Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
number of electrons we employ the parity projected partition function
formalism. Our main results and observations are as follows: (a) the dependence
of the supercurrent on the external flux and temperature in
both even and odd canonical ensembles derived here may strongly deviate
from that established for the grand canonical ensemble, (b) at sufficiently
low temperatures supercurrents in rings with odd number of electrons are
TT 20 Metall-Isolator-Übergänge
totally blocked provided QPC has only one conducting mode and (c) this
blocking effect gets partially lifted in structures with several or many conducting
modes. We also discuss possible realizations of the above effects
and demonstrate that our predictions can be directly tested in modern
experiments.
Zeit: Mittwoch 14:30–16:30 Raum: H18
TT 20.1 Mi 14:30 H18
Projektor-basiertes Renormierungsverfahren für das spinlose
Holstein-Modell — •Steffen Sykora 1 , Arnd Hübsch 1,2 und
Klaus W. Becker 1 — 1 Institut für Theoretische Physik, Technische
Universität Dresden, D-01062 Dresden — 2 Department of Physics,
University of California, Davis, CA 95616, USA
Mit Hilfe eines kürzlich vorgestellten Renormierungsverfahrens wird
der Metall-Nichtmetall-Übergang im eindimensionalen Holstein-Modell
bei Halbfüllung untersucht. In dieser Methode werden mit Hilfe einer
unitären Transformation sukzessiv Anregungsoperatoren der Elektron-
Phonon-Wechselwirkung von höheren zu tieferen Energien herausintegriert.
Als Ergebnis erhält man einen effektiven Hamiltonoperator für ein
quasifreies System aus renormierten ungekoppelten Elektronen und Phononen.
Für kleine Werte der Elektron-Phonon-Kopplung g findet man ein
stetiges Verhalten der elektronischen Einteilchenenergie ˜εk an der Fermikante
und somit eine metallische Phase. Dagegen öffnet sich für g größer
als ein kritischer Wert gc eine Lücke an der Fermikante, so daß eine isolierende
Phase entsteht. Gleichzeitig mit dem Auftreten der Lücke in ˜εk
wird die renormierte Phononenfrequenz ˜ωq am Rand der Brillouin-Zone
weich, was auf das Entstehen einer Peierls-Phase schließen läßt.
TT 20.2 Mi 14:45 H18
Weak ferromagnetism of the charge stripes in La5/3Sr1/3NiO4 —
•Rüdiger Klingeler 1 , Bernd Büchner 1 , Sang-Wook Cheong 2 ,
and Markus Hücker 3 — 1 Leibniz-Institut für Festkörper- und Werkstoffforschung
Dresden, PF 27 01 16, 01171 Dresden — 2 Department
of Physics and Astronomy, Rutgers University, Piscataway, New Jersey
08854 — 3 Physics Department, Brookhaven National Laboratory, Upton,
New York 11973
In doped nickelates the interplay of charge and spin degrees of freedom
results a long range charge order where static charge stripes and
spin stripes are present. We present measurements of the high field magnetization
and the specific heat of a La5/3Sr1/3NiO4 single crystal in order
to elucidate the thermodynamic properties of the stripe phase. The data
provide the magnetic phase diagram. The charge ordering temperature
TCO is nearly independent of the magnetic field and there is a significant
and highly anisotropic field dependence of the long range spin ordering
temperature TSO. High field magnetization data reveals a weak ferromagnetism
in the entire stripe ordered phase. We analyze these data in terms
of weak moments which are long range correlated. Double exchange interactions
are discussed as a possible origin of the weak ferromagnetism.
TT 20.3 Mi 15:00 H18
Magnetothermal Conductivity of Highly Oriented Pyrolytic
Graphite — •Konstantin Ulrich, Pablo Esquinazi,
Roberto Ocaña, and Heiko Kempa — Abteilung Supraleitung und
Magnetismus, Universität Leipzig, Linnestrasse 5, D-04103 Leipzig
The magnetic field (0T≤ B ≤ 9T) dependence of the longitudinal
thermal conductivity κ(T, B) of highly oriented pyrolytic graphite samples
was measured in the temperature range 0.2 K ≤ T ≤ 20 K for fields
parallel to the c−axis. With the measured longitudinal electrical resistivity
we show that the Wiedemann-Franz law is violated in the high-field
(quantum-Hall) regime, where large oscillations in the thermal conductivity
are measured at B > 2 T, as well as near the Metal-Insulator
transition (MIT) at B ∼ 0.1 T. For a strong anisotropic graphite sample
κ(B) reveals a kink-behavior at the MIT that looks similar to that predicted
by the magnetic-field induced gap theory due to the phenomenon
of magnetic catalysis. We discuss our results taking into account recently
published models for the behavior of Dirac-like Fermi quasiparticles.
