTRON GASTemperature effect on spin gap of Landau levels in a bilayer 2DEGThe spin-splitting of Landau level (LL) is wellknownto be enhanced by the exchange part of theCoulomb interaction of carriers in a two-dimensionalgas (2DEG). There are several techniques to measurethis spin gap in lateral transport measurements. Allof them were used in investigation of the spin gap atodd-integer LL filling factors ν. As for the value ofthe spin gap at the non-integer ν it is not accessible inlateral transport measurements and other techniquesshould be used, for example, tunnel spectroscopy. Recentlythe enhanced spin gap was observed in the electrontunnelling between 2DEGs [Popov etal, JETP,102, 677 (2006)]. The gap is determined from themagnetically induced voltage-shift of the resonant currentpeak. To measure the spin-gap adequately oneshould be sure that the voltage-shift originates fromthe LL pinning on the Fermi levels in the 2DEGs. Theconditions of the LL pinning had been considered theoretically[Popov, PRB, 73, 125310 (2006)] and areexpected to be met in the experiment [Popov et al.,JETP, 102, 677 (2006)]. In addition to the cyclotronand spin gaps the voltage shift of the current peakcan be originated from the so-call pseudogap. Thispseudogap shift ∆V r has been investigated earlier andfound to be temperature insensitive in the magneticfields below 12 T. Here we pay attention to the temperaturedependence of the spin gap which is observedequal in the both 2DEGs in spite of their different LLpopulation.The schematic conduction-band-bottom diagram ofthe tunnel diode is shown in the insert (a) in figure 40with the quantum subband levels in the 2DEGs. Theparameters of the 2DEGs are the following; the concentrationof the 2DEG with the level E 01 is n 1 =4 × 10 11 cm −2 ; the concentration of the 2DEG withlevel E 02 is n 2 = 6 × 10 11 cm −2 . The tunnel characteristicswere measured at 3 He liquid and vapour temperaturesranged from 0.4 K up to 10 K. Current peak orthe 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 maximumat V s = −14 mV associates with the second resonance,i.e., E 01 (V s ) = E 12 (V s ). In the magnetic fielddirected perpendicular the 2DEG planes the resonantfeatures had been shifted in voltage position (see circularsymbols showing positions V r in figure 40). Thisvoltage shift consists of two parts; first is the singleparticleone that can be described in the single-particlemodel (see sections of solid lines in figure 40) and thesecond part is the pseudogap shift ∆V r . The temperaturedependence of the pseudogap shift was investigatedearlier. There is a temperature dependence ofthe single-particle voltage shift of the first resonance.In particularly in the field range B ∈ (8 T; 14 T)the single-particle resonance voltage values are shownin figure 40 as a section of the solid line and equalV r = ω c − ∆ s , where ω c is a cyclotron frequency, ∆ sis a LL spin gap. We suppose linear magnetic dependenceof the LL spin gap, i.e. ∆ s = g ∗ µ B B here g ∗is an effective Landé factor, µ B is the Bohr magneton.Since the cyclotron energy does not depends upon thetemperature one should expect the temperature effectin the Landé factor. This dependence is shown in theinsert (b) in figure 40.To discuss the temperature effect on the spin gap oneshould keep in mind that the LL pinning takes placewhen the LL is partial populated. This means thatboth 2DEGs are in the metallic states. In this case theCoulomb interaction is screening and sensitive to thetemperature change.FIG. 40. Topography map of the second current derivativeas a function of a bias voltage and magnetic field. Theexperimental values of the resonant voltage V r are shownas circles. The values calculated in the single-particle modelare shown as sections of lines. In the insert (a) the diagramof the conduction-band bottom are shown with levels in thequantum wells. The diagram is shown for zero bias and zeroenergy corresponds to the Fermi level. In the insert (b) theeffective Landé factor is plotted versus temperature (seeblack circles). A solid line is shown as a eye-guide.J. C. PortalV. G. Popov (Institute of Microelectronics Technology of RAS, Chernogolovka, Russia)31

Semiconductors and Nanostructures

2011 SEMICONDUCTORS AND NANOSTRUCTURESInter-shell electron-hole exchange interaction in CdTe/ZnTequantum dotsStrong carrier confinement is one of the most characteristicfeatures of self-assembled semiconductor quantumdots (QDs). Due to this confinement, the interactionsbetween carriers become enhanced in comparisonwith higher-dimensional structures. Studies of thisinteraction may improve the predictive power of thesemi-empirical simulations of QD properties. Thesekind of studies can also have more direct applications,e.g. understanding and reduction of anisotropic part ofelectron-hole interaction can lead to entangled photonpair emission.Many studies have been devoted to exchange interactionbetween electron and hole in their ground states(s-shell), however the electron-hole exchange interactionis not limited to this case. For example, one canalso study properties of exchange interaction betweenp-shell electron and s-shell hole through recombinationof excitonic complexes containing 3 electrons, such asXX − or X 2− .The use of strong magnetic field in this experimentsallows to verify yet another property of electron-holeexchange interaction. In general, apart from discussediso- and anisotropic contributions, there is also nonzeroterm δ 2 related to splitting within the dark statesin the case of neutral exciton. Usually it is believedto be negligibly small of the order of single µeV. Inthe case of XX − transition, considerable ˜δ 2 transitionwould produce the second anticrossing (between thehighest of four visible lines and an optically inactivetransmition). For typical g-factors of electrons andholes, this anticrossing is expected to occur between15-20T. The collected data exhibited no sign of suchan anticrossing, proving validity of assumption of negligiblevalue of ˜δ 2 term.For more details see Kazimierczuk et al. Phys. Rev.B 84, 165319 (2011).Interestingly, such an exchange interaction can be satisfactorilydescribed using the same approach as in thewell known case of s-s exchange interactions, i.e., introducingisotropic part δ 0 and anisotropic part δ 1 . Incase of neutral exciton, these parameters are directlyrelated to the energy spectrum, being the splitting betweenbright and dark exciton and within bright statesrespectively.In case of XX − transition, the exchange parameterscannot be deduced directly from the PL spectrum ofa QD. In particular, the splitting between two linesof XX − emission in the PL spectrum reflect mainlythe dominating value of isotropic exchange ˜δ 0 , whilethe anisotropic part ˜δ 1 is much more difficult to determine.In this respect introducing the magnetic fieldalong the growth axis can reduce the isotropic contributionand allow for more precise determination of theanisotropic contribution. Experimentally, this is observedas a well pronounced anticrossing, as shown infigure 41. The value of the anisotropic exchange interactionis directly related to the strength of the anticrossing.Measurements of the field dependence of thePL signal thus allow to determine both contributionsseparately.FIG. 41. (a) A typical spectrum of a single CdTe/ZnTeQD at B = 0. (b) Magnetic field dependence (for anotherdot) of recombination of XX − transition exhibit only oneanti-crossing. The lack of second anti-crossing (with opticallyinactive transition) is the evidence of a small valueof ˜δ 2. (c) Scheme of the splitting of the related excitoniclevels in the magnetic field along the growth axis.M. Goryca, P. Kossacki, M. PotemskiT. Kazimierczuk (Institute of Experimental Physics, University of Warsaw, Poland)35

SEMICONDUCTORS AND NANOSTRUCTURES 2011Probing the anisotropy of quantum-dotsDuring the past years, spectroscopy of quantum dotshas been extensively used to probe the electronic propertiesof those nano-objects and to access to the distributionof charges inside the semiconductor. This isof a particular interest when looking at a single dotas it allows one to avoid the broadening of the signaldue to the average contribution of component fromdifferent energies. The evolution of the emission linesof a dot in magnetic field can reveal the behaviourof multiple charges and show interesting physics lyingunder the mixing of electronic states. When theenergies of a quantum dot reflect those of a two dimensionalharmonic oscillator, the magnetic field evolutionthat is obtained is following a Fock-Darwin spectrum.We comment here on several consequences of theanisotropy of the dots on the observed spectra.Samples consisted of low density GaAs/AlAs quantumdots. This allows us to study the dots individually. Weused optical fibres inside the magnet in order to avoidproblems due to the vibrations of the cooling system.The sample was excited by means of an Ar-laser at514 nm. The detection part of the set-up consisted oftwo detectors in a Hanbury Brown and Twiss configuration.The luminescence of the dots was collectedand separated in two paths through a beam-splitter.The two beam lines were then directed to two spectrometersallowing one to either analyze the spectra bymeans of a nitrogen-cooled CCD camera or to select aspecific region of interest before detection by low backgroundAvalanche Photo-diodes (APD: SPCM-AQR-16; Perkin-Elmer). The two APDs were connected toa time intervals analyzer and a simple start-stop experimentallowed us to check for single emitters beforerecording spectra. The polarization was controlled justbefore the spectrometers. A super-conducting magnetwas used to vary the magnetic field from 0 to 14 T.Figure 42 shows the polarization dependence of thephotoluminescence of the first excitonic lines of a singlequantum dot at B=0 T. The polarization dependanceof a specific spectral line does not only revealthe modulation of the intensity of this line as a functionof the polarization, but also the presence of twocomponents in opposition of phase at different energies.This splitting at 0 T is due to some anisotropy ofthe dot. However, the separation of the lines inducedby the anisotropy is very often too small to be detectedand only a few information can be extracted from thisvalue. We present in the following an additional effectwhen we populate more states of the dot.Figure 43 shows the high power excitation spectrum ofa single dot for different magnetic field ranging from0 to 14 T. We can observe the population of differentshells of the dot. Although the spectrum of thedot is quite similar to what one could expect for highmagnetic field, the absence of signal for the high energylines at lower magnetic field is a clear deviationfrom a standard Fock-Darwin diagram. This is due tochanges in selection rules of the population of excitedstates caused by the anisotropy of the dot.FIG. 43. Magnetic dependence of the photoluminescenceof a single quantum dot under high power excitation of thelaser. The general pattern of the spectrum differs from theconventional Fock-Darwin diagram due to the anisotropyof the dot.FIG. 42. Photoluminescence of a single quantum dot atlow excitation power at B=0 T. We can observe that therelative intensity of a line varies as a function of the detectedpolarisation. The measurement reveals the presenceof two components at different energies representative of asubstantial anisotropy of the dot.It has to be mentioned that if the two previously reportedeffects are representative of an anisotropy of thedot, they are rather qualitative. To get a deeper understandingof the phenomenon, one should also probe theexcited states via photoluminescence excitation (PLE)spectroscopy which would allows us to access to thereal excited states of the dot. We plan in a near futureto measure the PLE spectra of dots in magneticfield in order to compare them to high power excitationspectra. Simulation might then be required to geta complete picture.A.A.L. Nicolet, M. Potemski36

2011 SEMICONDUCTORS AND NANOSTRUCTURESMagnetotransport studies of GaAs/AlGaAs core-shell nanowireRadial core-shell nanowires (NWs) are promisingcandidate for the realization of innovative nanodevicesas well as for studying one dimensional (1D)physics. With its well-established properties in 2D heterostructuresand intrinsically high electron mobility,GaAs/AlGaAs system is of particular interest to realizecore-shell NW. Recently we develop an originalmethod using molecular beam epitaxy and electronbeamlithography to achieve reproducible modulationdopedGaAs/AlGaAs core-shell NWs embedded in aGaAs matrix [Lucot et al., Appl. Phys. Lett. 98,142114 (2011)]. The NWs are individually connectedas grown in a two-probe setup using the highly dopedgrowth substrate and a top diffused contact. The measuredconductance shows a non-Ohmic behaviour withtemperature and voltage-bias dependence described bya single scaling law and related to the energy cost foran electron from the 3D reservoir to tunnel into a 1Dor quasi-1D system.Information on the electron phase coherence is obtainedfrom quantum conductivity corrections due toweak localization effects or universal conductance fluctuation.Most of the NWs are totally coherent at verylow temperature (100 mK) and phase coherence length(l φ ) could be larger than 250 nm at 4 K. The temperaturedependence of l φ with T −1/3 indicates electronelectroninteraction being the major source of decoherence.An electron mean free path of l e = 7 − 12 nmwas estimated; thus the transport takes place in thediffusive regime. The l φ values of our NWs are comparableto the best values reported for InAs or InN NWsgrown by different growth methods.With the large measured coherence length and assumingthat the electron trajectories are restrictedto ringlike structures at the GaAs/AlGaAs interface,we expected to find evidence for regular magnetoconductanceoscillations due to electron interference ina magnetic field parallel to the wire axis. However,Aharonov-Bohm (h/e) and Altshuler-Aronov-Spivak(h/2e) oscillations are present only in the 3 most resistiveNWs we measured. For all the other NWs witha lower resistance, no clear periodic oscillations of themagnetoconductance are present and it still difficultto conclude about the localization of carriers. In factthe problem of carriers localization in this hexagonalGaAs/AlGaAs core-shell NWs (see inset figure 44) ismuch more complicated. Recent calculations [Bertoniet al., arXiv:1109.6616v1(2011)] show that the electronand hole gases can be tuned to different localizationsand symmetries inside the core as a function of the dopinglevel. Contrary to planar heterojunctions, conductionelectrons do not form a uniform 2D electron gas(2DEG) localized at the GaAs/AlGaAs interface, butrather show a transition between i) an isotropic, cylindricaldistribution deep in the GaAs core (low doping),and ii) a set of six tunnel-coupled quasi-1D channels atthe edges of the interface (high doping). In the case ofour Hexagonal wires, new possible trajectories, whichcould be neglected in the 1D limit, have to be considered.FIG. 44. Magnetoresistance of a single NW for a fieldapplied perpendicular or parallel to the NW axis. Insetshows the schematics of the core-shell NW geometry.In order to obtain more information about nature ofthe carriers and about their density we performed highmagnetic field magnetotransport up to 28 T. Pronouncedeffects of the magnetic field was observedonly for the resistive NWs. Typical magnetoresistancecurves R(H) of the most resistive NW are shown forvarious magnetic field orientation in figure 44. By comparingthe patterns one finds a totally anisotropic magnetoresistance.In the perpendicular configuration, noclear oscillations are present but monotonous increaseof the resistance with large plateaus between 3 and7 T or 13 and 22 T are observed. The Rotation ofthe magnetic field to the parallel configuration showsthe apparition of a pronounced conductance oscillationswith an increasing amplitude by increasing themagnetic field. This oscillatory behaviour is maximumin the parallel configuration and leaves thought of theShubnikov-de Haas effect. However, minima of the oscillationsare not periodic in 1/H and they do not followa clear evolution law when the NW axis rotatearound the magnetic field. Taking into account theseresults it remains difficult to obtain information aboutcarriers density. The electron/hole gas has probably amixed dimensionality, which is locally 1D or 2D.More experiments are required to explain the complextransport regime reached in the GaAs/AlGaAs coreshellNWs, but these first magnetotransport measurementat high magnetic field shows the richness of thephysics contained in this system.D. K. Maude, B. A. PiotD. Lucot, J. C. Harmand, G. Faini, D. Mailly (LPN-CNRS, Marcoussis, France)37

SEMICONDUCTORS AND NANOSTRUCTURES 2011Magnetically induced field effect in InAs nanowiresSemiconductor nanowires (SC NWs) are promising materialsas versatile building blocks for future nanoelectronics.SC NWs also represent unique systems for exploringelectronic transport phenomena at nanoscaleinerrant to the quasi1-D confinement when the diameterof the wire starts to be comparable with the Fermiwavelength.InAs NW appears as a natural choice for high performanceelectronic nanodevices; bulk InAs material isknown to present a small direct band gap, allowing asmall effective mass, a large Fermi wavelength and highelectron mobility at room temperature. The Fermi energyis also pinned in the conduction band which leadsto the formation of electron accumulation surface layer,giving rise to low resistive ohmic contacts.However, the mobility is strongly dependent on thediameter size; the radial charge carrier distribution remainsunknown as well as the strength of scattering potentialat the surface of the wires. InAs NWs are alsoexpected to develop a large electronic g factor leadingto strong spin-orbit interactions.The conductance versus backgate voltage unveils a tunablen-type conduction regime with a field effect mobilityof the order of 4000 cm 2 .v −1 .s −1 at 80 K for a 33 nmdiameter InAs NW (figure 45). The carrier density isin the range of 10 17 − 10 18 cm −3 and the electronicmean free path is around 70 nm, indicative of a weaklydiffusive regime.Below 30 K, large conductance fluctuations develop asa function of the backgate and magnetic field. Fromtheir magnitude and the correlation magnetic field, wededuce a phase coherence length of 120 nm, at 4.2 K.By applying a large magnetic field to the InAs NW,in the transverse configuration, we give experimentalevidence of a drastic magnetically induced switch-offthe conductance (figure 46). When increasing the electronicconcentration, the transition field is graduallyshifted to higher values. This phenomenon is currentlystudied by varying the temperature and the magneticfield orientation. Photo-current in high magnetic fieldis also in progress.FIG. 45. Conductance versus backgate voltage V g measuredfrom 300 K to 4.2 K on a 33 nm diameter InAsnanowire.We propose to address the aforementioned issues bymagnetotransport measurements in the high magneticfield regime on individually connected InAs NWs.FIG. 46. Magneto-conductance in a transverse configurationmeasured on a 33 nm diameter InAs nanowire for differentbackgate voltages.V. Prudkovsky, S. George, W. Escoffier, M. Goiran, B. RaquetR. Leturcq, P. Caroff (IEMN, Lille, France)38

2011 SEMICONDUCTORS AND NANOSTRUCTURESLocal and nonlocal magnetoresistance in two-dimensionaltopological insulatorRecently, a novel class of the topological state of aquantum matter has emerged, called topological insulator[Hasan and Kane, Rev. Mod. Phys. 82, 2045(2010)]. The first famous example of the two dimensionaltopological insulator (2DTI) is the integer quantumHall effect (QHE) state on the resistance minima .Another class of 2DTI is the quantum spin Hall effectstate, which can be realized in 2D system with strongspin-orbit interaction in the absence of the magneticfield. It has been shown that an “inverted” semiconductorHgTe/CdTe quantum well [König et al., Science318, 766 (2007)] possess the insulating phase havingthe gap in the bulk electron spectrum and a singlepair of counter propagating or helical edge states forthe two opposite spin polarizations.FIG. 47. (a) Local and nonlocal resistances at zero magneticfield as a function of the gate voltage for differentconfigurations. (b) Local resistance as a function of the perpendicularmagnetic field for different gate voltages. Redtraces a taken for electronic part of TI peak, blue traces aretaken for hole part of TI peak, (c) Nonlocal resistance asa function of the perpendicular magnetic field for differentgate voltages. T = 80 mK.In this work we present an experimental study of thetransport properties of “inverted” HgTe-based quantumwell with helical edge states strongly mixed byspin flip scattering. It is expected that the resistanceof the ballistic topological insulator around the bulkenergy gap in zero magnetic fields is quantized at valueh/2e 2 . However, our devices revel unusual behaviour,which is characterized by high resistance R >> h/e 2with metallic temperature dependence. The origin ofthe resistance is determined by the behaviour of thecontacts regions, similar to the conventional quantumHall effect. Contacts are assumed to be a thermalreservoirs, when full mixing of electron spin statescan occur. Note, that in contrast to the standardQHE, when mixing of edge states occurs within metallicohmic contacts, in our samples potential mixingis provided by 2D electron gas in the region outsideof the metallic gate due to finite bulk conductivity.The length of the edge states are determined by theperimeter of the sample part covered by metallic gate(mostly side branches) rather that the length of the microbridge.We calculate the length of 1D wires betweenprobe contacts and find that it exceeds 1.0 mm. Thehigh resistance R >> h/e 2 can be attributed to the3 basic mechanisms: (i) inelastic scattering; (ii) spinflipby Rashba spin-orbit mechanism; (iii) two-particleprocesses.Fig. 47 shows several traces for local and nonlocal resistancetaken at different configurations for 8 nm HgTewell. The application of the current between any pairof the probes creates net current along the sample edge,and can be detected by any other pair of the voltageprobes. In general, there is no difference betweenlocal and non local electrical measurements in topologicalinsulator. For example for nonlocal resistanceR I=1,10;V =3,4 , shown in figure 47(a) the application ofthe current between leads 1 and 10 produces currentalong the two paths; one longer: 1-2,2-3,3-4,4-5,5-6,7-8,8-9,9-10 and one shorter: 1-10. The occurrence ofthe nonlocal resistance of comparable amplitude impliesthat potential difference extend over macroscopicdistance ∼ 1 mm away from dissipative bulk currentpath. It is only possible, if the bulk conductivity iscompletely suppressed and conductivity of the TI sampleis determined by edge states.The application of a strong perpendicular magneticfield would be important to establish the physics oftopological insulators, and one may ask if they can alsoexhibit other interesting phenomena. Figure 47(b-c)shows the magnetoresistance in local and nonlocal configurationsfor different gate voltages, when the Fermilevel passes from electron to hole side of the peak. It isworth noting remarkable similarity between these twosets of curves. It confirms again that the transport inTI occurs only at the periphery of the sample.In summary, we demonstrate the similarity between localand nonlocal resistance measurements in TI regime.These data give the evidence that in realistic samplesedge-state transport truly exists over macroscopic distanceof ∼ 1 mm in the absence of the magnetic field.J. C. PortalG. M. Gusev (Instituto de Física da Universidade de São Paulo, SP, Brazil), Z.D.Kvon, O.A.Shegai,N.N.Mikhailov, S.A.Dvoretsky (Institute of Semiconductor Physics, Novosibirsk, Russia)39

SEMICONDUCTORS AND NANOSTRUCTURES 2011Quantum Hall effect in a quasi-three-dimensional HgTe filmStarting from the pioneering work of Khmelnitskii[Khmelnitskii, JETP Lett. 38, 552 (1983)], the possibilityof the existence of the quantum Hall effect (QHE)in a system with a three-dimensional energy spectrumremains one of the most intriguing and fundamentalproblems in the physics of the QHE. So far, thereare two experimental indications of the possibility ofthe QHE in a quasi-three- dimensional system withseveral occupied size-quantization levels, namely, theQHE in wide (60-100 nm) Al x Ga 1−x As/Al y Ga 1−y Asheterojunction-based parabolic wells and in heavilydoped thick (100 nm) GaAs layers. In this work, wepoint to an additional quasi-three-dimensional systema three-dimensional HgTe gapless semiconductor filmthat exhibits pronounced quantization of the Hall resistance.The experimental samples (inset in Fig. 1a) studied inthis work were 100 nm HgTe films grown by molecularbeam epitaxy on a (013) CdTe/ZnTe/GaAs substrate.It should be mentioned that the use of the describedorientation allowed us to grow thick high quality films.The thickness of the grown layers was controlled bymeasuring their ellipsometric parameters in situ, as describedin detail by Shvets et al. The magnetotransportmeasurements were carried out on the 50 µm wideHall bars with a distance between the voltage contactsof 100 and 250 µm fabricated from the grown films bystandard photolithography. Then, the ohmic contactswere prepared by etching indium. The measurementswere conducted in the temperature range of 4.2-0.2 Kin magnetic fields of up to 13 T with the use of thestandard lock-in detection at a frequency of 10-12 Hzand an excitation current through the sample of 10-100nA, which excluded heating effects.The main result of the work is shown in figure 48, whichpresents the dissipative ρ xx (B) and Hall ρ xy (B) componentsof the resistance tensor at the temperatureT = 0.3 K in magnetic fields of up to 13 T for twosamples with the different Hall electron densities in thefilm N s = 1.15 × 10 12 (sample 1) and 4.8 × 10 11 cm −2(sample 2). As is clearly seen, pronounced plateaus(ν = 7, 5, and 4) in the ρ xy (B) curve accompaniedby deep minima in ρ xx (B) appear for sample 1 abovethe magnetic field of 6 T. A similar picture takes placefor sample 2 for the plateaus with ν = 4 and 2. Themagnetic field position of the plateaus and minimain Fig. 1 allowed us to find the electron density inthe film N s = 1.15 × 10 12 (sample 1) and 4.8 × 10 11cm −2 (sample 2), which coincides with the above valuesfound from the Hall measurements. Thus, the datapresented in Fig. 1 unambiguously indicate the pronouncedquantization of the Hall resistance in thickHgTe films, whose spectrum is not two-dimensional.According to the calculation in the square-well model,the electrons in sample 1 (2) at the specified densitiesfill at least four (three) size-quantization levels. Theband structure of the quantum well nearly coincideswith that of bulk HgTe.Let us now turn to a more detailed analysis of thebehavior of the SdH oscillations. The oscillation periodin the inverse magnetic field gives for sample 1N s = 3.1 × 10 11 cm −2 , which is more that three timeslower than the value obtained from the Hall measurements.Similar discrepancy takes place for sample2, for which the two-dimensional formula yieldsN s = 1.85×10 11 cm −2 . Thus, the treatment of the SdHoscillations with the use of the formulas for the twodimensionalcarrier density leads to a strong discrepancywith the Hall data, which coincide with the QHEresults. This indicates the three-dimensional characterof the SdH oscillations in the semiclassical magneticfields. The carrier density found under the assumptionof the three-dimensional density of states yields thethree-dimensional density in sample 1 n = 0.93 × 10 17cm −3 . This agrees fairly well with a value of 1.15×10 17cm −3 found from the Hall measurements. The resultfor sample 2 is similar: the SdH oscillation periodand the Hall measurements yield n = 4.2 × 10 16 and4.8 × 10 16 cm −3 , respectively.FIG. 48. Magnetic field dependence of ρ xx(B) and ρ xy(B)in the 100 nm HgTe film at T = 300 mK: (a) Sample 1 withthe electron density N s = 1.15 × 10 12 cm −2 . The insetshows the cross section of the structure. (b) Sample 2 withthe electron density N s = 4.8 × 10 11 cm −2 .J.C.PortalE.B. Olshanetsky, Z.D. Kvon, S.S. Kobylkin, D.A. Kozlov, N.N. Mikhailov, S.A. Dvoretsky (Institute ofSemiconductor Physics, RAS SB, pr. Lavrenteva 13, Novosibirsk, 630090 Russia)40

2011 SEMICONDUCTORS AND NANOSTRUCTURESRatchet photovoltage measurements in Si/SiGe heterostructuresPhotogalvanic effects are well known in semiconductorphysics. Such properties are observable in systemswhich have are asymmetric i.e, asymmetry ofthe Coulombian potential in inhomogeneous materialor time inversion asymmetry. In biology, most of proteinsuse the asymmetry of polymer carbon chains tocarry the information or smaller molecules in humanbody. There are many examples in biology, fluidic andmechanical physics that are based on the photogalvanicalso called ratchet effect.One electronic version of the ratchet effect has been experimentallydemonstrated in semicircular antidot latticepatterned on a GaAlAs/GaAs heterostructure in2007 (Sassine et al. Phys. Rev. B, 78, 045431, 2008).As the heterostructure used in industry is based on silicon,experiments on Si/SiGe heterostructures were carriedout. The sample used for that study was a Si/SiGeheterostructure grown by reduced pressure chemicalvapor deposition (Kannan et al. App. Phys. Lett. 98,193505, 2011). Semicircular antidots were patternedon an area of 230 × 50m 2 2 in the center part of a Hallbar. The antidot lattices are fabricated with a hexagonalorganization and the following lattice parameters:radius of 120 nm and period of 600 nm was defined asshown in the inset of figure 49.The microwave frequency was fixed at 43.7 GHz with amaximum power. The polarization is also maintainedperpendicularly to the Hall bar length (cf. inset of figure49). A maximum photovoltage of about 3.5 mVwas observed and shifted in negative magnetic fieldin the longitudinal configuration as predicted theoreticallyin presence of strong electron electron interaction(Chepelianskii et al. Phys. Rev. E 78 041127, 2008).The photovoltage intensity disappears as the temperatureincreases due to the loss in 2DEG confinementand antidot depletion.A transverse photovoltage (perpendicular to the Hallbar) resulting from a strong electron-electron interac-FIG. 50. Magnetic field dependence of the transversephoto-voltage measured at 1.4 K.tion is presented on figure 50. In this configuration,the shape and the intensity of the photovoltage is alsovery sensitive to the asymmetrical shape of the antidots.A lower intensity is observed for the flat edge ofantidot (A-B side). On the opposite, a photovoltageintensity of about 3.5 mV is obtained on the C-D sideinvolving the semicircular side of the antidots.FIG. 49. Magnetic field dependence of the longitudinalphoto-voltage (contacts A-C) for different temperatures.In conclusion, we have presented the photovoltaic responseunder microwave illumination of 2DEG in presenceof a semicircular antidot lattice in Si/SiGe heterostructure.The photovoltage is found to be verysensitive to the antidot shape and electron electron interactionas theoretically predicted.E. S. Kannan, I. Bisotto, J.-C. PortalT. J. Beck and R. Murali (GeorgiaTech, Atlanta, USA)41

SEMICONDUCTORS AND NANOSTRUCTURES 2011Hall Spectroscopy of the Dipolar Spin Waves of NanomagnetsEddy currents generally give parasitic effects in highmagnetic field measurements. These effects are particularlystrong in high mobility electron systems wherethe large mean free path increases the phase relaxationlength and leads to the formation of persistent currentsin mesoscopic rings. Here we use the microwaveeddy currents, induced at the site of a two-dimensionalelectron gas by the dB/dt of micro-magnetic dots, toprobe confined spin waves. The magnetic dots are irradiatedwith microwaves whilst a large magnetic fieldis applied in the plane of the 2DEG. A dB/dt is producedwhenever a quantized spin wave mode comes inresonance. The signal is picked up as a photo-voltageinduced in the 2DEG by the stray magnetic field emanatingfrom each mode. This new technique not onlyallows probing the frequencies of individual modes butthe amplitude of resonances and their dependence onthe magnetic field tilt angle allows us to deduce the polarizationand orientation of the confined spin waves.Finally the technique is non invasive and highly sensitive.We report on the spin waves of individual magneticdots as small as ∼ 100 nm (Fig.51). These dimensionsare too small to be investigated by conventionaltechniques such as Brillouin light scattering.a value of the exchange stiffness constant of dysprosium,A = 1.5 × 10 −12 J.m −1 . The second type of spinwaves are dipolar magnetization waves which propagateacross the array through the magnetostatic interactionsbetween dots. These spin waves form a bandproportional to the strength of the dipolar interaction(up to 1.5 T) and which is evidenced as a broadeningof the ferromagnetic resonance: the resonance widthincreases proportionally to the dipolar interactions betweenstripes as the period of the array decreases. Theonset of the FMR corresponds to the excitation of dipolarspin waves of infinite wavelength (red dots). TheFMR cutoff corresponds to spin waves at the edge ofthe Brillouin zone (black dots). We derive an analyticalexpression of their dispersion curve and obtaina good fit of the ferromagnetic resonance broadeningfrom first principles.FIG. 51. AFM image of a dysprosium dot. The dot standsat the surface of a high mobility 2DEGThe data reveal two types of spin waves. Dipolarsurface spin waves manifest through a series ofsharp resonances at lower magnetic field [Jorzick et al.,Phys.Rev.Lett. 88, 047204 (2010)]. These waves areconfined in the minima of the demagnetization fieldwhich create spin wave magnetic wells near the polesurfaces. Dipolar surface spin waves are localized andinteract very little with neighbouring dots in arrayedstructures as demonstrated by the narrow resonancesin Fig.52 (blue circles). We calculate the eigenfrequenciesof their quantized modes and obtain a qualitativeexplanation of the low field resonances. The fit yieldsFIG. 52. (a) Frequency dependence of microwave resonancesin the photo-voltage of an array of magnetic dots.The fan diagram shows the FMR peak. The photo-voltagedips at low field (blue circles) are resonances with dipolarsurface spin waves quantized by the magnetic wells nearthe pole surfaces. (b) Power dependence of the resonantdips.Our experiments show that photo-voltage measurementsin hybrid semiconductor-ferromagnetic structuresprovide a sensitive and non-invasive tool for probingthe spin waves of small magnets (10 − 500 nm).J-.C. PortalS. Littlejohn, A. Nogaret (Univ. Bath), H. E. Beere, D. A. Ritchie (Univ. Cambridge)42

2011 SEMICONDUCTORS AND NANOSTRUCTURESMagnetoresistance in MOSFET saturation regime of operationThe effect of magnetic field in the MOSFET saturationregime of operation has been studied for thefirst time on bulk MOSFETs high-κ/metal gate (EOT2.4nm) obtained from ST Microelectronics with channellengths from 1µm down to 50 nm and 10µm width.Study of MOSFETs in the saturation regime allowsperformance to be analyzed in its actual context (I onbeing measured at large V d drain voltage) unlike measurementsin the linear regime (small V d ). I on is governedby high field effects in the channel althoughthere is a correlation between the linear and saturationregimes of MOSFET operation. Both linear and saturationregime extraction have their own merits anddrawbacks and therefore both are necessary in orderto have a complete understanding of MOSFET performance.and we can see that in saturation, µ MRsat is almostindependent of the drain voltage V d . Figure 54 showsµ MRsat versus µ MRlin for different channel lengthsand we can see that linear and saturation region µ MRare correlated. Using the source drift velocity (v sats )model for drain current in saturation, v sats was extractedfor different channel lengths (at B = 0) fromthe plateau of g m /C ox W at high V d where g m is thetransconductance, C ox is the equivalent gate capacitanceand W is the channel width. The extracted v satsand µ MRsat at V d = 1.1 V and V g = 1.1 V are plottedagainst channel length in figure 55. The v sats extractedusing this method is well below the bulk Si velocitysaturation value (≃ 10 7 cm/s) even for 50 nm atthis temperature. The µ MRsat and v sats are correlatedand µ MRsat decreases with increasing v sats . Thereforemagnetoresistance measurement is useful even in thesaturation regime MOSFET operation and a deeperstudy in this regime is on-going to exploit full advantagesof MR extraction technique for the future technologynodes.FIG. 53. Extracted µ MR in cm 2 /Vs versus V d at 270 Kfor V g = 1 V for different channel lengths.The advantages of magnetoresistance (MR) measurementin the linear regime is that we can directlymeasure the mobility without any extraction limitations,especially in determining the physical dimensions,which makes it attractive for future technologynodes. Here, for the first time, the effect of magneticfield in the saturation regime is investigated. Itis found that there exist an equivalent µ MRsat , whichfollows the linear regime µ MR with channel length andtemperature. µ MR is extracted using the expression,FIG. 54. µ MRsat vs µ MRlin (in cm 2 /Vs) at 270K fromdifferent channel lengths are shown. µ MRsat is taken fromV d = 1.1 V and µ MRlin from V d = 50 mV.I D0 − I DBI DB= µ 2 B 2 ,where I D0 is the drain current at 0 magnetic field. Bis the magnetic field applied normal to the inversioncharge plane (x-y plane). I DB is the correspondingdrain current at field B for the same drain and gatevoltages V d and V g .Figure 53 shows the extracted µ MR plotted against V dfor different channel lengths at gate voltage V g = 1 V at270 K. We can therefore extract µ MR as a function ofdrain voltage from linear µ MRlin to saturation µ MRsatFIG. 55. Extracted µ MRsat in cm 2 /V.s and v sats (in10 6 cm/s) at 0 magnetic field at 270K for different channellengths is shown. Figure shows correlation between µ MRsatand v sats.D. K. Maude, B. A. PiotN. Subramanian, M. Mouis, G. Ghibaudo (IMEP-LAHC, Minatec, Grenoble, France)43

SEMICONDUCTORS AND NANOSTRUCTURES 2011Origin of the negative magnetoresistance in 2D polycrystalline SnO 2thin filmsThe analysis of high-field magnetoresistance (MR)data, obtained on our samples of SnO 2 polycrystallinethin films, provides a possibility to justify the mechanismof electron-electron interaction (EEI) in disorderedsystems [Altshuler et al., Modern problems incondensed matter sciences 10, North Holland 1985]since the highly controllable degree of disorder in oursamples, provides the possibility to tune the range ofrealization of quantum interference under applied magneticfields (MF).The experimental MF dependences of magnetoresistance(MR), measured at different fixed temperatures,are negative in the full range of applied MF. In thestrongly localized regime, the total MR can be writtenas the sum of a positive component (PMR) whichis induced by the MF shrinkage of the localized wavesfunctions, and a negative part (NMR), related to thepresence of the under barrier scattering of hoppingelectrons by intermediate hopping sites and to interferencesbetween different hopping trajectories (Nguyen,Spivak, Shkolvskii,- NSS-model).Usually, for 3D semiconductors the NMR is suppressedwith decrease in temperature after the crossover fromMott-type hopping to Efros-Shklovskii (ES) hoppingover the states within the Coulomb gap. At the sametime, the amplitude of NMR increases as the temperaturedecreases in the Mott-type hopping regimeand decreases with the temperature in the ES hoppingregime. Regarding our samples, the NMR amplitudesincreases as the temperature decreases, thus excludingthe ES hopping regime.In figure 56 the normalized magnetoconductance(G 0 = e22π 2 ) (MC) are presented. The black solid2lines are the extensions to high MF of the fittingcurves obtained for the weak localizations (WL) modelonly [Hikami et al., Prog. Theoretical Phys. 63,707 (1980)]. It is clear that these black curves donot capture the behaviour of MC at high MF. Thered solid line represents the asymptotic behaviourof correction to MC from WL for B ≫ B tr , whereB tr = 4eDτ= 0.34 T, D and τ being the diffusioncoefficient and the elastic scattering time respectively.The total MC is written as a sum of the Drude component,a term of WL and a term of EEI for 2D system[B.L. Altshuler et al., Quantum Theory of Solids, MirPublishers. Moscow 1982. pp. 130-237.], and presentedon figure 57.At vanishing field (B < 1 T) the fit follows the WLmodel. Here the red solid line represent the 2D EEIcontribution to MC, the black one represents theasymptotic behaviour of the WL contribution forB ≫ B tr , and the blue solid line is the extrapolationto high field of the low-field WL correction to MC.Reasonable values of obtained parameters and goodfitting results suggest the samples we study are on themetallic side of the metal-insulator transition (MIT)and charge transport is of diffusive nature.In conclusion, the theory of quantum corrections toMC due to WL and EEI gives more or less reasonabledescription of the observed charge transport characteristicsof not intentionally doped SnO 2 polycrystallinefilms which behave as a quasi-two dimensional electrongas on the metallic side of the MIT.FIG. 56. Normalized magnetoconductance measured atdifferent temperatures. Black solid line are the extrapolationresults of the weak localizations (WL) contribution(B

