Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
(x≈1/3, 3/4 and y≈0, 2/3, 4/3). The observed phonon scattering is analyzed
in terms of lattice strain due to the structural misfit and disorder.
Hydration, due to the intercalation of H2O layers, releases a part of this
strain. We will show experimental evidence for a pseudo gap formation in
the electronic scattering continuum. Furthermore, our Raman data suggest
a connection between disorder on the partly occupied Na sites, the
split off of the a1g level from the other t2g states of Co 4+ and superconductivity.
Work supported by DFG SPP1073, NATO PST.CLG.9777766,
INTAS 01-278, CREST of JST (Japanese Science and Technology Corporation)
and MRSEC Program of NSF under award number DMR 02-
13282.
TT 14.14 Di 18:40 H18
Beziehung zwischen orbitaler Besetzung und Magnetismus in
Ca2−xSrxRuO4 — P. Steffens, O. Friedt, O. Schumann, R.
Müller, J. Baier, M. Kriener, T. Zabel, S. Nakatsuji, Y. Maeno,
T. Lorenz, A. Freimuth, M. Braden, •P. Steffens, O.
Friedt, O. Schumann, R. Müller, J. Baier, M. Kriener, T.
Zabel, S. Nakatsuji, Y. Maeno, T. Lorenz, A. Freimuth und
M. Braden — II. Physikalisches Institut, Universität zu Köln
TT 15 Korrelierte Elektronen: Theorie I
Das Phasendiagram von Ca2−xSrxRuO4 ist sehr vielfältig, obwohl sich
Ca und Sr nur durch ihre Ionengröße unterscheiden[1-3]. Es reicht von
dem Spin-triplet Supraleiter Sr2RuO4 bis hin zum antiferromagnetisch
geordneten Mott-Isolator Ca2RuO4. Insbesondere in dem noch metallischen
Konzentrationsbereich nahe der elektronischen Lokalisierung beobachtet
man verschiedene konkurrierende magnetische Instabilitäten, die
durch nur kleine strukturelle Verzerrungen bestimmt zu sein scheinen.
So kann man sowohl mittels eines Magnetfeldes als auch mit äußerem
Druck zwischen verschiedenen magnetischen Phasen schalten, und dabei
immer strukturelle Änderungen induzieren. Wir diskutieren das Wechselspiel
zwischen orbitaler Besetzung und magnetischen Eigenschaften
anhand von Neutronen-Streuuntersuchungen und Messungen von thermischer
Ausdehnung, spezifischer Wärme und Magnetostriktion.
[1] S. Nakatsuji et al., PRL 84, 2666 (2000) [2] M. Braden et al., PRB
58, 847 (1998); O. Friedt et al., PRB 63, 174432 (2001) [3] S. Nakatsuji
et al., PRL 90, 137202 (2003)
Zeit: Dienstag 14:30–16:30 Raum: H19
TT 15.1 Di 14:30 H19
The coherence scale of the two-dimensional Kondo Lattice
model. — •Fakher F. Assaad — Institut für Theoretische
Physik und Astrophysik, Universität Würzburg, Am Hubland D-97074
Würzburg
A doped hole in the two-dimensional half-filled Kondo lattice model
with exchange J and hopping t has momentum (π, π) irrespective of the
coupling J/t. The quasiparticle residue of the doped hole, Z(π,π), tracks
the Kondo scale, TK, of the corresponding single impurity model. Those
results stem from high precision quantum Monte Carlo simulations on
lattices up to 12 × 12. Accounting for small dopings away from halffilling
within a rigid band approximation, this result implies that the
effective mass of the charge carriers at the Fermi level tracks 1/TK or
equivalently that the coherence temperature Tcoh ∝ TK. This results is
consistent with the large-N saddle point of the SU(N) symmetric Kondo
lattice model.
