Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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-