Metals, Superconductors and StronglyCorrelated Systems

2011 METALS, SUPERCONDUCTORS...Quantum oscillations the organic metal θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 )The Fermi surface (FS) of the strongly two-dimensional(2D) organic metal θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) displayedin figure 58 achieves a linear chain of coupled 2D holeorbits which is an experimental realization of the famousmodel proposed by Pippard in the early sixtiesto compute magnetic breakdown (MB).These formulae are in agreement with the dHvA oscillationspectra of the strongly 2D organic metal θ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ) (see figure 59), not only for thebasic α and MB-induced β orbits, but also for frequencycombinations including the “forbidden orbit”β − α, assuming an effective Landé factor g ∗ = 2.FIG. 58. Fermi surface of the two-dimensional organicmetal θ-(ET) 4CoBr 4(C 6H 4Cl 2). The basic α and magneticbreakdown-induced β orbits are displayed.FIG. 60. (a) Mass plots and (b) Dingle plots of β − α(circles), β + α (squares) and 2β − α (diamonds). Solidlines are bests fits of the data, calculated with m α = 1.81,m β = 3.52, B 0 = 35 T, T D = 0.79 K and g ∗ = 2, with aprefactor as only free parameter.FIG. 59. Fourier analysis in the field range 35 − 55.3 Tof the magnetic torque data reported in the inset. Solidtriangles are calculated with F α = 0.944 kT and F β =4.60 kT.Analytical formulae for de Haas-van Alphen (dHvA)oscillations in such FS have been derived systematicallytaking into account the chemical potential oscillationsin magnetic field (for more details, see [Audouardet al., arXiv:1110.3696]). Although corrective termsare observed, basic (α) and magnetic breakdowninduced(β and 2β - α) orbits can be accounted forby the Lifshits-Kosevich (LK) and Falicov-Stachowiaksemiclassical models in the explored field and temperatureranges. Oppositely, the “forbidden orbit” β - αamplitude is described by a non-LK equation involvinga product of two classical orbit amplitudes. Furthermore,strongly non-monotonic field and temperaturedependence may be observed for the second harmonicsof basic frequencies such as 2α and the magneticbreakdown orbit β + α, depending on the value of thespin damping factors.FIG. 61. Fourier analysis in the field range 35 − 55.3 Tof the tunnel diode oscillator data (in-plane magnetoresistance)displayed in the inset. Solid triangles are calculatedwith F α = 0.944 kT and F β = 4.60 kT.In contrast, magnetoresistance oscillations spectrameasured through a contactless tunnel diode oscillatortechnique exhibit a tremendous number of Fouriercomponents (see figure 60) suggesting, besides quantuminterference contributions, more severe deviationsfrom the LK model that remains to be explained withinthe Shubnikov-de Haas theory.A. Audouard, D. Vignolles, L. Drigo, F. Duc, G. BallonJ.-Y. Fortin (Institut Jean Lamour, Université de Nancy I), R. B. Lyubovskii, G. V. Shilov, E. I. Zhilyaeva,R. N. Lyubovskaya (IPCP, Chernogolovka, Russia), E. Canadell (ICMAB, Barcelona, Spain)47

METALS, SUPERCONDUCTORS... 2011dHvA investigation of the MAX phase compound Ti 3 SiC 2The MAX phases are a class of compounds thathave unusual combinations of properties making thempromising candidates for a wide variety of applications.Their chemical formula can be written as M n+1 AX n ,where n = 1, 2, 3, M is a transition metal, A is an sp element,and X=C or N. These materials consist of layersof A atoms with intercalated layers of binary carbideor nitride octahedra. They crystalize with a hexagonalstructure with large anisotropy of the ratio c/a ofthe lattice constants, which in turn is expected to leadto a large anisotropy in the physical properties. MAXphases are good electrical conductors, with a resistivitythat increases linearly with temperature. Indeed,their electronic properties are often better than thoseof the corresponding pure transition metal, with thehighest conductivity obtained so far for Ti 3 SiC 2 .occur in the melt at a temperature T = 1650 ◦ C. Thehighly anisotropic crystal growth, occurring in a stepflow mode (see figure 62) limited by the carbon supply,results in the formation of crystalline flakes oriented inthe basal plane. The samples are extracted from thesolidified melt in a second step.FIG. 63. (a) Oscillatory dHvA torque signal of Ti 3SiC 2measured with B ‖ c at T = 10 mK. (b)Fourier transformof the oscillatory torque versus 1/B showing a large numberof frequencies. The inset shows the calculated Fermi surfaceof Ti 3SiC 2.FIG. 62. SEM image of a Ti3SiC2 sample illustrating thestep flow growth mode.Although detailed calculations of the Fermi surface ofMAX phases exist [Chaput et al. Phys. Rev. B 75,035107 (2007)] there has been no experimental investigationto date due to the unavailability of single crystals.Here we present preliminary results on the MAXphase Ti 3 SiC 2 for which single crystals have recentlybecome available.The Ti 3 SiC 2 single crystals have been grown in ahigh temperature Czochralski crystal pulling-furnaceheated by induction, starting from a high purity siliconand titanium melt around a Ti composition x = 34 at%Mercier et al., Phys. Rev. B 83, 075411 (2011). Carbonis supplied by partial dissolution of the graphitecrucible. Spontaneous nucleation and crystal growthFor the measurements a Ti 3 SiC 2 single crystal wasmounted on a CuBe capacitive cantilever and cooledto mK temperatures in the mixing chamber of a dilutionrefrigerator. The oscillatory dHvA torque (τ osc )as a function of inverse magnetic field over a small fieldrange can be seen in figure 63(a). This was measuredwith the magnetic field B ‖ c. A number of frequenciesare clearly present in the raw data. This is confirmed inthe Fourier transform of τ osc (1/B) in figure 63(b). Thelarge number of frequencies present simply reflects thecomplex nature of the calculated Fermi surface whichcan be seen in the inset of figure 63(b). We are in theprocess of performing an in-situ rotation to map outthe Fermi surface together with temperature dependentmeasurements at selected angles to extract theeffective masses for the different carrier pockets.B. A. Piot, I. Sheikin, D. K. MaudeT. Ouisse, Didier Chaussende (Laboratoire des Matériaux et du Génie Physique, INP-CNRS, Grenoble),L. Chaput (Institut Jean Lamour, CNRS-Nancy University)48

2011 METALS, SUPERCONDUCTORS...Magnetic-field-induced stripe order in underdoped YBa 2 Cu 3 O yWhat happens when a superconductor is exposed toa strong magnetic field? In type-II superconductors,vortices of the superconducting current define a hexagonallattice of tubes into which the magnetic field penetrates.The higher the magnetic field, the larger thenumber of vortices and since each vortex core is a nonsuperconducting,so-called normal, region, superconductivitydisappears once the cores overlap throughoutthe sample. In high temperature copper-oxide superconductors,however, this may not be the whole storybecause the normal state is everything but normal.The electronic properties of this state have been investigatedfor the last 25 years but the problem is socomplex that no consensus has been reached yet.High magnetic fields are the most powerful way forpromoting the vortex cores at the expense of superconductivityand thus extending the temperature rangeover which the normal state can be probed. Ideally,when superconductivity is entirely suppressed, experimentscan address the question of what ground statethe system would have if it were not superconducting.Recently, a series of high field experiments at theLNCMI Toulouse have given a new twist to the field ofhigh T c superconductivity. The discovery of quantumoscillations [Doiron-Leyraud et al., Nature 447, 565(2007)] and subsequent Hall effect measurements havedemonstrated that the Fermi surface of underdopedYBa 2 Cu 3 O y is reconstructed into small electron pockets.Motivated by these results, we have performedCu nuclear magnetic resonance (NMR) experiments insingle crystals of YBa 2 Cu 3 O y in magnetic fields strongenough to significantly weaken superconductivity (upto 33.5 Tesla).Our most spectacular result (figure 64) is the discoveryof the splitting of the high frequency quadrupole satellite(Cu2F) on cooling below ∼50 K (for YBa 2 Cu 3 O 6.5 )in a field of 28.5 T, for those nuclei located belowoxygen-filled chains [Wu et al., Nature 477, 191(2011)]. This splitting is a manifestation of charge orderoccurring in the CuO 2 planes. While (only) twocharge modulation patterns are compatible with theNMR splitting, the physical context indicates that thischarge order is most likely the same 4a-periodic chargestripe order as observed in stripe-ordered cuprates suchas La 1.8−x Eu 0.2 Sr x CuO 4 . Furthermore, we unravelthree remarkable aspects of this stripe order:- In contrast with previously known stripe-order inLa 1.8−x Eu 0.2 Sr x CuO 4 , charge order is not accompaniedby any spin order here, as demonstrated by themeasurement of different NMR observables (signal intensity,line shift and relaxation rates).- The temperature onset for the charge order coincideswith the drop and the sign change of the Hall and Seebeckcoefficients, demonstrating a direct link betweenthe Fermi surface reconstruction and the translationalsymmetry breaking due to charge order.FIG. 64. Low temperature splitting of the 63 Cu(2F) highfrequency quadrupole satellite of YBa 2Cu 3O 6.5 in a field of28.5 T.- The charge order develops only when superconductivityis weakened by the magnetic field (charge orderis absent for fields below about 10 T and it does notexist if a field of 28.5 T is applied in the CuO 2 planes).This discovery demonstrates that there is a close competitionbetween charge-stripe order and superconductivity.Actually, the closeness of the values of the transitiontemperatures T charge and T c is an indication thatboth phases have very close energies.Stripe order is clearly stabilized by the presence ofan anisotropy of the crystal lattice (chains here).However, the fact that this ordering occurs at thesame electronic density in materials as different asLa 1.8−x Eu 0.2 Sr x CuO 4 and YBa 2 Cu 3 O y implies thatthe phenomenon has to arise from an intrinsic electronictendency in CuO 2 planes. Our results thus indicatethat the tendency towards charge order is anintrinsic part of the cuprate physics and that chargestripes could be the generic non-superconductingground-state of underdoped cuprates. Further, ourwork provides the strongest evidence to date for theexistence of competing orders. This observation is ofparamount importance because a number of theoriesattribute the anomalous normal state of the cuprates(the pseudogap state) to the proximity of an, hithertohidden, form of electronic order which would be incompetition with superconductivity. In this scenario,the superconducting pairing could originate from fluctuationsassociated with the critical point where thiscompeting order vanishes at T = 0 in the temperaturevs. doping phase diagram.T. Wu, H. Mayaffre, S. Krämer, M. Horvatić, C. Berthier, M.-H. JulienR. Liang, W. N. Hardy, D. A. Bonn (University of British Columbia, Vancouver))49

METALS, SUPERCONDUCTORS... 2011Nuclear magnetic resonance study of (EDT-TTF-CONMe 2 ) 2 BrLow dimensional organic conductors are valuable forunderstanding Mott-Hubbard physics, as the Coulombcorrelations in these systems are strong and their effectis further enhanced by the low-dimensionality of theircrystal and electronic structures. Subtle changes inthe structure can lead to a radical modification of theelectronic ground state ranging from insulator to superconductor.The fragile balance between the on-siteCoulomb interaction and the transfer integral can betuned easily by physical or chemical pressure. The organicconductors contain molecules with unpaired carriersin π-molecular orbitals with an open shell configuration.Strong molecular overlap in some preferreddirections allows the delocalization of the π electronsin these directions, which provides their quasi-1D orquasi-2D character. However, when the localizationis promoted, in particular for the quasi-1D systems, a1D antiferromagnetic (AF) Heisenberg spin chain canbe generated. An important characteristic of thesesystems is the commensurate band filling. In suchfilled systems 1/4- and 1/2-umpklapp electron scatteringprocesses play an important role in the electronlocalization. However, the relative importance of theseprocesses in the localization is still debated either in favorof the 1/2- [Emery et al., Phys. Rev. Lett. 48,1039 (1982)], or the 1/4-umklapp process [Giamarchi,Physica B. 230, 975 (1997)].FIG. 65. Left: Organic molecule and their 3D stacking inEDTBr. The carbon site enriched with 13 C is indicated by‘∗’. Right: Evolution of the 13 C NMR spectra as a functionof temperature at 5.5 kbar and 9 T.The prototypical quasi 1D conductors, e.g.(TMTTF) 2 X (TMTTF = tetramethyltetrathiafulvalene,X = Br, PF 6 , etc.), are slightly dimerizeddue to non-uniform stacking, which makes them effectivelyhalf-filled with the presence of both 1/2-and 1/4-umklapp processes. Here we focus on (EDT-TTF-CONMe 2 ) 2 X (X = AsF 6 , Br), which is a genuinequarter-filled system due to its uniform stacking [Heuzeet al., Adv. Mater. 15, 1251 (2003)].We performed 13 C Nuclear Magnetic Resonance(NMR) measurements in 13 C enriched (EDT-TTF-CONMe 2 ) 2 Br (henceforth called as EDTBr) underpressure up to 9 kbar, and at an external magneticfield (H 0 ) of 9 T. A single crystal of 0.05×0.3×3 mm 3size was placed in an NMR micro coil, made from50 µm Cu wire and fixed with epoxy in an epoxyholder. The whole assembly was put into a Cu-Bepressure cell with “Fluorinert” as the pressure transmittingmedium.FIG. 66. Phase diagram of EDTBr. The solid lines representthe phase diagram determined by resistivity and 1 HT 1 measurements [Auban-Senzier et al., Phys. Rev. Lett.102, 257001 (2009)]. The points are obtained from thiswork (see text), and the dashed lines are guide to eye.The unit cell of EDTBr has only one inequivalent 13 Csite when H 0 is orthogonal to the stacking axis. Inthe charge-ordered (CO) insulator state (figure 66),because of the charge disproportionation in the COstate, there are two types of 13 C sites with differentchemical shifts: one near the charge rich region, andthe other near the charge depleted region. At higherpressure, the system undergoes a phase transition tothe metallic state where these two type of 13 C sitesbecome equivalent and the spin-lattice relaxation rateT1 −1 follows the Korringa law. The charge-order transitionin the phase diagram was determined from theonset of the NMR line splitting (□ in figure 66). However,on lowering the temperature in the CO state, thesplitting reaches a maximum, before the lines graduallybroaden and collapse (see figure 65). This mayindicate an onset of a new phase (◦ in figure 66). Below6.5 kbar, in the CO state at low temperatures theT1 −1 shows a maximum, which we attribute to the AFfluctuations related to a phase transition to the AFphase (⋆). Above 6.5 kbar, outside the strongly localizedregime, we observe a pronounced maximum in theT1 −1 at low temperatures (△) indicating the presenceof strong spin fluctuations. This peak appears well belowthe transition seen by transport measurements andchallenges us on the nature of the ground state.S. Mukhopadhyay, H. Mayaffre, M. Horvatić, C. BerthierC. Mézière, P. Batail (UMR 6200 CNRS / Univ. Angers), P. Auban-Senzier, C. R. Pasquier (UMR 8502CNRS / Univ. Paris-Sud)50

2011 METALS, SUPERCONDUCTORS...Fermi surface reconstruction across the 26 Tesla metamagnetictransition in CeRh 2 Si 2Field-induced metamagnetic transitions are fairly commonin f-electron compounds, both magnetic and nonmagnetic.However, the fate of the Fermi surface andeffective mass across such a transition remains somewhatcontroversial. Theoretically, in non-magneticmaterials, where the f-electrons are itinerant belowthe transition, a drastic change of the Fermi surfaceis expected at the metamagnetic transition due to thelocalization of the f-electrons. The effective mass isexpected to diverge in some cases. In contrast, the f-electrons are localized already at zero field in magneticcompounds. As the f-electrons remain localized abovethe metamagnetic transition, neither the Fermi surface,nor the effective mass is expected to change acrossthe transition in this case at a first glance. However,magnetic order modifies the crystallographic Brillouinzone, which in turn modifies the Fermi surface. Sincethe magnetic order is suppressed above the transition,the original crystallographic Brillouin zone is restored,which leads to the reconstruction of the Fermi surface.It is not quite clear what should happen to the effectivemass in such case.been previously observed well below the metamagnetictransition [S. Araki et al, Phys. Rev. B 64, 224417(2001)]. Some of the observed de Haas-van Alphenfrequencies and their angular dependence are well accountedfor by the 4f-localized band structure calculationsperformed for LaRh 2 Si 2 . In addition, however,there are numerous low frequencies corresponding tosmall Fermi surfaces. These Fermi surfaces originatefrom the magnetic Brillouin zone, which is eight timessmaller in volume than its crystallographic counterpartin this compound. The effective masses are rathersmall being of the order of a few bare electron masses.CeRh 2 Si 2 crystallizes in a ThCr 2 Si 2 -type tetragonalstructure. It becomes antiferromagnetic below T N1 =36 K, where the magnetic moments are arrangedalong the c-axis. Furthermore, its magnetic structurechanges again at T N2 = 24 K. When a magnetic fieldis applied along the crystallographic c-axis, a complextwo-step metamagnetic transition occurs at about 26 Tat low temperatures (figure 67).FIG. 68. Fourier spectra of the dHvA oscillatory signalbelow (upper panel) and above (lower panel) the metamagnetictransition observed in CeRh 2Si 2. The temperatureand orientation of the magnetic field are the same asin figure 67.FIG. 67. Metamagnetic transition in CeRh 2Si 2 as observedin magnetic torque measurements at T = 30 mKand magnetic field applied at 2 ◦ from [001] to [100] direction.The insets show the oscillatory torque both below andabove the transition after subtracting the non-oscillatingbackground.The de Haas-van Alphen oscillations in CeRh 2 Si 2 haveOur de Haas-van Alphen measurements were performedon a high quality CeRh 2 Si 2 single crystal upto 35 T. The measurements revealed two high frequencies,8.6 and 16.3 kT, above the transition (figure 68).These frequencies are in good quantitative agreementwith the 4f-localized theoretical band structure calculationsperformed for a non-magnetic ground state.The frequencies, however, are not observed below thetransition as the corresponding Fermi surfaces are toobig to fit inside the magnetic Brillouin zone.I. SheikinD. Aoki (DSM-INAC-CEA, Grenoble, France), K. Götze (Dresden Pulsed Magnetic Field Laboratory, Dresden,Germany)51

METALS, SUPERCONDUCTORS... 2011Field-induced Lifshitz transition in CeIrIn 5CeIrIn 5 belongs to the Ce 1-1-5 subclass of heavyfermion compounds. It crystallizes in the tetragonalHoCoGa 5 structure, which is derived from cubic CeIn 3intercalated with IrIn 2 layers along the tetragonal c-axis. The low temperature specific heat coefficient ofCeIrIn 5 is strongly enhanced, γ = 750 mJ/K 2 mol. Theground state of the material is non-magnetic.A metamagnetic-like transition was reported to occurin CeIrIn 5 at low temperature near 30 T for the[001] field orientation. The transition is characterizedby a non-linear increase in magnetization and a λ-likeanomaly in the specific heat. The results of the previousresistivity and de Haas-van Alphen (dHvA) effectstudy [Capan et al., Phys. Rev. B 80, 094518 (2009)]are not really conclusive, and the physics of the metamagnetictransition in CeIrIn 5 remains obscure.spectra both below and above the transition for fieldat 10 ◦ from [001]. The FFT spectrum of the oscillationsbelow the transition was obtained from a widefield range. This allowed us to directly resolve a nonlinearsplitting of most of the fundamental frequencies.Since the field range above the transition is limited, theFFT does not provide the same high resolution as belowthe transition. That is why we complimented ouranalysis of the oscillations above the transition by themaximum entropy method (MEM). Most of the fundamentalfrequencies observed below the transition arestill present and are not significantly changed above it.The only notable exception is the highest β 1 -frequencythat disappears entirely above the transition.Figure 69 shows torque as a function of magnetic fieldfor several orientations around the [001] direction. Themetamagnetic transition (indicated by arrows) manifestsitself as a distinct kink. The transition occursslightly below 28 T for field along the c-axis and shiftsto higher field with increasing the angle.FIG. 69. Field dependence of the magnetic torque inCeIrIn 5 is shown around the field-induced metamagnetictransition (indicated by arrows). The curves are verticallyshifted for clarity.While the high frequency dHvA oscillations are barelyvisible on the as measured curves of fig 69, one canclearly see a new low frequency that appears above thetransition even without subtracting the background.For a magnetic field applied at 1 ◦ from the [001] direction,the low frequency value is 367 T. The correspondingeffective mass is huge, being 54 bare electronmasses. This is much higher than the masses reportedfor all the branches observed at lower field. Thus, avery small but extremely heavy pocket of the Fermisurface emerges above the metamagnetic transition.Figure 70 shows the lowest temperature oscillatoryFIG. 70. Spectra of the dHvA oscillations below (a)and above (b) the metamagnetic transition in CeIrIn 5 withmagnetic field at 10 ◦ from the [001] direction. For the oscillationsabove the transition (b), both the spectra obtainedfrom FFT (black) and MEM (red) are shown.Both the emergence of a new small heavy pocket ofthe Fermi surface and a complete disappearance of thehighest dHvA frequency above the transition imply afield-induced Lifshitz transition, which seems to be atthe origin of the metamagnetic transition in this compound.Field dependence of most of the dHvA frequenciesbelow the metamagnetic transition also confirmsthe Lifshitz transition scenario.I. SheikinD. Aoki (DSM-INAC-CEA, Grenoble, France)52

2011 METALS, SUPERCONDUCTORS...Direct observation of spin-dependent effective masses in CeCoIn 5In heavy fermion compounds, theory predicts differenteffective masses for the majority and minorityspan bands. Experimentally, however, such a spinsplittingwith different effective masses is difficult toobserve. CeCoIn 5 is a notable exception where thespin-dependent effective mass was observed at relativelylow field [McCollam et al., Phys. Rev. Lett. 94,186401 (2005)]. The authors performed dHvA measurementsup to 18 T and down to very low temperatureof 6 mK. By fitting the temperature dependenceof the oscillatory amplitude with “spin-dependent”Lifshitz-Kosevich formula, they obtained two differentmasses for spin-up and spin-down bands. It should beemphasized, however, that McCollam et al. have notobserved the spin-splitting of the oscillatory signal directly.The spin-split effective masses were extractedfrom the analysis of the temperature dependence of theoscillatory signal of a single frequency.Our dHvA measurements in CeCoIn 5 were performedat field up to 28 Tesla and temperature down to 40 mK.While the base temperature of our experiment was notnearly as low as that of McCollam et al., a larger fieldrange of our measurements resulted in a much betterdHvA frequency resolution. The oscillatory signal obtainedwith magnetic field applied at 2.5 ◦ from c toa-axis at T = 40 mK is shown in figure 71(a). Thecorresponding Fourier transform for a field range from15 to 28 T is shown in figure 71(b). The splittingof the α 1 -frequency is quite clear without any furtheranalysis. The values of the split frequencies, 5.42 and5.49 kT, are very close to each other and differ by onlyabout 1%.At a first glance, the α 2 peak does not appear to besplit. Its broadening and characteristic asymmetricshape, however, indicate that it consists of two veryclose peaks. This can be seen in the inset of figure 71(b) showing a zoom of the frequencies originating fromthe α sheet of the Fermi surface. Indeed, an attempt tofit the α 2 peak with a double-peak Gaussian function(the red line) produces an excellent result and revealstwo close satellites (the green lines). The frequenciesof the satellites, 4.80 and 4.85 kT, also differ by about1%.FIG. 72. Mass plot for the spin-split frequencies α 1 (red)and α 2 (black) in CeCoIn 5. The orientation of the magneticfield is the same as in figure 71. Open and close symbolscorrespond to the different spin components.FIG. 71. dHvA oscillatory signal (a) and its Fourierspectrum from 15 to 28 T (b) observed in CeCoIn 5 withmagnetic field applied at 2.5 ◦ from [100] to [100] direction.The spin-splitting of the α 1 and β 2 frequencies can be seenclearly. The inset zooms on the frequencies originating fromthe α sheet of the Fermi surface. The characteristic shapeand broadening of the α 2 peak implies that it is composedof two close satellites. The satellites (green lines) are indeedrevealed by fitting the peak by a two-peak Gaussianfunction (red line).Figure 72 shows the mass plot of the split frequenciesα 1 and α 2 from figure 71. The spin-dependent effectivemasses of the α 1 orbit, (11.1 ± 0.3)m 0 and (16.3 ±0.4)m 0 , differ by about 50%. However, the differencebetween the effective masses of the α 2 satellites, α 21and α 22 , (23.4±1.3)m 0 and (9.9±0.3)m 0 is more thana factor of two. This ratio is similar to that found byMcCollam et al.I. SheikinD. Aoki (DSM-INAC-CEA, Grenoble, France)53

METALS, SUPERCONDUCTORS... 2011Fermi-surface evolution in Yb-substituted CeCoIn 5CeCoIn 5 is a heavy-fermion Pauli-limited unconventionalsuperconductor with the highest superconductingtransition temperature for Ce-based compounds ofT c = 2.3 K. The particular interest in the superconductivityof this compound arises from the fact thatunder pressure or chemical substitution the superconductingpair correlations develop in the vicinity of aquantum critical point (QCP). Theories to describesuch a feature are at present highly controversial andneed to be improved. In many strongly correlated electronsystems, hosting a QCP, doping has been used asa tuning parameter for the investigation of quantumcriticality and non-Fermi-liquid behavior. CeCoIn 5 isespecially well suited for such a study, since the stoichiometryis stable and the substitution on the differentlattice sites is possible. The diluting on the rareearthsite has strong influence on the Kondo-latticecoherence and Cooper-pair breaking. However, forYb-substituted CeCoIn 5 the Kondo-coherence temperatureand the superconducting transition temperaturedo not scale. In its divalent state, Yb is expected to actas a non-magnetic dilution destroying the Kondo coherenceand superconductivity. Recent angle-resolvedphotoemission spectroscopy (ARPES) data [Booth etal., Phys. Rev. B 83, 235117 (2011)] and thermodynamicalmeasurements [Shu et al., Phys. Rev. Lett.106, 156403 (2011)] revealed an intermediate valenceof Yb in Ce 1−x Yb x CoIn 5 that might explain the observedunique features.In order to better understand the electronic propertiesof Ce 1−x Yb x CoIn 5 , we performed a systematic studyof the Fermi-surface evolution with Yb substitution, x,via de Haas-van Alphen (dHvA) measurements. Ourfirst dHvA data were obtained at the HLD by useof a dilution refrigerator offering a base temperatureof 20 mK and magnetic fields up to 20 T. The measurementsperformed on a sample with 10%Yb concentration,x = 0.1, clearly revealed that even higherfields are required in order to resolve a more completeFermi-surface picture. For the sample with x = 0.2 thefield was not sufficient to observe any quantum oscillations.Applying magnetic fields up to 35 T at LNCMI-Grenoble allowed us to resolve many dHvA frequenciesfor both concentrations.In figure 73 (a) and (b), the measured angular dependenceof the dHvA frequencies for the Yb concentrationsx = 0.1 and 0.2 are shown. From that, someprincipal very important features of the Fermi-surfacetopology could be extracted. For instance, the lowfrequencydHvA oscillations labeled as γ and ɛ havebeen detected by rotations around the b as well asaround the c axis. These branches correlate with smallclosed three-dimensional Fermi surfaces. By rotatingthe samples away from the c axis we observed a1/ cos(Θ)-like behavior for the frequencies α 1 , α 2 , andα 3 which arise from one band and can be interpreted asthe signature for a strongly corrugated Fermi cylinder.The β branch can be assigned to an orbit originatingfrom a complicated multiconnected Fermi surface. Forthe measured samples, the well-known Fermi surfacesof CeCoIn 5 (x = 0) have changed only slightly.The cyclotron effective masses have been determinedfor both compounds from the temperature dependenceof the dHvA oscillations. For x = 0.1, the observedheavy effective masses remain strongly renormalizeddue to many-body interaction. Contrary, for x = 0.2the effective masses are reduced considerably to valuesbetween 1.8 and 16 times the free electron mass.(a)(b)FIG. 73. Angular dependence of the dHvA frequenciesfor Ce 1−xYb xCoIn 5 with Yb concentrations of x = 0.1 (a)and x = 0.2 (b) measured at HLD (closed symbols) andat LNCMI-Grenoble (open symbols). The solid lines areguides for the eyes.I. SheikinO. Ignatchik, A. Polyakov, B. Bergk (Dresden High Magnetic Field Laboratory, Dresden, Germany)54

2011 METALS, SUPERCONDUCTORS...Interplay of magnetism, Fermi surface reconstructions, andhidden-order in the heavy-fermion material URu 2 Si 2URu 2 Si 2 is surely one of the most mysterious of theheavy-fermion compounds. Despite more than twentyyears of experimental and theoretical works, the orderparameter of the transition at T 0 = 17.5 K is stillunknown. The state below T 0 remains called “hiddenorder phase” and the stakes are still to identify the energyscales driving the system to this phase. In a highmagnetic field applied along the easy-axis c, the hiddenorder of URu 2 Si 2 is destabilized and is replaced bya polarized paramagnetic regime above 39 T, after acascade of first-order transitions between 35 and 39 T[Sugiyama et al., J. Phys. Soc. Jpn. 68, 3394 (1999),Kim et al., Phys. Rev. Lett. 91, 256401 (2003)].We have performed a new set of magnetoresistivityand magnetization measurements on high-quality singlecrystals in pulsed magnetic fields up to 60 Tesla.Several samples of different qualities have been investigated,the magnetic field being applied along the maincrystallographic axes a and c and temperatures beingset between 1.5 and 65 K. Transversal and longitudinalmagnetoresistivity experiments and magnetization experimentswith the compensated-coil and torque techniqueshave been carried out.From the combination of magnetoresistivity and magnetizationdata, we extract the magnetic field - temperature(H, T ) phase diagram for H ‖ c shown infigure 74, for the first time in the extended scales [0-55 T; 0-60 K]. We show that the quantum critical area[35 T-39 T] is initiated by the vanishing of a crossovertemperature, which reaches 40-50 K at zero-field. Thiscrossover, which probably results from inter-site correlations,is a precursor of the hidden order phase.The orbital effect in the magnetoresistivity has beenstudied by comparing samples of different purities. Astrongly-sample-dependent contribution to the magnetoresistivityρ x,x , which is related to the field-inducedcyclotron motion of the conducting electrons, peaks at29 T at low temperature (figure 75). Indeed, the maximalvalue of ρ x,x is bigger when the sample qualityis higher and it becomes almost sample-independentat temperatures above 6 K (see Inset of figure 75).At low-temperature, the strong decrease of ρ x,x above29 T indicates that the Fermi surface is modified whenapproaching the polarized paramagnetic regime.FIG. 74. Magnetic field-temperature phase diagram ofURu 2Si 2, for H ‖ c, extracted from our magnetization andmagnetoresistivity measurements.FIG. 75. High field transversal magnetoresistivity at T =1.5 K of three samples of different qualities of URu 2Si 2(samples ♯1 and ♯2 studied here, and a third sample studiedin Levallois et al., EPL 85, 27003 (2009). The inset showsthe magnetoresistivity of samples ♯1 and ♯2 at T = 6 and40 K.Our study permitted to emphasize the interplay betweenthe magnetic properties and that of the Fermisurface and the necessity to use a dual “localizeditinerant”description of the f-electrons for a futureunderstanding of hidden order in URu 2 Si 2 .G. Scheerer, W. Knafo, G. Ballon, D. VignollesD. Aoki, J. Flouquet (DSM-INAC-CEA, Grenoble)A. Mari (LCC, Toulouse)55

METALS, SUPERCONDUCTORS... 2011Gap like structure in a nonsuperconducting layered oxycarbonateBi 2+x Sr 4−x Cu 2 CO 3 O 8+δ single crystalIn the normal state of underdoped high-T c superconductorsthere is a pseudogap state, in which the densityof states is depleted upon decreasing temperaturebelow a characteristic temperature T ∗ ≫ T c . Whetherthe pseudogap state is the precursor to superconductivityor a state that competes with superconductivity hasbeen a matter of long-standing debates. We attemptto clear up this question and to find the pseudogap in anonsuperconducting cuprate without superconductingfluctuations or local incoherent pairs and to study theeffect of the magnetic field on this gap using the tunnelingmethod. Measurements were performed on layeredoxycarbonate Bi 2+x Sr 4−x Cu 2 CO 3 O 8+δ single crystals.The crystals were mirror-smooth faceted plates withthe most developed ab planes.FIG. 76. (a)Temperature dependence of the in-plane ρ abresistivity for the oxycarbonate single crystal. The datapoints show ρ ab for various magnetic fields (H ‖ c). Theinset plots ρ ab (H) for the same single crystal at varioustemperatures with H ‖ c. (b) Tunneling conductancesdI/dV (V ) for the tunnel break junction at T = 1.8 K forvarious magnetic fields H ‖ c. The curves have been shiftedvertically relatively the lowest curve.The actual cationic composition of the used sampleswas measured at 50 − 70 different points on the crystaland the scatter in the data was less than 2%. TheX-ray diffraction analysis confirmed the presence ofa single phase in our single crystals. The half-widthof the X-ray rocking curves for the crystals was lessthan 0.1 ◦ . These data demonstrate the high structuralquality and high homogeneity of the samples on a microscopicscale. We were able to make superconductingand nonsuperconducting single crystals by varyingthe Bi content and consequently the doping level.Figure 76(a) displays the temperature dependence ofthe in-plane ρ ab resistivity for the oxycarbonate singlecrystal, with a logarithmic scale for the horizontal axis.The data points show the resistivity data at H = 10 Tand H = 16 T (open and closed circles, respectively)with the magnetic field H ‖ c -axis. At zero magneticfield, the sample shows an insulating behaviorand remains in its normal state down to 10 mK. Atultra low temperatures, T = 10 − 70 mK, ρ ab showsa downward deviation (saturation) from the insulatingbehavior. A similar behavior of the in-plane resistancehas been observed by us in Bi2201 in normal state. Theinset in Fig. 76(a) plots the transverse in-plane magnetoresistanceρ ab (H) for the same single crystal atvarious temperatures from 10 mK to 0.5 K with H ‖ c.At ultra low temperatures, the magnetic field dependenceshows a small negative magnetoresistance andbecomes positive with increasing temperature above0.3 K. The observed magnetoresistance behavior of theoxycarbonate single crystal is in complete agreementwith that of the negative transverse in-plane magnetoresistancereported in investigations of nonsuperconductingBi2201 single crystals [Jing et al. Phys. Rev.Lett. 67, 761 (1991)]. The authors explained theseresults by localization and the gradual suppression oflocalization effects by the magnetic field. Since our resultsare in agreement with these experiments, it is reasonableto assume that they have the same physical origin.Figure 76(b) displays the tunneling conductancesdI/dV (V ) for the tunnel break junction at T = 1.8 K.For clarity, the curves have been shifted vertically relativelythe lowest curve. It is significant that there areno changes in the dI/dV (V ) curves with magnetic field,notably the gap like structure does not change either inshape or in the voltage position (dashed vertical line).It is simply not possible to relate the observed gap likestructure in our dI/dV spectra to the superconductinggap: Firstly, because any “tails” of superconductivityin sample are lacking, Secondly, the voltage position ofthe gap peak in dI/dV (V ) remained unchanged in theapplied magnetic fields, whereas, the superconductinggap peak must shift to lower voltages with increasingfield. It seems reasonable to assume that the gap likestructure observed here in the dI/dV spectra is associatedwith the proper pseudogap in the electronic densityof states, which exists in our nonsuperconductingcuprate. In this case such a pseudogap is either unrelatedto superconductivity at all or the existence ofsuch a pseudogap is a necessary condition for the subsequentoccurrence of superconductivity with increasingcarrier density in the sample.D. K. Maude, B. A. PiotS. I. Vedeneev (P.N. Lebedev Physical Institute, Russian Academy of Science, Moscow, Russia)56

2011 METALS, SUPERCONDUCTORS...Extended criticality in the electrical resistivity of La-doped Bi2201The phase diagram of high temperature superconductorshas intrigued condensed matter physicists aroundthe globe for over two decades now but many outstandingissues remain, including: (i) Is there a quantumcritical point at some location in the phase diagram?(ii) If so, do quantum fluctuations associatedwith proximity to the quantum critical point dominateboth the normal state and superconducting statephysics of the cuprates as they do in other quantumcritical systems? (iii) Is there a relation between theclosing of the pseudogap and quantum criticality? (iv)Is there a link between superconductivity and the normalstate resistivity as there is in conventional superconductors?In 2008, we measured the in-plane electrical resistivityρ ab (T ) of the high temperature superconductorLa 2−x Sr x CuO 4 (LSCO) at the LNCMI-T in Toulouse,concentrating on the limiting low-T behaviour insidethe superconducting dome. These measurements established,for the first time, the presence of a criticalpoint in the electrical resistivity at a doping levelp c = 0.19 that is consistent with other bulk measurements[Cooper et al., Science 323, 603 (2009)]. Moreimportantly, our data revealed that the electrical resistivityin cuprates displays critical behaviour, characterizedby an extended region of T -linear resistivityover a broad region of the phase diagram, that wasunlike any known quantum critical system and thusestablished transport in cuprates as a new paradigmin the field of strongly correlated electron physics. Intriguingly,the magnitude of the T -linear componentin the electrical resistivity appeared to scale with T cimplying a correlation between the superconductivityand this anomalous scattering term.In order to explore the possibility that this ‘extendedcritical behaviour’ was generic to all cuprates, andlinked to the proximity to the Mott insulator, ratherthan to a van Hove singularity, we decided to repeatour study on a related single-layer cuprate famiily LadopedBi 2 Sr 2 CuO 6+δ (La-Bi2201). Single crystals ofLa-Bi2201 with p values ranging from 0.13 to 0.29 wereprocured through collaboration with Prof. Takeuchifrom Sendai University, Japan and had an importantadditional advantage that they had already been thoroughlyexplored by angle-resolved photoemission spectroscopy(ARPES) [Kondo et al., Nature 457, 296(2009)], thus allowing a study of the evolution of theFermiology and the transport properties in the samematerials. Our measurements were carried out bothin-house and at the LNCMI-T in Toulouse, in pulsedfields up to 60 Tesla, in order to destroy the superconductivityand probe the resistivity down to as low atemperature as possible.Figure 77 shows a comparison of the doping evolutionof the temperature-dependent coefficients of ρ ab (T ) inLSCO (Green) and La-Bi2201 (Blue). The top panelshows the doping dependence of α 1 , the coefficient ofthe T -linear resistivity component, while the bottompanel shows the doping dependence of α 2 , the coefficientof the T 2 resistivity component. In both cases,the coefficients are obtained from least-square fits ofthe ρ ab (T ) curves for T < 200 K to the expressionρ ab (T ) = α 0 +α 1 T +α 2 T 2 . The evolution of the two coefficientsis remarkably similar in the two families, confirmingthat this transport behaviour is indeed genericto the cuprates and is independent of Fermiology.FIG. 77. (Top panel) Doping dependence of α 1, the coefficientof the T -linear resistivity component. (Bottom panel)Doping dependence of α 2, the coefficient of the T 2 resistivitycomponent. In both panels, solid squares are coefficientsobtained from least-square fits of the ρ ab (T ) curves for T

