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Plenarvorträge - DPG-Tagungen

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Tiefe Temperaturen Dienstag<br />

TT 13.10 Di 18:30 H20<br />

Measurements of coherent transition radiation by Hilberttransform<br />

spectroscopy with Josephson junctions — •Andre<br />

Kaestner 1 , Frank Ludwig 2 , Frank Ludwig 1 , and Meinhard<br />

Schilling 1 — 1 Institut für elektrische Messtechnik und Grundlagen<br />

der Elektrotechnik, TU Braunschweig, Hans-Sommer-Straße 66,<br />

D-38106 Braunschweig — 2 Deutsches-Elektronen-Synchrotron DESY,<br />

Notkestraße 85, D-22603 Hamburg<br />

We present measurements of coherent transition radiation from 50<br />

to 500 GHz using the method of Hilbert-transform spectroscopy. The<br />

Josephson junctions out of YBa2Cu3O7 (YBCO) used for Hilberttransform<br />

spectroscopy are prepared on symmetric 24 o LaAlO3 bicrystal<br />

substrates. For better coupling of high frequency radiation they are<br />

equiped with integrated wideband antennas out of YBCO. We obtain for<br />

a 6 micrometer wide junction an ICRN-product of 170 microvolt at 77 K<br />

and 2.0 millivolt at 10 K. The set-up is optimized for low insertion loss<br />

over a wide frequency range from 50 GHz to 2 THz. The measurements<br />

of coherent transition radiation are compared with similar measurements<br />

done at the Tesla Test Facility for the same set-up.<br />

TT 14 FV-internes Symposium ”Orbital Physics”<br />

Zeit: Dienstag 14:30–18:55 Raum: H18<br />

Hauptvortrag TT 14.1 Di 14:30 H18<br />

Lattice instability of frustrated orbital and spin systems —<br />

•Maxim Mostovoy — Materials Science Center, University of Groningen,<br />

The Netherlands<br />

In materials with strong exchange interactions between electrons occupying<br />

degenerate orbitals the cooperative Jahn-Teller effect may be suppressed,<br />

as it is often impossible to order orbitals on all pairs of neighboring<br />

transition metal ions in a unique optimal way. This frustration<br />

may be relieved by several competing mechanisms, e.g., quantum orbital<br />

fluctuations or ‘Peierls-like’ lattice distortions. I will discuss similarities<br />

and differences between frustrated Jahn-Teller materials and frustrated<br />

spin systems, such as antiferromagnets with a pyrocholore and triangular<br />

lattice.<br />

Fachvortrag TT 14.2 Di 15:00 H18<br />

Magnetism, Charge Order and Giant Magnetoresistance in<br />

SrFeO3−δ — •C. Ulrich 1 , A. Lebon 1 , P. Adler 2 , C. Bernhard 1 ,<br />

A. Boris 1 , A. Pimenov 1 , A. Maljuk 1 , C.T. Lin 1 , and B. Keimer 1<br />

— 1 Max-Planck-Institut FKF, Stuttgart — 2 Institut für Anorganische<br />

Chemie, Universität Karlsruhe<br />

The relationship between spin ordering, charge ordering, and magneto–<br />

transport effects has recently received much attention in the context of<br />

the colossal magnetoresistance (CMR) phenomenon in manganites. In the<br />

3D perovskite SrFeO3, Fe 4+ is isoelectric to the Jahn–Teller ion Mn 3+ in<br />

the manganite system. However, while the parent compound of the CMR<br />

manganites, LaMnO3, is insulating and shows a cooperative Jahn–Teller<br />

effect, its analogue SrFeO3 remains metallic at all temperatures and exhibits<br />

a helical antiferromagnetic spin structure below 130 K. Therefore,<br />

SrFeO3 is right at the borderline between an itinerant and a strongly<br />

localized electron system.<br />

We have studies three single crystals of SrFeO3−δ with different oxygen<br />

content. Cubic SrFeO3.00 is metallic in the entire temperature range<br />

and shows a large negative magnetoresistance effect at 55 K. Tetragonal<br />

SrFeO2.87 shows semiconducting behavior below a second magnetic phase<br />

transition at 75 K. Our Mössbauer studies reveal that this phase transition<br />

is accompanied by a charge ordering of Fe 3.5+ into Fe 3+ and Fe 4+ . An<br />

orthorhombic sample SrFeO2.81 on the other side showed semiconducting<br />

behavior and a pronounced positive MR effect at 70 K.<br />

TT 14.3 Di 15:20 H18<br />

Frustrated orbital ground state in thiospinels — •J. Hemberger,<br />

H.-A. Krug von Nidda, V. Fritsch, N. Büttgen, E.-W. Scheidt,<br />

P. Lunkenheimer, R. Fichtl, V. Tsurkan, and A. Loidl — Center<br />

for Electronic Correlations and Magnetism, Institute of Physics, University<br />

of Augsburg<br />

We studied the ground state properties of the normal cubic spinel compounds<br />