TT 20.4 Mi 15:15 H18
X-ray magnetic and natural circular dichroism at the vicinity
of the Co low-spin to high-spin transition in REBaCo2O5.5
(RE= Dy; Gd) system — •S. Gold 1 , E. Goering 1 , P. Lemmens 2 ,
A. Podlesnyak 3 , G. Bychkov 4 , J. Deisenhofer 5 , A. Loidl 5
und G. Schütz 1 — 1 MPI für Metallforschung, Stuttgart — 2 MPI für
Festkörperforschung, Stuttgart — 3 PSI, ETHZ&PSI, Villingen — 4 Inst.
of Solid State & Semicond. Phys., Nat. Acad. Sci. Minsk — 5 EP5,
Universität Augsburg
The magnetic perovskite REBaCo2O5.5 (RE= Dy; Gd) system shows
a rich magnetic and electronic phase diagram, with a MI transition at
higher temperatures and ferromagnetic ordering just below room temperature.
With a further reduction of temperature the ferromagnetic magnetization
is strongly reduced. This has been discussed in terms of a
Co spin state transition and/or antiferromagnetism. A double peak like
structure could be observed in the Co L2,3 XMCD. We interpret this double
peak by spectroscopic distinguishable octahedral and pyramidal Co
sites. The Co L2,3 XMCD has been separated from the overlapping Ba
M4,5 spectra by a recently developed XMCD analysis tool called moment
analysis. This analysis exhibits an unusual large orbital/spin-moment ratio.
In addition, we could observe an extreme large X-ray natural circular
dichroism (XNCD) signal, which is about two orders of magnitude larger
compared to other published results and about 20% of the corresponding
white line intensity. This suggests a large non centrosymmetric charge
distribution at the Co site. Nevertheless, the orgin of this extreme large
XNCD effect is not clear up to now.
TT 20.5 Mi 15:30 H18
Pressure-induced metal-insulator transition in rare earth nickelates
RNiO3 — •R. Lengsdorf 1 , J.A. Alonso 2 , M.J. Martinez-
Lope 2 , M. Amboage Castro 2 , and M.M. Abd-Elmeguid 1 — 1 II.
Physikalisches Institut Universität zu Köln, Zülpicherstr. 77, 50937 Köln,
Germany — 2 Instituto de Ciencia de Materiales de Madrid, Consejo
Superior de Investigaciones Cientificas, Cantoblanco, E-28049 Madrid,
Spain
We report on the effect of pressure up to 25 GPa on the electronic
and structural properties (electrical resistance and x-ray diffraction) of
two selected antiferromagnetic charge transfer insulators EuNiO3 (orthorhombic,
TN = 220 K) and LuNiO3 (monoclinic, TN = 130 K) which
undergo a thermal-induced insulator-metal transition (IMT) at TIM =
460 K and 599 K, respectively. The IMT in LuNiO3 is connected with
a monoclinic-to-orthorhombic structural phase transition, whereas the
orthorhombic structure of EuNiO3 remains unchanged.
In both systems we find a pressure-induced IMT at a critical pressure
of about 6 GPa, while the crystal structure remains unchanged. Structural
phase transitions are observed at much higher pressures: EuNiO3
orthorhombic to rhombohedral at 12 GPa and LuNiO3 at 16 GPa. The
experimental results are explained in terms of a pressure-induced increase
of the 3d-2p-3d orbital hybridization as a result of increasing the Ni-O-
Ni bonding angle with increasing pressure. The influence of the charge
transfer between localized Ni and O states on the pressure-induced IMT
in LuNiO3 is also discussed.
TT 20.6 Mi 15:45 H18
Low temperature instability and magnetic entropy in
La7/8Sr1/8MnO3 — •Jochen Geck 1 , Peter Wochner 2 , Sven
Kiele 3 , Rüdiger Klingeler 4 , Pascal Reutler 4 , and Bernd
Büchner 4 — 1 II. Physikalisches Institut der RWTH Aachen —
2 Max-Planck-Institut für Metallforschung, Stuttgart — 3 HASYLAB
am DESY, Hamburg — 4 Leibniz-Institut für Festkörper- und
Werkstoffforschung, Dresden
Specific heat and magnetization measurements reveal that the entropy
changes at the metal-insulator transition of La1−xSrxMnO3 with x ∼ 1/8