METALS, SUPERCONDUCTORS... 2011Fermi surface reconstruction by stripe order in cupratesuperconductorsThe observation of quantum oscillations and negativeHall effect in the cuprate superconductor YBa 2 Cu 3 O y(YBCO) convincingly showed that the Fermi surface inunderdoped cuprates near a doping p of 1/8 includesa small electron pocket [Doiron-Leyraud et al., Nature447, 565 (2007) and LeBoeuf et al., Nature 450, 533(2007)], in stark contrast with the large hole Fermisurface seen in overdoped cuprates. Because a smallelectron Fermi surface is not part of the band structureof YBCO, its origin must be traced back to a Fermisurface reconstruction caused by the appearance of adensity-wave order.The nature of that density-wave order and its relationto high temperature superconductivity are currentlythe subjects of fierce debates. We recently examinedthis question through a series of experiments performedat the Grenoble high-magnetic field laboratory,from which we conclude that stripe order is the causeof Fermi surface reconstruction in hole-doped cuprates[Laliberté et al., Nat. Commun. 2, 432 (2011)].In figure 78, we show the Seebeck coefficient of YBCOand Eu-LSCO as a function of temperature and fordifferent dopings, in the normal state reached by theapplication of a large magnetic field (28 T). As seen inboth materials, the Seebeck coefficient goes from positiveat high temperature to negative at low temperature,this being a signature of a high-mobility electronpocket in the Fermi surface. The temperature at whichthe sign change occurs depends on doping. At p = 0.08,however, the Seebeck coefficient is always positive andno sign change is observed in both materials, showingthat the electron pocket has disappeared.In figure 79 we show the sign change temperature T0S ofthe Seebeck coefficient as a function of doping. In bothmaterials, this quantity defines a dome centered at p= 1/8. The striking similarity between the two materialsis compelling evidence that the Fermi surfacereconstruction has a common root. In Eu-LSCO thisis clearly caused by stripe order, a form of incommensuratespin and charge density-wave, which has beendetected in the charge channel at a temperature T CO(shown in figure 79).By analogy, Fermi surface reconstruction in YBCOmust also be caused by stripe order. Recent NMRexperiments in high-magnetic field by the group ofMarc-Henri Julien in Grenoble have recently identifiedcharge-stripe order in YBCO at p = 0.10 and 0.12,confirming our interpretation.FIG. 78. Seebeck coefficient S plotted as S/T as a functionof temperature for YBCO and Eu-LSCO at differentdopings as shown. High magnetic fields were necessary toreach the normal state at low temperature (dots are 28 Tdata, squares are zero field data).FIG. 79. Temperature-doping phase diagrams of YBCOand Eu-LSCO, showing the Seebeck and Hall effect signchange temperatures T0 S (red dots) and T0 H (blue circles),respectively. For Eu-LSCO we show the charge-order temperatureT CO (diamonds) as measured by X-rays [Fink etal., Phys. Rev. B 83, 092503 (2011)].For further information see LeBoeuf et al., Phys. Rev.B 83, 054506 (2011) and Chang et al., Phys. Rev. B84 014507 (2011).L. Malone, I. Sheikin, C. ProustJ. Chang, N. Doiron-Leyraud, F. Laliberté, R. Daou, E. Hassinger, L. Taillefer (Physics Department,University of Sherbrooke, Canada), K. Behnia (ESPCI, Paris) B. J. Ramshaw, R. Liang, D. Bonn, W.Hardy (Department of Physics and Astronomy, University of British Columbia, Vancouver) S. Pyon, T.Takayama, H. Takagi (Department of Advanced Materials, University of Tokyo, Kashiwa, Japan)58

2011 METALS, SUPERCONDUCTORS...Mapping the phase diagram of high-T c superconductorsStarting in 2007, the observation of quantum oscillationsin the underdoped cuprate YBa 2 Cu 3 O 6.51and overdoped cuprate Tl 2 Ba 2 CuO 6+δ together withnegative Hall and Seebeck coefficients in underdopedYBa 2 Cu 3 O 6+δ have suggested that a Fermi surface reconstructionof the large hole pocket (observed in theoverdoped side and in agreement with band structurecalculation) into small electron pocket(s) occurs withinthe underdoped side of the phase diagram. In order toget more insight on this reconstruction mechanism, theevolution of different properties (quantum oscillationsfrequency and irreversibility field) upon doping havebeen studied. First of all, the oscillatory part of theout of plane magnetoresistance R c , the frequency ofwhich is related to the size of the Fermi pocket, hasbeen measured. This oscillatory signal becomes verysmall as the doping level departs from high qualityoxygen ordered YBa 2 Cu 3 O 6.51 . While there is a subtlechange in the frequency of oscillations, a carefulanalysis by indexing the maximum of the oscillationsversus 1/B yields the doping dependence of the frequencyshown in figure 80.as a smooth crossover rather than a real phase transition.In that respect, the experimental determinationof the upper critical field have been the subjectto intensive work and is still under debate. As a contrary,determination of irreversibility field is a straightforwardprocedure as it corresponds to the hysteresisending point on magnetization curve. Magnetizationfor different doping has been measured with the cantilevermethod. Doping dependance of irreversibilityfield is plotted in figure 81. A clear minimum is evidencearound p ∼ 1/8.FIG. 81. Doping dependence of irreversibility field (symbols)and T c (blue line) for YBa 2Cu 3O 6+δ .FIG. 80. Doping dependence of the frequency of quantumoscillations for YBa 2Cu 3O 6+δ . The red point atp = 0.14 corresponds to in-plane resistivity measurementsin YBa 2Cu 4O 8.This experiment shows that there is a clear trend ofincreasing frequency as the doping increases, that is tosay the size of the small Fermi pocket slightly increaseswhen the system is doped. This point puts some constraintson the different Fermi surface reconstructionscenarios.Besides the evolution of the oscillation frequency, wehave also followed the evolution of the irreversibilityfield. Due to the importance of thermal fluctuation,the transition between the (vortex liquid) superconductingstate and the normal state can be regardedThis minimum lies in the same doping range as the60 K plateau observed in the doping dependance of T c(blue line - right scale of figure 81). Our observation istherefore a confirmation of a weakness of superconductivityat 1/8 in YBa 2 Cu 3 O 6+δ . This weakness could bedue to a stripe phase that is stabilized at this particulardoping level due to a commensurability betweenthe charge and the lattice periodicities (1 charge every4 lattice constants). Until recently, such a stripe phasehave been only identified in some other members of thecuprates family (namely La 2−x Ba x CuO 4 , Nd and Eudoped La 2−x Sr x CuO 4 ).In a recent high magnetic field copper NMR experimentin YBa 2 Cu 3 O 6.54 Wu et al., (Nature 477, 191(2011)) have succeeded to evidence a splitting in theNMR spectrum which appears below ∼ 50 K for a fieldof 28.5 T. This result has been interpreted as the consequenceof a small and unidirectional charge densitymodulation (periodicity of 4a) without any spin modulation.This first evidence for a charge (stripe) orderin YBa 2 Cu 3 O 6.54 is consistent with the present measurementsof the irreversibility field and will help tounderstand the mechanism responsible for the Fermisurface reconstruction.S. Lepault, S. Badoux, D. LeBoeuf, B. Vignolle, D. Vignolles, C. ProustB. J. Ramshaw, R. Liang, D. A. Bonn, W. N. Hardy (University of British Columbia, Vancouver, Canada)59

METALS, SUPERCONDUCTORS... 2011The upper critical magnetic field of (Ba 0.68 K 0.32 )Fe 2 As 2 andBa(Fe 0.93 Co 0.07 ) 2 As 2 single crystalsAlthough the Fermi surface of superconducting ironpnictides are definitely quasi-two-dimensional, data regardinganisotropy of the upper critical field, H c2 ,are quite puzzling. To get more information aboutthis problem, temperature dependence of H c2 for thehole-doped (Ba 0.68 K 0.32 )Fe 2 As 2 and electron-dopedBa(Fe 0.93 Co 0.07 ) 2 As 2 have been measured up to 60 Tboth for magnetic field parallel and perpendicular tothe conducting plane. Given the very small samples resistance,a contact-less tunnel diode oscillator (TDO)-based measurement technique was used [Drigo et al.,Eur. Phys. J. Applied Phys. 52 10101 (2010)] inwhich, at first order, frequency variations are proportionalto the London penetration depth.good single crystals quality. The deduced temperaturedependence of H c2 is reported in figure 84. Inbrief (for more details, see [Gasparov et al., JETP Letters93 667 (2011)]), the temperature dependence ofH c2 for (Ba 0.68 K 0.32 )Fe 2 As 2 is accounted for by theconventional Werthamer Helfand Hohnenberg (WHH)model including Pauli paramagnetism contribution forboth H parallel and H perpendicular to the conductingplane. Best fits yield the Pauli limit H P = 134 T,in good agreement with ARPES data. The deducedcoherence length is higher than the interlayer spacingindicating a 3D superconductivity.FIG. 82. Magnetic field dependence of the TDO frequencyfor (Ba 0.68K 0.32)Fe 2As 2 with magnetic field appliedperpendicular to the conducting plane (H ‖ c).FIG. 84. Temperature dependence of the uppercritical fields, H c2, for (a) (Ba 0.68K 0.32)Fe 2As 2 and (b)Ba(Fe 0.93Co 0.07) 2As 2 for H‖ c (circles) and H⊥ c (squares).Solid squares stand for 4-point resistance measurements.Dotted and dashed lines are best fits of the WHH modelneglecting Pauli-limiting. Solid lines in (a) include a Paulipair breaking contribution with only one free parameter,H P = 134 T, for both field orientations. The solid curvein (b) is a two-band fit. Insets display the temperaturedependentanisotropy of γ = H c2(H‖ c) / H c2(H⊥ c).FIG. 83. Magnetic field dependence of the TDO frequencyfor Ba(Fe 0.93Co 0.07) 2As 2 with magnetic field applied perpendicularto the conducting plane (H ‖ c).Examples displayed in figures 82 and 83 evidence narrowsuperconducting transitions, in line with the veryIn contrast, Pauli paramagnetic pair breaking can beneglected for Ba(Fe 0.93 Co 0.07 ) 2 As 2 . For this electrondopedcompound, the data support a temperature dependenceof H c2 that can be accounted for by the WHHmodel and by a two-gap behaviour for H parallel andH perpendicular, respectively, to the conducting plane.L. Drigo, A. AudouardV. A. Gasparov (ISSP, Chernogolovka, Russia), D. L. Sun, C. T. Lin (MPI, Stuttgart, Germany), S. L.Budko, P. C. Canfield (Ames Laboratory, Iowa, USA), J. Wosnitza (HLD, Dresden, Germany)60

2011 METALS, SUPERCONDUCTORS...Fermi surface of 111 iron pnictide superconductors: LiFeP, LiFeAsThe purpose of our experiment was to study the Fermisurface of the ‘111’ iron-based superconductors, LiFeX(X=As,P) which are two of only a few iron-based superconductorswhich superconduct at ambient pressurein their undoped stoichiometric form. Unlike the analogous‘1111’ compounds (LaFeAsO, LaFePO), hereboth LiFeAs with T c = 18.5 K and LiFeP with T c = 5 Ksuperconduct. Also it has been found that LiFeP hassuperconducting gap nodes whereas LiFeAs does not.So these materials provide a key test of microscopictheories which seek to explain superconductivity iniron pnictides. The idea is to develop a unified theorylinking the Fermi surface parameters to the superconductinggap structure. The calculated Fermi surfacesof LiFeAs and LiFeP are very similar so withoutincluding the many body physics there is no obviousreason why their superconducting properties should beso different. Experimental determination of the differencesin actual Fermi surfaces should therefore helpto highlight the interactions which are important forsuperconductivity.de Haas-van Alphen (dHvA) data for both compoundswhich allowed us to determine the bulk Fermi surfaces.These pulsed field measurements were supplementedwith later measurements in dc field in Nijmegen andTallahassee which allowed us to determine more preciselythe effective masses and also the field angle dependenceof the dHvA frequencies with higher precision.The combined results have allowed us to determinealmost completely the Fermi surface of LiFeP.The topology is in good agreement with band structurecalculations but the effective mass enhancementvaries significantly between sheets, being anomalouslysmall for one of the hole bands. We think this is causedby the mixed orbital character of this sheet and is likelythe driving the nodal superconducting pairing state inthis materials. For LiFeAs we were only able to observethe electron Fermi surface sheets but these hada very high mass enhancement, much larger than forLiFeP. This establishes the trend that the same interactionsdriving the mass enhancement are also responsiblefor superconductivity as LiFeAs has a higher massenhancement and T c compared to LiFeP. The observedsheets in LiFeAs were in very good agreement withband structure. The results were reported in [Putzkeet al., ArXiv/1107.4375].FIG. 85. dHvA torque oscillations for LiFeP and LiFeAs(top panels) at T = 1.5 K measured in pulsed field. FFTs ofthis data showing multiple extremal orbits (peaks in FFT)are shown in the bottom panels.During an experimental run in Toulouse in Februaryand May 2011, we were able to obtain high qualityFIG. 86. (a) Density function theory (DFT) calculationsof the Fermi surface of LiFeP. (b) Experimental dHvA frequency(F cos θ) vs. field angle (θ) for LiFeP and LiFeAs.Triangle = pulsed field data, other symbols = dc field data(Tallahassee). θ = 0 corresponds to B‖c. The solid linesare calculations based on the DFT Fermi surfaces withsmall band energy shifts.D. Vignolles D. LeBoeufC. Putzke, I. Guillamón, A. Carrington (University of Bristol, UK), A.I. Coldea, M.D. Watson (Universityof Oxford, UK) , A. McCollam (HMFL, Nijmegen, The Netherlands), I.I. Mazin (NRL, Washington, USA)S. Kasahara, T. Terashima, T. Shibauchi, Y. Matsuda (Kyoto University, Japan)61

Magnetic Systems

2011 MAGNETIC SYSTEMSNMR study of the “pinwheel” kagome compound Rb 2 Cu 3 SnF 12Kagome Heisenberg antiferromagnetic lattice has forlong been the focus of numerous research topics. Itshigh frustration brings about many phases as a possibleground state of both quantum and classical considerationsof the system, such as various spin liquids,valence bond crystals, √ 3 × √ 3 order, etc. InRb 2 Cu 3 SnF 12 , Cu 2+ spins (S = 1/2) form a distortedkagome lattice where, although the magneticcouplings differ in magnitude, the ground state of thesystem remains non-magnetic. The symmetry of thesystem is hexagonal (R¯3), and the distortion can beseen as 6 elongated kagome hexagons surrounding aregular one. This modifies the bonding angles of theCu 2+ −F − −Cu 2+ bonds to 4 different values whichin turn creates 4 different exchange couplings. Thestrongest coupling (J 1 = 222 K) causes pairs of spins toform singlet dimers in a “pinwheel” pattern around thekagome hexagon, and the remaining three couplings(J 2 = 0.95J 1 , J 3 = 0.85J 1 , J 4 = 0.55J 1 ) define the energystructure (dynamics) of the system. The groundstate is thus a valence bond solid (VBS) in which thesinglet state is separated from the triplet by a spin gap∆ = 28 K ≈ J 1 /10, strongly renormalized by similarfrustrating nearest-neighbour couplings. The applicationof magnetic field above H C = 13 T (perpendicularto the kagome planes) apparently starts to fill thetriplet band, which is observed as a strong increaseof magnetization. As is always the case in kagomelattices, a Dzyaloshinskii-Moriya (DM) interaction betweenthe spins is present, which perturbes the symmetricHeisenberg interaction and broadens the transitionat H C .We have performed a 63,65 Cu NMR study on a singlecrystal of Rb 2 Cu 3 SnF 12 (size 4 × 3.5 × 1 mm 3 ),for magnetic fields high enough to observe the systemcrossover towards the high-field regime. The coppersites are situated in the center of CuF 6 octahedrafor which the Cu 2+ ion ground state wave function isd x2 −y2, z being the tetragonal axis. In this configurationtypical values of the NMR hyperfine couplingsare known. In Rb 2 Cu 3 SnF 12 , the octahedra are tiltedaway by roughly ±11 ◦ from the c axis (perpendicular tothe kagome plane), so the ratio of hyperfine couplingsin parallel (‖ c) and perpendicular (⊥c) orientationswill come close to one order of magnitude. An NQRstudy [Tashiro et al., J. Phys.: Conf. Ser. 320, 012052(2011)] showed there are 4 different quadrupolar frequencies,all in the frequency range 48 - 55 MHz. Usingthese two informations, we were able to interpretthe 63,65 Cu NMR spectra measured at several magneticfield values, see figure 87. Because of the largequadrupolar couplings, the 63,65 Cu Zeeman states arestrongly perturbed which shifts the frequencies of centraltransitions. Therefore, it was necessary to performa direct-diagonalization analysis in order to resolvethe central lines and accompanying satellites, but alsoto compensate for the quadrupolar shifts and isolatethe contribution of the spin polarizations. Throughsuch analysis, and comparison to the measurements ofthe macroscopic susceptibility, we found that the spectracan only be accounted for if there exist positivelyand negatively polarized sites. This indicates that theground state of the system is more complex than theone based on “simple” singlet dimer states.FIG. 87. Complete spectrum of copper at 11.57 T, overlappingwith a set of 87 Rb lines.27 Al signal was used forprecise determination of the magnetic field.In order to follow the behaviour of singlet-triplet gapwith the magnetic field we performed measurements ofthe nuclear spin-lattice relaxation rate T1 −1 . Throughit we probe the spin fluctuations which determine thelow energy excitations of the VBS state. Keeping themagnetic field constant, the temperature dependenceof T1 −1 below 6 K showed an activated e −∆/T behavior,allowing us to determine the value of the singlet-tripletenergy gap. As the field is raised towards 13 T, the gapvalue is seen to be decreasing, and passing through abroad minimum after which it increases again at higherfields. The large residual gap value ∆(H C ) ≈ ∆(0)/2is a clear evidence for anticrossing of singlet-triplet energybands, which calls for the revision of the nature ofa DM vector proposed by a recent neutron scatteringstudy. Still, further experiments at higher magneticfields are required in order to fully determine the completefield dependence of the gap and resolve the behaviorof the local spin polarizations beyond the criticalfield.M. S. Grbić, M. Horvatić, C. BerthierH. Tanaka (Department of Physics, Tokyo Institute of Technology, Tokyo, Japan)65

MAGNETIC SYSTEMS 2011NMR study of the Bose-Einstein condensation in DTNIn a number of antiferromagnetic quantum spin systemsthe magnetic field (H) induced low-temperature3D ordered phase is associated to a Bose-Einstein condensation(BEC) of magnons. The hallmark of thephenomenon is considered to be the shape of the phaseboundary close to the critical field (H c ), T c (H) ∝|H c − H| 2/3 , but there are other requirements whichseverely reduce the number of systems that are reallyrepresentative of BEC. Such system should have rotationalsymmetry with respect to the applied magneticfield, the spin gap should close to zero at H c , and remainzero in the BEC phase. One apparently idealBEC system is the S = 1 spin chain compound NiCl 2 -4SC(NH 2 ) 2 , abbreviated as DTN, when the magneticfield is applied along chains (c axis), which is thehard axis of the single-ion anisotropy of the spins.DTN has moderately high upper critical field value,H c2 = 12.3 T, and its 3D ordered phase extends upto Tcmax = 1.2 K, so that a standard dilution refrigeratorprovides convenient temperature (T ) range downto Tcmax /20. In our previous study of DTN we haveinvestigated the critical behaviour of the spin dynamicsby 1 H NMR measurements close to H c and in the1D regime at temperatures well above Tcmax . Here wepresent the low-temperature study of DTN, in order tocharacterize its BEC phase.The phase boundary of the BEC phase has been determinedfrom the NMR line splitting, induced by thecanting of the spins in the 3D ordered phase, which isfurthermore found to be concomitant with the peakin the T2 −1 rate related to the critical fluctuations.In contrast to a previous observation at H c2 by acsusceptibilitymeasurements, our NMR data are consistentwith the expected (H c2 − H) 2/3 dependence.Extrapolating the data to T = 0 we obtained the preciseexperimental H c2 value, which is an important referencepoint for the understanding of the T1 −1 relaxationrate data reflecting the spin dynamics. Figure 88presents the low temperature T1 −1 data taken outsidethe BEC phase. At fields above H c2 the temperaturedependence is found to be activated and the fits tothe data allowed a precise gap determination at severalmagnetic field values. On decreasing the field thegap is found to be closing linearly with (H − H c2 ), asexpected. However, very close to H c2 a small residualgap ∆(H c2 ) = 0.4 K ≈ Tc max /3 remains open. Onepossible explanation is that this gap is a consequenceof a small misorientation of the sample. Indeed, analyzingthe line positions in the complete NMR spectrum,we could estimate that the field was tilted byθ = 3 ◦ from the c axis of the sample. In this casea perfect orientation of the sample should close thegap. Another possibility is that the observed ∆(H c2 )is intrinsic to the sample and reflects some neglectedinteractions (exchange couplings). This case is clearlyunfavourable to the BEC-type description of the system,and we plan to measure the angle dependence ofthe ∆(H c2 ) in order to define its origin.We have also measured the T1 −1 relaxation rate insidethe BEC phase, to see if the observed behaviouris related to the spin fluctuations related to the expectedGoldstone mode, or some “unwanted” gapopens. The magnetic field dependence at low temperature,T1 −1 (H, T = 0.12 K), as well as temperaturedependence T1 −1 (T ) taken at several field values donot apparently correspond to a gap opening, but theypoint to some multi-magnon process. The T1 −1 (T ) dependenceis found to be nearly field independent andsimilar to the one observed in a Cu 2 (C 5 H 12 N 2 ) 2 Cl 4ladder system [Mayaffre et al., Phys. Rev. Lett. 85,4795 (2000)] which has the rotational symmetry brokenby the Dzyaloshinskii-Moriya interactions, incompatiblewith BEC. To obtain further insight into the realnature of the 3D ordered phase in DTN we plan tocheck whether the observed spin dynamics depend onthe sample tilt angle θ, and proceed with theoreticalanalysis of our NMR data.FIG. 88. Temperature dependence of the proton NMRrelaxation rate T −11 in DTN at several values of magneticfield tilted by θ = 1 ◦ and 3 ◦ from the c axis. T −11 measuresthe spin fluctuations which show a gapped behaviour (fitsgiven by solid lines) at low temperature and above the uppercritical field H c2 = 12.32 T. While the gap is linearlyclosing as H c2 is approached from above, a small residualgap of 0.4 K remains open at H c2.S. Mukhopadhyay, R. Blinder, M. Horvatić, C. BerthierM. S. Grbić (University of Zagreb, Croatia), A. Paduan-Filho (Universidade de São Paulo, Brazil)66

2011 MAGNETIC SYSTEMSMagnetic properties of Y 0.9 Gd 0.1 Fe 2 D 4.2 compoundRFe 2 Laves phase can absorb up to 5D(H)/mole. TheYFe 2 D 4.2 deuteride which crystallizes in a monoclinicsuperstructure below 323 K is ferromagnetic at lowtemperature. Upon heating, this compound undergoesa sharp first-order magnetovolumic transition atT MO = 84 K (volume decrease of 0.55%) associatedwith a transition from the ferromagnetic to an antiferromagneticordering. The Néel temperature is locatedat 131 K. Above this temperature the compound is consideredas a paramagnet. It is worth noting that theF-AF transition is extremely sensitive to external appliedmagnetic field and presents an itinerant-electronmetamagnetic behaviour (IEM). T MO is also tuned bya volume change created by an external pressure, anH for D isotope substitution, or upon replacing Y withanother rare earth element.iv) The transition field (H T R ) is defined as the fieldcorresponding to the maximum of (dM T /dH) and deducedfrom each isothermal M T (H) curves (inset figure90). ∆H T R /∆T slope is larger for the Gd deuteride(2.20kOe/K) than for the non-substituted compound(1.37 kOe/K) (figure 91); v) when the metamagnetictransition is observed, there is a field for whichthe magnetization of the Gd substituted compound isequal to that of the Y deuteride. That means the Gdsublattice is fully demagnetized under high fields.FIG. 90. Isothermal magnetization curves of the Y 0.9Gd 0.1Fe 2 D 4.2 and YFe 2D 4.2 compounds near 140 K. Theinset shows the field variation of the differential magneticsusceptibility deduced from the M(H) curves.FIG. 89. Isothermal magnetization curves of Y 0.9Gd 0.1Fe 2 D 4.2 and YFe 2D 4.2 at 4.2 K. Field dependenceof the Gd sublattice (see text).Y 0.9 Gd 0.1 Fe 2 D 4.2 was prepared by solid-gas reactionand the crystallographic structure checked by powderx ray diffraction. The magnetization measurementswere performed in high continuous magnetic field upto 31 T. The results reported here indicate that only asmall amount (10 at.%) of Gd for Y substitution alterssignificantly both the structural and magnetic propertiesof the deuterides; i) the cell volume is largerthan for the non-substituted Gd compound. T MOis then shifted from 84 to 110 K; ii) a field-inducedspin reorientation of the Gd moments is observed at4.2 K in field larger than 25.1 T (figure 89) (this transitionis however not observed above 4.2 K); iii) athigher temperature the field-induced transition is attributedto the antiferro-ferro transitions of the Fe sublatticeslike in the YFe 2 D 4.2 compound (figure 90);FIG. 91. Transition field (full circles) and differentialmagnetic susceptibility (open circles) versus temperature ofthe Y 0.9 Gd 0.1Fe 2 D 4.2 compound. Temperature variationof the transition field (squares) for the YFe 2D 4.2.M. GuillotV. Paul-Boncour (CMTR, ICMPE, CNRS and Univ. Paris XII, France), T. Mazet (Institut Jean Lamour,University of Nancy, France)67

MAGNETIC SYSTEMS 2011Magnetic properties of nanocrystalline Dy 3 Fe 5 O 12 garnetRare earth iron garnets (REIG) with the chemical formulaR 3 Fe 5 O 12 (where R is a rare earth cation) arepromising materials for use in high performance microwaveand electrochemical devices owing to theirhigh resistivity, high Curie temperature, and highchemical stability. Here we report the synthesis ofsingle phase Dy 3 Fe 5 O 12 powders by the ball millingtechnique, and a study of its structural and magneticproperties.When the grain size decreases from 100 to 22 nm lowermagnetization values are observed whatever the fieldis (figure 93. Two values of the spontaneous magnetizationmay be determined: i) M Spont (1) was deducedfrom the extrapolation to zero applied field ofthe M(H) variations in the 20-60 kOe range; ii) M Spont(2) results from the extrapolation to H = 0 of the Mpart in the 120 − 280 kOe high field range (using aquadratic function in both H ranges). On the otherhand in the 150 − 280 kOe field range the magnetizationcurves of the different samples are well described(within ±1%) by the following approach law to saturationM = M Sat (1 − b/H 2 ). When the grain size isreduced below 100 nm M Spont (1) and M Spont (2) decrease.At 100 K we note that M Spont (1) and M Spont(2) are a linear function of the grain size (figure 94).On the contrary M Sat appears sensitive to the grainsize only when the grains are smaller than 50 nm (figure94).FIG. 92. (a) X-ray diffraction of bulk and ball-milledDy 3Fe 5O 12 for different milling time (hours) and (b) SEMsurfacemorphology of the as-prepared (bulk) Dy 3Fe 5O 12.FIG. 93. First quadrant magnetization curves for thevarious hours milled Dy 3Fe 5O 12 nanoparticles at 4.2 K.Dy 3 Fe 5 O 12 particles were synthesized in polycrystallineform by the solid-state reaction method froma mixture of α-Fe 2 O 3 and Dy 2 O 3 in nominal compositionsof 5 : 3. Milling of the as-prepared particleswas carried out for different durations in a controlledatmosphere using a planetary ball mill. The crystalstructure was characterized by x-ray powder diffractionusing Cu-K α radiation: Figure 92(a) indicatesthat the as-prepared and milled samples have the cubicstructure and the lattice parameters in agreementwith the literature values. The average grain size wasdetermined by using the Scherrer formula from the fullwidth at half maximum of the (420) diffraction peak.The average grain size for the as-prepared, 10h, 20hand 30h milled samples were 100, 50, 32 and 22 nmrespectively.FIG. 94. Saturation and spontaneous magnetization versusthe grain size for the bulk and milled Dy 3Fe 5O 12 samplesat 4.2 K (full symbols) and 100K (empty symbols).57 Fe Mössbauer spectra were recorded at 300 K and77 K for the as-prepared and 30h (22 nm) milled samples.The Fe 2+ content was found equal to about14 − 15 at.% in the 22 nm milled sample, to about 10at.% in the 32 nm milled sample and to about 6 at.% inthe 10, 50 nm milled sample respectively. In the threemilled samples the Fe 2+ content values are not modifiedby the temperature 300 K and 77 K. However, itworth noting that 15 at.% Fe 2+ content correspondsto less than 3% of oxygen vacancies.M. GuillotC. N. Chinnasamy, V. G. Harris (Dept. of Electrical and Computer Engineering, Northeastern University,Boston, USA), J. M. Grenèche (Laboratoire de Physique de L’Etat Condensé, CNRS-Université du Maine,Le Mans, France)68

2011 MAGNETIC SYSTEMSHigh-field magnetization study of ErCo 2The cubic Laves-phase RCo 2 compounds (R=RareEarth) are of particular interest, since they displayinteresting magnetic properties which arise from thecoexistence and interaction of the well localized rareearth4f electrons and of the itinerant 3d electrons ofCo. ErCo 2 shows a first-order transition from the ferrimagneticphase to the paramagnetic phase upon heatingat T C ≃ 32 K. At low temperature the Er magneticmoments (8.8µB atom −1 ) form a ferromagnetic sublattice.On the other hand the Co magnetic moment is0.7 − 1.0µB. Recently it has been suggested that Coclusterswith short range correlations exist in the paramagneticphase of ErCo 2 at T c ≃ 32 < T < T f ≃ 55K. This new magnetic configuration has been identifiedas “parimagnetism” T f being denoted as the flippingtemperature.are close to a linear field variation and M sat cannot bethen determined. We now consider the iso-field variationsM H (T ). A displacement of T c towards highertemperature takes place progressively whereas the Mjump associated to T c becomes less and less intense.In the inset of figure 97 four examples of the derivativeof d(M H (T ))/dT versus T are reported for the differentH values (denoted H CT R ). The iso-field transitiontemperature denoted as T CT R is defined as the T valuethat corresponds to the maximum of absolute value ofthe derivative d(M H (T ))/dT for each H value. Strictlyspeaking T CT R is identical to T c in zero applied field. The variation of H CT R versus T CT R is shown infigure 97. Below about 60 kOe H CT R increases quasilinearly with T CT R which varies from 38 to about 50 K.But when H CT R > 100 kOe T CT R remains constant atT CT R = (62 ± 5) K.FIG. 95. Isothermal magnetization curves of ErCo 2 at4.2K. The inset shows the field variation of the magnetizationat different temperatures.FIG. 96. Temperature variations of the remanent, spontaneousand saturation magnetization. The vertical linesunderlines the different temperature values.The isothermal M T (H) curves are reported in figure 95from which the remanent (M rem ), the spontaneous(M spont ), and the saturation (M sat ) magnetisation canbe deduced. Their temperature dependence can beseen figure 96. M rem decreases very rapidly in the temperaturerange 37 − 53 K and disappears completelyabove this temperature. This suggests that in the“parimagnetic” domain a weak but unambiguous remanentmagnetization is observed. On the other handM spont exists up to about 100 K. Its T decreasing issimilar to the variation of T f versus temperature. Inother word M spont that is very weakly temperature dependentin the ferrimagnetic range only is also relatedto the “parimagnetic” structure in the 35−100 K rangeand gets gradually smaller and smaller when T increases.In contrast the temperature variation of M satis relatively weak. When T > 100 K the M T (H) curvesFIG. 97. Critical Magnetic field versus the Transitiontemperature in ErCo 2.M. GuillotY. Oner (Department of Physics, Istanbul Technical University, Turkey)69

MAGNETIC SYSTEMS 2011High field-dependence of the surface spin magnetization in core-shellferrite nanoparticlesFerrofluids or magnetic liquids are colloidal dispersionsof nano-sized magnetic particles in a non-magnetic carrierwith numerous applications from complex engineeringto biomedical applications such as MRI imagingor magnetothermy [Jang et al., Angew. Chem. Int.Ed. 48, 1234 (2009)]. The development of these newtechnological applications requires the deep knowledgeof how the properties of magnetic nanoparticles (NPs)differ from bulk ones when the size decreases down tothe nanometric range (3 to 10 nm in diameter). Muchattention has been given to these magnetic properties[Battle et al., J. Phys. D 35, R15 (2002)]. Indeed,the spatial confinement induces finite-size effects in themagnetic core and the existence of an interface leads toan incomplete and distorted atomic surrounding in thesurface shell. Surface effects are related to the brokenexchange bonds, which reduce the coordination of surfacecations and modify the super-exchange near thesurface. Because of their chemical core-shell structure,ferrite NPs chemically synthesized by co-precipitationin aqueous media and surrounded by a thin layerof magnetically disordered maghemite [Gomes et al.,J. Phys. Chem. C 112, 6220 (2008)] presents enhancedsurface effects. Under-field Mössbauer spectroscopyand magnetization measurements [Sousa etal., J. Appl. Phys. 106, 093901 (2009)], performed upto a few Tesla have shown that these NPs have also acore-shell magnetic structure. This structure consistsof a well-ordered ferrimagnetic core surrounded by asurface layer of spins, fluctuating at high temperaturesand freezing in a disordered manner at low temperatures.In a sample, the respective volume fractions ofthe cores (Φ core ) and of the shells (Φ shell ) are determinedby chemical titration and the nanoparticle diameterdistribution is known by X-ray diffraction D XRand Transmission Electron Microscopy (log-normal d 0 ,σ) - see Table II. The shell thickness is evaluated to0.7 nm, which is very close to the ferrite lattice parameter.name D XR d 0 σ Φ core Φ shell m s coreA 9.0 nm 8.0 nm 0.2 0.75 0.25 515 kA/mB 3.3 nm 2.8 nm 0.3 0.44 0.56 212 kA/mTABLE II. Sample characteristicsMeasurements under pulsed magnetic field up to B =52 T at low temperatures 1.5 K, 5 K and 10 K have beenperformed. Powders of nanoparticles of different sizeswith a core based on soft ferrites of different nature(Mn, Cu and Ni ferrites) have been tested. We presenthere the results obtained for the two Mn ferrite samplesof Table II.Whatever the sample, a strong surface contribution isobserved in high fields as illustrated in figure 98. Thisrepresentation compares the behavior of the highestbranch of the hysteresis cycle (from high field to zerofield) for the two samples of table II, same ferrite withFIG. 98. ZFC magnetization M/Φ core of powders samplesbased on magnetic nanoparticles with a core of Mn ferrite,normalized by the volume fraction of the cores. Here thehighest part of the hysteresis cycle (from high field to zerofield) is presented. Red circles correspond to SQUID ZFCmeasurements (from 0−7 T - A. Sulpice - CRETA - CNRS- Grenoble); blue diamonds (sample A) and green squares(sample B) correspond to present LNCMI-Toulouse measurements(0 − 52 T).different NP core diameter. In both cases the magnetizationM/Φ core in high field does not saturates becauseof the surface spins which are badly coupled to that ofthe core. In figure 98, the smaller are the nanoparticles,the stronger is the effect. If M is written as the sumof two terms M core and M shell , it is possible to extractM shell in very high field as then M core /Φ core saturatesto a given value m s core already known from previousmeasurements at higher temperatures. In [Aquino etal., Phys. Rev. B 72, 184435 (2005)] m s core was determinedfrom the extrapolation of the Bloch law forthe core (as given in table II). Here we obtain at B =52 T, M shell /Φ shell of the order of 300 kA/m for bothsamples A and B, which is a reasonable order of magnitude.However for the smallest nanoparticles no signof M shell saturation is seen in the experiment at 52 T.G. Ballon and W. KnafoF. Gomes da Silva, E. Dubois, R. Perzynski (UPMC-PECSA, Paris, France), R. Aquino, J. Depeyrot (UnB,Brasilia, Brazil)70