TT 15.2 Di 14:45 H19
Quasi-particle structure and ferromagnetism in the Kondolattice
model — •Tilmann Hickel and Wolfgang Nolting —
Humboldt-Universität, Institut für Physik, Newtonstr. 15, 12489 Berlin
The Kondo-lattice model (KLM) describes an indirect exchange interaction
of localized magnetic moments, mediated by itinerant conduction
electrons. The quantum-mechanical character of these local spins,
which has an substantial impact on the magnetic properties of the concerned
materials, prohibits a straight-forward application of standard
many-body approaches to the model Hamiltonian. We shall show that
the projection-operator method (POM) is instead a valuable analytical
technique to obtain substantial statements on one-particle properties of
the KLM. It allows the derivation of perturbational results of the same
quality as obtained for fermionic models as the Hubbard model. We will
use the POM to study the ferromagnetic phase of the KLM and its dependence
on material parameters self-consistently. In each step we make
sure, that the subsystems of conduction electrons and localized moments
are approximated on the same level.
TT 15.3 Di 15:00 H19
Spectral Densities of Single Impurity Models by Density-
Matrix Renormalization — •Götz S. Uhrig 1 , Carsten Raas 1 ,
and Frithjof B. Anders 2 — 1 Institut für Theoretische Physik,
Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany —
2 Institut für Theoretische Physik, Universität Bremen, 28334 Bremen,
Germany
The quantitative control of the dynamic correlations of single impurity
models is essential in many active fields. We analyze the local propagator
with a constant energy resolution by dynamical density-matrix renormalization
(D-DMRG). This approach is particularly useful for higher-lying
excitations. In contrast to other approaches, sharp dominant resonances
at high energies are found. We analyze their line shapes and discuss their
origin and importance. An analysis of the Kondo energy scale is also
presented.
The D-DMRG provides the density of states convolved with a
Lorentzian of width η. Choosing η is a trade-off between run-time
and energy resolution. Thus, accurate deconvolution strategies are
essential. We analyze several deconvolution methods to calculate
continuous spectral densities and propose a novel non-linear algorithm,
the least-bias algorithm.
Our findings provide a well-controlled numerical approach to impurity
problems which can serve as the impurity solver in dynamical mean
field theory (DMFT) calculations as well as in calculations of more complex
impurities like quantum dots, molecules, or the effective problems
of extended DMFT schemes.
TT 15.4 Di 15:15 H19
Elektronische Eigenschaften des Anderson-Gitters - ein Zugang
durch Flussgleichungen — •Karsten Meyer und Klaus W. Becker
— Insitut für Theoretische Physik, TU Dresden
Die von Wegner [1] vorgeschlagene Flussgleichungsmethode wird zur
Untersuchung des Anderson-Gitters verwendet. Die elektronische Struktur
ist dabei durch zwei Quasiteilchenbänder gekennzeichnet, die durch
eine Lücke voneinander getrennt sind. Die Ergebnisse werden mit den
Resultaten aus anderen theoretischen Zugängen verglichen.
Für das System lässt sich eine charakteristische Temperatur T ∗ definieren,
die für kleine Hybridisierungsstärken exponentiell klein wird. T ∗ ist
tendenziell gegenüber der Kondo-Temperatur des Störstellen-Anderson-
Modells abgesenkt. Ein solcher Effekt ist mit dem von Nozières [2] diskutierten
Exhaustion-Szenarium konsistent.
[1] F. Wegner, Ann. Phys. 3, 77 (1994)
[2] P. Noziéres, Eur. Phys. J. B 3, 447 (1998)
TT 15.5 Di 15:30 H19
Low-energy charge excitations on frustrated lattices — •Erich
Runge — Max Planck Institute for the Physics of Complex Systems,
Dresden, Germany
We study strongly interacting electrons on frustrated lattices of complex
topology. Often, these systems have a huge phase space for low-lying
excitations. In the case of LiV2O4, this leads to heavy-fermion behavior.
However, the nature of the low-energy excitations is generally not well
understood. It has even been predicted that fractionally charged quasiparticles
exist in two and three dimensions [1].
We discuss in detail the interplay of the frustrated kinetic energy
and the Coulomb repulsion between nearest-neighbor sites. An effective
Hamiltonian for the low-energy excitations is derived, which involves ring
exchanges and shows a huge density of states of non-magnetic excitations.
It can serve as a prototype for a new class of effective interactions and
quasi-particles.
Large-scale numerical results for the spectral function and related
quantities are presented as well.
[1] Peter Fulde, Karlo Penc, and Nic Shannon, Fractional charges in py-