AB2S4 (A = Fe, Mn and B = Sc, Cr) by means of specific heat,<br />

magnetization and susceptibility measurements, as well as dielectric and<br />

optical spectroscopy. In all compounds frustration strongly influences the<br />

interplay of orbital and magnetic degrees of freedom and corresponding<br />

order phenomena. Our results indicate the existence of distinguished spinorbital<br />

scenarios: In the case of nonmagnetic B-sites (Sc) we find strong<br />

magnetic coupling (Curie-Weiss temperature θCW = −45 K for A = Fe)<br />

without any indications of order down to 50 mK. The ground states of<br />

these compounds have to be characterized as a spin liquid in MnSc2S4<br />

and a spin-orbital liquid in FeSc2S4, where Fe 2+ is Jahn-Teller active but<br />

orbitally frustrated. In the case of magnetic B-sites (Cr) spin-order is<br />

induced and the low-temperature states have to be described as orbital<br />

liquid, orbital glass, or finally orbital order.<br />

TT 14.4 Di 15:35 H18<br />

Isotropic spin waves in the paramagnetic spin liquid state of<br />

FeSc2S4 — •Alexander Krimmel 1 , Vladimir Tsurkan 1 , Alois<br />

Loidl 1 , Michael Mücksch 1,2 , and Marek Koza 2 — 1 Institut für<br />

Physik, Univ. Augsburg, D-86159 Augsburg, Germany — 2 Institut Laue<br />

Langevin, BP156, 38042 Grenoble Cedex 9, France<br />

FeSc2S4 crystallizes in the normal cubic spinel structure with magnetic<br />

Fe 2+ ions (3d 6 , high spin configuration, µeff = 5.12µB) on the A-site. The<br />

Jahn-Teller ions Fe 2+ are orbitally frustrated. Specific heat and magnetic<br />

measurements showed no signs of long range magnetic order down to 50<br />

mK. The ground state properties of FeSc2S4 can be described by a spinorbit<br />

liquid. Here we report on inelastic neutron scattering experiments<br />

on polycrystalline FeSc2S4. Even at 80 K, the paramagnetic spectral response<br />

is centered around 0.6 ˚A −1 corresponding to the antiferromagnetic<br />

Bragg position. On cooling, a well defined isotropic magnon dispersion<br />

evolves but the system remains in a cooperative paramagnetic state without<br />

magnetic Bragg peaks. These results are discussed within a scenario<br />

of collective magnetic excitations in an orbital liquid state.<br />

TT 14.5 Di 15:50 H18<br />

Orbital degree of freedom in manganites determined by O1s<br />

and Mn2p NEXAFS: crystal field versus orbital coupling in<br />

single-layered La1−xSr1+xMnO4 — •Michael Merz 1 , P. Reutler<br />

2 , B. Büchner 2 , Y. U. Idzerda 3 , S. Tokumitsu 4 , N. Nücker 4 ,<br />

and S. Schuppler 4 — 1 Institut für Kristallographie, RWTH Aachen<br />

— 2 Institut für Festkörper- und Werkstoffforschung, Dresden — 3 Naval<br />

Research Laboratory, Washington — 4 Forschungszentrum Karlsruhe, Institut<br />

für Festkörperphysik<br />

Manganites exhibit a manifold of highly interesting magnetic and electronic<br />

phases, with charge- and orbital-ordered states being at the center<br />

of interest. Especially single-layered La1−xSr1+xMnO4 is a prototypical<br />

example for such ordering effects. For this compound a ferro-orbital ordering<br />

of |3z2 − r2 > states is expected from the strong crystal field, yet<br />

anti-ferro type orbital coupling might lead to an admixture of |x2 −y2 ><br />

states.<br />

Using polarization-dependent O1s and Mn2p near-edge x-ray absorption<br />

spectroscopy on untwinned La1−xSr1+xMnO4 single crystals (0 ≤<br />

x ≤ 0.5), the doping- and temperature-dependent occupancies of the<br />

O2p and Mn3d orbitals were studied. According to the spectra, Sr doping<br />

not only provides holes to the system but also induces a continuous<br />

transfer of electrons from out-of-plane |3z2−r2 > to in-plane |3x2−r2 ><br />

and |3y2 − r2 > states. The data clearly demonstrate that electrons reside<br />

for all doping levels on mixed α|3z2 − r2 > +β|x2 − y2 > states, i.<br />

e., antiferro-coupled canted orbitals where α and β strongly depend on<br />

the Sr content.<br />

TT 14.6 Di 16:05 H18<br />

Magnetische Polaronen: Magnetismus der dotierten Schichtmanganate<br />

La1−xSr1+xMnO4 (x=1/8) — Davide Cattani 1 ,<br />

Christoph Baumann 2,1 , Pascal Reutler 2 , •Rüdiger Klingeler<br />

2 , Alexandre Revcolevschi 3 und Bernd Büchner 2 —<br />

1 Università di Parma, I-43100 Parma — 2 Leibniz-Institut für Festkörperund<br />

Werkstoffforschung Dresden, PF 270116, D-01171 Dresden —<br />

3 Laboratoire de Chimie des Solides, Université Paris-Sud, F-91405<br />

Orsay Cedex<br />

In den einfach geschichteten Manganaten La1−xSr1+xMnO4 zeigen Messungen<br />

der thermischen Ausdehnung im Fall lochfreier (x=0) oder gering<br />

dotierter Systeme (x=1/8) eine grosse, stark anisotrope magnetoelastische<br />

Kopplung beim Einsetzen der langreichweitig antiferromagne-

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