2011 MAGNETIC SYSTEMSMagnetic field induced phase transition in a ferronematic liquidcrystalThe major technical application of liquid crystals istheir use in the nowadays widespread liquid crystaldisplays. In these devices the most characteristic featureof liquid-crystalline systems is exploited; a strongresponse of their molecular and supermolecular organizationto a small external perturbation. Due to theanisotropy of the dielectric permittivity of liquid crystalstheir molecular orientation can be easily controlledby relatively weak electric fields. As for the use ofmagnetic fields, their values have to be rather large(B ∼ 1 T) because of the small value of the anisotropyof diamagnetic susceptibility (χ a ∼ 10 −7 ).It has long been known that the possibility exists inliquid crystals for an external field to substantially alterthe nematic-isotropic transition temperature. However,the effect could not be induced by magnetic fieldH. The principal reason is that the estimated criticalfields are well over 100 T for traditional liquidcrystal materials. According to our previous magnetodielectricstudies [Kopčanský et al., IEEE Transactionson Magnetics 47, 4409 (2011)], the magnetic nanoparticlesinfluence the critical magnetic field of liquid crystals.Figure 100 shows the change of the temperatureof isotropic-nematic phase transition in thecalamitic liquid crystal 4-(trans-4’-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) doped with rod-likemagnetic particles. At the magnetic field of 12T theshift about 0.25 ◦ C was found.Our results have proven that ferronematics composedof calamitic liquid crystals and rod-like magneticnanoparticles can be effective in demonstrating themagnetic field induced isotropic-nematic phase transition.This is the first observation a magnetic fieldinduced isotropic-nematic phase transition in such systems.Due to doping, the magnetic field induced shiftof the phase transition from the isotropic to nematicphase decreases from 100 T to 10 T.FIG. 99.Ferronematic liquid crystal.Ferronematics (figure 99) are a manifestation of theidea of Brochard and de Gennes, who suggested thatdoping liquid crystals by fine magnetic particles mayenhance their sensitivity to magnetic fields. There aremany reports showing that doping of a nematic liquidcrystal with small amount of nanoparticles affects theimportant properties of nematic materials resulting ina decrease of structural transitions threshold. In thelast two decades interest in these materials has grownsubstantially; not only because of the interesting physicalproblems, but also for the promise to provide anoptical device technology based on magnetic switching.FIG. 100. Nematic-isotropic transition temperature versusapplied magnetic field squared (symbols). The solidline represents a least squares linear fit to the data.X. Chaud, E. BeaugnonP. Kopčanský, N. Tomašovičová, M. Timko, M. Koneracká, V. Závišová, Z. Mitróová (Institute of ExperimentalPhysics, SAS, Košice, Slovakia), P. Chometon (CRETA, Grenoble, France)71

Molecular Magnetism

2011 MOLECULAR MAGNETISMMultifunctional oxalate-based molecular magnetsMost of the studies related to multifunctionalityare implying bidimensional coordination networks.We have recently obtained a compound of formula(NH 4 ) 4 [MnCr 2 (ox) 6 )].4H 2 O where the threedimensional(3D) anionic network exhibits a structurewhich is reminiscent of β−quartz [Pardo et al., J. Am.Chem. Soc. 133, 15328 (2011)].by metalloligand leads to a strong increase of the diameterof the channels and to their chemical functionalisation.Accordingly, one family of channels is occupiedby ammonium ions and water molecules. Remarkablephysico-chemical properties directly derivesfrom the organization of the material; a long range ferromagneticordering at low temperature is provided bythe host anionic network whereas the counter-ions andthe guest molecules give rise to room temperature highprotonic conductivity (figure 101).We are currently exploring the formation of oxalatebridgedarchitectures in presence of cations basedon cyclic amidinium moieties. The synthetic versatilityof these organic cations is transferred to theheterometallic networks leading to a large varietyof structures ranging from 0D to 3D shown in figure102. Using a non H-bond donor bisamidiniumcation, two heterometallic oxalate-bridged dinuclearanions were obtained [Maxim et al., New J. Chem.35, 1254 (2011)]. The anion being the combinationof a tris(oxalato)metallate(III) moiety with atetra(aqua)manganese(II) entity. Interestingly, theyappear as metallotectons, at the crossroad between thelinear trinuclear complexes obtained last year [Pardoet al., Dalton Trans. 39, 4951 (2010)] and classical 2Dand 3D oxalate based networks. A 3D H-bonded networkis formed between the crystallisation and coordinationwater molecules and the terminal and bridgingoxalate ligands preventing the formation of compoundsof higher nuclearity or dimensionality.FIG. 101. (a) Complex-plane impedance for(NH 4) 4[MnCr 2(ox) 6)].4H 2O at 295 K at 96% RH. The insetshows the Arrhenius-type plot at various temperatures.(b) Relative room humidity (%RH) dependence of the conductivity(σ) at 295 K.Compared to quartz, the replacement of oxide bridgesFIG. 102. Synthetic strategies for obtaining oxalatebridgedmaterials associated with a amidinium-basedcations.C. Maxim, C. TrainE. Pardo (University of Valencia, Spain), M. Verdaguer (IPCM, UPMC, Paris), S. Ferlay (ULP, Strasbourg),S. I. Ohkoshi (University of Tokyo, Japan)75

MOLECULAR MAGNETISM 2011Π− stacked radicals question modelsOrganics radicals are fascinating objects because ofboth their improbable stability and their importancein biological processes and materials science. Stableradicals as building blocks towards magnetic moleculebasedmaterials are complementary compared to d andf metal ions due the intrinsically delocalised spin densityand to the numerous possibilities of binding. Comparedto the most studied nitroxide-based radicals, verdazylradicals offer natural possibilities of π− stacking.We are presenting two examples of these possibilitiesbased on a global approach combining the experimentaland theoretical explorations.K. This study underlines the common microscopic originsof exchange interaction and SCO phenomenon inspin delocalised systems. This close relationship is furtherconfirmed by theoretical studies that emphasizethe sensitivity of magnetic interactions to the relativeposition of the radicals.FIG. 104. Lateral and top views of the stacking of theradicals in the Svdpy:hq adduct.FIG. 103. Thermal variation of the χ M .T product ofCuCl(Ovdim).CH 3OH. The solid line is the best fit of theexperimental data using a spin transition model.In a attempt to coordinate copper(II) to a verdazylradical starting from the reduced form of the radical,the methanolic solution rapidly turns deep brown, astrong indication that the radical indeed formed in thesolution. The resolution of the crystallographic structurerevealed the formation of a linear copper(I) complexof formula CuCl(Ovdim)CH 3 OH (Ovdim = 1,5-dimethyl-3-(2’-imidazolyl)-6-oxoverdazyl). The formationof the radical thus occurred together with the reductionof the copper(II) ion. Structural inspectionsperformed at five different temperatures reveal the existenceof stacks of complexes arranged in a head-totailmanner where the verdazyl moieties are alternativelylocated above one another in a face-to-face arrangementor above an imidazole ring [Norel et al.,Angew. Chem. Int. Ed. 123, 7266 (2011)]. Itwas not possible to reproduce the thermal variation ofthe magnetic susceptibility using standard chain modelas in the case of 1,5-dimethyl-3-(2’-hydroxyphenyl)-6-oxoverdazyl described herein last year [Norel et al.,Polyhedron 29, 342 (2010)]. On the contrary a quantitativeagreement was obtained using a spin cross-over(SCO) model (figure 103). The pertinence of usingsuch a model was confirmed by the strong contractionof the face-to-face arrangement, which is dominatingthe magnetic properties, between 320 and 120This sensitivity is well-exemplified in the 1:1 adductof 1,5-dimethyl-3-(2’-pyridyl)-6-thiooxoverdazyl radical(Svdpy) with hydroquinone (hq) noted Svdpy:hqThe single-crystal X-ray diffraction of this adduct atroom temperature (RT) shows that the radicals exhibita slight curvature that leads to the formation ofalternating head-over-tail π−stacked 1D chains (figure104). Temperature-dependent X-ray measurementsreveal that the lateral slippages between the radicalsof the stacks vary from 64 to 78 pm and 54 to40 pm between 100 and 303 K. Surprisingly, despitethe alternation of the inter-radicals distances and lateralslippages, the magnetic susceptibility data can befitted with excellent agreement using a regular onedimensionalantiferromagnetic (1D AF) chain modelwith J = −5.9 cm −1 . On the contrary, wavefunctionbasedcalculations indicate an alternation of the magneticinteractions parameters correlated with the structuralanalysis at RT. Moreover, they demonstrate thatthe thermal slippage of the radicals induces a switchingof the physical behaviour since the exchange interactionchanges from antiferromagnetic (−0.9 cm −1 )at 100 K to ferromagnetic (1.4 cm −1 ) at 303 K. Inthe present case, the theoretical approach reveals amuch richer magnetic behaviour than the analysis ofthe magnetic susceptibility data. Through the questionsthat they raise, these results confirm the importanceof the strong interplay between the experimentaland theoretical approaches of these systems and ultimatelyquestion the relevance of a spin-coupled picturebased on temperature-independent parameters.N. Bréfuel, C. TrainV. Robert (ULP, Strasbourg), G. Pilet (UCL, Lyon), J. Pécaut (CEA, Grenoble)76

2011 MOLECULAR MAGNETISMSpectroscopic study of magnetic exchange between highlyanisotropic spin centresMagnetic anisotropy in molecular magnetic systemssuch as single molecule magnets (SMMs) mostly stemsfrom single ion anisotropy. Thus, metallic ionslike six-coordinate Co(II) complexes, with a largeanisotropy due to contributions from a first-order angularmomentum, are very appealing building blocksfor SMMs. [Co 2 (H 2 O)(O 2 C t Bu) 4 (HO 2 C t Bu) 2 (py) 2 ]represents one of the simplest cases in which a firstorderorbital contribution to the angular momentumis present in a magnetically exchange coupled system.The single ion 4 T 1g ground state is split into sixKramers doublets by the low-symmetry crystal fieldand the spin-orbit coupling, with a splitting of ca.200 cm −1 between the lowest doublet and the nextone. Thus low-temperature data is interpreted usingan effective spin-1/2 description. Magnetic measurementsindicate that the two s eff = 1/2 spins coupleferromagnetically to give a total S eff = 1 ground stateand an S eff = 0 excited state. The zero-field energylevelstructure has been obtained from inelastic neutronscattering (INS). The data is consistent with awell-isolated four-level scheme, with the S eff =0 lyingin the S eff =1 multiplet (figure 105). The completelifting of the degeneracy implies that the exchange interactionis rhombic. However, EPR spectroscopy isnecessary to bring out the highly anisotropic Zeemaninteraction.values of the Co(II) ions. EPR spectra have been modelledusing the spin-Hamiltonian:∑H = s 1 .J.s 2 + µ 0 B.g i .s ii=1,2where g i are the local effective g-matrices for thes eff = 1/2 centres and J is an anisotropic exchangematrix. The two Co(II) ions, and hence g Co1 andg Co2 , are symmetry equivalent, being related by a crystallographictwo-fold rotation axis bisecting the Co-O-Co angle. In order to avoid over-parameterization,many parameters were fixed from INS or from ab-initiocalculations. The principal values of J are fixed toreproduce the experimental zero-field energies, givingJ x,y,z = +0.32, +1.60, −3.36 cm −1 . The principal axesof J were obtained from single-crystal magnetic studies,whereas the ones of g i (figure 105) from ab initiomethods. Starting from the calculated values, the principalsingle ion g-values were refined to obtain goodsimulations of all the EPR spectra (figure 106), givinga highly rhombic g tensor with principal valuesg x,y,z = 2.3, 3.9, 6.2. These simulations were also usedto check the correct assessment of the principal axesas well as the sign combination for J i . They are remarkablygood given the simplicity of the model andthe extreme sensitivity to the parameters, e.g., to theorientations of the hugely anisotropic g i matrices. Importantly,they reproduce the spread of the spectraover a wide frequency and hence magnetic field range(34 − 345 GHz) [Boeer et al., Angew. Chem. 50, 4007(2011)].FIG. 105. Molecular structure of the complex, with principalaxes of g i Co blue, O red, C gray, N dark blue, Homitted for clarity.Multifrequency EPR spectra up to 345 GHz have beenmeasured at low temperature and reveal up to twelvetransitions covering the full field range (figure 106),thus allowing the determination of the effective g-FIG. 106. Variable-frequency EPR spectra of a pelletsample at 4 K (2 K at 345 GHz): experimental (black) andcalculated spectra (red) with the model described in thetext.A. L. BarraA. B. Boeer, D. Collison, E. J. L. McInnes, G. A. Timco, R. E. P. Winpenny (University of Manchester,U.K.), L. F. Chibotaru, L. Ungur (University of Leuven, Belgium)77

MOLECULAR MAGNETISM 2011Magnetic bistability of isolated giant-spin centers in a diamagneticcrystalline matrixAt low temperature, single-molecule magnets (SMMs)present an energy barrier for the reversal of theirspin, making them magnetically bistable systems. Atthe lowest temperatures, when magnetic relaxation involvesquantum tunnelling (QT) , weak intermolecularmagnetic interactions are involved in the relaxationprocess and make its understanding much more complex.To get rid of this contribution, magnetically dilutedsamples have been studied. Here the studied systemis not only magnetically diluted but all moleculesare iso-oriented.where O 0 4 is a Stevens operator, while D, E and B 0 4are the crystal field parameters. It can be noted thatlinewidth does not vary with dilution and a distributionof both D and E values resulting from local straininducedeffects has to be considered , to account for thebroadening of the lines at the extrema of the spectra.To probe the effect of magnetic dilution in the quantumregime, single-crystal measurements at 40 mK using amicroSQUID magnetometer were performed with themagnetic field applied along the easy axis. Whereasboth pure and diluted phases display pronounced stepsat evenly-spaced field values in their hysteresis loopsdue to resonant QT, the most striking difference isthe enhanced relaxation in zero-field upon dilution (figure108). The observation of faster zero-field relaxationin magnetically-diluted SMMs demonstrates that intermolecularinteractions can play a favorable role inSMM behavior, leading to a better memory effect. Thisfeature needs to be carefully considered when designingspintronic devices based on individual molecules.[Vergnani et al., Chem. Eur. J. accepted].FIG. 107. HF-EPR spectra recorded at 230 GHz and10 K on polycrystalline samples of 1C 6H 6 and 3a-cC 6H 6together with a calculated spectrum (upper curve).[Fe 4 (L) 2 (dpm) 6 ] (1), Fe 4 , have been diluted in its diamagneticGa 4 analogue, [Ga 4 (L) 2 (dpm) 6 ] (2), whereH 3 L = 2-hydroxymethyl-2-phenylpropane-1,3-diol andHdpm = dipivaloylmethane. Solid solutions of 1C 6 H 6 ,2C 6 H 6 and [(Fe 4 ) x (Ga 4 ) 1−x (L) 2 (dpm) 6 ]C 6 H 6 weresuccessfully prepared with x = 0.01 (3aC 6 H 6 ), 0.02(3bC 6 H 6 ), 0.05 (3cC 6 H 6 ). Diluted samples exhibitmagnetic properties (magnetization and susceptibilitymeasurements) typical for Fe 4 complexes, with anS = 5 spin ground state with easy-axis anisotropy.High-frequency EPR (HF-EPR) spectra recorded on1C 6 H 6 and 3a-cC 6 H 6 at 230 GHz in the T range from20 to 5 K exhibit the same fine structure arising froma zero-field split S = 5 ground state (figure 107). Thisestablishes the occurrence of intact tetrairon(III) complexesin the diluted samples. The spectra could befitted with D = −0.416 cm −1 , E = 0.016 cm −1 andB4 0 = 6 × 10 −6 cm −1 (g = 2.0) using the spin Hamiltonian:FIG. 108. Hysteresis cycles recorded by applying the magneticfield along the easy axis of single crystals of 1C 6H 6(upper panel) and 3aC 6H 6 (lower panel) at 40 mK anddifferent field-sweep rates.H = µ B B.g.S + DS 2 z + E(S 2 x − S 2 y) + B 0 4O 0 4 (3)A. L. Barra, P. NeugebauerA. Cornia, L. Vergnani, M. J. Rodriguez-Douton (University of Modena, Italy), R. Sessoli, L. Sorace(University of Florence, Italy), W. Wernsdorfer (Institut Néel,CNRS)78

Applied Superconductivity

2011 APPLIED SUPERCONDUCTIVITYTests of HTS model coils under very high fieldThere are numerous applications for high temperaturesuperconducting (HTS) magnets such as, for example,to generate very high magnet flux densities, HTS insertsfor the high energy physics, SMES and pulsedcurrent sources, and fully superconducting magnets toreduce energy costs. There are also opportunities suchas the commercial availability of YBCO coated conductorsin reasonable length (> 100 m), the high criticalcurrent density (J c ) obtained under high magneticfields, the outstanding strength (> 600 MPa) of thecoated conductors made by the IBAD (ion beam assisteddeposition) route thanks to the use of Hastelloy Rsubstrates. Preliminary measurements have showncritical currents I c above 500 A at 15 T and T = 4 Kon commercial YBCO coated conductors. With thesehigh critical current and tensile strength, these conductorsare very promising for making HTS superconductingcoils able to generate very high field. Indeed aworld record magnetic field of 35.4 T for a HTS insertin a 31 T background magnetic field has been recentlyreported [Trociewitz et al., Appl. Phys. Lett. 99,202506 (2011)].of high magnetic field and induced stress without havingto build a complete insert. For that purpose, alarge cryostat (figure 109) has been designed and builtto fit into the φ = 160 mm room temperature bore of a20 T 20 MW resistive magnet. The samples are bathedin liquid helium at 4.2 K or cooled by conduction bya large cooper exchanger supplied with vaporized heliumgas through a needle valve. The temperature ofthe gas can be also regulated upstream at the evaporationstage. The helium gas is transferred from thecryostat to the sample holder through a conical fittingat the bottom of the cryostat. The temperature can becontrolled from 4 − 300 K. The regulation is done by a336 Lakeshore temperature controller, managing 2 setsof heater and 3 calibrated Cernox gauges. Coils fittingwithin the following dimensions can be tested; minimuminner diameter 26 mm, maximum outer diameter115 mm, and maximum height 140 mm.HTS coils were prepared with a commercially availableSuperPower R YBCO CC tape (SCS4050-4 mmwide, 0.1 mm thick) with a high strength Hastelloy RC276 substrate that guarantees a yield strength above600 MPa. A double pancake coil was recently testedwith 2 windings of 10 turns of this tape on an 80 mmdiameter Cu mandrel for better thermal stability. TheYBCO layer is kept facing inwards. Figure 110 showsa view of the sample holder and the coil mounted on it.The coil was cooled and maintained in liquid helium(4.2 K). Currents were ramped into the coil at 10 A/sup to 200 A at 10 T, then 16 T without detecting anytransition. The magnetic field is applied so as to orientatethe Lorentz forces outwards. It was increased upto 18 T and several current ramps were performed withan increasing maximum current by step of 50 A from200 A to 350 A with no transition. Finally, the YBCOtape broke close to a large copper block used as currentlead while carrying 400 A at 18 T. The tape does notappear to be burned, and the failure is probably dueto the stress concentration. A first estimation suggeststhat the coil should yield for a hoop stress above 700MPa! Further analysis is now under way.FIG. 109. CAD drawing of the HTS coil test bench (left).The view includes the sample holder with its exchanger.Picture of the cryostat as mounted on the 20 MW site M8at LNCMI (right).Very few characterizations exist above 15 T [Braccini etal., Supercond. Sci. Technol. 24, 035001 (2011)] andLNCMI is one of the few facilities in the world wheresuch measurement can be performed. Various solutionscan be tested by investigating small HTS coils used as amodel to evaluate the most severe constraints in termsFIG. 110. Double pancake coil mounted on the sampleholder. The white arrow shows the broken tape after failureat 18 T above 700 MPa.X. Chaud, R. Suran, J. Aubril, F. DebrayA. J. Vialle (Institut Néel, Grenoble, France), P. Tixador (Institut Néel / G2Elab, Grenoble, France)81

APPLIED SUPERCONDUCTIVITY 2011Transport J c measurements of YBCO coated conductors above 20 TLNCMI is involved in the characterization of hightemperature superconductors under very high magneticfield for applied superconductivity or fundamentalphysics. Several magnetic configurations are availablefor applied superconductivity: 50 mm room temperaturediameter (24 T on 10 MW and 30 T on20 MW sites) and 160 mm room temperature diameter(20 T on 20 MW sites). Because of the largedemand for very high field resistive magnets, accessto the 10 MW is easier. Standard cryostats for measuringthe large critical currents involved, which fit ina 50 mm room temperature diameter offer a limitedworking space with usually the possibility to measureonly at 4 K in liquid helium. A variable temperatureinsert designed to fit into such cryostat has been developedfor LNCMI (figure 111).noise.Because we are using fast current ramps (10 to 1000A/s) to reduce the heating at the current injectionpoint and to avoid burning the sample, and becauseof heat diffusion time and response time of the heatsensor, even when placed on the tape, it is very difficultduring a measurement to discriminate a possibleheating of the tape from the heating at transition.Therefore, to validate our measurement, we performedRound Robin tests with samples already testedat CERN and CEA. We found good agreement up to300 A. To go further requires avoiding or controllingthe heat propagation from the current leads and currentinjection points. This can be done by reducing thesize of the superconducting section to reduce the measurementcurrent to below 300 A (see e.g. [Bracciniet al., Supercond. Sci. Technol. 24, 035001 (2011)])or by reinforcing the current leads by superconductingtapes. Also, the use of very high field helps to decreaseslightly the performance of the wire, so that they areeasier to measure.FIG. 111. Variable temperature insert with a short sampleholder. Liquid helium is vaporized into an exchanger, thenthe gas temperature is regulated by a furnace below thesample holder.It integrates a flow cryostat function into the sampleholder. The liquid helium is directly supplied froma liquid helium dewar to an exchange box located atthe bottom of the holder; the helium flux then goesinto an intermediate furnace just below the sampleholder. The sample holder itself can be terminatedin two different configurations. One allows measurementsof short samples (∼3 cm) with the possibility ofan angular positioning from 0 to 90 ◦ of the sample withregards to the magnetic field (in-plane to out-of-planefield orientation). The second confiuration enablesmeasurements of VAMAS type coils. VAMAS standsfor Versailles project on advanced materials and standards.This initiative provides some technical basis tostandardize measurements, testing, specifications, andstandards. In the present case, a VAMAS type coil representsa 1 m length of conductor wound onto a 34 mmmandrel (figure 112). This insures a sufficient length toinject the current into the conductor and to detect thesupra-to-normal transition with the 1µV/cm criterion,which is more difficult with short samples because ofFIG. 112. A VAMAS coil made of a YBCO coated conductortape mounted on the VTI.Figure 113 shows preliminary result for critical currentmeasurement on a VAMAS coil of a YBCO coated conductortape at 20 K from 20 to 26 T.FIG. 113. Critical current on a VAMAS coil of a YBCOcoated conductor tape at 20 K from 20 to 26 T.X. Chaud, R. Suran, F. DebrayP. Brosse-Maron (Institut Néel, Grenoble, France), P. Tixador (Institut Néel / G2Elab, Grenoble, France),J. Fleiter (CERN, Geneva, Switzerland)82

2011 APPLIED SUPERCONDUCTIVITYSuperconductivity of MgB 2 /Nb/Cu wires with amorphous carbondopingFor practical superconducting materials, the criticalcurrent density J c is a very important parameter, especiallyat high magnetic fields. However, the pureMgB 2 generally shows lower J c values due to its lowerupper critical field (B c2 ) and poor flux pinning. Inorder to improve J c (B) properties, a lot of methods,such as ball-milling method, magnetic field annealing,irradiation, and chemical doping, have been studied.Compared to other methods, chemical doping, especiallywith carbon or carbide doped, was consideredas the effective and convenient method to enhance theJ c (B) properties of MgB 2 . However, there are few reportsabout the effects of amorphous carbon doped onJ c (B) properties of Nb/Cu composite clad MgB 2 wires.In this work, we report the preparation and propertiesof MgB 2 /Nb/Cu wires with amorphous carbon doped.We show that the combination of carbon substitutioneffect highly dispersed doped particles and grain refinementcontributed to the improvement of J c (B) performance.the filled composite tube was drawn to wires of 1 mmin diameter. There was no annealing during the fabricationprocess. After cold deformation, the compositewires were cut into short pieces around 10 cm in lengthand sintered at 700 ◦ C for 2 hours in flowing argon, thenfurnace-cooled to room temperature.Figure 114 is the scanning electron micrograph (SEM)and the surface-scanning energy dispersive Spectroscopy(EDX) mapping taken from the superconductorcore of x=0.08 doped MgB 2 /Nb/Cu wire. MgB 2core shows a granular microstructure. The distributionsof the Mg, B, and C atoms are uniform overall.The positive effects of this homogeneous dopant wouldincrease the J c (B) properties of MgB 2 /Nb/Cu wires.The measurements of the critical current density J cversus B of MgB 2−x C x /Nb/Cu wires were performedby a standard four probe method at 4.2 K up to 15 T.As shown in Figure 115, the undoped wire possesseshigher transport J c than the C doped wires at lowerfield. But the J c value decrease with increasing themagnetic field faster than C doped samples. We thinkthe carbon entering the MgB 2 lattice substitutes forBoron and leads to the decreased of T c while significantlyenhances the J c of MgB 2 /Nb/Cu wires at highfield. The highest J c (B) performance was obtained forthe sample of MgB 1.92 C 0.08 , the J c values decreasedwith increasing the C doping level further. The J cvalues as high as 1.4 × 10 5 A/cm 2 (4.2 K, 5 T) and3.3 × 10 4 A/cm 2 (4.2 K, 10 T) have been achieved forMgB 1.92 C 0.08 wires.FIG. 114. SEM image (a) and EDX mapping (b,c,d) ofthe core of a MgB 1.92C 0.08 wire.MgB 2 wires were fabricated using an in situ powderin-tube(PIT) process with Nb as the barrier and Cu asthe stabilizer. The mixture of Mg (325 mesh), submicronamorphous boron powder, and submicron amorphouscarbon powder with stoichiometry of MgB 2−x C x(x = 0.00, 0.05, 0.08, 0.10, 0.15), was well mixed andgrounded in an agate mortar for 30 minutes in a vacuumglove box and argon atmosphere. The mixturepowders were filled in Nb/Cu composite tubes. ThenFIG. 115. Transport J c measured at T = 4.2 Kfor MgB 2−xC x/Nb/Cu wires with different carbon content(x = 0 − 0.15).X. ChaudS.N. Zhang, Q.Y.Wang, G. Yan (Northwest Institute for Non-Ferrous Metal Research, Xi’an, China), A.Sulpice (CRETA / Institut Néel, Grenoble, France)83

APPLIED SUPERCONDUCTIVITY 2011Superconductivity of Ca doped Bi-2212 single filament tapesBi-2212 has great potential for the applications suchas the superconducting insert coils of a high field(∼30 T) magnet. However, due to its intrinsic latticestructure, Bi-2212 exhibits strong anisotropic property,extremely short coherence length, leading to therapid decrease of critical current density, J c , with appliedmagnetic field. Thus, it is very important toenhance its flux pinning properties. In this study,nonstoichiometric Ca content was introduced into Bi-2212 lattice during the co-precipitation process. Dueto the different pinning mechanism both parallel andperpendicular to the ab-plane of Bi-2212 lattice, I cmeasurements were performed in under the magneticfield range of 0 to 20 T with the field directionparallel and perpendicular to the tape surface, respectively.Bi 2.1 Sr 1.96 Ca x Cu 2.0 O 8+δ precursors withx = 0.90, 0.95, 1.00, 1.05 were prepared by the coprecipitationprocess with the starting materials ofBi 2 O 3 , SrCO 3 , CaCO 3 , and CuO (> 99.9%). Aftera series of calcination processes in air at 800 ◦ C/12 h,820 ◦ C/20 h, and 850 ◦ C/20 h with intermediate grinding,the precursors were densely packed into Ag tubes.With the drawing and rolling process step by step, singlefilament tapes with the thickness of ∼ 300 m, anda width of 4 mm were obtained. Partial melting processhas been performed with the T max of 893 ◦ C anddwell time of 20 minutes. Then samples were cooleddown with the cooling rate of 5 ◦ C/h to the annealingtemperature of 840 ◦ C and kept at 840 ◦ C for 20 h.Figure 116 is the scanning electron micrograph (SEM)of the superconductor core of Bi-2212 single filamenttapes with the Ca content of 0.90 to 1.05. The influenceof Ca content on the secondary phase distribution canbe clearly observed. With the increasing of Ca content,the content of Bi-2201 (Bi 2 Sr 2 CuO y ) phase decreasedand both the content and particle size of alkali earthcuprates (AEC) increased. On the other hand, boththe energy dispersive spectroscopy (EDX) and X-raydiffraction (XRD) were performed and the differenceof Ca content in Bi-2212 phase has been ensured.The measurements of J c versus B of Bi-2212 tapes withdifferent Ca content were performed by a standard fourprobe method at 4.2 K up to 20 T at LNCMI-G. Asshown in figure 117, both the transport J c values andnormalized J c (J c /J 0 ) are higher for the stoichiometrictape than the other tapes due to the less scattering effect.While, as shown in figure 118, the perpendicularfield exhibited stronger effect on the suppress of superconductingproperties and the tapes with Ca contentof 1.05 and 0.95 showed higher J c than the stoichiometricsample. We think that the introduction of nonstoichiometricCa content in Bi-2212 lattice caused theformation of point defect and therefore lattice distortion,which can be recognized as the pinning centers.Although the intrinsic pinning of Bi-2212 along the c-axis is so strong that the point defect pinning is notthat effective, the pinning effect can obviously enhancethe J c -B properties when the magnetic field is perpendicularto the ab plane considering of the intrinsicallyweak pinning effect on the ab plane.FIG. 116. SEM images of single filament Bi-2212 tapeswith Ca content of 0.90, 0.95, 1.00 and 1.05.FIG. 117. J c and normalized J c/J 0 under ‖ magnetic fieldfor single filament Bi-2212 tapes with different Ca content.FIG. 118. J c and normalized J c/J 0 under ⊥ magnetic fieldfor single filament Bi-2212 tapes with different Ca content.X. ChaudS.N. Zhang, Q.Y.Wang, G. Yan (Northwest Institute for Non-Ferrous Metal Research, Xi’an, China), A.Sulpice (CRETA / Institut Néel, Grenoble, France)84

2011 APPLIED SUPERCONDUCTIVITYHigh magnetic field HTS SuperSMES protection developmentThis experimentation is an important part of an ANRproject involving LNCMI, IN, CEA-IRFU and Nexans,called SuperSMES, to investigate the use of thehigh T c material to enhance the energy density of superconductingmagnetic energy storage (SMES). Someof our studies focus on the development of protection.Indeed, in case of an accidental quench, current mustbe switched off quickly in order to avoid destruction ofthe storage coil. However, triggering the discharge isdifficult since the resistive voltage remains low. Thus,investigations focus on quench phenomena and particularlyin which directions and at which velocities thequench propagates in the coil.mandrel linked to a thermal exchanger supplied withhelium for thermalising the coils at variable temperature.The junctions between coils are made on theoutside by using a 12 mm SuperPower R YBCO CoatedConductor tape.The coil must be quenched in nominal configuration.In this first experimental step, our protocol proposedworking at 4.2 K, with a current flowing in the coilof 200 A DC and under a high static magnetic fieldof about 18 Tesla. The coil was tested at LNCMI, onthe M8 site (160 mm room temperature bore, 20 MW,18 T). Specific instrumentation was developed includinga virtual instrumentation device based on an independentADwin-Gold II acquisition card which isa high-capacity compact real-time system. In orderto avoid coil destruction caused by an uncontrolledquench, protection system was implemented. This systemsurveys the resistance value of the HTS around theheater comparing it with its value at 300 K. As soonas this limit is reached, the coil current is switched off.First results are illustrated in the figure 120. It onlyconcerns the nominal conditions. One cycle is definedby a current ramp until 200 A, maintained at this valueduring a few seconds. During this dwell time, a 4.5 Acurrent was pulsed for 10 ms duration into one of theheaters. The coil did not quench during this first test.This means it has delivered during the test duration a0.7 − 0.8 T magnetic field in an 18 T background. Dueto energy winter break, tests have been interrupted andwill continued in March.FIG. 119. View of the HTS pancake coils mounted ona large copper piece for conduction thermalisation. Theassembly is ready for test under 18 T into the large 160 mmcryostat mounted on M8.A specific highly instrumented HTS coil was built byCEA as seen on figure 119. It consists of six pancakecoils associated as two double coils (a continuoustape, no junction) placed in between two singlecoils. The pancake coils are made of 216 turns (36turns each one) of SuperPower R YBCO Coated Conductorbare tapes (SCS4050-4 mm wide, 0.1 mm thick)co-wound with a Cu tape (100 or 200µm) isolated onone face. The substrate of these tapes is made of highstrength Hastelloy R C276 providing a yield strengthabove 600 MPa necessary to sustain the high Laplaceforces under very high magnetic field. On each doublepancake coils, one coil has been instrumented witha stainless steel heater. Heaters create a controlledlocal hot spot, i.e. just sufficient to initiate quench.Several connections allow measuring potentials in theradial and orthoradial directions all around the heaterin order to follow the quench propagation. The pancakesare assembled on a specially designed big copperFIG. 120. Curves representing first results on the HTStest coil. During the current ramp and dwell on the coil,and despite current injection in the heater, the HTS coilremains superconducting.X. Chaud, J. Spitznagel, J. Aubril, Y. Miyoshi, F. DebrayT. Lécrevisse, J.M. Rey (CEA-IRFU, Saclay, France), A.J. Vialle (Institut Néel, Grenoble, France), B.Vincent, P. Tixador (Institut Néel / G2Elab / CRETA, Grenoble, France)85

APPLIED SUPERCONDUCTIVITY 2011Superconducting critical current measurements of MgB 2 basedconductorsColumbus Superconductors is active in manufacturinglong length of superconducting wires and tapes basedon magnesium diboride. Thanks to the feasibility ofthe ex-situ method Columbus is able to supply differentconductors for a wide range of applications. Theresearch activities concern wires and tapes for highmagnetic field application such as magnets for MRIsystems and coils for nuclear fusion experiments. Thebest way to improve the behavior in presence of magneticfield is to improve the starting MgB 2 powdersthrough doping.FIG. 121. Image of 19 doped filaments with copper stripsstabilization (top), 750µm thickness (bottom).magnetic field and to test the effects of different sheathmaterials (see figures 121 and 122). Moreover we measuredthe critical current of tapes manufactured withdoped powders and stabilized with copper strips (seefigure 123).The samples have been measured up to 10.5 Tesla andthe measurements, in some cases, has been limited torelatively high field values due to problems related tothe sample length, ≃ 7 cm, as in the case of the 37filaments not stabilized ((figure 123)(top)).The applications of one, or two, copper strips (≃ 200micron thickness) by tin soldering does not reduce thesuperconducting critical current. Nowadays, the techniqueof soldering copper strips seems to be one of themost effective to have the right quantity of stabilizingmaterial without decreasing the superconductingproperties.From these results we have selected the best process todope the MgB 2 powders having a large improvementin terms of superconducting critical current in presenceof applied magnetic field. For the next measurementsession we intend to modify the actual sample holderjust to be able to measure also windings to avoid currentlength transfer problems measuring at field lowerthan 5 Tesla.FIG. 122.diameter.Image of 37 doped filaments round wire, 1.1mmThe target of the experiments is to evaluate the magneticfield dependence of the superconducting criticalcurrent density of MgB 2 based multifilamentary wiresand tapes. We investigated the effects of different dopingmethods in order to enhance the pinning propertiesof MgB 2 . On the other side we had also to verify the effectsof different configurations of the multifilamentaryconductors on the critical current density in presence ofFIG. 123. Critical current versus magnetic field for differentdoping and number of copper strips used for stabilization.X. ChaudA. Tumino, S. Brisigotti and G. Grasso (Columbus Superconductors SpA, Genoa, Italy)86

Magneto-Science

2011 MAGNETO-SCIENCEVacuum Magnetic Birefringence (BMV) ExperimentA further steps towards the measurement of the magneticbirefringence of the vacuum using pulsed fieldshas been realized. In 2010-2011, our apparatus hasbeen calibrated using nitrogen gas and a detailed errorbudget has been evaluated. Finally, our vacuum upperlimit has been improved.Apparatus calibrationIn order to calibrate our apparatus and to evaluateits present sensitivity we have measured the magneticbirefringence ∆n of molecular nitrogen. These measurementshave been performed at different pressureP from 2.1 × 10 −3 to 32.1 × 10 −3 atm and are summarizedin figure 124.FIG. 124. Magnetic birefringence of molecular nitrogenas a function of pressure. The solid line corresponds to thelinear fit of experimental data.In this range, nitrogen can be considered as an ideal gasand the pressure dependence of its birefringence is thuslinear. We have checked that our data are correctlyfitted by a linear equation. Its ∆n axis-intercept isconsistent with zero within the errors. Its slope givesthe normalized magnetic birefringence ∆n u at B = 1 Tand P = 1 atm which can be given as:∆n u = (−2.00 ± 0.08 ± 0.06) × 10 −13 atm −1 T −2 .The uncertainty 0.08×10 −13 atm −1 T −2 on the slope isgiven by the least squares method and represents theA-type total uncertainty at 1σ.Our value of the normalized birefringence is comparedin Table III to other experimental published values atλ = 1064 nm.Ref.∆n u × 10 −13(at P = 1 atm and B = 1 T)PVLAS -2.17 ± 0.21Q&A -2.02 ± 0.16 ± 0.08This work -2.00 ± 0.08 ± 0.06TABLE III. Comparison between our value of the nitrogennormalized magnetic birefringence and other experimentalpublished values at λ = 1064 nm.This shows that our value agrees perfectly well withother existing measurements. Our total uncertaintycan be calculated by adding quadratically the A-typeand B-type uncertainties, and we get 10 −14 atm −1 T −2 .This is 1.8 more precise than the other results. It thereforeprovides a successful calibration of the whole apparatus.Upper limit on vacuum magnetic birefringencemeasurementsOnce the calibration performed we have evaluated ourupper limit in vacuum which is crucial for us since thegoal of our experiment is to measure for the first timethe vacuum magnetic birefringence. We therefore performedmeasurements in vacuum with the present apparatus.Our best upper limit for the value of the vacuummagnetic birefringence is ∆n < 5.0 × 10 −20 T −2per 4 ms pulse. During operation, the pressure insidethe UHV system was better than 10 −10 atm. The totalresidual magnetic birefringence is of the order of6 × 10 −23 T −2 , which is well below of our current sensitivity.The upper limit per pulse we got both in gases and invacuum is the best ever reached for this kind of measurement.For sake of comparison, the vacuum birefringencelimit reported by the PVLAS collaborationis ∆n ≤ 2.1 × 10 −20 T −2 for an integration time oft int = 65 200 s. This corresponds to an upper limit on∆n of 8.5 × 10 −17 T −2 in 4 ms of integration. Thisproves that pulsed fields are a powerful tool for magneticbirefringence measurements.PerspectivesLong terms perspectives depend on the possibility tohave magnetic fields higher than the one we can reachfor the moment on the experiment and which is ofthe order of 10 T. We have designed a new pulsedcoil, called XXL-coil, which has already reached a fieldhigher than 30 T when a current higher than 27 000 Ais injected. This corresponds to more than 300 T 2 m.Let’s finally discuss the improvement necessary to getto the final goal. We want to perform at most 1000pulses to get a sensitivity of ∆n = 4 × 10 −24 T −2 , necessaryto observe the vacuum birefringence. We thereforeneed a sensitivity better than 1.3 × 10 −22 T −2 perpulse. Two XXL-coils will allow us to improve our currentsensitivity of a factor 100. And a factor smallerthan 10 on optical sensitivity will be achievable byplacing on a better acoustic insulation and workingon a more robust locking system. This will especiallyreduce the noise of the transmitted measured light intensities.In the near future, the apparatus will bemodified in order to host the XXL-coils and thereforethe final version of the experiment will be ready foroperation.P. Berceau, R. Battesti, M. Fouché, C. Rizzo89

MAGNETO-SCIENCE 2011Electrodeposition of selenide semiconductors under magnetic fieldThe concept of synthesis of semiconductors by an electrochemicalroute is an extremely interesting perspectivedue to its very low cost. The composition, morphology,structure and thereby quality of depositedsemiconducting thin films strongly depends on the appliedparameters of the electrolysis. Analysis of theelectrosynthesis of semiconductors such as the selenidesindicated that the process of the deposition is controlledby the diffusion of one of the species in the solution.It is reported that applied magnetic field parallelto the electrode surface leads to enhancement of masstransfer by additional convection arises as an effect ofLorenz force and magnetohydrodynamic effect.This effect can be additionally applied during the electrosynthesisof II-VI semiconductors and improve therate of deposition process. The disadvantage of theelectrochemical method is the morphology and structureof the coatings, which is caused by the hydrogenevolution. This competitive reaction affects also on theefficiency of the electrolysis.It is reported that the magnetohydrodynamic effect additionallyaffects on the desorption of hydrogen andimprove the smoothness of the deposits. The influenceof the magnetic field on the process of electrodepositionof semiconductors was checked by the carrying outthe process of electrolysis in the electromagnet up to12 Tesla. Magnetic field was oriented parallel to theelectrode surface in order to check the influence of magneticfield on the composition, structure, morphologyof the CdSe and ZnSe films.Additionally, the rate of deposition and efficiency ofthe electrolysis was checked. The complex analysis ofthe effect of magnetic field up to 12 T on the processof electrochemical deposition of selenides were carriedout. The study embrace the influence of parallel directedmagnetic field on the rate of deposition and efficiencyof the electrolysis and composition , structureand morphology of the deposited materials.The results confirmed that the applied magnetic fieldincrease of the rate of deposition of selenides accordingto the theory of the magnetohydrodynamic effect (seefigure 125(a)). The efficiency of the electrosynthesis ofthe compounds were very high and reach about 100%.The composition analysis of zinc selenide films characterizedthe content of selenium above 50 at.%, but theXRD analysis confirmed the presence of zinc selenidephase.FIG. 125. (a) Mass of the coatings after 1 hour electrolysis.(b)Morphology of the CdSe coatings after 1 hourelectrolysis.The cadmium selenide films we characterized the equalatomic amount of selenium and cadmium and the XRDanalysis confirmed the presence of cadmium selenidephase. The exemplary modification of morphology ofthe coatings affected by the magnetic field is presentedin figure 125(b).F. DebrayR. Kowalik, P. Zabinski (AGH University of Science and Technology, Krakow, Poland)90

2011 MAGNETO-SCIENCEHydrogen evolution in high magnetic fieldMetals or alloys of low over-voltage of hydrogen evolutionare characterized by a low coefficient of Tafellines inclination within the range of activation wherethe process of hydrogen evolution is controlled by thespeed of electrode reactions taking place. Catalyticactivity of materials used in the production of electrodesfor hydrogen evolution are influenced mainly bythe composition, morphology, structure and physicochemicalproperties. Catalytic activity of cobalt withinthe range of hydrogen evolution is comparable to theactivity of nickel. It enables application of alloys oncobalt matrix for hydrogen evolution, too. Additionof elements such as oxygen or carbon can effectivelyprevent preferential solubility of molybdenum or iron.Addition of carbon to alloys of Co-Mo deposited in theelectrolytic way also enhances their catalytic activitywithin the range of hydrogen evolution in 8 M NaOH at90 ◦ C. A magnetic field can cause additional convectionin the diffusive layer at the electrode surface throughthe magnetohydrodynamic effect. The additional convectionenhances laminar flow at the electrode surface,and subsequently a decrease of the diffusive layer thickness.This can change the alloy composition, structureand morphology and therefore affects the electrocatalyticproperties of such alloys. Here we describe27.6 at.% of molybdenum for Co-Mo and Co-Mo-C alloys,respectively. Magnetic field causes an increaseof molybdenum content both for two-component andthree-component alloys. The structural tests of thedeposited coatings show that all the alloys feature anamorphous structure. However, for three-componentalloys of Co-Mo-C deposited in the magnetic field ofintensity at 6 Tesla, there were small peaks originatingfrom the hexagonal crystalline phase against a backgroundof a diffuse peak from the amorphous phase.This suggests a reconstruction of a crystalline structureof the alloy and finally an alloy with mixture ofcrystalline and amorphous phases is obtained.FIG. 126. Content of Mo in the obtained alloys in relationto the applied value of the magnetic field.Co-Mo and Co-Mo-C alloys deposited in the magneticfield (B = 1 − 12 T) with the force lines parallel tothe working electrode. Attention was focused on theinfluence of molybdenum and carbon on catalytic activityof the alloys obtained in the process of hydrogenevolution in 8 M NaOH at 90 ◦ C as well as on the influenceof the magnetohydrodynamic effect. Additionalconvection induced by the latter enhances the laminarflow at the electrode surface, and a decrease of thediffusive layer thickness. The effect is caused by theLorentz force on an ion moving perpendicularly to thedirection of the magnetic field action. The results ofcomposition of Co-Mo and Co-Mo-C alloys depositedat different values of the magnetic induction vector areto be found in figure 126.Without magnetic field the alloys contained 27.2 andFIG. 127. The results of over-voltage measurements ofhydrogen evolution for particular alloys.The magnetic field applied during an alloy depositionaffects its electro-catalytic properties in the hydrogenevolution process. Deposition of Co-Mo alloy in a magneticfield caused a decrease of Tafel inclination coefficientfrom 87 to about 25 mV/dec. This indicates achange of the hydrogen evolution mechanism from amixed one to a mechanism where the discharging processis so fast that the slowest stage is recombinationof adsorbed atoms of hydrogen with creation of H 2 .The best electro-catalytic properties in the process ofhydrogen evolution in hot, concentrated NaOH werefeatured by a Co-Mo alloy of the content of 30.5 % atof Mo (9 T). The other alloys, beside the one depositedwithout a magnetic field, demonstrated slightly worseelectro-catalytic properties. The observed differencesin the Tafel lines inclination confirm a high influenceof the applied magnetic field and content of carbonon the mechanism of hydrogen evolution on depositedcoatings.F. DebrayP. Zabinski, R. Kowalik, (AGH University of Science and Technology, Krakow, Poland)91

MAGNETO-SCIENCE 2011Morphology and phase composition of electrodeposited Co-Ni alloysunder high magnetic fieldOver the years, cobalt layers and cobalt-based alloyshave been very explored, mainly because of their magneticproperties and wide applications. It is well knownthat magnetic field applied during electrodeposition affectsstructure and morphologies of deposits and thereforeprovokes some changes in the physical propertiesof the coatings. In the present paper, we will focuson the effects of high magnetic field on the grain size,roughness and phase composition of nanocrystallineCoNi film.coefficient Y(hkl) of a plane (hkl) for the film can begiven by,Y (hkl) =I(hkl)/I b (hkl)∑ (1/N) × [I(hkl)/Ib (hkl)] ,where I(hkl) is the measured XRD peak intensity ofthe plane (hkl) of the film, Ib(hkl) the standard XRDpeak intensity of the plane (hkl) and N the number ofthe reflection diffraction peak.Before annealing, the texture coefficient Y(111)changes with and without applied magnetic field. Afterannealing, the orientation (111) becomes more andmore predominant with high magnetic field. Work inprogress is focused on the effect of grain size and phasecomposition evolution on the magnetic properties ofthe alloys.FIG. 128. AFM images of the Co-Ni alloy surfaces fordifferent applied magnetic fields during electrodeposition .Electrodeposition process has been carried out indouble-wall cell maintained at 50oC with circulatingthermostatic water. The sulfate bath contains 0.3M ofCo 2+ and 0.7 M of Ni 2+ .The pH was adjusted at 4.7.Electrochemical experiments were carried out with aconventional three-electrode cell. The working electrode(WE) was a titanium disk with an area of 1 cm 2embedded in epoxy resin. The substrate was made ofglass covered by a thin film of ITO (1000 Å). Figure 128shows AFM images and evolution of the morphology asa function of applied magnetic field. The dependenceof grain size and roughness of CoNi film on the magneticflux density is shown in figure 129. Up to 9 T,the lateral feature size and interfacial roughness increasedwhereas they decreased for B amplitude equalto 12 T, in this case the nodule size was less than 30nm (figure 129).Then afterwards the nanostructuredCo-Ni alloys have been annealed under controlled atmosphereat 673 K during 4 hours. Figure 130 showssome results on XRD measurements for these samplesbefore and after annealing.Using the measured XRD peak intensities, the textureFIG. 129. Evolution of grain size and roughness of Co-Nialloys obtained under high magnetic field.FIG. 130. Texture coefficient Y (111) of Co-Ni alloy as afunction of applied magnetic field before and after annealing.F. DebrayA. Levesque, D. Li, A. Franczak, J. P. Chopart (LACMDTI, University of Reims, France), Q. Wang (EPM,Northeastern University, Shenyang, China)92

Instrumentation

2011 INSTRUMENTATIONTowards the introduction of nuclear magnetic resonance as amethod available in pulsed high-field magnetsNuclear Magnetic Resonance (NMR) counts to themost powerful analytical methods relaying on magneticfields. Particularly it provides insights into the electronicenvironment of the nucleus under investigation- for example about chemical bonding. Beside thereis a great number of interesting questions which couldprofit from NMR experiments in pulsed high-field magnets(normal state of certain high-temperature superconductors,quantum critical points in heavy fermionsystems, field-driven spin- and charge density waves inorganic conductors, ...), there have been only experimentsdemonstrating the feasibility of NMR in pulsedfields, up to now. Our aim is to introduce NMR asa new analytical method available in the pulsed highfieldmagnets at the LNCMI and to provide first examplesof it’s usefulness to contribute to the solutionof scientific questions.These results show the feasibility of NMR experimentsin pulsed field at the LNCMI, but the spectral widthof about 400 ppm is still too high for most scientificapplications. Also the signal-to-noise ratio of about 10needs to be approved. In order to find a solution tothese problems a new probe was constructed which allowsexact positioning of small samples. In order to improvethe signal-to-noise ratio, the preamplifier,whichis of critical influence, will be operated at cryogenictemperature.Thanks to the recent improvements of thecapacitor bank which allows for an exacter determinationof the voltage, future NMR experiments can alsobe performed at lower field change rates nearer to thefield maximum, what will reduce field distortions byeddy currents.FIG. 131. Time profile of the magnetic field pulse and thevoltage detected at the NMR receiver channel Tx. The redline indicates the magnetic field value B 0 which correspondto the irradiated frequency f 0 and the gyro magnetic ratio γ.FIG. 132. Simplified schematic overview of the spectrometersetup. RF pulses are cut out from the carrier frequencyusing a RF switch controlled by the pulse generator. TheRF pulses are amplified and fed into the NMR probe viathe duplexer, dividing excitation and reception chain. Thesignal is first amplified by the preamplifier and than mixeddown and filtered. The frequency of frequency generator 2differs from that of generator 1 by 10.7 MHz. At last theresulting signal is digitalized by an oscilloscope.In figure 131 the methology for NMR experiments inpulsed fields is explained. During the field pulse, RFpulses are applied to the resonance circuit containingthe sample coil. Every time the current magnetic fieldmatches the Larmor frequency f 0 ∝ B 0 a NMR signalcan be detected. The NMR signal is mixed downto 10.7 MHz and than digitalized. The spectrometersetup is shown in figure 132.First results of 27 Al-NMR in pulsed field at 46 T havebeen recorded (see figure 133). The sample (2 mm diameter,6 mm length) was aluminum powder at liquidnitrogen temperature. In total 266 RF pulses of 1.7 µslength, every 100.9 µs were applied. During detectionthe field change rate was still of the order of 400 T/s.The sample position was adjusted based on a pick-upcoil measurement of the field profile with a precision of±1 mm.FIG. 133.27 Al-Spectrum of aluminum powder recordedat 46 T and liquid nitrogen temperature. The spectrumrefers to the irradiated frequency.H. Stork, P. Bontemps, G. L. J. Rikken95

INSTRUMENTATION 2011A flexible probe dedicated for Nuclear Magnetic Resonanceexperiments in pulsed fieldsSince 2010 efforts have been performed in order toimplement nuclear magnetic resonance (NMR) experimentsin pulsed magnetic fields at the LNCMI. WhileNMR signals in pulsed fields could be successfully observed,the performance of the previous NMR proberegarding line width and signal-to-noise ratio were stillinsufficient for being of real scientific use. Beside overcomingweaknesses which have been recognized in theprevious design, the intention was to design a probewhich is flexible enough to be easily adjustable to avariety of future experiments.matching inductance L M and tuning capacity C t( see figure 134 ) the matching can be performedat a given resonance frequency in-situ at cryogenictemperatures.• Superconducting magnet option. Thelength of the probe has been adapted in order tofit in the 14 T superconducting magnet availableat the LNCMI-T. At that field of B 0 ≈ 14 T1 H-NMR experiments can be performed at Larmorfrequencies f 0 = γB 0 near the Larmorfrequency of low-γ nuclei in the pulsed magnet(e.g. 63 Cu: γ = 11.285 MHz/T compared toγ = 42.5774 MHz/T for 1 H). This allows to optimizethe probe, the settings (like pulse length)and the spectrometer in a reasonable time.• Low Noise pre-Amplifier (LNA) at cryogenictemperature. The option to place thepreamplifier in the He-cryostat is foreseen in thenew probe design. The pre-amplifier strongly influencesoverall spectrometer signal-to-noise ratio.Cooling HEMT transistors significantly reducesthermal noise (see figure 135). The amplifiermust respond to the following criteria whichare; being able to stand high temperature variations,comply to necessary space restrictions andit also must have a fast recovery time. For thecryo-LNA, a specialized protection circuit (λ/4line, surface mount PIN diodes, bandpass filter)in 50Ω microstrip technology is developed.• Precise positioning of the sample. NMR signalsimulations show a need for a positioning accuracyof ±0.1 mm axially and ±0.2 mm horizontally.Therefore the centering of the probe wasameliorated and it can be positioned axially bya linear stage.FIG. 134. Overview of the new dedicated nuclear magneticResonance probe. The main part of the probe is theresonant circuit including the sample coil, a variable tuningcapacity and a matching inductance. The resonant circuitis connected to a LNA by a 40-cm-coaxial cable. It is protectedfrom RF pulse excitation by a lambda/4 line andPIN diodes. A following band pass filter serves to protectthe LNA from DC offsets and high-frequency distortionsinduced by diodes.Key features of the new probe design are:• Variable matching. With the previous probe itwas a difficult and time-consuming task to matchthe impedance of the resonant circuit properly to50Ω the more because the impedance changeswhen the probe is cooled down. With variableFIG. 135. Noise figures of the LNA for 300 and 77 K (4.2 Knoise figure setup is on study). Between 300 K and 77 Kthe noise figure is reduced by 46% across the bandwidth.At working frequency (530 MHz), the LNA noise figure isonly 0.23 dB, which is better than that of most commercialLNAs at this frequency and ambient temperature.P. Bontemps, N. Bruyant, H. Stork, G. L. J. Rikken96

2011 INSTRUMENTATIONPulsed field setup for single crystal X-ray diffractionOf all x-ray techniques, single crystal diffraction withmonochromatic beams offers by far the highest resolution,the strongest absolute signal, and the best signalto noise ratio. It is therefore the method of choicefor small crystallographic changes (such as magnetoelasticeffects), and for weak signals, such as magneticscattering or scattering of orbital order. This type ofexperiment, however, is more difficult to perform inpulsed magnetic field than other x-ray diffraction techniques,such as white beam Laue diffraction, or powderdiffraction; it requires precise alignment of the sample,and is therefore sensitive to vibration that might beintroduced by electromagnetic forces induced by thefield pulse. The scattering process is summarized bythe Bragg law,nλ = 2d sin θ,where λ is the wavelength of the incident X-ray beam,d the inter-reticular distance and θ the angle betweenthe reticular plane and the incident X-rays. Due to thehigh quality of the synchrotron beam delivered at theESRF in Grenoble, the angular resolution is as highas 0.001 ◦ . A small rotation of the sample will thendramatically decrease the scattered intensity. In thefollowing we will present some improvements made onthe 30 T split-coil pulsed magnet setup in order toreduce the level of vibration induced by the shot.Technical improvementsIn figure 136 we present the design of the new He cryostatcapsule (black) and of the corresponding sampleholder (blue). Both parts are in Torlon, a polymerfairly transparent to X-ray, eliminating the possibilityof induced current on the sample holder.As one can see, we introduce a conical part on both elementsallowing us to physically lock the sample holderwith the He cryostat. No sample stick is therefore necessaryduring the shots, and we developed an optionalmeasurement stick uncoupled from the sample holder.This first step should ensure stability of the sampleorientation over time and remove all thermal bridgewith the experimental hutch. Additionally, we performseveral modifications of both LN2 cryostat anddiffractometer in order to reduce the induced currentsgenerated by the magnetic pulse. To do so, we replacethe cryostat base aluminium plate by its equivalentin G10 (glass fiber epoxy composite). These changesshould eliminate most of the eddy currents and greatlyimprove the stability of samples during pulses.ResultsThe new capsule is fully efficient with a negligible rotationof the sample 5 s after a 30 T pulse (down from0.1 ◦ ). Moreover, temperatures as low as 1.6 K wereeasily reached under irradiation by ESRF beam. Infigure 137 we present the evolution of the Si (4 0 0)scattered intensity during a 8 kV magnetic pulse before(red) and after (black) the replacement of the aluminiumbase plate by its G10 counterpart. This changealone reduces the intensity drop on the photodiode bya factor 4 corresponding to a factor 3 on the sample’srotation. The same measurement at maximum field(17.5 kV, 30 T) has been carried out. As for now, thesample is stable during the field ramp up and thereis still room for improvement. To go further, we planto deeply modify the ID06 diffractometer by removingseveral aluminium parts such as X and Y sampletranslations.FIG. 136. Sketch of the new He cryostat capsule achievingstrong coupling between the sample holder and the cryostat.The measurement stick is independent from the sampledirect environment.FIG. 137. Scattered intensity versus time of the Si (4 0 0)Bragg reflection (F W HM = 0.005 ◦ ) at 8 kV with Al (red)and G10 base plates (black). As one can see, reducing eddycurrents increases the quality of the measurement.X. Fabrèges, F. Duc, P. Frings, M. NardoneT. Roth and C. Detlefs (ESRF, Grenoble, France)97

INSTRUMENTATION 2011Sensor validation for polyhelix in-situ instrumentationHigh field polyhelix inserts need to be optimized toincrease their field factor in the frame of “hybrid” development.Up to now in-situ measurements are verypoor; electrical potentials are measured for each set ofdouble helices, each insert being composed of 6, 12 or14 helices. Consequently, an estimation of the meantemperature of two consecutive helices is obtained bythis measurement, taking into account the copper alloycoefficient temperature. In order to further optimizethe helices, we have searched for sensors able to withstandthe severe magnet running conditions; magneticfield up to 35 T, deionized water at high flow rate up to30 m.s −1 , hydraulic pressure (10 − 30 bars). The priorityis the local measurement of the temperature ofa helix and water velocity in the corresponding channel.For temperature measurement two sensors wereselected. The first is fixed to the endpoint of an opticalfiber. It uses the temperature-dependent birefringenceof specially selected crystal as temperature transductionmechanism (figure 138). The operating range is−40 to +250 ◦ C with a precision of ±1 ◦ C suitable formagnet operation.20 T. Tests up to 35 T are planned before insertionin a polyhelix. For SAW sensor validation, additionaltests have to be performed.FIG. 138. End point thermometer on OPSens optical fiberwhich is waterproof throughout.FIG. 140. Test of the stability of the sensor under magneticfield up to 20 T under a ramp of the magnetic field(broken line), and the sensor response at fixed temperature(solid line).The second family consists of “Surface Acoustic Wave”(SAW) sensors based on electro-acoustic properties ofpiezoelectric materials such as Nb-Li or Zn oxide (figure139).FIG. 139.SAW temperature sensor from SENSEOR.FIG. 141. Mock up for validation of microflowmeter withdoppler effect.They are working without power supply because energytransfer and data communication is achieved usinga 433 MHz radio frequency. An experimental heatingcylinder fitted with sensors was prepared and testedunder a maximum magnetic field of 20 T (figure 140).Thermometric optic fiber sensors are qualified up toFor velocity measurements, we have tested doppler microvelocimeteron a 200µm thick water channel up to20 m.s −1 (figure 141). The sensor will be installed onour hydraulic test loop in 2012 to obtain experimentaldata for the numeric modeling of radially cooledhelices.F. Debray, J. L. De Marinis, J. Matera, J. Spitznagel, P. Sala98

2011 INSTRUMENTATIONTransient recording system for magnet sitesThe goal of this system is to record magnet “fast” parameters(currents and voltages) during any abnormalevent, independently of the user control desk and themagnet general control and monitoring system. Thesystem was set into operation on all the magnet sitesduring 2011.points/channel FIFO buffer (20 s) providing selectablepre-trigger (to analyze what happened before the abnormalevent). Triggering occurs when the absolutevalue of dI/dt or dV/dt exceeds their respective maximumlevel. When it happens, the buffer is saved to abinary file on disk, then a new buffer is created immediately.A data reading program reads recordedbinary data files, configuration file (gains, magnetstype,...), displays and plots physical parameters values.A conversion program converts complete or timeselectable part of the binary file to a physical parameterASCII file, using relative or absolute time origin.Most recorded transients are triggered by dI/dt, andoften result from the magnet general control systememergency procedures leading to a magnet cut off (figure143). Some of them can be initiated by powersupplies instabilities or magnet local short circuit dueto aging (mostly dV/dt trigger in this case) and do notlead to magnet cut off (figure 144).FIG. 143. Example of a transient linked to Helix powerinstability which has led to a cut off.FIG. 142.Magnet current analog network.Hardware: On every high field magnet site, a PCcomputer is used with 1 Adlink DAQ2205 (16 bit) card.Measurement channels are: 4 magnet currents, 4 magnetcurrent references and up to 16 winding voltages.Voltages are measured directly on site and adaptedto the ±10 V bipolar analog inputs. Currents andcurrent references are measured by DCCT devices directlyon the 4 main power supplies (24 MW) and sentby 4 transmitters to the site computer receivers (figure142) using a (0 − 20 mA) current loop analog network(unipolar 0 +10V inputs).Software: Programs have been written for recording,reading and converting data. Recording program uses“Scatter Gather DMA” mode of the acquisition cardat 5 KHz sampling frequency. It creates a 100, 000FIG. 144. Example of a transient on an outer Bitterprobably due to aging and not leading to a cut off.Statistics of the events recorded by this new systemis a powerful tool to anticipate major degradation ofany part of the high field magnets. It will be a verypowerful tool for the magnet safety when operating in“hybrid” conditions.P. Sala, J. L. De Marinis99

INSTRUMENTATION 2011Cryogenic development for pulsed magnetic fieldsStandard nitrogen cryostatsStandard nitrogen cryostats for cooling of pulsed magnetsare part of the basic equipment provided in ourexperimental sites. During this year, three additionalstandard liquid nitrogen cryostats were implemented(figure 145).Faraday cage for magneto-optic experiments inthe megagauss generatorA concentric double wall Faraday cage has been completed.This enclosure provides a shielded environmentfor the cooled HgCdTe photodiode detector used forthe signal acquisition. It aims at reducing the electricalnoise resulting from the 70 kV trigger discharge ofthe generator. Built from rolled 8 mm thick plates ofAW-5083 type weldable aluminium alloy, the Faradaycage is equipped with two dismountable top flanges allowingthe refill of the detector with liquid nitrogen(figure 147).FIG. 145.One of the newly installed nitrogen cryostats.Liquid nitrogen cryostat for the Vacuum MagneticBirefringence experimentA new type of dismountable LN2 cryostat for the coolingof a new generation of transverses magnets intendedfor the “Vacuum Magnetic Birefringence” experiment(BMV) was designed (figure 146, left). Thecryostats will allow the passage of the insulation vacuumthrough the horizontal bore of 18 mm diameterof the magnet, over a length of 800 mm (figure 146,right), and the assembly of an optical resonant cavityalong this space.FIG. 147.Faraday cage, external top flange removed.The inner enclosure yield a noise attenuation greaterthan 100 dB in the frequency range between 10 kHzand 100 MHz. The inner cage was placed in a secondenclosure, built on the same basis, and insulated fromit with PVC wedges. The outer cage was connectedto the main Faraday cage surrounding the megagaussgenerator. This arrangement avoids any galvanic contactbetween the triax cable shielding, connected to theinner cage, and the main Faraday cage. The efficiencyof the shielding system can be seen in figure 148.FIG. 146. (Left) The liquid nitrogen cryostat of the vacuummagnetic birefringence experiment. (Right) Cross sectionview of the cryostat.FIG. 148. Trigger noise (black) measured on the photodiodewithout the double cage compared with that obtainedusing the new shielding configuration (red).M. Nardone, A. Zitouni, P. Delescluse, T. Domps, J.-M. Lagarrigue, L. Bendichou, T. Schiavo100

2011 INSTRUMENTATION3 He refrigerator development for pulsed and dc magnetic fields3 He cryostat for experiments in high pulsedmagnetic fieldsA top loading 3 He cryostat for electronic transport,magnetization, and magneto-optics measurements underpulsed magnetic fields up to 60 T has been designed,built and tested. This unit incorporates designelements of a previously developed cryostat usedsince 2001 for experiments in a 11 mm bore, 60 T coil[Vedeneev et al., Phys. Rev. B 73, 014528 (2006)].The new cryostat (see figure 149) was designed to fitinto the 2 nd generation of 25 mm bore, rapid cooling,60 T coil. Diameters of the 4 He Dewar extension andof the 3 He bath have been increased from 7 to 20 mmand from 4 to 11 mm, respectively. This new environmentmakes easier the centering of each cryogenicstage. The 11 mm diameter of the 3 He bath allows theinsertion of bigger sample holders intended for varioustypes of measurements. In order to avoid heating byeddy currents resulting from the sudden change of themagnetic flux during the pulse, the dismountable 3 Hechamber was made with a glass fiber tube while thesample holder was machined from Peek pieces. Cryogenicstest were planned in two steps, the first task wasto check the performance and the second one was tocompare the behaviour in the field relative to the firstcryostat. After condensing of about 0.2 mol of 3 He at atemperature of 1.4 K and after about 30 min of pumping,a base temperature of 330 mK was obtained. Thetemperature could be maintained over three hours beforecomplete evaporation of the bath. Additional testsare under way. Preliminary experiments are plannedfor early 2012.The cryostat features (top to bottom) are as follows:conventional staged bath pumping line, condenser and4.5 cm 3 liquid 3 He bath surrounded by a vacuum chambersealed with an indium wire flange. Placed inthe vacuum, the cold finger of the cryostat consistsof a 6 mm diameter and 80 mm length silver rod, M6threaded at its vacuum side end. At the other end,a 1 mm thick layer of silver powder is sintered, over alength of 25 mm, in order to increase the area immersedin the cold liquid and, therefore, enhance the heat exchangebetween liquid 3 He and cold finger. The coldfinger is soft welded, in its middle, through the bottomend of the 3 He chamber.FIG. 150.The 3 He refrigerator for dc magnetic fields.Silver was chosen instead of the more usual (andcheaper) copper as raw material for the cold finger becausethe contribution of the nuclear magnetic momentto the heat capacity in copper is 20 times greaterthan in silver, resulting in a poor thermal diffusivityof copper at low temperature in high magnetic field.Connected to a 18 m 3 two stage vane pump, the refrigeratorwas tested in the “one shot” mode. 0.12 mol of3 He were condensed at 1.4 K. A base temperature of302 mK, measured with a calibrated RuO 2 type sensormounted on the cold finger, was observed after about50 min of pumping and maintained over a period of tenhours (figure 151, left). Hold time could have been increasedby condensing more 3 He but at the cost of alonger cooling down time. Cooling power is displayedin figure 151, right.FIG. 149. The 3 He cryostat for pulsed fields with thevacuum chamber mounted.3 He refrigerator for thermoelectric measurementsin steady magnetic fields up to 34 TDesign, assembling and testing of a cold finger type3 He refrigerator (see figure 150) developed within theframework of the ANR project DELICE have been performed.The main requirements were; sample undervacuum, rapid cooling down, base temperature closeto 300 mK during a time longer than the duration ofthe slower sweeps of magnetic field considered (about2 hours).FIG. 151. (Left) Hold time test. The dashed arrow indicatesthe end of condensation and the start of pumping onthe 3 He bath while dotted arrow corresponds to the deliberatestop of pumping and return to atmospheric pressureof the 4 He bath in contact with the condenser. (Right)Cooling power from 0.3 to 0.65 K.M. Nardone, A. Zitouni, P. Delescluse101

INSTRUMENTATION 2011Dilution fridge for use in pulsed magnetic fields up to 60 TA 3 He- 4 He dilution refrigerator (see figure 152), devotedto electronic transport and magnetization measurementsunder pulsed magnetic fields up to 60 T,was developed and finalized. Three main requirementswere requested: a base temperature of 50 mK, a reductionof eddy currents in the mixing chamber zone anda straightforward construction. A particular attentionwas paid both to the ease of use and reliability of therefrigerator. For this purpose, a counterflow heat exchangerhas been inserted in the pumping line abovethe still where the circulating 3 He is cooled taking advantageof the enthalpy of the cold gas pumped intothe still. This arrangement eliminates the need for theusual 1 K pot, enhances the hold time of the main 4 Hebath and avoid any risk of blockage of the 4 He pot fillline. With such an arrangement, the time necessary forcondensing the 1.3 mol of 3 He- 4 He mixture is less than3h30 for an optimal pressure of 2.5 bars maintained atthe injection side with the help of a compressor duringcondensation. A simple continuous type heat exchangerconnects the still bottom to the upper part ofthe mixing chamber.The glass fibre mixing chamber avoids any heating byeddy currents of the sample environment during thepulse, excepting that occurring in the sample itself. Atemperature rise of about 100 mK occurs progressivelyfew seconds after the shot, only, due to the metallicparts of the refrigerator located above the mixingchamber. Nevertheless, the refrigerator fully recoversits base temperature in less than 30 minutes comparedto the time interval of about 1 hour needed to coolagain the pulsed coil between two shots at 60 T. Withoutheating, the still, pumped with an 18 m 3 /h rotaryvane pump, stabilize at around 0.6 K while theavailable cooling power measured in experimental conditionsreaches 3.5µW at 100 mK. During experimentsthe dilution fridge can be operated from its 50 mKbase temperature to 0.7 K by a simple resistive heaterlocated in the mixing chamber.very low temperature permitted us to study a stronglysample-dependentmagnetoresistance anomaly peakedat 30 T. It is the consequence of a Fermi surface reconstructionoccurring before that the hidden-orderis replaced by a polarized paramagnetic regime above39 T. Quantum oscillations are also reported, confirmingthat the Fermi surface is modified at high fields.FIG. 152. The in-house dilution refrigerator for use inpulsed magnetic fields.In summary, the technical choices made during developmenthave enabled the realization of a simple andreliable fridge that can be used routinely for experimentsperformed in collaboration with the LNCMI-Tvisitors.As an example, the magnetoresistance of two singlecrystals of URu 2 Si 2 has been measured using thisdilution refrigerator in magnetic fields up to 60 Tand in temperatures between 50 mK and 800 mK (seefigure 153). Interest for the heavy-fermion systemURu 2 Si 2 is motivated by the presence of a “hiddenorder”state which develops below 17.5 K and whosenature is still unknown (despite more than 30 yearsof intensive research). This set of new experiments atFIG. 153. Transversal magnetoresistance of the heavyfermionsystem URu 2Si 2 measured in a pulsed magneticfield applied along c. To increase the in resolution, theresistance was averaged over several shots.M. Nardone, A. Zitouni, W. Knafo, G. Scheerer, C. Proust, C. JaudetD. Aoki, J. Flouquet (DSM-INAC-CEA, Grenoble, France), T. Matsuda (JAEA Tokai, Japan)102

2011 INSTRUMENTATIONA new test station to measure the critical current ofsuperconducting strandsA test station for critical current measurements of superconductingstrands with lengths up to 1.6 m hasbeen developed at LNCMI-Grenoble (see figures 154and 155). A modular cryostat has been designed forits integration within the existing infrastructures of thelaboratory including the 24 MW sites of resistive magnetsfor continuous magnetic fields up to 35 T. Thesample holder can carry up to 10 strands wound in coilsof 44 mm diameter and stacked up around a commoncurrent lead, which allows using the expensive magnettime more efficiently.To reduce the thermal load, a novel integrated switchingunit containing 10 high current contactors operatingin liquid helium has been implemented in orderto connect alternately the remaining terminal of eachstrand sample to the second current lead. First resultsof critical current measured on Nb-Ti strands ina 10 T superconducting solenoid are in good agreementwith those obtained at the CERN reference test facility.They allow fine assessment of the sensitivity andprecision of the overall measurement chain.FIG. 154. (a) Cross-sectional view of cryostat schematicallyinserted in a resistive or superconducting magnet .The cryostat is mounted on an elevator to center the sampleunder test with respect to the superconducting magnet.(b) Sample holder .FIG. 155. (a) Switching unit with high current electromagneticcontactors. (b) Function principle of contactorrelay. The current to the coil might be reversed in order toguarantee proper activation of the contactor.W. Joss, S. Krämer, R. Pfister, P. Pugnat, L. Ronayette, and H. Xiao103

Magnet Development

2011 MAGNET DEVELOPMENTPulsed high magnetic field coilsProduction of user coilsAs in previous years, we focused for user coils on coilsbuilt from a copper alloy with Zylon fiber reinforcement.Three types of user magnets are available forexperiments, 1 × 60 T 1 MJ 13 mm bore, 5 × 60 T5 MJ 28 mm bore and 2 × 70 T 6 MJ 13 mm borewith pulse durations (t max ) of respectively 27, 57 and32 ms. We produced in total 5 user coils, 3 high-fieldcoils and 3 special coils. As an improvement of thiscoil-type, we decided to combine copper stainless steelmacro-composite wire and Zylon. The stress vs. strainproperties of this wire are compatible with Zylon andoffer an excellent combination for high magnetic fields.This home-made high strength wire has proved its efficiency,one 60 T 1 MJ 12 mm bore coil has madethousands shots and is still in use. An 80 T insert,to be used in a two coil system, has been realized andtested in our 12 MJ 33 T outer coil reaching 81.3 T.This coil is now available for user experiments up to80 T.coils with an improved design will be wound at thebeginning of 2012.Within the framework of the MAGFINS project westudied, optimized and designed a new coil for large angleconical access with a cooling channel to increase theduty-cycle. This coil will be used for neutron diffractionexperiments at the ILL. Mechanical design is performedin close collaboration with the ILL cryogenicsservice, responsible for the N 2 and 4 He cryostats.FIG. 157. The MAGFINS coil: the coil will be woundaround a double cone structure inside which will be vacuumwhereas the coil is immersed in N 2,liq .FIG. 156. Field as a function of time for the dual coilsystem. The inset shows the pulse shape of the inner coil.Special coilsWe have designed and tested a prototype of a copperstainless steel “Polyhelix” coil. This design is developedto increase the ratio of field duration over coolingtime, which is a key point for such experiments as neutrondiffraction or X-ray diffraction and spectroscopy.The first prototype failed at the top extremity due tothe important torque on the helices but proved thepossibility of short cool down times.In collaboration with the Laboratoire pour l’Utilisationdes Lasers Intenses (LULI) we have designed a 40 Tsplit-coil that will be wound and tested next year. Themain objective is the combination of pulsed field andpulsed laser to perform plasma physics experiments.We have pursued our efforts on the production of intensetransverse magnetic field for optical experiments(BMV). Two so-called “XXL” coils were wound butboth failed due to insulation problems. Two new XXLIn order to be able to do NMR measurements the homogeneityof our standard 60 T coils had to be improved.Several options are under study either a longercylindrical coil or a coil with a non-homogeneous currentdistribution.Technical developments & outlookLayer transitions are a weak point in coil constructionsince the cylindrical symmetry is disturbed and thepossibilities for fiber reinforcement at this position arelimited. These weaknesses are felt most when the layertransitions are in relatively strong field as happens forinsert coils as well as for the (close to the center) layertransitionsof a split-pair magnet. We are investigatingseveral techniques to make these transitions more reliable.Coils tend quite often to fail in the mid-plane ofthe innermost half. Since the cooling annulus presentsa natural separation between two parts of the coil weare investigating the idea to build the modular standard60 T coils in a way that would permit to replaceonly the innermost part in case of a coil failure.The procurement of a 6 MJ rapid transportable generatorwill open the possibility to operate monolithic80 T coils anywhere (e.g. on synchrotron beamlines)and 90+ T dual coil systems. We will continue to buildsmall-scale coils in order to investigate the possibilitiesof high-strength conductors and zylon reinforcement toincrease maximum field and lifetime.P. Frings, J. Billette, J. Béard, F. Giquel, J-M. Lagarrigue, M. Suleiman107

MAGNET DEVELOPMENT 2011Cu/Nb nanocomposite wires processed by severe plasticdeformation for applications in high pulsed magnetsThe generation of high pulsed magnetic fields(B > 80 T) requires magnet winding materials withhigh electrical conductivity to minimize ohmic heatingeffects. At high field strength, the Lorentz forcesresults in extreme Von Mises stresses on the windingmaterial. Multi-functional materials with high electricalconductivity and high strength are thereforeneeded to safely meet these harsh operating conditionsin non-destructive magnets. The LNCMI togetherwith the Pprime Institute and CEA are involved inthis challenge and large efforts have been devoted tothe elaboration and to the characterization of plasticitymechanisms in copper-based nanocomposite wires,in particular Cu/Nb. Different techniques have beenapplied in addition to the classical macroscopic tensiletests; nanoindentation, in-situ deformation in transmissionelectron microscopy (TEM) and very recentlyin-situ deformation under neutron beam [Thilly etal., Appl. Phys. Lett. 88, 191906 (2006);Vidal etal., Scripta Mat. 60, 171 (2009)] and synchrotronbeam [Thilly et al., Appl. Phys. Lett. 90, 241907(2007); Thilly et al., Acta Mat. 57, 3157 (2009);Dubois et al., Acta Mat. 58, 6504 (2010)]. Resultson the plasticity mechanisms observed in the fcc/bccCu/Nb nanofilamentary wires have been obtained andwe derive guidelines for further optimization. Thetypical resistivity values at 77 K are between 0.4 and0.6 µΩcm. The decrease of the electrical conductivitywith a reduction in diameter results from theincrease of dislocation density, the dislocations beingscattering centers for electrons, and also from the sizereduction of the finest Cu channels, the thickness ofwhich becoming comparable to the mean free path ofelectrons at 77 K (∼200 nm). The resistivity satisfiesthe requirements for magnet applications.Macroscopic tensile testsNanofilamentary Cu/Nb wires have been tested atroom temperature (RT) and 77 K: when the Nb filamentdiameter d Nb is smaller than 500 nm, they exhibitUltimate Tensile Strength (UTS) very much higherthan a prediction based on a rule of mixture calculation.As example, σ UT S = 1.5 GPa at RT and1.9 GPa at 77 K for a conductor with a total diameterof 2.5 mm, with 85 4 filaments (X Nb = 28 %)with diameter of 142 nm and inter-filamentary spacingd Cu−0 = 43 nm. Such ultra high strength canonly be explained by modifications of plasticity mechanismsinduced by the nanostructuration of the differentphases. In particular, the microstructural evolution ofthe Nb nanofilaments toward a single crystalline structurewith simple crystallographic orientation (〈110〉)suggests a possible whisker-like behaviour. For the Cumatrix, size effects are expected but cannot be distinguishedfrom filaments effect in the macroscopic data.Similar high strength was obtained in Cu/Nb/Cu systemwith a strong size effect when decreasing the Nbnanotube thickness.Plasticity mechanismsAll the experimental techniques applied to the Cu/Nbsystem allowed for a better understanding of themechanical properties of the multi-scale Cu-basednanocomposites. All the results reported from macroscopicand microscopic microstructural and mechanicalinvestigations are evidencing size effects on the plasticitymechanisms in the nanostructured counterpartsof the nanofilamentary wires: for microstructural dimension,d, below 100-200 nm, the traditional deformationmechanisms involving intra-granular dislocationsources and dislocation pile-ups at grain boundary(GB) cease to operate because of space restriction;a single dislocation regime is acting, with dislocationsources within GBs or inter-phase interfaces.This mechanism can be related to the Orowan scalinglaw:σ y−Orowan ∝ (1/d) · ln(d/b)where σ y−Orowan is the yield stress and b the lengthof the Burgers vector. For the Nb nanofilaments, awhisker-like behaviour was evidenced with the associatedexponential dependence. The demonstrated validityof these scaling laws allows for their use as guidelinesto further improvement of nanomaterials mechanicalproperties.Co-deformation propertiesThe complex micro-structure of the Cu/Nb nanofilamentarywires has a strong impact on the macroscopicdeformation properties. The largest Cu channels arethe first regions to enter in the plastic regime, whileall other regions are still elastic. Then the finest Cuchannels start to yield at very high stress; a strongload transfer from the Cu matrix onto the Nb nanofilamentsappears. The most interesting property is theextremely high elastic limit of Nb nanofibres, close totheir theoretical strength. It allows them to sustain allthe plastic deformation of the Cu matrix via massivedislocation storage at Cu-Nb interfaces. All combined,these co-deformation mechanisms bring two essentialproperties to the multi-scale nanostructured Cu/Nbwires: ductility and strength. The latter is associatedto the Cu/Nb nanocomposite regions while theformer is brought by the large Cu channels.F. Lecouturier, N. Ferreira, L. Bendichou, P. DelescluseL. Thilly, J. B. Dubois, P. O. Renault (Institut Pprime, Poitiers), H. van Swygenhoven, S. van Petegem(PSI, Zurich Switzerland), P. Olier (CEA-DEN-LTMEX, Saclay), M. Di Michiel (ESRF, Grenoble)108

2011 MAGNET DEVELOPMENTPulsed energy suppliesThe 14 MJ capacitor bankThe 14 MJ capacitor bank functioned without any majorproblems. The 20 chargers (200 mA at 24 kV) havebeen replaced by 4 Technix chargers (833 mA at 24 kV)with an optional 2 chargers extra. All the chargersare put in parallel and can charge between 1 and 10modules. The advantages are enormous; much morespace in the 14 MJ room (nearly all the light-blue cabinetshave disappeared) so now the 1 MJ transportablegenerator can be installed in the same room (accessand installation greatly simplified). When using notall the 10 modules (this is most of the time the case)the charging speed is increased, and last but not leastfailure of a charger does not influence the availabilityof the installation and only increase the chargingtime slightly. This major modification was carefullyplanned; electronic circuits to adapt the programmablelogic controller (PLC) to the programming input of thecharger, as well as in-house-assembled anti-return HVdiodes were ready and tested before the summer shutdown.After the imposed holidays in the first half ofAugust the point of no return was reached when westarted to remove the old chargers and control units.The installation of the new chargers went rather welland the installation was working properly before thefirst users arrived by the end of September.FIG. 158. A view of the 14 MJ, 24 kV, 65 kA generatorin the basement, nearly all the light-blue charger cabinetshave disappeared.This long shut-down was also used to implement anew grounding scheme for the generators. Previouslythe grounding of the 14 MJ generator was totally isolatedfrom all the metallic structures and the EDFground except for one common ground connection.This grounding scheme is safe as long there is no othergalvanic connection possible to any metallic structure.With the installation becoming more and more complexthis condition became more and more difficult torealize and to guarantee. After a thorough analysisit was decided to modify the grounding to the morecommon scheme of a grounding grid where all metallicparts are connected via redundant connections to thecapacitor ground and to the EDF ground. This schemecannot be invalidated by an addition or removal of asingle connection and is thus considered safer and morestable in a complex situation.The 1 MJ & 290 kJ transportable banksThe transportable generators are now in full operation.The only modification is that we have replaced theindustrial-PC by a standard notebook. The successfuloperation of these unique units at the ILL, ESRF andLULI drew attention from other institutes. A use ofthe installation at SLS in Zürich has been planned for2012 and SOLEIL is interested as well.The 6 MJ transportable capacitor bankFinally we obtained permission to put out a call fortender for a 6 MJ transportable generator. This, muchdelayed, acquisition will permit the laboratory to usemonolithic coils in the 80 T range, which is impossibleat the moment due to a too slow pulse of the 14 MJgenerator and too little energy of the 1 MJ generator.Software for generator and instrument controlAccording to the trend in the scientific community, wedecided to use the Python language as much as possiblefor the supervision tasks and the graphical userinterfaces. As a first step, we developed a new driverfor the “BMV” chargers. Since, a large number of toolshave been written to drive pulse generators, recorders,multimeters and other devices.Building and SafetyDue to the delay with the new building it is likely thatthe new 6 MJ generator will have to be placed outsideonce it arrives. Special safety procedures will have tobe imposed in order to use this generator in the old testboxes instead of more secure boxes of the new building.Safety remains a concern as far as this type of installationis not common in industry. An estimate based onthe TNT equivalent of the stored energy is generallyaccepted as an overestimation of the risk. The exactmagnitude of the risk is not easy to evaluate, but experiencewith coil-failures and other incidents at theLNCMI and other installations seem to suggest thatonly a small amount of the stored energy is likely tobe transformed into kinetic energy of the fragments.OutlookApart from the building and a correct dimensioning ofthe safety measures the most important task will bethe upgrade of the two small “BMV” capacitor banks.This upgrade includes an increase in energy as well asan increase in the maximum available and acceptablecurrent. The hard-wired control logic of the 290 kJgenerator will be replaced by a PLC and a µ-PC inorder to make the interface of this installation identicalto that of the 1 MJ generator.P. Frings, B. Griffe, J-P. Nicolin, T. Schiavo, L. Drigo109

MAGNET DEVELOPMENT 2011Helix development for DC high field magnetsSince November 2010, the use of 30000 A polyhelix iseffective on each of the three 24 MW sites of the facility.14, 12 and 6 polyhelix inserts are in operationproviding respectively 35 T in 34 mm, 30 T in 50 mm,and 20 T in 160 mm when in operation in a 10 T backgroundmagnetic field provides by a set of two Bitterstacks. Up to these magnetic field values, polyhelix insertshave exhibited a remarkable reliability; betweenSeptember 2009 and December 2011 the 35 T magnethas cumulated 2500 hours of magnet time with a meanpower of 8 MW (4 MW on the insert and 4 MW onthe outer Bitter stacks) without any degradation onthe magnet. In the frame of the hybrid project (43 Twith possible further upgrade), it is necessary to pushthe polyhelix technology to its limit. Consequently itis interesting to test new inserts without interferingwith the congested planning of the three 24 MW sites.For this reason the construction of a dedicated housingto run polyhelix inserts without the surrounding Bitterstacks was decided. Figure 159 shows the housingunder construction.The new site will be operational March 2012, after thewinter shutdown. In a first step spare inserts of the35 T magnet will be put into operation. In this caseinserts will not be limited mechanically, allowing themto work at higher current to explore running conditionsunder higher temperatures. This magnet will providesto users an easy access to 25 T helping to decrease thepressure on the high field sites. In parallel, and withthe aim to explore the behavior of helices near theiryield strength, we have ordered copper alloy tubes withhigher electrical conductivity and consequently loweryield strength. The aim will be to approach 30 T witha maximal power of 12 MW and to run the magnetnear its yield strength to study the influence on thelife time. This high magnetic field version is plannedfor the last quarter of 2012.FIG. 160. The ”mixed” insert to be tested in 2012.FIG. 159. View of the new dedicated housing for polyhelixinserts. The new site will be operational March 2012.In parallel to this process, the laboratory is continuingthe development of radially cooled helices. A “mixed”insert (1 radially cooled helix placed at the center of aset of longitudinally cooled helix), figure 160, will betested during 2012.C. Auternaud, F. Debray, J. L. De Marinis, J. Dumas, T. Disparti, M. Kamke, J. Matera, C. Mollard, R.Pfister, D. Ponton, P. Sala, J. Spitznagel, C. Trophime, J. M. Tudela, E. Verney, N. Vidal110

2011 MAGNET DEVELOPMENTCopper/stainless steel polyhelix magnetsThe good and flexible compromise obtained betweenmechanical strength and electrical conductivity(see figure 161) suggests that copper/stainless steel(Cu/SS) composite conductors can improve the performancesof polyhelix-type magnets, thereby increasingthe strength of static magnetic fields. We designed amachined by electrical discharge machining (EDM).The outer surface of the internal helix is insulated withKapton polyimide film.The testing was based on the principle of thecoilin/coilex system. It was performed by positioningthe pair of helices, energized by a fast 100 kJgenerator, in an external coil (B = 20 T), powered bythe 14 MJ, 24 KV capacitor bank of LNCMI-T. Themagnet was operated in liquid nitrogen. The coolingtime between two shots was less than 1 minute. Thelast shot before the damaging one has produced a fieldof 25.7 T: 17.1 T in the external coil and 8.6 T in thepair of helices (see figure 163).FIG. 161. Yield strength versus conductivity for materialsused in dc magnets and pulsed magnets.pair of helices based on a Cu/SS structure consistedof a copper internal tube and a 304L stainless steelexternal tube. Taking a yield strength for low coldworked304L of 850 MPa at 77 K, a maximum field of31 T can be generated.FIG. 163. Magnetic field during the test of a copper/stainlesssteel pair of helices.During the next shot, increasing the magnetic field amplitude,the failure of the coil occurred without collateraldamage. Post-mortem analysis has evidencedthat the failure results from an electrical arc betweenthe two helices near the first turn. That is probablycaused by insulation breakdown due to axial movementof the edge of the coils.FIG. 162. The pair of copper/stainless steel helices afterelectrical discharge machining.The raw materials, i.e. copper and stainless steeltubes, have been machined with very tight tolerances.Each polyhelix has been assembled under a hydraulicpress. The composite helices (see figure 162) have beenWe have demonstrated that copper/stainless steel compositestructures can be straightforwardly adapted tothe polyhelix concept. A first prototype has been designed,built and tested and the first result is promising.New developments to increase the magnetic field, dealingwith the materials selection as well as the assembly,are in progress in order to realize a high duty cycle 40 Tmobile pulsed field user platform dedicated to neutronand X-ray diffraction experiments.F. Lecouturier, N. Ferreira, L. Bendichou, J. Billette, J. Béard, F. Debray, T. Disparti, J. M. Tudela111

MAGNET DEVELOPMENT 2011First electron cyclotron resonance ion source using radially cooledpolyhelicesThanks to the high plasma density available inElectron Cyclotron Resonance Ion Sources (ECRIS),they can deliver high ionic currents. The plasma densityof these devices varies as the square of the resonancefrequency, i.e. as the square of the magneticfield. The present highest ECR frequency usedin the world is 28 GHz. With the joint (LPSC-LNCMI) development of the SEISM (Sixty gigahertzElectron cyclotron resonance, Ion Source usingmegawatt Magnets), we plan to operate an ECRIS at60 GHz in order to extract four times more ionic currentthan in other sources worldwide. To reach thisgoal, the plasma has to be magnetically confined withan induction about 4 times higher than the ECR’s one.Figure 164 shows a cross-section view of the SEISMprototype where the grey cylinder, on the axis, is theplasma chamber with its extraction electrodes on theright.ECRIS at high frequencies (28-60 GHz) require a veryspecific environment with high safety constraints (Xrays,microwaves, high voltage). LPSC with its acceleratorand ion sources pole has all the infrastructureallowing the operation of such an equipment. AnEQUIPEX (COLOSSECRIS) has been proposed byLPSC in 2011 and gathering LPSC, LNCMI, theFIG. 165. Axial magnetic induction measurements onaxis (146.25 A: flux variation integration; 15000 A : Hallprobes)FIG. 164.Cross-section of the SEISM ECRIS prototype.It is magnetized by the four central copper helices poweredby the means of semi rigid current leads formedby multiple copper foils. Magnetic measurements withHall probes have been performed up to 15000 A atthe LNCMI facility. To validate our measurements,we have measured the axial magnetic field at low current(146.25 A) with a new flux variation integrationsystem. Figure 165 shows the comparison betweenthese two measurements. The high current inductionvalues have been divided by a factor 105 in orderto fit the Bmax value obtained at low intensity.A good agreement between the two measurements interms of intensity and maximas positions is observed(Fig 165). The detailed study of B shows that theresonance zone looks like an ellipsoid with its greataxis in the radial direction, and the agreement withthe previous simulations is satisfying. The measureddata show that the magnetic field topology and magnitudesare suitable to use the magnetic structure fora 28 GHz ECRIS. The beams extracted from this newprototype will have to be characterized during the nextexperiments. The future: The ion beam studies with’Grand Accelerateur National d’Ions Lourds’ and the’Institut de Physique Nucleaire de Lyon’. This projectissued from COLOSSUS, consists of two equipments;a high performance 28 GHz superconducting source forthe SPIRAL2 project at GANIL, and a R&D platformfor split magnets, interesting for future high frequencyECR ion sources at LPSC or extreme conditions sciencesat ESRF and ILL (X-rays or neutrons with highfields).FIG. 166. COLOSSECRIS superconducting bus betweenLNCMI and LPSC, extension towards ESRF and ILLThe core of the platform consists in 2 semi-rigidcryostats equipped with MgB 2 superconducting cablesat 20 K allowing the transport of the power(4 × 15000 A - 400 V) from LNCMI to LPSC, with afuture possible extension towards ESRF and ILL dueto the modularity of the system (figure 166). A highintensity beam line and a magnetic field measurementstand will be available for the various scientific programmes.R. Barbier, F. Debray, J. Matera, P. Sala, C. Trophime, C. Grandclement, P. PetmezakisJ. Jacob, C. Fourel, T. Lamy, M. Marie-Jeanne, P. Sole, J-L Vieux-Rochaz (LPSC, Grenoble, France)112

2011 MAGNET DEVELOPMENTUse of high field magnets for magnetic levitationThe principle of magnetic levitation in static magneticfield consists in compensating the gravity by applyingmagnetic volume force to a pure material directed antiparallelto the gravitational force. A diamagnetic orparamagnetic material of small volume V , having amagnetic susceptibility χ, exposed to a magnetic fieldB is submitted to magnetic volumic forces f m given by,f m = 12µ 0χ grad(B 2 )with µ 0 the magnetic permeability of vacuum. If weset G = grad(B 2 ), the exact compensation of gravityg occurs when,the Bitters is set to its maximum of 31 kA) for differentpoints of levitation [Mailfert et al., J. Phys. Conf.Ser. 97 012199 (2008)]. The volume at 5% of residualgravity is then obtained from the surface delimited bythe isovalue of ‖ɛ‖ = ‖G − G 0 ‖/‖G 0 ‖ in a cylindricalgrid (cf. figure 167 and figure 168) by using theGuldin-Pappin theorem. The values are reproduced intable IV. All the calculations were performed using analyticalexpressions of B under the assumptions thatthe magnets are axisymmetric and the current distributionis of Bitter type.G = G 0 = 2 µ 0 g ρ.χThe following values of ‖G 0 ‖ : −1000 T 2 /m, −2800T 2 /m and −4000 T 2 /m are necessary to achieve compensationin hydrogen, water and helium respectively.In a 24 MW 34 mm bore diameter magnet we can reachup to ‖G‖ = ±6000 T 2 /m. Our high magnetic fieldinserts may thus be used for magnetic levitation purposes.However due to constraints of the experimentalsetup (bore diameter must be greater than 170 mm)we cannot use this magnet. So we have considered analternative configuration consisting of the two existingM9 bitters and a set of 6 helices (the outermost ofthe actual 14 helices insert). For this magnet we haveconsidered its application to the levitation of liquid hydrogen.FIG. 168. Detailed view of the levitation zone for liquidH 2.TABLE IV. Theoretical volume levitated at 5% residualgravity. Z m1 denotes the coordinates of the levitationpoint, B 0 the magnetic at this point and P the power consumptionof the magnet.V ol(5%)V ol specrepresents the ratio oflevitated volume at 5% over the volume of a cylindrical experimentalcell with a 3 cm radius, 10 cm height and whosebase is at z = Z m1.Z m1 [cm] B 0 [T] I H [kA] P [MW] Vol(5%) [cm 3 V ol(5%)]V ol spec[%]12.56 12.53 15.33315 14.4 351.44 25.013.57 12.153 15.488 14.4 355.76 19.214.84 11.74 16.126 14.6 294.73 13.3FIG. 167. Isovalue of ɛ in a cylindrical grid - The magnetsare represented in grey. The bore diameter is about180 mm.The approach used for this study was to find the currentI H in the polyhelices insert (while the current inThis study has been carried out in collaboration withCEA/INAC-SBT. Further studies will be considered toinvestigate the impact of the “real 3D” magnet geometryon the levitation zone. Experiments are plannedfor the first semester of 2012.C. Trophime, F. DebrayD. Chatain, C. Jeandey, F. Millet (DSM-INAC-CEA, Grenoble)A. Mailfert (LEM, Nancy University)113

MAGNET DEVELOPMENT 2011Final design of the new Grenoble hybrid magnetA CEA-CNRS French collaboration is currently developinga new hybrid magnet. Its funding has been completedin 2011 within the framework of the EquipexLaSUP. This hybrid magnet combines a resistive insertcomposed of Bitter and polyhelix coils and a newlarge bore superconductor outsert to produce an overallcontinuous magnetic of 42+ T in a 34 mm warmaperture.The design of the superconducting coil outsert has beencompleted after thorough studies and successful experimentalvalidation phases. Based on the novel developmentof a Nb-Ti/Cu Rutherford Cable On ConduitConductor (RCOCC) cooled down to 1.8 K by themean of a bath of superfluid helium at atmosphericpressure, the superconducting coil aims to produce acontinuous magnetic field of 8.5 T in a 1.1 m cold borediameter.The mechanical behavior of the cold mass has beenstudied under two loads; cooling down from room temperatureto operating temperature and Laplace forcesat operating temperature. Computations have beenhandled in 2D and 3D to take into account the axisymmetricdiscretization of the tie rods. Table V showsthe maximal stresses induced in each component underthese two loads. The stress level in the coil due toLaplace forces is large compared to the ones inducedby the cooling down. In any case, the maximal VonMises stresses are below the allowable stresses, exceptat 9 T in the copper, with a maximal stress of 213 MPa,slightly greater than the allowable 211 MPa. The totalshear stress in the insulation reaches a maximum of18 MPa at 8.5 T and 19 MPa at 9 T, below the usualvalues of the allowable shear stresses.Electromagnetic interactions between superconductingand resistive coils can generate very high forces if theyare not precisely aligned along the main axis or if theydo not respect symmetry with the middle plan. Thiscould occur if an incident happens on the resistive coils,a partial short-circuit for example. The worst scenario,a short-circuit of one half of the resistive coils, has beenstudied; it is a very complex and long calculation butit can be solved analytically as we considered the mutualinductances and the resistances to be constant. Itshows vertical forces up to 231 kN on the superconductingcoil, large enough to raise the magnet from itsbase plate.Large superconducting magnets are usually designedto fulfil the “Stekly” criterion, ensuring that the powerevacuated by conduction is systematically larger thanthe power generated by Joule heating effect during apossible transition of the superconductor. This is thecase in all parts of the winding of the superconductingoutsert of the hybrid magnet and such a criterion providesan unconditional stability of the conductor byenabling the spontaneous return of the conductor tothe superconducting state after a “local” transition.At a nominal current of 7100 A, the energy stored inthe superconducting coil is equal to 76 MJ. In case ofquench this energy will be dumped into a resistor of70 mΩ limiting the maximum temperature to below100 K and the maximum voltage to 500 V.FIG. 169. Three dimensional representation of the outermostsuperconducting part of the hybrid magnet.Supercon. cable Cu-Ag stabiliser InsulationCool down 73 62 50Laplace force 141 169 96Total 148 164 116Allowed value - 211 -TABLE V. Maximal stresses in MPa under loading of themagnet at 8.5 T. The allowed value is 2/3 of the yield stress.F. Debray, W. Joss, R. Pfister, P. Pugnat, L. Ronayette, C. Trophime, and H. XiaoG. Aubert, C. Berriaud, R. Berthier, P. Fazilleau, B. Hervieu, FP. Juster, M. Massinger, C. Mayri, C.Pes, Y. Queinec, (CEA/Saclay, Gif-sur-Yvette, France)114

2011 PROPOSALSProposals for magnet time – Projects carried out in 2011APPLIED SUPERCONDUCTORSMeasurement of HTS strands and cable.A. Ballarino (CERN, European Organization for Nuclear Research, Geneva, Switzerland), G. De Rijk (CERN,European Organization for Nuclear Research, Geneva, Switzerland), J. Fleiter (CERN, European Organizationfor Nuclear Research, Geneva, Switzerland).Test of Rutherford Cable On Conduit Conductor (RCOCC) for the Hybrid ProjetW. Joss (LNCMI-CNRS, Grenoble, France)Magnetic field dependence of transport properties for doped Bi-2212 round wiresA. Sulpice (CRETA-CNRS, Grenoble, France), S. Zhang (Northwest Institute for Nonferrous Metal Research,Xi’an, China), Ch. Li (Northwest Institute for Nonferrous Metal Research, Xi’an, China)Magnetic field dependence of superconducting critical current of MgB 2 based wires and tapesA. Tumino (Columbus Superconductors SpA, Genova, Italy), S. Brisigotti (Columbus Superconductors SpA,Genova, Italy), V. Cubeda, M. Palombo (Columbus Superconductors SpA, Genova, Italy)Solenoid inserts testing for dipole developmentJ-M. Rey (CEA-IRFU, Gif sur Yvette, France) P. Tixador (CNRS-IN, INP, Grenoble, France) Th. Lecrevisse(CEA-IRFU, Gif sur Yvette, France) M. Devaux (CEA-IRFU, Gif sur Yvette, France) X. Chaud(CNRS-LNCMI, Grenoble, France)MAGNETISMComposition and temperature dependence of the metamagnetic transition in La(Fe,Co) 12 B 6 systemO. Isnard, Diop L.(CNRS, France) G. Ballon (LNCMI-CNRS, Toulouse, France).High-field magnetization of multiferroic HoMnO3X. Fabreges (LNCMI, France) Petit S. (Laboratoire Lon Brillouin, France) W. Knafo, G. Ballon(LNCMI-CNRS, Toulouse, France).High-field magnetoresistance measurements on ultra-pure URu 2 Si 2 single crystalsG. Scheerer, (LNCMI-CNRS, Toulouse, France) D. Aoki , J. Flouquet (CEA Grenoble, FRANCE).Measurements under high magnetic fields of the spin flop transitions in quasi two dimensional (2D)antiferromagnetO. Isnard (CNRS, France) E. G. Souza dos Santos (UFRGS, Brazil) L. Diop (UJF, France) G. Ballon(LNCMI-CNRS, Toulouse, France).Effect of the spin flop on the polarization of multiferroic BiFeO 3M. Viret, R. Moubah (CEA Saclay, France) B. Raquet (LNCMI-CNRS, Toulouse, France).Field Induced Phases in Class of Prototypical Magnetic Bose Glass CompoundsD. Huvonen, S. Zhao (ETH Zurich, Switzerland) G. Ballon (LNCMI-CNRS, Toulouse, France).High-field magnetization of U-based ferromagnetic superconductorsF. Hardy F., C. Meingast (Karlsruher Institut fur Technologie, Germany) D. Aoki, J. Flouquet (CEAGrenoble, FRANCE) W. Knafo, G. Ballon (LNCMI-CNRS, Toulouse, France).High magnetic-field phase diagram of CeRhIn 5−x Sn x115

PROPOSALS 2011G. Knebel, G. Lapertot, J. Buhot, D. Aoki, J. Flouquet (CEA Grenoble, FRANCE) W. Knafo, D. Vignolles(LNCMI-CNRS, Toulouse, France).High-field magnetoresistance of U-based ferromagnetic superconductorsD. Aoki, J. Flouquet (CEA Grenoble, FRANCE) W. Knafo, G. Scheerer(LNCMI-CNRS, Toulouse, France).High-field magnetoresistance of the hidden-order system URu 2 Si 2N. Gales, D. Aoki, J. Flouquet (CEA Grenoble, FRANCE) W. Knafo, C. Proust (LNCMI-CNRS, Toulouse,France).High magnetic field study of magnetic ground state of tetragonal iron chalcogenidesR. Viennois (Institut Gerhardt, Universite Montpellier 2, France) E. Giannini (Universit de Genve,Switzerland) G. Ballon (LNCMI-CNRS, Toulouse, France).Global ferrimagnetic ordering in Gd 60 Mn 30 X 10 (X = In, Al) due to the antiferromagnetic coupling between Gdand Mn atomsC. Mayer, B. Chev (ICMCB , France) G. Ballon (LNCMI-CNRS, Toulouse, France).Magnetization in pure and impurity doped BiCuPO Spin laddersJ. Bobroff, N. Laflorencie (Laboratoire de Physique des Solides, Universite Paris Sud, CNRS, France) A.Mahajan (IIT Bombay, Inde) B. Vignolle, W. Knafo(LNCMI-CNRS, Toulouse, France).Quantum critical end point in UGe 2 .V. Taufour (CEA, Grenoble, France), L. Malone (LNCMI-CNRS, Grenoble, France), H. Kotegawa (KobeUniversity, Kobe, Japan), D. Aoki (CEA, Grenoble, France), G. Knebel (CEA, Grenoble, France), J. Flouquet(CEA, Grenoble, France), I. Sheikin (LNCMI-CNRS, Grenoble, France)Bose-Einstein condensation of degenerate incommensurate magnons in BiCu 2 PO 6H. Ronnow (EPFL, Lausanne, Switzerland), C. Ruegg (London Center for Nanotechnology, London, UK), S.Wang (Paul Scherrer Institute, Villigen, Switzerland), F. Casola (ETHZ, Zurich, Switzerland), T. Shiroka(ETHZ, Zurich, Switzerland)Study of quantum spin liquid under high magnetic field.M. Yamashita (Kyoto University, Kyito, Japan)Magnetization of Permanent Magnet Structures for NMR and MRI.D. Sakellariou (DSM/IRAMIS/SIS2M/LSDRM CEA Saclay, Gif sur Yvette, France)Magnetic field induced isotropic nematic phase transition in ferromagneticsP. Kopcansky (Institute of experimental physics, SAS, Kosice, Slovakia)Electromagnon in magnetoelectric epsilon-Fe 2 O 3V. Goian (Institute of Physics, Academy of Science, Prague, Czech Republic)Anisotropy of magnetic field induced isotropic-nematic phase transition in ferronematicsP. Kopcansky (Institute of experimental physics, SAS, Kosice, Slovakia)MAGNETIC RESONANCENMR investigation of the putative FFLO phase in κ-(BEDT-TTF) 2 Cu(NCS) 2 .H. Mayaffre (LNCMI-CNRS, J. Fourier University, Grenoble, France), V. Mitrovic (Brown University,Providence, USA), C. Berthier (LNCMI-CNRS, Grenoble, France), M. Horvatic (LNCMI-CNRS, Grenoble,116

2011 PROPOSALSFrance)High-field NMR investigation of the disordered antiferromagnet Bi(Cu (1−x) Zn x ) 2 PO 6 for x=0.01/0.05.F. Casola (ETHZ, Zurich, Switzerland), T. Shiroka (ETHZ, Zurich, Switzerland), H.R. Ott (ETHZ, Zurich,Switzerland), Ch. Ruegg (London Center for Nanotechnology, London, UK), M. Kenzelmann, J.F. Mesot (PaulScherrer Institute, Villigen, Switzerland)Towards sensitivity enhanced high resolution NMR at 30 T.S. Krämer (LNCMI-CNRS, Grenoble, France), H. Stork (LNCMI-CNRS, Grenoble, France), M. Horvatic(LNCMI-CNRS, Grenoble, France), C. Berthier (LNCMI-CNRS, Grenoble, France)High-field part of the phase diagram of novel quasi-1D Ising-like quantum antiferromagnet.M. Klanjsek (Jozef Stefan Institute, Ljubljana, Slovenia), M. Horvatic (LNCMI-CNRS, Grenoble, France), C.Berthier (LNCMI-CNRS, Grenoble, France), S. Krämer (LNCMI-CNRS, Grenoble, France), S. Mukhopadhyay(LNCMI-CNRS, Grenoble, France)NMR study of the field dependence of stripe order in YBCO superconductors.M-H. Julien (LNCMI-CNRS, Grenoble, France)NMR study of the quantum criticality of BiCu 2 PO 6F. Casola (ETH Zurich, Zurich, Switzerland), T. Shiroka (ETH Zurich, Zurich, Switzerland), C. Raegg (PaulScherrer Institute, Villigen, Switzerland), Sh. Wang (Paul Scherrer Institute, Villigen, Switzerland), H.Ronnow (Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland), J-F. Mesot (Paul ScherrerInstitute, Villigen, Switzerland), H-R. Ott (ETH Zurich, Zurich, Switzerland)RF induced flux lattice annealing (RIFLA) in the Superconductor LuNi 2 B 2 CG. Clark (Department of Physics and Astronomy, UCLA, Los Angeles, USA)NMR investigation of disorder effects in the Bi(ZnxCu 1−x ) 2 PO 6 quantum magnetF. Casola (ETH, Zurich, Switzerland)HF-EPR study of layered carbon nanomaterialsP. Neugebauer (J.W.Goethe University of Frankfurt, Frankfurt, Germany)HF-EPR of one-electron oxidized copper(II) salophen complexesF. Thomas (J. Fourier University, Grenoble, France)HF-EPR study of R 2 RiboNucleotide Reductase from bacillus CereusK.K. Andersson (University of Oslo, Oslo, Norway)METALS AND SUPERCONDUCTORSQuantum oscillations under pressure in cuprates superconductorsB. Ramsaw, J. Day, D. Bonn (University of British Columbia, Canada) S. Badoux, D. Vignolle, C. Proust(LNCMI-CNRS, Toulouse, France).Fermi surface investigation of κ-(BEDT-TTF) 2 -Cu[N(CN) 2 ]-Br x -Cl 1−xP. Batail (CIMI, France) S. Lepault., D. Vignolles, C. Proust (LNCMI-CNRS, Toulouse, France).Upper critical magnetic field in single-crystals of Eu 0.5 K 0.5 Fe 2 As 2 and K x Fe 2 Se 2 compoundsV. Gasparov (Institute of Solid State Physics Russian Academy of Sciences, Russia) L. Drigo, A.Adouard(LNCMI-CNRS, Toulouse, France).117

PROPOSALS 2011Ultrasound measurement in pulsed magnetic fields : hidden phase transition in high-T c superconductorsD. Bonn (University of British Columbia, Canada) C. Proust, B. Vignolle, D. Leboeuf(LNCMI-CNRS,Toulouse, France).Two-dimensional superconducting electron gas at oxide interfacesN. Bergeal, J. Lesueur, J. Biscaras (LPEM CNRS ESPCI ParisTech, France) C. Proust (LNCMI-CNRS,Toulouse, France).Electric field effect studies of magnetotransport at LAALO 3 /SRTIO 3 interfacesS. Gariglio, A. Cavaglia,A. Fete, C. Cancellieri, J.M. Triscone (DPMC, University of Geneva, Switzerland) C.Proust (LNCMI-CNRS, Toulouse, France).Charge transport and torque magnetometry in graphite up to 70 TB. Fauque, K. Behnia (ESPCI, France) D. Vignolles, C. Proust (LNCMI-CNRS, Toulouse, France).Enhancement of quasiparticle mass close to maximum Tc in BaFe 2 (As/P) 2A. Carrington, A. Coldea (Bristol University, UK) C. Proust (LNCMI-CNRS, Toulouse, France).Pauli and orbital effects in the q-1D organic compound (TSeT) 2 ClV. Laukhin, E. Canadell(ICMAB, Spain) D. Vignolle, A. Audouard(LNCMI-CNRS, Toulouse, France).Negative c-axis magnetoresistance in graphiteY. Kopelevich (Instituto de Fisica Gleb Wataghin, Bresil) B. Jouault ( Groupe d’Etude des Seminconducteurs,France) W. Escoffier(LNCMI-CNRS, Toulouse, France).Evolution of magnetotransport properties with doping in underdoped YBCOB. Ramsaw, D. Bonn, L. Taillefer (University of British Columbia, Canada) D. Leboeuf, S. Lepault, D.Vignolles, C. Proust (LNCMI-CNRS, Toulouse, France).Normal state transport properties of Hg1201L. Taillefer, N. Doiron-Leyrau, J. Chang, F. Laliberte, O. Cyr-Choi(Université de Sherbrooke, Canada) M.Greven (University of Minnesota, USA) C. Proust (LNCMI-CNRS, Toulouse, France).In plane magnetotransport in electron-Doped high-T c superconductorsM. Kartsovnik, T. Helm, W. Biberacher (Walther-Meissner-Institut, Germany) C. Proust (LNCMI-CNRS,Toulouse, France).superconductor/insulator transition in LaSrCuOB. Leridon (CNRS, France) B. Vignolle (LNCMI-CNRS, Toulouse, France).Ultrasound measurement in pulsed magnetic fields : elemental Bismuth beyond the quantum limit and hiddenphase transition in high-T c superconductorsD. Bonn (University of British Columbia, Canada) Fauque B. (ESPCI, France) D. Leboeuf, B. Vignolle, C.Proust (LNCMI-CNRS, Toulouse, France).Quantum Oscillations in high purity, underdoped cuprate superconductorsD. Bonn D. , B. Ramsaw, R. Liang, J. Day (University of British Columbia, Canada) C. Proust(LNCMI-CNRS, Toulouse, France).Quantum oscillations in LaAlO 3 /SrTiO 3 interfaces in the two dimensional conduction regimeA. Caviglia, S. Gariglio, C. Cancellieri, J.M. Triscone (DPMC, University of Geneva, Switzerland) C. Proust(LNCMI-CNRS, Toulouse, France).High-field magnetostriction and magnetization of heavy-fermion systemsD. Aoki, J. Flouquet (CEA Grenoble, FRANCE) W. Knafo, G. Ballon(LNCMI-CNRS, Toulouse, France).Quantum oscillations in cuprates: the case of the electron-doped materialR. Greenz (U. of Maryland, USA) D. Leboeuf, S. Lepault, D. Vignolles, B. Vignolle, C. Proust(LNCMI-CNRS, Toulouse, France).118

2011 PROPOSALSFermi surface investigation of high T c superconductorsD. Bonn (University of British Columbia, Canada) S. Lepault, D. Vignolles, B. Vignolle, C. Proust(LNCMI-CNRS, Toulouse, France).Search for metamagnetism and field induced quantum criticality in YbCu 2 Si 2 in pulsed magnetic fields.D. Braithwaite, A. Fernandez-Panella, L. Malone (CEA, France) W. Knafo (LNCMI-CNRS, Toulouse, France).De Haas-van Alphen study of the valence fluctuation compound EuIr 2 Si 2 .O. Ignatchik (Dresden High Magnetic Field Laboratory, Dresden, Germany), A. Polyakov (Dresden HighMagnetic Field Laboratory, Dresden, Germany), J. Wosnitza (Dresden High Magnetic Field Laboratory,Dresden, Germany), S. Seiro (MPI for Chemical Physics of Solids, Dresden, Germany), Ch. Geibel (MPI forChemical Physics of Solids, Dresden, Germany)Normal state magnetotransport anisotropy of the electron-doped high-Tc superconductor Nd 2−x Ce x CuO 4 .M. Kartsovnik (Walther-Meissner-Institut, Garching, Germany), T. Helm (Walther-Meissner-Institut,Garching, Germany), W. Biberacher (Walther-Meissner-Institut, Garching, Germany)Fermi surface properties in the ferromagnetic superconductor UCoGe.D. Aoki (CEA, Grenoble, France), I. Sheikin (LNCMI-CNRS, Grenoble, France)Field effect in Ferromagnetic Superconductors.L. Malone (LNCMI-CNRS, Grenoble, France), J. Flouquet (CEA, Grenoble, France), D. Aoki (CEA, Grenoble,France), W. Knafo (LNCMI-CNRS, Toulouse, France)Superconducting-insulating transition in BSCCO thin films.H. Raffy (LPS-CNRS - Orsay, France), I. Matei (Institut Néel-CNRS, Grenoble, France), V. Jovanovic(Institute of Physics, Belgrade, Serbia), P. Monceau (Institut Néel-CNRS, Grenoble, France), G. Remenyi(Institut Néel-CNRS, Grenoble, France)Specific heat on iron pnictides and related compounds.C. Marcenat (CEA, Grenoble, France), T. Klein (Institut Néel-CNRS, Grenoble, France), A. De Muer(LNCMI-CNRS, Grenoble, France), K. Hasselbach (Institut Néel-Cnrs, Grenoble, France), W.K. Kwok(Argonne National Laboratory, Argonne, Illinois USA), H.H. Wen (Institute of Physics, Beijing, China), Z.S.Wang (Institute of Physics, Beijing, China), P. Toulemonde (Institut Néel-Cnrs, Grenoble, France), D.Braithwaite (CEA, Grenoble, France)Nernst effet in bismuth for a field oriented along the bisectrix axis.K. Behnia (LPEM-ESPCI, Paris, France), B. Fauqué (LPEM-ESPCI, Paris, France), Z. Zhu (LPEM-ESPCI,Paris, France)Shubnikov de Haas effect in URhGe.E. Yelland (University of Edinburgh and University of St. Andrews, UK), A. Huxley (University of Edinburgh,Edinburgh, UK), J. Barraclough (University of ST. Andrews, ST. Andrews, UK)High-field thermoelectric response in Fe 1+y Se x Te 1−x .A.Pourret (CEA, Grenoble, France), L. Malone (LNCMI-CNRS, Grenoble, France), D. Braithwaite (CEA,Grenoble, France), G. Knebel (CEA, Grenoble, France), K. Behnia (LPEM-ESPCI, Paris, France), B. Fauqué(LPEM-ESPCI, Paris, France)Thermoelectric effects on very underdoped YBCO.L. Taillefer (Sherbrooke University, Sherbrooke, Canada), N. Doiron-Leyraud (Sherbrooke University,Sherbrooke, Canada), E. Hassinger (Sherbrooke University, Sherbrooke, Canada), F. Laliberté (SherbrookeUniversity, Sherbrooke, Canada), O. Cyr-Choiniare (Sherbrooke University, Sherbrooke, Canada), D. Bonn119

PROPOSALS 2011(University of British Columbia, Vancouver, Canada)High-field de Haas-van Alphen effect in non-centrosymmetric CeCoGe 3 and LaCoGe 3 : field-dependent splittingof the dHvA frequenciesI. Sheikin (LNCMI-CNRS, Grenoble, France), R. Settai (University of Osaka, Osaka, Japan)De Haas-van Alphen study of the isostructural compounds Ce 1−x Yb x CoIn 5O. Ignatchik (Dresden High Magnetic Field Laboratory, Dresden, Germany), A. Polyakov (Dresden HighMagnetic Field Laboratory, Dresden, Germany)Quantum metal phase in disordered superconductorsD. Shahar (Weizmann Institute of Science, Rehovot, Israel), B. Sacepe (Néel Institute-CNRS/UJF, Grenoble,France), A. Pourret (SPSMS, CEA-INAC and Université Joseph Fourier, Grenoble, France)Field induced Fermi surface transition in URhGeE. Yelland (University of Edinburgh and University of St Andrews, St Andrews, UK), A. Huxley (University ofEdinburgh, Edinburgh, UK), J. Barraclough (University of St Andrews, St Andrews, UK)Magnetotransport oscillations close to the extreme quantum limit at the LaAlO 3 /SrTiO 3 interfaceS. Gariglio (DPMC, University of Geneva, Geneva, Switzerland), A. Fete (DPMC, University of Geneva,Geneva, Switzerland), J-M. Triscone (DPMC, University of Geneva, Geneva, Switzerland), D. Stornaiuolo(DPMC, University of Geneva, Geneva, Switzerland), D. Li (DPMC, University of Geneva, Geneva,Switzerland)Field induced dimensional crossover in URhGeE. Yelland (University of Edinburgh and University of St Andrews, St Andrews, UK), A. Huxley (University ofEdinburgh, Edinburgh, UK), J. Barraclough (University of St Andrews, St Andrews, UK)Ultrasound measurements in the pseudogap phase of high-T c cuprates in high magnetic field D.Leboeuf (LNCMI-CNRS, Toulouse, France), C. Proust (LNCMI-CNRS, Toulouse, France), D. Bonn (UBC,Vancouver, Canada)Effect of Disorder on a Pauli Limited SuperconductivityV. Mitrovic (Physics Department, Brown University, Providence, USA)Probing the flux lines lattice motions in iron-pnictides superconductors by high magnetic field NMRL. Bossoni (University of Pavia-CNISM, Pavia, Italy)SEMICONDUCTORSQuantum PhaseTransitions in Corbino geometry in GrapheneE. Diez, C. Sanchez-Fabres Cobaleda,(Laboratorio de Bajas Temperaturas, Spain) W. Escoffier(LNCMI-CNRS,Toulouse, France).High magnetic field magneto-conductance in individual InAs nanowiresR. Leturcq, P. Caroff (IEMN, France) B. Raquet, V. Prudkovsky, M. Goiran (LNCMI-CNRS, Toulouse,France).Magneto-transport investigation of individual semiconductor heterostructured nanowiresF. Rossella, V. Bellani (Dep. of Physics - University of Pavia, Italy) B. Raquet, M. Goiran (LNCMI-CNRS,Toulouse, France).SdH oscillations in single and few-layer MoS 2120

2011 PROPOSALSB. Radisavljevic, A. Kis, J.S. Heron (EPFL-STI-LANES, Switzerland) W. Escoffier (LNCMI-CNRS, Toulouse,France).Landau Level spectroscopy of graphiteP. Plochocka(LNCMI-G, France) O. Portugall, G. Rikken(LNCMI-CNRS, Toulouse, France).Influence of Disorder on the N = 0 Landau Level Degeneracy Lifting in Graphene Beyond ν = 2V. Bellani.,F. Rossella (Dep. of Physics - University of Pavia, Italy) C. Sanchez-Fabres Cobaleda,(Laboratoriode Bajas Temperaturas, Spain) J.M. Poumirol, R. Ribero, W. Escoffier, A. Kumar, B. Raquet, M. Goiran(LNCMI-CNRS, Toulouse, France).Magnetoresistance and Shubnikov de-Haas effect in aligned multi-walled carbon nanotubes in high pulsedmagnetic fieldsV; Samuilov (Department of Materials Science and Engineering, State University of NewYork at Stony Brook,USA) J. Galibert (LNCMI-CNRS, Toulouse, France).Quantum Hall effect in tri-layer grapheneJ.M. Poumirol, R. Ribero, W. Escoffier, A. Kumar, B. Raquet, M. Goiran(LNCMI-CNRS, Toulouse, France).Quantum limit and surface states in topological insulator bismuth selenideJ. Paglione, N. Butch (Center for Nanophysics and Advanced Materials, University of Maryland, USA) C.Proust (LNCMI-CNRS, Toulouse, France).Transport properties of thin graphite single crystals in the ultra-quantum magnetic fieldsY. Latyshev, A. Orlov (Institute of Radio-Engineering and Electronics, RAS, Russia) P. Monceau (InstitutNéel, France) W. Escoffier (LNCMI-CNRS, Toulouse, France).Investigation of compensation decay and freezing-out of quasimetal conductivity channels in narrow-gapHgCdTe epilayers grown by molecular beam epitaxyB. Aronzon, N. Talipov, G. Cherekova, I. Diensov (RRC, Russia) J. Galibert (LNCMI-CNRS, Toulouse,France).Dirac Fermion confinement in Graphene NanoribbonsJ.M. Poumirol, R. Ribero, B. Raquet (LNCMI-CNRS, Toulouse, France).From Peapods to DWCNT : Study of the electronic transport of C60@SWCNT under UV irradiationP. Puech, M. Monthioux (CEMES, France) V. Prudkovsky, B. Raquet, W. Escoffier, M. Goiran(LNCMI-CNRS, Toulouse, France).Disorder and magnetic edges current in Graphene NanoribbonsS. Roche (CIN2, Spain) H. Dai (Staford, USA) J.M. Poumirol, R. Ribero, W. Escoffier, B. Raquet(LNCMI-CNRS, Toulouse, France).Kosterlitz-Thouless and FQHE in Corbino geometry in GrapheneE. Diez, M. Amando, C. Sanchez-Fabres Cobaleda (Laboratorio de Bajas Temperaturas, Spain) W.Escoffier(LNCMI-CNRS, Toulouse, France).Single-object electrical transport and photoconductivity measurements under pulsed magnetic fieldB. Raquet, W. Escoffier, M. Goiran (LNCMI-CNRS, Toulouse, France).FQHE and KT-transition in clean grapheneM. Amado, E. Diez (Laboratorio de Bajas Temperaturas, Spain) J.M. Poumirol, R. Ribero, W. Escoffier, B.Raquet(LNCMI-CNRS, Toulouse, France).Landau levels in the optical spectra of CuInSe 2 .M. Yakushev (Department of Physics, University of Strathclyde, Glasgow, UK), F. Luckert (Department ofPhysics, University of Strathclyde, Glasgow, UK), R. Martin (Department of Physics, University of121

PROPOSALS 2011Strathclyde, Glasgow, UK)Polarized Magneto-Raman Scattering of Graphene Monolayers.P. Kossacki (Insitute of Experimental Physics, University of Warsaw, Warsaw, Poland), C. Faugeras(LNCMI-CNRS, Grenoble, France), M. Kuhne (LNCMI-CNRS, Grenoble, France), M. Orlita (LNCMI-CNRS,Grenoble, France), M. Potemski (LNCMI-CNRS, Grenoble, France)Magneto-optical properties of MoS 2 monolayers.P. Leszczynski (LNCMI-CNRS, Grenoble, France), C. Faugeras (LNCMI-CNRS, Grenoble, France), A. Nicolet(LNCMI-CNRS, Grenoble, France), P. Kossacki (Institute of Experimental Physics, University of Warsaw,Warsaw, Poland), M. Orlita (LNCMI-CNRS, Grenoble, France), M. Potemski (LNCMI-CNRS, Grenoble,France)Complete Valley-Spin Phase Diagram of the Two-dimensional Electron Gas.K. Takashina (Department of Physics, University of Bath, Bath, UK) V. Renard (Institut Nel-CNRS, Grenoble,France), Y. Hirayama (Graduate School of Science, Tokoku University, Sendai, Japan), Y. Niida (GraduateSchool of Science, Tokoku University, Sendai, Japan), A. Fujiwara (NTT Basic Research Laboratories,Kanagawa, Japan), B. Piot (LNCMI-CNRS, Grenoble, France), D. Maude (LNCMI-CNRS, Grenoble, France)Corbino samples in graphene: Role of edge currents at high magnetic field degeneracy break.E. Diez (Laboratorio de Bajas Temeraturas, Universidad de Salamanca, Salamanca, Spain), C. Sanchez-Fabrs(Laboratorio de Bajas Temeraturas, Universidad de Salamanca, Salamanca, Spain), F. Rossella (Dipartimentodi Fisica A. Volta, Universita de Pavia, Pavia, Italy)Studies of Aharonov-Bohm effect on single antidot graphite structures.Y. Latyshev (Institute of Radio-Engineering and Electronics, RAS, Moscow, Russia), A. Orlov (Institute ofRadio-Engineering and Electronics, RAS, Moscow, Russia), P. Monceau (Institut Néel-CNRS, Grenoble,France)Quantum Hall effect in epitaxial grapheme.P. Vasek (Institute of Physics of the ASCR, Praha, Czech Republic), P. Svoboda (Institute of Physics of theASCR, Praha, Czech Republic), L. Smrcka (Institute of Physics of the ASCR, Praha, Czech Republic), N.Goncharuk (Institute of Physics of the ASCR, Praha, Czech Republic)High-field magneto-optical properties of Bi 2 Se 3 .M. Orlita (LNCMI-CNRS, Grenoble, France), M. Potemski (LNCMI-CNRS, Grenoble, France), C. Faugeras(LNCMI-CNRS, Grenoble, France), G. Martinez (LNCMI-CNRS, Grenoble, France)Metrology of the QHE in epitaxial grapheme.B. Jouault (Groupe d’Etude des Semiconducteurs, Montpellier University, Montpellier, France), B. Jabakhanji(Groupe d’Etude des Semiconducteurs, Montpellier University, Montpellier, France)Optical studies of single nanotubes.J. Alexander-Webber (Physics Department, Oxford University, UK), R. Nicholas (Physics Department, OxfordUniversity, UK)Landau level spectroscopy of mono- and few-layer graphene epitaxially grown on Si-terminated surface of SiC.L. Nadvornik (Charles University, Prague, Czech Republic), M. Orlita (LNCMI-CNRS, Grenoble, France), G.Martinez (LNCMI-CNRS, Grenoble, France), C. Faugeras (LNCMI-CNRS, Grenoble, France), A. Witowski(Warsaw University, Warsaw, Poland), M. Potemski (LNCMI-CNRS, Grenoble, France), W. Strupinski122

2011 PROPOSALS(Institute of Electronic Materiels Technology, Warsaw, Poland)Cyclotron-assisted exciton hopping between free and acceptor-bound trions observed inmagneto-photoluminescence of a two-dimensional hole gas.L. Bryja (Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland), J. Jadczak (Institute ofPhysics, Wroclaw University of Technology, Wroclaw, Poland), P. Plochocka (LNCMI-CNRS, Grenoble,France)Voltage control spin polarization in InAs self-assembled quantum dots.G.E. Marques (Departamento de Fisica, Universidade Federal de Sao Carlos, Sao Carlos, Brazil), Y. GalvaoGobato (Departamento de Fisica, Universidade Federal de Sao Carlos, Sao Carlos, Brazil), M. Daldin Teodoro(Departamento de Fisica, Universidade Federal de Sao Carlos, Sao Carlos, Brazil), M. Orlita (LNCMI-CNRS,Grenoble, France)Electronic Raman scattering of graphite in high magnetic fields.C. Faugeras (LNCMI-CNRS, Grenoble, France), P. Kossacki (LNCMI-CNRS, Grenoble, France), M. Potemski(LNCMI-CNRS, Grenoble, France), M. Amado (Laboratorio de Bajas Temperaturas, Universidad deSalamanca, Salamanca, Spain)Cyclotron resonance in HgTe/HgCdTe quantum wells at high magnetic fields.M. Orlita (LNCMI-CNRS, Grenoble, France), C. Faugeras (LNCMI-CNRS, Grenoble, France), M. Potemski(LNCMI-CNRS, Grenoble, France), G. Martinez (LNCMI-CNRS, Grenoble, France)Quantum Hall effect in epitaxial graphene.C. Berger (Georgia-Tech, Atlanta, USA), C. Faugeras (LNCMI-CNRS, Grenoble, France), M. Potemski(LNCMI-CNRS, Grenoble, France), B. Piot (LNCMI-CNRS, Grenoble, France)Magneto-optical spectroscopy study of graphite in tilted magnetic fieldsL. Smrcka (Institute of Physics of the ASCR, v.v.i, Praha, Czech Republic), N. Goncharuk (Institute ofPhysics of the ASCR, v.v.i, Praha, Czech Republic), P. Vasek (Institute of Physics of the ASCR, v.v.i, Praha,Czech Republic), P. Svoboda (Institute of Physics of the ASCR, v.v.i, Praha, Czech Republic)High field magnetotransport in the topological insulator Bi 2 Se 3 .B. Piot (LNCMI-CNRS, Grenoble, France), D. Maude (LNCMI-CNRS, Grenoble, France), G. Martinez(LNCMI-CNRS, Grenoble, France)Metal Insulator transition in Bilayer and tri-layer graphene at high magnetic fields.E. Diez (Laboratorio de Bajas Temperaturas, Universidad de Salamanca, Salamanca, Spain), C.Sanchez-Fabres Cobaleda (Laboratorio de Bajas Temperaturas, Universidad de Salamanca, Salamanca, Spain)Magneto-optical studies of topological insulators.G. Martinez (LNCMI-CNRS, Grenoble, France), M. Orlita (LNCMI-CNRS, Grenoble, France)Staged fractional quantum Hall effect in a wide parabolic well superimposed with superlattice.G. Gusev (Instituto de Fisica da Universidade de Sao Paulo, Sao Paulo, Brasil), J-Cl. Portal (LNCMI-CNRS,Grenoble, INSA, Toulouse, France)Optical properties of AFM imaged single nanotubes.123

PROPOSALS 2011R.J. Nicholas (Physics Department, Oxford University, Oxford, UK), J. Alexander-Webber (PhysicsDepartment, Oxford University, Oxford, UK)Quantum interference of Dirac fermions on graphite/graphene nanostructures with a periodic arrays ofantidots.Y. Latyshev (Institute of Radie-Engineering and Electronics, RAS, Moscow, Russia), A. Orlov (Institute ofRadie-Engineering and Electronics, RAS, Moscow, Russia), P. Monceau (Néel Institute-CNRS, Grenoble,France)Magnetotransport measurement of GaAs/AlGaAs core-shell nanowireD. Mailly (CNRS-LPN, Marcoussis France),Magneto-optical investigation of CVD grapheneA. Witowski (University of Warsaw, Warsaw, Poland)Cyclotron resonance in highly doped epitaxial grapheneI. Crassee (University of Geneva, Geneva, Switzerland), A. Kuzmenko (University of Geneva, Geneva,Switzerland)Optical detection of Mn +2 resonance in CdMnTe quantum wellsM. Goryca (University of Warsaw, Warsaw, Poland)Transport in disordered two-dimensional topological insulatorG. Gusev (University of Sao Paulo, Sao Paulo, Brasil)Spin polarization of the exotic ν = 5/2 fractional quantum Hall stateI. Bar-Joseph, M. Stern (Weizmann Institute, Rehovot, Israel), B. Piot, P. Plochocka, D. Maude(LNCMI-Grenoble, France)Dipolar spin waves of molecular magnetsA. Nogaret (University of Bath, Bath, UK)THz transmission spectroscopy of HgTe QWsF. Teppe (University of Montpellier, Montpellier, France)Cyclotron resonance in 2D semimetal based on HgTe quantum wellD. Kozlov (Institute of Semiconductor Physics, Novosibirsk, Russia)Low-energy excitations in graphiteP. Neugebauer (J.W. Goethe University of Frankfurt, Frankfurt, Germany)Magnetoplasmon excitations in AlGaN/GaN-based grating-gate Terahertz modulatorsF. Teppe (University of Montpellier, Montpellier, France)Tunnel spectroscopy of intersubband plasmons in two-dimensional electron gasesV. Popov (Institute of Microelectronics Technology of Russian Academy of Science, Moscow, Russia)SOFT MATTER AND OTHERS124

2011 PROPOSALSHigh field dependence of the surface spin magnetization in core-shell ferrite nano-particlesR. Perzynski (Universite Pierre et Marie Curie, Paris, France) G. Ballon (LNCMI-CNRS, Toulouse, France).Electrodeposition of selenide semiconductors under magnetic field.R. Kowalik (AGH University of Science and Technology, Cracow, Poland), P. Zabinski (AGH University ofScience and Technology, Cracow, Poland), A. Jarek (AGH University of Science and Technology, Cracow,Poland), K. Mech (AGH University of Science and Technology, Cracow, Poland)Hydrogen evolution in high magnetic field.P. Zabinski (AGH University of Science and Technology, Cracow, Poland), A. Jarek (AGH University ofScience and Technology, Cracow, Poland)Oxide and DMS electrodeposition under high magnetic field.J-P. Chopart (LACMDTI, UFR SCIENCES, Reims, France), A-L. Daltin (LACMDTI, UFR SCIENCES,Reims, France), P. Baudart (LACMDTI, UFR SCIENCES, Reims, France)Magnetic field investigation of the single Ni colour centre in synthetic diamond.P. Plochocka (LNCMI-CNRS, Grenoble, France), I. Breslavetz (LNCMI-CNRS, Grenoble, France), E.Gheeraert (Institut Néel-CNRS and Université Joseph Fourier, Grenoble, France)Field quality Investigation.P. Sala (LNCMI-CNRS, Grenoble, France), F. Debray (LNCMI-CNRS, Grenoble, France)Growth of Co-Fe magnetic thin film by pulsed electrodeposition under high magnetic film.P. Baudart (LACMDTI, UFR Sciences, Reims, France), A. Levesque (LACMDTI, UFR Sciences, Reims,France), D. Li (LACMDTI, UFR Sciences, Reims, France), J-P. Chopart (LACMDTI, UFR Sciences, Reims,France)Generation of orthogonal aligned fibers in hybrid hydrogels for corneal regenerationY. Yang (Keele University, Stoke-on-Trent, UK)125

THESES 2011PhD theses 20111. Jan KuncHigh mobility two-dimensional electron gas in CdTe quantum wells: High magnetic field studies.Doctorat de l’Université Joseph Fourier, Grenoble, en co-tutelle avec Charles Université, Prague.Thèse soutenue le 14 février 2011.2. Nicolas UbrigOptical properties of carbon based materials in high magnetic fields.Doctorat de l’Université Toulouse III - Paul Sabatier.Thèse soutenue le 18 mars 2011.3. Taras DauzhenkaCouches minces d’oxyde d’étain : la localisation faible et les effets de linteraction.Doctorat de l’Université Toulouse III - Paul Sabatier, en co-tutelle avec l’Université d’Etat de Belarus,Minsk .Thèse soutenue le 19 mai 2011.4. Jean-Marie PoumirolEtude des propriétés électroniques du graphène et des matériaux à base de graphène sous champ magnètiqueintense.Doctorat de l’Université Toulouse, délivré par l’Institut National des Sciences Appliquées de Toulouse,discipline Nanophysique.Thèse soutenue le 22 juillet 2011.5. Sergey KrishtopenkoSpin splitting and collective effects in InAs/AlSb quantum well heterostructures.Doctorat de l’Université Toulouse III - Paul Sabatier, en co-tutelle avec l’Institut de Physique des Microstructures(IPM) de Nizhni-Novgorod.Thèse soutenue le 10 novembre 2011.6. Mateusz GorycaSpin dynamics in low-dimensional semiconductor structures.Doctorat de l’Université Joseph Fourier, Grenoble, en co-tutelle avec l’Université de Varsovie.Thèse soutenue le 16 décembre 2011.126

2011 THESESHabilitations à diriger la recherche 20111. Cyril ProustFermi surface reconstruction in underdoped cuprates.Université Toulouse III - Paul Sabatier.Soutenue le 12 janvier 2011.2. Clement FaugerasMagneto-spectroscopie optique du graphene et du graphite.Université Joseph Fourier, Grenoble.Soutenue le 16 mars 2011.3. Paulina PlochockaUsing magneto-spectroscopy to investigate two dimensional Dirac and normal fermions.Université Joseph Fourier, Grenoble.Soutenue le 6 décembre 2011.4. Ilya SheikinQuantum oscillations in f-electron compounds.Université Joseph Fourier, Grenoble.Soutenue le 19 décembre 2011.127

PUBLICATIONS 2011List of Publications 2011[1] E. Abou-Hamad, P. Bontemps, and G. L. J. A. Rikken, “NMR in pulsed magnetic field,” Solid State NuclearMagnetic Resonance 40, 42–44 (2011).[2] F. Aimo, S. Krämer, M. Klanjšek, M. Horvatić, and C. Berthier, “Magnetic structure of azurite above the magnetizationplateau at 1 of saturation,” Physical Review B 84, 012401 (2011).3[3] T. Alboussiere, P. Cardin, F. Debray, P. La Rizza, J. P. Masson, F. Plunian, A. Ribeiro, and D. Schmitt, “Experimentalevidence of Alfven wave propagation in a Gallium alloy,” Physics of Fluids 23, 096601 (2011).[4] Dai Aoki, Tatsuma D. Matsuda, Frédéric Hardy, Christoph Meingast, Valentin Taufour, Elena Hassinger, IlyaSheikin, Carley Paulsen, Georg Knebel, Hisashi Kotegawa, and Jacques Flouquet, “Superconductivity Reinforcedby Magnetic Field and the Magnetic Instability in Uranium Ferromagnets,” Journal of the Physical Society of Japan80SA, SA008 (2011).[5] Dai Aoki, Ilya Sheikin, Tatsuma D. Matsuda, Valentin Taufour, Georg Knebel, and Jacques Flouquet, “FirstObservation of Quantum Oscillations in the Ferromagnetic Superconductor UCoGe,” Journal Of The PhysicalSociety Of Japan 80 (2011), 10.1143/JPSJ.80.013705.[6] A. Audouard, F. Duc, D. Vignolles, R. B. Lyubovskii, L. Vendier, G. V. Shilov, E. I. Zhilyaeva, R. N. Lyubovskaya,and E. Canadell, “Temperature- and pressure-dependent metallic states in (BEDT-TTF) 8[Hg 4Br 12(C 6H 5Br) 2],”Physical Review B 84, 045101 (2011).[7] A. B. A. Baranga, R. Battesti, M. Fouché, C. Rizzo, and G. L. J. A. Rikken, “Observation of the inverse Cotton-Mouton effect,” Europhysics Letters 94, 44005 (2011).[8] Anne-Laure Barra, Anders Dossing, Thorbjorn Morsing, and Johan Vibenholt, “A high field and frequency electronparamagnetic resonance study of the S=2 chromium(II) complex trans-[Cr(NCCH 3) 4(FBF 3) 2] and investigationof its reaction with dioxygen: Crystal structure of [(CH 3CN) 5CrOCr(NCCH 3) 5](BF 4) 4 · 2CH 3CN,” InorganicaChimica Acta 373, 266–269 (2011).[9] P. Berceau, R. Battesti, M. Fouché, and C. Rizzo, “The vacuum magnetic birefringence experiment: a test forquantum electrodynamics,” Canadian Journal Of Physics 89, 153–158 (2011).[10] Paul Berceau, Rmy Battesti, Mathilde Fouch, Paul Frings, Marc Nardone, Oliver Portugall, Geert Rikken, andCarlo Rizzo, “Quantum vacuum magnetic birefringence,” Hyperfine Interactions , 1–6 (2011), 10.1007/s10751-011-0517-z.[11] B. Bergk, A. Demuer, I. Sheikin, Y. Wang, J. Wosnitza, Y. Nakazawa, and R. Lortz, “Magnetic torque evidence forthe Fulde-Ferrell-Larkin-Ovchinnikov state in the layered organic superconductor κ − (BEDT − TTF) 2Cu(NCS) 2,”Physical Review B 83, 064506 (2011).[12] I. Bisotto, E. S. Kannan, S. Sassine, R. Murali, T. J. Beck, L. Jalabert, and J-C Portal, “Microwave basednanogenerator using the ratchet effect in Si/SiGe heterostructures,” Nanotechnology 22, 245401 (2011).[13] Angelika B. Boeer, Anne-Laure Barra, Liviu F. Chibotaru, David Collison, Eric J. L. McInnes, Richard A. Mole, GiovannaG. Simeoni, Grigore A. Timco, Liviu Ungur, Tobias Unruh, and Richard E. P. Winpenny, “A SpectroscopicInvestigation of Magnetic Exchange Between Highly Anisotropic Spin Centers,” Angewandte Chemie-InternationalEdition 50, 4007–4011 (2011).[14] Pierre Bouillot, Corinna Kollath, Andreas M. Läuchli, Mikhail Zvonarev, Benedikt Thielemann, Christian Rüegg,Edmond Orignac, Roberta Citro, Martin Klanjšek, Claude Berthier, Mladen Horvatić, and Thierry Giamarchi,“Statics and dynamics of weakly coupled antiferromagnetic spin- 1 ladders in a magnetic field,” Physical Review B283, 054407 (2011).[15] J. Chang, Nicolas Doiron-Leyraud, Francis Laliberte, R. Daou, David LeBoeuf, B. J. Ramshaw, Ruixing Liang,D. A. Bonn, W. N. Hardy, Cyril Proust, I. Sheikin, K. Behnia, and Louis Taillefer, “Nernst effect in the cupratesuperconductor YBa 2Cu 3O y: Broken rotational and translational symmetries,” Physical Review B 84, 014507(2011).[16] Cayetano Cobaleda, Francesco Rossella, Sergio Pezzini, Enrique Diez, Vittorio Bellani, Duncan Maude, and PeterBlake, “Quantum hall effect in inhomogeneous trilayer graphene,” Physica E: Low-dimensional Systems andNanostructures 44, 530 – 533 (2011).[17] T. A. Dauzhenka, V. K. Ksenevich, I. A. Bashmakov, and J. Galibert, “Origin of negative magnetoresistance inpolycrystalline SnO 2 films,” Physical Review B 83, 165309 (2011).[18] O. Drachenko, H. Schneider, M. Helm, D. Kozlov, V. Gavrilenko, J. Wosnitza, and J. Leotin, “Modification to thecentral-cell correction of germanium acceptors,” Physical Review B 84, 245207 (2011).[19] O. Drachenko, S. Winnerl, H. Schneider, M. Helm, J. Wosnitza, and J. Leotin, “Compact magnetospectrometer forpulsed magnets based on infrared quantum cascade lasers,” Review Of Scientific Instruments 82, 033108 (2011).[20] C. A. Duarte, L. E. G. Armas, E. C. F. da Silva, G. M. Gusev, A. K. Bakarov, S. Wiedmann, and J. C. Portal,“Fractional quantum Hall effect in second subband of a 2DES,” Europhysics Letters 94, 37010 (2011).[21] F. El Hallak, P. Rosa, P. Vidal, I. Sheikin, M. Dressel, and J. van Slageren, “Giant magnetisation step in Fe 2:Molecular nanomagnets in the weak exchange limit,” Europhysics Letters 95, 57002 (2011).[22] X. Fabrèges, I. Mirebeau, S. Petit, P. Bonville, and A. A. Belik, “Frustration-driven magnetic order in hexagonalInMnO 3,” Physical Review B 84, 054455 (2011).[23] C. Faugeras, M. Amado, P. Kossacki, M. Orlita, M. Kuehne, A. A. L. Nicolet, Yu. I. Latyshev, and M. Potemski,128

2011 PUBLICATIONS“Magneto-Raman Scattering of Graphene on Graphite: Electronic and Phonon Excitations,” Physical ReviewLetters 107, 036807 (2011).[24] H. V. A. Galeti, Y. Galvao Gobato, V. O. Gordo, L. F. dos Santos, M. J. S. P. Brasil, V. López-Richard, G. E.Marques, M. Orlita, J. Kunc, D. K. Maude, M. Henini, and R. J. Airey, “Magneto-optical investigation of twodimensionalgases in n-type resonant tunneling diodes,” Semiconductor Science and Technology 27, 015018 (2012).[25] V. Gasparov, L. Drigo, A. Audouard, D. Sun, C. Lin, S. Budko, P. Canfield, F. Wolff-Fabris, and J. Wosnitza,“Upper critical magnetic field in Ba 0.68K 0.32Fe 2As 2 and Ba(Fe 0.93Co 0.07) 2As 2,” JETP Letters 93, 667–672 (2011).[26] V. Gavrilenko, S. Krishtopenko, and M. Goiran, “Electron-electron interaction and spin-orbit coupling in InAs/AlSbheterostructures with a two-dimensional electron gas,” Semiconductors 45, 110–117 (2011).[27] Y. Galvao Gobato, H. V. A. Galeti, L. F. dos Santos, V. Lopez-Richard, D. F. Cesar, G. E. Marques, M. J. S. P.Brasil, M. Orlita, J. Kunc, D. K. Maude, M. Henini, and R. J. Airey, “Spin injection from two-dimensional electronand hole gases in resonant tunneling diodes,” Applied Physics Letters 99, 233507 (2011).[28] M. Goiran, M. Millot, J. M. Poumirol, I. Gherasoiu, W. Walukiewicz, and J. Leotin, “Electron cyclotron effectivemass in indium nitride,” Applied Physics Letters 98, 079903 (2011).[29] Abdessamad Grirrane, Antonio Pastor, Agustin Galindo, Eleuterio Alvarez, Carlo Mealli, Andrea Ienco, AnnabellaOrlandini, Patrick Rosa, Andrea Caneschi, Anne-Laure Barra, and Javier Fernandez Sanz, “Thiodiacetate-Manganese Chemistry with N ligands: Unique Control of the Supramolecular Arrangement over the Metal CoordinationMode,” Chemistry-A European Journal 17, 10599–10616 (2011).[30] G. M. Gusev, Z. D. Kvon, O. A. Shegai, N. N. Mikhailov, S. A. Dvoretsky, and J. C. Portal, “Transport indisordered two-dimensional topological insulators,” Phys. Rev. B 84, 121302 (2011).[31] G. M. Gusev, S. Wiedmann, O. E. Raichev, A. K. Bakarov, and J. C. Portal, “Evidence for zero-differentialresistance states in electronic bilayers,” Physical Review B 83, 041306 (2011).[32] F. G. G. Hernandez, G. M. Gusev, Z. D. Kvon, and J. C. Portal, “Linear and nonlinear transport in a smallcharge-tunable open quantum ring,” Physical Review B 84, 075332 (2011).[33] N. E. Hussey, R. A. Cooper, X. F. Xu, Y. Wang, I. Mouzopoulou, B. Vignolle, and C. Proust, “Dichotomy inthe T-linear resistivity in hole-doped cuprates,” Philosophical Transactions Of The Royal Society A-MathematicalPhysical And Engineering Sciences 369, 1626–1639 (2011).[34] A. V. Ikonnikov, M. S. Zholudev, K. E. Spirin, A. A. Lastovkin, K. V. Maremyanin, V. Ya. Aleshkin, V. I. Gavrilenko,O. Drachenko, M. Helm, J. Wosnitza, M. Goiran, N. N. Mikhailov, S. A. Dvoretskii, F. Teppe, N. Diakonova,C. Consejo, B. Chenaud, and W. Knap, “Cyclotron resonance and interband optical transitions in HgTe/CdTe(01 3) quantum well heterostructures,” Semiconductor Science and Technology 26, 125011 (2011).[35] J Jadczak, L Bryja, A Wójs, G Bartsch, D R Yakovlev, M Bayer, P Plochocka, M Potemski, D Reuter, andA Wieck, “Strong temperature destabilization of free exciton recombination in a two-dimensional structures withhole gas,” Journal of Physics: Conference Series 334, 012050 (2011).[36] J. Jadczak, L. Bryja, A. Wojs, G. Bartsch, D. R. Yakovlev, M. Bayer, P. Plochocka, M. Potemski, D. Reuter,and A. Wieck, “Exciton Exchange between Nearly-Free and Acceptor-Bound States of a Positive Trion Assisted byCyclotron Excitation,” Acta Physica Polonica A 119, 600–601 (2011), 39th Conference on the Physics of Semiconductors,Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[37] S. Jandl, S. Mansouri, A. A. Mukhin, V. YuIvanov, A. Balbashov, M. M. Gospodino, V. Nekvasil, and M. Orlita,“Study of crystal-field excitations and Raman active phonons in o-DyMnO 3,” Journal Of Magnetism And MagneticMaterials 323, 1104–1108 (2011).[38] M. Jeong, F. Bert, P. Mendels, F. Duc, J. C. Trombe, M. A. de Vries, and A. Harrison, “Field-Induced Freezing ofa Quantum Spin Liquid on the Kagome Lattice,” Physical Review Letters 107, 237201 (2011).[39] Johannes Jobst, Daniel Waldmann, Florian Speck, Roland Hirner, Duncan K. Maude, Thomas Seyller, and Heiko B.Weber, “Transport properties of high-quality epitaxial graphene on 6H-SiC(0001),” Solid State Communications151, 1061–1064 (2011).[40] E. S. Kannan, I. Bisotto, J. C. Portal, R. Murali, and T. J. Beck, “Photovoltage induced by ratchet effect inSi/SiGe heterostructures under microwave irradiation,” Applied Physics Letters 98, 3590255 (2011).[41] M. V. Kartsovnik, T. Helm, C. Putzke, F. Wolff-Fabris, I. Sheikin, S. Lepault, C. Proust, D. Vignolles, N. Bittner,W. Biberacher, A. Erb, J. Wosnitza, and R. Gross, “Fermi surface of the electron-doped cuprate superconductorNd 2−xCe xCuO 4 probed by high-field magnetotransport,” New Journal Of Physics 13, 015001 (2011).[42] H. Katsuno, H. Ohta, O. Portugall, N. Ubrig, M. Fujisawa, F. Elmasry, S. Okubo, and Y. Fujiwara, “Energystructure of Er-2O center in GaAs:Er,O studied by high magnetic field photoluminescence measurement,” JournalOf Luminescence 131, 2294–2298 (2011).[43] T. Kazimierczuk, J. A. Gaj, A. Golnik, M. Goryca, M. Nawrocki, M. Koperski, T. Smolenski, J. Suffczynski,P. Wojnar, and P. Kossacki, “Excitation Mechanisms of CdTe/ZnTe Quantum Dots under Non-Resonant andQuasi-Resonant Regime,” Acta Physica Polonica A 119, 588–591 (2011), 39th Conference on the Physics of Semiconductors,Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[44] L. Kilanski, W. Dobrowolski, E. Dynowska, M. Wojcik, B. J. Kowalski, N. Nedelko, A. Slawska-Waniewska,D. K. Maude, S. A. Varnavskiy, I. V. Fedorchenko, and S. F. Marenkin, “Colossal linear magnetoresistance ina CdGeAs 2:MnAs micro-composite ferromagnet,” Solid State Communications 151, 870–873 (2011).[45] P. Kopcansky, N. Tomasovicova, M. Koneracka, M. Timko, V. Zavisova, A. Dzarova, J. Jadzyn, E. Beaugnon, andX. Chaud, “Phase Transitions in Liquid Crystal Doped with Magnetic Particles of Different Shapes,” InternationalJournal Of Thermophysics 32, 807–817 (2011).[46] M. Koperski, M. Goryca, T. Kazimierczuk, P. Kossacki, P. Wojnar, and J. A. Gaj, “Magnetoluminescence of a CdTeQuantum Dot with a Single Manganese Ion in Voigt Configuration,” Acta Physica Polonica A 119, 618–620 (2011),39th Conference on the Physics of Semiconductors, Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[47] P. Kossacki, C. Faugeras, M. Kühne, M. Orlita, A. A. L. Nicolet, J. M. Schneider, D. M. Basko, Yu. I. Latyshev,and M. Potemski, “Electronic excitations and electron-phonon coupling in bulk graphite through Raman scattering129

PUBLICATIONS 2011in high magnetic fields,” Physical Review B 84, 235138 (2011).[48] D. A. Kozlov, Z. D. Kvon, N. N. Mikhailov, S. A. Dvoretskii, and J. C. Portal, “Cyclotron Resonance in aTwo-Dimensional Semimetal Based on a HgTe Quantum Well,” JETP Letters 93, 170–173 (2011).[49] R. B. G. Kramer, V. S. Egorov, V. A. Gasparov, A. G. M. Jansen, and W. Joss, “Condon domain phase diagramfor silver,” Low Temperature Physics 37, 39–44 (2011).[50] S. S. Krishtopenko, V. I. Gavrilenko, and M. Goiran, “Theory of g-factor enhancement in narrow-gap quantumwell heterostructures,” Journal Of Physics-Condensed Matter 23, 385601 (2011).[51] V. K. Ksenevich, N. I. Gorbachuk, T. A. Dauzhenka, I. A. Bashmakov, N. A. Poklonski, and A. D. Wieck,“AC-Conductivity of Thin Polycrystalline Tin Dioxide Films,” Acta Physica Polonica A 119, 146–147 (2011).[52] M. Kubisa, K. Ryczko, J. Jadczak, L. Bryja, J. Misiewicz, and M. Potemski, “Nonlinear Zeeman Splitting of Holesin Doped GaAs Heterostructures,” Acta Physica Polonica A 119, 609–611 (2011), 39th Conference on the Physicsof Semiconductors, Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[53] A. Kumar, W. Escoffier, J. M. Poumirol, C. Faugeras, D. P. Arovas, M. M. Fogler, F. Guinea, S. Roche, M. Goiran,and B. Raquet, “Integer Quantum Hall Effect in Trilayer Graphene,” Physical Review Letters 107, 126806 (2011).[54] F. Laliberte, J. Chang, N. Doiron-Leyraud, E. Hassinger, R. Daou, M. Rondeau, B. J. Ramshaw, R. Liang, D. A.Bonn, W. N. Hardy, S. Pyon, T. Takayama, H. Takagi, I. Sheikin, L. Malone, C. Proust, K. Behnia, and LouisTaillefer, “Fermi-surface reconstruction by stripe order in cuprate superconductors,” Nature Communications 2,432 (2011).[55] Vladimir N. Laukhin, Alain Audouard, David Vignolles, Enric Canadell, Tatyana G. Prokhorova, and Eduard B.Yagubskii, “Magnetoresistance oscillations up to 32 K in the organic metal β”-(ET) 4(H 3O)[Fe(C 2O 4) 3]C 6H 4Cl 2,”Low Temperature Physics 37, 749–754 (2011), Translation of Fizika Nizkikh Temperatur 37, 943-949 (2011).[56] D. LeBoeuf, N. Doiron-Leyraud, B. Vignolle, M. Sutherland, B. J. Ramshaw, J. Levallois, R. Daou, F. Laliberté,O. Cyr-Choinière, J. Chang, Y. J. Jo, L. Balicas, R. Liang, D. A. Bonn, W. N. Hardy, C. Proust, and L. Taillefer,“Lifshitz critical point in the cuprate superconductor YBa 2Cu 3O y from high-field Hall effect measurements,”Physical Review B 83, 054506 (2011).[57] P. Lejay, E. Canevet, S. K. Srivastava, B. Grenier, M. Klanjsek, and C. Berthier, “Crystal growth and magneticproperty of MCo 2V 2O 8 (M=Sr and Ba),” Journal Of Crystal Growth 317, 128–131 (2011).[58] Gregory P. Lousberg, J-F Fagnard, X. Chaud, M. Ausloos, P. Vanderbemden, and B. Vanderheyden, “Magneticproperties of drilled bulk high-temperature superconductors filled with a ferromagnetic powder,” SuperconductorScience & Technology 24, 035008 (2011).[59] O Makarovsky, O Thomas, A Patané, R P Campion, C T Foxon, E E Vdovin, D K Maude, A G Balanov, andL Eaves, “Electronic energy levels, wavefunctions and potential landscape of nanostructures probed by magnetotunnellingspectroscopy,” Journal of Physics: Conference Series 334, 012010 (2011).[60] L. Malone, T. D. Matusda, A. Antunes, G. Knebel, V. Taufour, D. Aoki, K. Behnia, C. Proust, and J. Flouquet,“Thermoelectric evidence for high-field anomalies in the hidden order phase of URu 2Si 2,” Physical Review B 83,245117 (2011).[61] Catalin Maxim, Sylvie Ferlay, and Cyrille Train, “Binuclear heterometallic M(III)-Mn(II) (M = Fe, Cr) oxalatebridgedcomplexes associated with a bisamidinium dication: a structural and magnetic study,” New Journal OfChemistry 35, 1254–1259 (2011).[62] C. Mayer, S. Gorsse, G. Ballon, R. Caballero-Flores, V. Franco, and B. Chevalier, “Tunable magnetocaloric effectin Gd-based glassy ribbons,” Journal of Applied Physics 110, 053920 (2011).[63] M. Millot, N. Ubrig, J.-M. Poumirol, I. Gherasoiu, W. Walukiewicz, S. George, O. Portugall, J. Léotin, M. Goiran,and J.-M. Broto, “Determination of effective mass in InN by high-field oscillatory magnetoabsorption spectroscopy,”Physical Review B 83, 125204 (2011).[64] Zuzana Mitroova, Natalia Tomasovicova, Milan Timko, Martina Koneracka, Jozef Kovac, Jan Jadzyn, Ivo Vavra,Nandor Eber, Tibor Toth-Katona, Eric Beaugnon, Xavier Chaud, and Peter Kopcansky, “The sensitivity of liquidcrystal doped with functionalized carbon nanotubes to external magnetic fields,” New Journal Of Chemistry 35,1260–1264 (2011).[65] A. Nogaret, F. Nasirpouri, J-C Portal, H. E. Beere, D. A. Ritchie, A. T. Hindmarch, and C. H. Marrows, “Doublespin resonance in a spatially periodic magnetic field with zero average,” Europhysics Letters 94, 28001 (2011).[66] Lucie Norel, Jean-Baptiste Rota, Lise-Marie Chamoreau, Guillaume Pilet, Vincent Robert, and Cyrille Train,“Spin Transition and Exchange Interaction: Janus Visions of Supramolecular Spin Coupling between Face-to-FaceVerdazyl Radicals,” Angewandte Chemie-International Edition 50, 7128–7131 (2011).[67] Maximilian Nothaft, Steffen Höhla, Aurélien Nicolet, Fedor Jelezko, Norbert Frühauf, Jens Pflaum, and JörgWrachtrup, “Optical Sensing of Current Dynamics in Organic Light-Emitting Devices at the Nanometer Scale,”ChemPhysChem 12, 2590–2595 (2011).[68] E. B. Olshanetsky, Z. D. Kvon, S. S. Kobylkin, D. A. Kozlov, N. N. Mikhailov, S. A. Dvoretskii, and J. C. Portal,“Quantum Hall Effect in a Quasi-Three-Dimensional HgTe Film,” JETP Letters 93, 526–529 (2011).[69] M. Orlita, C. Faugeras, J. Borysiuk, J. M. Baranowski, W. Strupinski, M. Sprinkle, C. Berger, W. A. de Heer, D. M.Basko, G. Martinez, and M. Potemski, “Magneto-optics of bilayer inclusions in multilayered epitaxial graphene onthe carbon face of SiC,” Physical Review B 83, 125302 (2011).[70] M. Orlita, C. Faugeras, R. Grill, A. Wysmolek, W. Strupinski, C. Berger, W. A. de Heer, G. Martinez, andM. Potemski, “Carrier Scattering from Dynamical Magnetoconductivity in Quasineutral Epitaxial Graphene,” PhysicalReview Letters 107, 216603 (2011).[71] M. Orlita, K. Masztalerz, C. Faugeras, M. Potemski, E. G. Novik, C. Bruene, H. Buhmann, and L. W. Molenkamp,“Fine structure of zero-mode Landau levels in HgTe/Hg xCd 1−xTe quantum wells,” Physical Review B 83, 115307(2011).[72] Emilio Pardo, Cyrille Train, Geoffrey Gontard, Kamal Boubekeur, Oscar Fabelo, Hongbo Liu, Brahim Dkhil,Francesc Lloret, Kosuke Nakagawa, Hiroko Tokoro, Shin-ichi Ohkoshi, and Michel Verdaguer, “High Proton Con-130

2011 PUBLICATIONSduction in a Chiral Ferromagnetic Metal-Organic Quartz-like Framework,” Journal Of The American ChemicalSociety 133, 15328–15331 (2011).[73] M. L. Peres, V. A. Chitta, D. K. Maude, N. F. Oliveira, Jr., P. H. O. Rappl, A. Y. Ueta, and E. Abramof,“Spin-Orbit Coupling in n-Type PbTe/PbEuTe Quantum Wells,” Acta Physica Polonica A 119, 602–605 (2011),39th Conference on the Physics of Semiconductors, Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[74] A. P. Petrović, R. Lortz, G. Santi, C. Berthod, C. Dubois, M. Decroux, A. Demuer, A. B. Antunes, A. Paré,D. Salloum, P. Gougeon, M. Potel, and Ø. Fischer, “Multiband Superconductivity in the Chevrel Phases SnMo 6S 8and PbMo 6S 8,” Physical Review Letters 106, 017003 (2011).[75] Z. Pribulova, J. Kacmarcik, C. Marcenat, P. Szabo, T. Klein, A. Demuer, P. Rodiere, D. J. Jang, H. S. Lee, H. G.Lee, S.-I. Lee, and P. Samuely, “Superconducting energy gap in MgCNi 3 single crystals: Point-contact spectroscopyand specific-heat measurements,” Physical Review B 83, 104511 (2011).[76] P. Puech, S. Nanot, B. Raquet, J.-M. Broto, M. Millot, A.W. Anwar, E. Flahaut, and W. Bacsa, “ComparativeRaman spectroscopy of individual and bundled double wall carbon nanotubes,” Physica Status Solidi B 248, 974–979(2011).[77] Yu. A. Pusep, G. M. Gusev, A. K. Bakarov, A. I. Toropov, and J. C. Portal, “Magnetotransport in a wide parabolicwell superimposed with a superlattice,” Journal Of Applied Physics 109, 3576134 (2011), 30th International Conferenceon the Physics of Semiconductors (ICPS-30), Seoul, SOUTH KOREA, JUL 25-30, 2010.[78] B. J. Ramshaw, B. Vignolle, J. Day, R. X. Liang, W. N. Hardy, C. Proust, and D. A. Bonn, “Angle dependenceof quantum oscillations in YBa 2Cu 3O 6.59 shows free-spin behaviour of quasiparticles,” Nature Physics 7, 234–238(2011).[79] J. M. Rey, M. Bruchon, X. Chaud, F. Debray, T. Lecrevisse, E. Mossang, and P. Tixador, “Geometry Optimizationfor SMES Solenoids Using HTS Ribbons,” IEEE Transactions On Applied Superconductivity 21, 1670–1673 (2011).[80] R. Ribeiro, J. M. Poumirol, A. Cresti, W. Escoffier, M. Goiran, J. M. Broto, S. Roche, and B. Raquet, “Unveilingthe Magnetic Structure of Graphene Nanoribbons,” Physical Review Letters 107, 086601 (2011).[81] G. L. J. A. Rikken, “A New Twist on Spintronics,” Science 331, 864–865 (2011).[82] G. L. J. A. Rikken and B. A. van Tiggelen, “Measurement of the Abraham Force and Its Predicted QED Correctionsin Crossed Electric and Magnetic Fields,” Physical Review Letters 107, 170401 (2011).[83] Maria Jesus Rodriguez-Douton, Matteo Mannini, Lidia Armelao, Anne-Laure Barra, Erik Tancini, Roberta Sessoli,and Andrea Cornia, “One-step covalent grafting of Fe 4 single-molecule magnet monolayers on gold,” ChemicalCommunications 47, 1467–1469 (2011).[84] P. M. C. Rourke, I. Mouzopoulou, X. F. Xu, C. Panagopoulos, Y. Wang, B. Vignolle, C. Proust, E. V. Kurganova,U. Zeitler, Y. Tanabe, T. Adachi, Y. Koike, and N. E. Hussey, “Phase-fluctuating superconductivity in overdopedLa 2−xSr xCuO 4,” Nature Physics 7, 455–458 (2011).[85] F. Rullier-Albenque, H. Alloul, and G. Rikken, “High-field studies of superconducting fluctuations in high-T ccuprates: Evidence for a small gap distinct from the large pseudogap,” Physical Review B 84, 014522 (2011).[86] Lara F. dos Santos, Yara Galvao Gobato, Marcio D. Teodoro, Victor Lopez-Richard, Gilmar E. Marques, MariaJ. S. P. Brasil, Milan Orlita, Jan Kunc, Duncan K. Maude, Mohamed Henini, and Robert J. Airey, “Circularpolarization in a non-magnetic resonant tunneling device,” Nanoscale Research Letters 6, 101 (2011).[87] J. Scola, Y. Dumont, N. Keller, M. Vallee, J. G. Caputo, I. Sheikin, P. Lejay, and A. Pautrat, “Incomplete spinreorientation in yttrium orthoferrite,” Physical Review B 84, 104429 (2011).[88] Rikio Settai, Keisuke Katayama, Dai Aoki, Ilya Sheikin, Georg Knebel, Jacques Flouquet, and Yoshichika Onuki,“Field-Induced Antiferromagnetic State in Non-centrosymmetric Superconductor CeIrSi 3,” Journal Of The PhysicalSociety Of Japan 80, 094703 (2011).[89] Ilya Sheikin, Pierre Rodiere, Rikio Settai, and Yoshichika Ōnuki, “High-Field de Haas–van Alphen Effect in Non-Centrosymmetric CeCoGe 3 and LaCoGe 3,” Journal of the Physical Society of Japan 80SA, SA020 (2011).[90] Donglu Shi, Peng He, Peng Zhao, Fang Fang Guo, Feng Wang, Chris Huth, Xavier Chaud, Sergey L. Bud’ko, andJie Lian, “Magnetic alignment of Ni/Co-coated carbon nanotubes in polystyrene composites,” Composites PartB-Engineering 42, 1532–1538 (2011).[91] T. Smolenski, T. Kazimierczuk, M. Goryca, P. Kossacki, J. A. Gaj, P. Wojnar, K. Fronc, M. Korkusinski, andP. Hawrylak, “Influence of Configuration Mixing on Energies and Recombination Dynamics of Excitonic States inCdTe/ZnTe Quantum Dots,” Acta Physica Polonica A 119, 615–617 (2011), 39th Conference on the Physics ofSemiconductors, Jaszowied Int Sch, Krynica-Zdroj, POLAND, JUN 19-24, 2010.[92] Radostina Stoyanova, Anne-Laure Barra, Meglena Yoncheva, Elitza Kuzmanova, and Ekaterina Zhecheva, “Localstructure of Mn 4+ and Fe 3+ spin probes in layered LiAlO 2 oxide by modelling of zero-field splitting parameters,”Dalton Transactions 40, 9106–9115 (2011).[93] A. Sytcheva, U. Löw, S. Yasin, J. Wosnitza, S. Zherlitsyn, P. Thalmeier, T. Goto, P. Wyder, and B. Lüthi, “AcousticFaraday effect in Tb 3Ga 5O 12,” Physical Review B 81, 214415 (2010).[94] P. Thakur, Amit Kumar, S. Gautam, and K.H. Chae, “Electronic charge transfer in cobalt doped fullerene thinfilms and effect of energetic ion impacts by x-ray absorption spectroscopy,” Thin Solid Films 519, 8401 – 8405(2011), first International Conference of the Asian Union of Magnetics Societies (ICAUMS 2010).[95] Ane B. Tomter, Giorgio Zoppellaro, Florian Schmitzberger, Niels H. Andersen, Anne-Laure Barra, Henrik Engman,Pr Nordlund, and K. Kristoffer Andersson, “HF-EPR, Raman, UV/VIS Light Spectroscopic, and DFT Studies ofthe Ribonucleotide Reductase R2 Tyrosyl Radical from Epstein-Barr Virus,” PLoS ONE 6, e25022 (2011).[96] Cyrille Train, Michel Gruselle, and Michel Verdaguer, “The fruitful introduction of chirality and control of absoluteconfigurations in molecular magnets,” Chemical Society Reviews 40, 3297–3312 (2011).[97] V. Tripathi, K. Dhochak, B. A. Aronzon, V. V. Rylkov, A. B. Davydov, B. Raquet, M. Goiran, and K. I. Kugel,“Charge inhomogeneities and transport in semiconductor heterostructures with a Mn δ-layer,” Physical Review B84, 075305 (2011).[98] A. H. Trojnar, M. Korkusiński, E. S. Kadantsev, P. Hawrylak, M. Goryca, T. Kazimierczuk, P. Kossacki, P. Wojnar,131

PUBLICATIONS 2011and M. Potemski, “Quantum Interference in Exciton-Mn Spin Interactions in a CdTe Semiconductor Quantum Dot,”Physical Review Letters 107, 207403 (2011).[99] N. Ubrig, P. Plochocka, P. Kossacki, M. Orlita, D. K. Maude, O. Portugall, and G. L. J. A. Rikken, “High-fieldmagnetotransmission investigation of natural graphite,” Physical Review B 83, 073401 (2011).[100] B. Vignolle, D. Vignolles, D. LeBoeuf, S. Lepault, B. Ramshaw, R. X. Liang, D. A. Bonn, W. N. Hardy, N. Doiron-Leyraud, A. Carrington, N. E. Hussey, L. Taillefer, and C. Proust, “Quantum oscillations and the Fermi surfaceof high-temperature cuprate superconductors,” Comptes Rendus Physique 12, 446–460 (2011).[101] D. Vignolles, A. Audouard, V. N. Laukhin, E. Canadell, T. G. Prokhorova, and E. B. Yagubskii, “Quantuminterference and Shubnikov-de Haas oscillations in β”-(ET) 4(H 3O)[Fe(C 2O 4) 3]C 6H 4Cl 2 under pressure,” SyntheticMetals 160, 2467–2470 (2010).[102] G. H. Wagnière and G. L. J. A. Rikken, “Chirality and magnetism II: Free electron on an infinite helix, inverseFaraday effect and inverse magnetochiral effect,” Chemical Physics Letters 502, 126–129 (2011).[103] S Wiedmann, G M Gusev, A K Bakarov, and J C Portal, “Emergent fractional quantum Hall effect at evendenominator ν = 3/2 in a triple quantum well in tilted magnetic fields,” Journal of Physics: Conference Series 334,012026 (2011).[104] S Wiedmann, G M Gusev, O E Raichev, A K Bakarov, and J C Portal, “Zero-resistance states in bilayer electronsystems induced by microwave irradiation,” Journal of Physics: Conference Series 334, 012014 (2011).[105] S. Wiedmann, G. M. Gusev, O. E. Raichev, S. Kraemer, A. K. Bakarov, and J. C. Portal, “Microwave-inducedHall resistance in bilayer electron systems,” Physical Review B 83, 195317 (2011).[106] S. Winnerl, M. Orlita, P. Plochocka, P. Kossacki, M. Potemski, T. Winzer, E. Malic, A. Knorr, M. Sprinkle,C. Berger, W. A. de Heer, H. Schneider, and M. Helm, “Carrier Relaxation in Epitaxial Graphene PhotoexcitedNear the Dirac Point,” Physical Review Letters 107, 237401 (2011).[107] Tao Wu, Hadrien Mayaffre, Steffen Kraemer, Mladen Horvatic, Claude Berthier, W. N. Hardy, Ruixing Liang, D. A.Bonn, and Marc-Henri Julien, “Magnetic-field-induced charge-stripe order in the high-temperature superconductorYBa 2Cu 3O y,” Nature 477, 191–194 (2011).[108] Michael V. Yakushev, Franziska Luckert, Clement Faugeras, Anatoli V. Karotki, Alexander V. Mudryi, andRobert W. Martin, “Excited States of the A and B Free Excitons in CuInSe 2,” Japanese Journal Of AppliedPhysics 50, 05FC03 (2011).[109] Q. Zhang, W. Knafo, P. Adelmann, P. Schweiss, K. Grube, N. Qureshi, Th. Wolf, H. v. Löhneysen, and C. Meingast,“Complex magnetoelastic properties in the frustrated kagome-staircase compounds (Co 1−xNi x) 3V 2O 8,” PhysicalReview B 84, 184429 (2011).[110] X. Q. Zhou, B. Schmidt, C. Proust, G. Gervais, L. N. Pfeiffer, K. W. West, and S. Das Sarma, “Quantum-ClassicalCrossover and Apparent Metal-Insulator Transition in a Weakly Interacting 2D Fermi Liquid,” Physical ReviewLetters 107, 086804 (2011).132

2011 PATENTSList of patents 20111. Bobine ameliorée apte à générer un champ magnètique intense et procédé de fabrication de ladite bobineFrench patent n ◦ FR 29 59059First deposed : 19 April 2010Inventors: F. Debray, J. Dumas, R. Pfister, C. Trophime, J. M. Tudela, N. Vidal2. Electroacoustic transducer and electroacoustic system comprising such an electroacoustic transducer, basedon our discovery regarding the functioning of the eardrumFrench patent n ◦ FR 11 54671First deposed: 27 May 2011Inventors: Bernard Auriol, Jérôme Béard, Jean-Marc Broto, Didier Descouens3. Process and device to measure an electric tension relative to a collagen fiber, to help to diagnose a possibledeterioration and to evaluate the functional quality of collagen fibersFrench patent n ◦ FR 11 54672First deposed: 27 May 2011Inventors: Bernard Auriol, Jérôme Béard, Jean-Marc Broto, Didier Descouens4. Dispositif pour caractériser un faisceau laser pulséFrench patent n ◦ FR 10 57007First deposed : 3 September 2010Inventors: A. Ben-Amar Baranga, R. Battesti, M. Fouché, C. Rizzo, G. L. J. A. Rikken5. Fil conducteur composite à base de nanotubes et nanofibresFrench patent n ◦ FR 10 60409First deposed : 13 December 2010Inventors: L. Thilly, F. Lecouturier, J.-B. Dubois, N. Ferreira, P.-O. Renault, P. Olier.133

2011Collaborating external laboratoriesBelarusBelarus State University, Dept Phys. SC & Nanoelectronics, BY-Minsk: 44Belarus State Univ. Research Institute of Physicochemical Problems, BY-Minsk: 44BelgiumUniversity of Leuven: 77BrazilInstituto de Física da Universidade de São Paulo : 27,29,39Universidade de São Paulo: 66UnB, Brasilia: 70CanadaMcGill University, Montreal: 26University of British Columbia, Vancouver: 49,58,59Physics Department, University of Sherbrooke, Canada: 58ChinaEPM, Northeastern University, Shenyang: 92Northwest Institute for Non-Ferrous Metal Research, Xi’an: 83,84CroatiaUniversity of Zagreb: 66Czech RepublicCharles University, Prague: 8Institute of Physics ASCR, v.v.i.,Prague: 10FranceCEA-DEN-LTMEX, Saclay: 108CEA-Grenoble: 76CEA-IRFU, Saclay: 85CEA-LETI, Grenoble, France: 7CEA/Saclay, Gif-sur-Yvette: 114CMTR, ICMPE, CNRS and Univ. Paris XII: 67CRETA, Grenoble: 71,83,84,85DSM-INAC-CEA, Grenoble: 51,52,53,55,113EPSCI, Paris: 58ESRF, Grenoble: 97,108IEMN, Lille: 38Institut Jean Lamour, CNRS-Nancy University: 47,48,67Institute Néel, CNRS, Grenoble: 21,22,30,78,81,82,85Institut Néel / G2Elab, Grenoble: 81,82,85Institut Pprime, Poitiers: 108IMEP-LAHC, Minatec, Grenoble: 43IPCM, UPMC, Paris: 75L2C, Montpellier:13Laboratoire des Matériaux et du Génie Physique, INP-CNRS, Grenoble: 48Laboratoire de Physique de L’Etat Condensé, CNRS-Université du Maine, Le Mans: 68LACMDTI, University of Reims: 92LCC, Toulouse: 55LEM, Nancy University: 113LPMMC-UJF-CNRS, Grenoble: 18,19,20LPN-CNRS, Marcoussis: 37LPSC, Grenoble: 112UCL, Lyon: 76134

2011ULP, Strasbourg: 75,76UPMC-PECSA, Paris: 70UMR 6200 CNRS / Univ. Angers: 50UMR 8502 CNRS / Univ. Paris-Sud: 50GermanyDresden Pulsed Magnetic Field Laboratory, Dresden: 51,54,60Helmholtz-Zentrum Dresden-Rossendorf: 9MPI, Stuttgart: 60Paul Drude Institute, Berlin: 28Technische Universität Berlin: 9IsraelWeizmann Institute of Science, Rehovot: 25ItalyColumbus Superconductors SpA, Genoa: 86Universita di Catania: 18University of Florence: 78University of Modena: 78University of Pavia, Pavia: 11JapanDepartment of Advanced Materials, University of Tokyo, Kashiwa: 58Department of Physics, Tokyo Institute of Technology, Tokyo: 65Kyoto University, Kyoto: 61National Institute for Materials Science, Tsukuba: 22NTT Basic Research Laboratories, NTT Corporation, Atsugi: 30Tohoku University, Sendai: 30University of Tokyo: 75PolandAGH University of Science and Technology, Krakow: 90,91Institute of Experimental Physics, Warsaw University: 8,9,35ITME Warsaw: 8,10RussiaIPCP, Chernogolovka: 47Institute of Microelectronics Technology of RAS, Chernogolovka: 31Institute of Semiconductor Physics, Novosibirsk: 27,29,39,40Kotelnikov Institute of Radio-Engineering and Electronics RAS, Moscow: 19,20,21P.N. Lebedev Physical Institute, Russian Academy of Science, Moscow: 56SlovakiaInstitute of Experimental Physics, SAS, Košice: 71SpainCI2N and ICREA, Barcelona: 7,12ICMAB, Barcelona: 47ICMM, Madrid: 12University of Salamanca, Salamanca: 11SwedenLinköping University: 10135

2011SwitzerlandCERN, Geneva: 82PSI, Zurich: 108The NetherlandsHMFL, Nijmegen: 61TurkeyDepartment of Physics, Istanbul Technical University: 69UkraineInstitute of Semiconductor Physics, NAS of Ukraine, Kiev: 27,29United KingdomClarendon Laboratory, Oxford University, Oxford: 14,15,16University of Bath, Bath: 42University of Bristol, Bristol: 57,61University of Cambridge, Cambridge: 42University of Manchester: 77University of Oxford, Oxford: 61USAAmes Laboratory, Ames, Iowa: 60Dept. of Electrical and Computer Engineering, Northeastern University, Boston: 68GeorgiaTech, Atlanta: 8,9,41NRL, Washington: 61Princeton University, Princeton: 26University of California, San Diego: 12University of Maryland, College Park: 26136

2011Author index(Contributors of LNCMI to this annual report)AUBRIL J. :81,85LEBOEUF D. : 59,61AUDOUARD A. : 47,60LECOUTURIER F. : 108,111AUTERNAUD C. : 110LEPAULT S. : 59BADOUX S. : 59MALONE L. : 58BALLON G. : 47,55,70MARTINEZ G. : 8,28BARBIER R. : 112MATERA J. : 98,110,112BARRA A. L. : 18,77,78MAUDE D.K. : 10, 11,13,14,15,16,17,25,30,37,43,48,56BATTESTI R. : 89MAXIM C. : 75BÉARD J. : 107,111MAYAFFRE H. : 49,50BEAUGNON E. : 71MIYOSHI Y.: 85BENDICHOU L. : 100,108,111MOLLARD C. : 110BERCEAU P. : 89MUKHOPADHYAY S. : 50,66BERTHIER C. : 49,50,65,66NARDONE M. : 97,100,101,102BILLETTE J. : 107,111NEUGEBAUER P. : 18,78BISSOTO I. : 41NICOLET A. L. : 19,20,36BLINDER R. : 66NICOLIN J.-P. : 109BONTEMPS P. : 95,96ORLITA M. : 8,9,10,18,19,20,28BREFUEL N. : 76PETMEZAKIS P. : 112BROTO J.M. : 7PIOT, B.A. : 11,13,21,14,15,17,25,30,43,37,48,56BRUYANT N. : 1,96PFISTER R. : 103,110,114CHAUD X. : 71,81,82,83,84,85,86PLOCHOCKA P. : 9,14,15,16,22,25DAUZHENKA T.A. : 44PONTON D. : 110DEBRAY F. : 81,82,85,90,91,92,98,110,111,112,113,114 PORTAL J.C. : 27,29,31,39,40,41,42DELESCLUSE P.: 100,101,108PORTUGALL O. : 14,15,16,22DE MARINIS J. L. : 98,99,110POTEMSKI M. : 8,9,18,19,20,35,36DISPARTI T. : 110,111POUMIROL J.M. : 7,12DOMPS T.: 100PROUST C. : 26,57,58,59,102DRIGO L. : 47,60,109PRUDKOVSKY V. : 38DUC F. : 47,97PUGNAT P. : 103,114DUMAS J. : 110RAQUET B. : 7,12,38ESCOFFIER W. : 7,12,38RIBEIRO R. : 7FABREGES X. : 97RIKKEN G.L.J.A. : 14,15,16,22,95,96FAUGERAS C. : 8,12,18,19,20,28RIZZO C. : 89FERREIRA N. : 108,111RONAYETTE L. : 103, 114FOUCHÉ M. : 89SALA P. : 98,99,112FRINGS P. : 97,107,109SCHEERER G. : 55,102GALIBERT J. : 44SCHIAVO T. : 100,109GEORGE S. : 38SCHNEIDER J.M. : 14,15,17GIQUEL F. : 107SHEIKIN I. : 17,48,51,52,53,54,58GOIRAN M. : 7,12,38SOLANE P.Y. : 14,15,16,22GORYCA M. : 35SPITZNAGEL J. : 85,98,110GRANDCLEMENT C. : 112STORK H. : 95,96GRBIC H. : 65SULEIMAN M. : 107GRIFFE B. : 109SURAN R. : 81,82GUILLOT M. : 67,68,69TRAIN C. : 75,76HORVATIC M. : 49,50,65,66TROPHIME C. : 110,112,113,114JAUDET C. : ,102TUDELA J.M. :110,111JOSS W. : 103,114VANNIER C. : 1JULIEN M.H. : 49VERNEY E. : 110KAMKE M. : 110VIDAL N. : 110KANNAN E. S. : 41VIGNOLLE B. : 57,59KNAFO W. : 55,70,102VIGNOLLES D. : 47,55,59,61KOSSACKI P. : 19,20,35WARTH-MARTIN C. : 1KRÄMER S. : 29,49,103WIEDMANN S. : 27,29KÜHNE M. : 19,20WU T. : 49KUMAR A. : 12XIAO H. : 103, 114LAGARRIGUE J.-M. : 100,107ZITOUNI A. : 100,101,102137

INDEX1D confinement, 382D Fermi liquid, 262DEGpolaronic effects, 28RDNMR, 25Aharonov-Bohm effectgraphite, 21AntiferromagneticKagome lattice, 65Bi 2+x Sr 4−x Cu 2 CO 3 O 8+δ , 56C-EDT-TTF-CONMe 2 ) 2 Br, 50Capacitor bank, 109CeRh 2 Si 2 , 51COLOSSECRIS superconducting bus, 112Copper/stainless steel composite, 111Cyclotron resonancegraphite, 18dHvAθ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ), 47CeCoIn 5 , 54CeIrIn 5 , 52CeRh 2 Si 2 , 51graphite, 17Ti 3 SiC 2 , 48DiamondNi colour centre, 22Dilution refrigeratorpulsed magnetic field, 102Dirac fermion confinement, 7ElectrodepositionCdSe, 90Co-Ni alloys, 92ZnSe, 90Epitaxial grapheneC-terminated surface, 8quantum Hall effect, 13Scattering time, 8EPR[Co 2 (H 2 O)(O 2 C t Bu) 4 (HO 2 C t Bu) 2 (py) 2 ], 77[Fe 4 (L) 2 (dpm) 6 ], 78Fermi surfaceθ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ), 47CeCoIn 5 , 54LiFeP, 61URu 2 Si 2 , 55YBa 2 Cu 3 O y , 58YBa 2 Cu 3 O 6+δ , 59Ferronematic, 71Fractional quantum Hall effectRDNMR at ν = 5/2, 25GrapheneCarrier dynamics, 9Dirac point, 9epitaxial, 8, 10few layer graphene, 11Multilayer epitaxial graphene, 9nanoribbons, 7trilayer, 12GraphiteAharonov-Bohm effect, 21dHvA, 17electron-hole asymmetry, 14–16Fermi surface, 17Lifshitz transition, 18magneto-phonon effect, 20Megagauss fields, 16Raman scattering, 19, 20splitting of optical transitions, 15SWM Hamiltonian, 14He3 refrigeratordc magnetic field, 101pulsed magnetic field, 101Heavy fermionsCeCoIn 5 , 53CeCoIn 5 , 54CeIrIn 5 , 52URu 2 Si 2 , 55High strength wiresCu/Nb, 108HT c superconductorsCa doped Bi-2212, 84La-doped Bi2201, 57pseudogap, 56YBa 2 Cu 3 O y , 58YBa 2 Cu 3 O 6+δ , 59HTS coilYBCO, 85YBCO pancake coils, 81Hybrid magnetdesign, 114new test station, 103Lifshitz transitionCeIrIn 5 , 52MAGFINS coil, 107Magnet for levitation, 113Magnet transients, 99Magnetic levitationH 2 O, H 2 He, 113Magnetic nanoparticlesDy 3 Fe 5 O 12 , 68manganese ferrite, 70MagnetizationErCo 2 , 69Y 0.9 Gd 0.1 Fe 2 D 4.2 , 67Magnetohydrodynamics, 91MAX phaseTi 3 SiC 2 , 48MegagaussFaraday cage, 100Metal-insulator transition, 30MetalsTi 3 SiC 2 , 48Metamagnetic transition138

2011CeIrIn 5 , 52CeRh 2 Si 2 , 51MgB 2wires, 86Molecular magnet[Co 2 (H 2 O)(O 2 C t Bu) 4 (HO 2 C t Bu) 2 (py) 2 ], 77[Fe 4 (L) 2 (dpm) 6 ], 78ab initio calculation, 76conduction, 75oxalate, 75verdazyl radical, 76Nano-crystalCo-Ni alloys, 92Nano-magnetismdipolar spin waves, 42dysprosium dot, 42Nanocomposite wiresNb nanofilaments, 108NanowiresInAs, 38Nitrogen cryostat, 100Vacuum Magnetic Birefringence, 100NMRBose-Einstein condensation, 66DTN, 66in pulsed fields, 95probe for pulsed fields, 96Number of projects, 1Organic conductorsquasi 1D conductor , 50ParimagnetismErCo 2 , 69Phase transitionAntiferromagnetic Transition, 50Metal-insulator transition, 50Polyhelixinsert, 110pulsed and dc coils, 111pulsed magnet, 107Power consumption, 1Quantum dotsGaAs/AlAs, 36Quantum Hall effectHgTe film, 40HIROS, 27microwave induced ZRS, 29quasi 3D, 40RDNMR, 25trilayer graphene, 12wide quantum well, 27, 29Quantum phase transitionURu 2 Si 2 , 55Quantum-classical crossover, 26Radial core-shell nanowires, 37Rb 2 Cu 3 SnF 12 , 65SEISM ECRIS prototype, 112Silicon-on-insulator (SOI), 30Superconducting pnictides, 60SuperconductorsMgB 2 , 83, 86PnictideLiFeAs, 61LiFeP, 61Quantum criticality, 57Stripe order, 58, 59stripe order, 49underdoped YBa 2 Cu 3 O y , 49SuperSMES, 85Thin filmspolycrystalline SnO 2 , 44Topological insulatorHgTe quantum well, 39Tunnel diode oscillator, 60Tunnelling spectroscopyBi 2+x Sr 4−x Cu 2 CO 3 O 8+δ break junction, 56Landau level, 31spin gap, 31Two-dimensional organic metalθ-(ET) 4 CoBr 4 (C 6 H 4 Cl 2 ), 47Users of LNCMI, 1Vacuum Magnetic birefringence, 89VAMAS, 82X-raypulsed fields, 97XXL coils, 107Zylon reinforcement, 107139

CONTINUOUS MAGNETIC FIELDSLNCMI-G, CNRS25 rue des Martyrs, B.P. 16638042 GRENOBLE cedex 9 - FranceTel. : +33 (0)4 76 88 10 48Fax : +33 (0)4 76 88 10 01PULSED MAGNETIC FIELDSLNCMI-T, CNRS143 avenue de Rangueil31400 TOULOUSE - FranceTel. : +33 (0)5 62 17 28 60Fax : +33 (0)5 62 17 28 16lncmi.direction@lncmi.cnrs.frhttp://www.lncmi.cnrs.frCréation : Anne-Claire LECOMTE, www.studiographisme.fr - Impression : COQUAND - LA TYPOPhotos : CNRS Photothèque ; LNCMI ; CNRS Photothèque / Martin HYTCH ; CNRS Photothèque / Denis MOREL ;LNCMI / Baptiste VIGNOLLE ; CNRS Photothèque / Stéphane BECHU.