Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
TT 6.14 Mo 17:30 H18
Signal transport in one-dimensional interacting Fermi systems
— •Peter Schmitteckert — Institut für Theorie derKondensierten
Materie, Universität Karlsruhe, 76128 Karlsruhe, Germany
We have extented the density matrix renormalization group method
(DMRG) to calculate the time evolution of a short wave paket in an
one-dimensional system of interacting fermions at zero temperature.
In particular we demonstrate that it is not sufficient to target for the
low energy sector (a few low lying states) of the system to simulate the
dynamic of short pulses, although the overlap between the perturbed
system and the low energy sector of the unperturbed system is close to
one. Therefore, special care has to be taken in the selection of the target
states within the DMRG.
TT 7 Korrelierte Elektronen: Schwere Fermionen
Zeit: Montag 14:30–17:00 Raum: H19
TT 7.1 Mo 14:30 H19
Leitfähigkeit von Systemen schwerer Fermionen mit nichtmagnetischen
Störstellen — •Claas Grenzebach und Gerd Czycholl
— Institut für Theoretische Physik, Universität Bremen
Systeme schwerer Fermionen werden durch das periodische Andersonmodell
(PAM) beschrieben, nichtmagnetische Störstellen jeweils durch
energetisch sehr hoch liegende (ǫf → ∞) und damit unbesetzte f-
Niveaus.
Das PAM wird mittels der dynamischen Molekularfeldtheorie
(DMFT) auf ein effektives SIAM abgebildet. Dieses wird mit modifizierter
Störungsrechnung (MPT) behandelt, einer Näherung, die exakt bis
zu 2. Ordnung in U ist und den atomaren Limes reproduziert. Dabei wird
für die Störstellen-(Unordnungs-)Streuung durch die Kondo- ” Löcher“
die CPA (coherent potential approximation) verwendet, was konsistent
mit der DMFT-Abbildung ist.
Für verschiedene Störstellenkonzentrationen werden in diesem Rahmen
statische und dynamische Leitfähigkeiten berechnet.
TT 7.2 Mo 14:45 H19
Renormalization of the periodic Anderson model: an alternative
analytical approach to heavy Fermion behavior — •Klaus
Becker 1 and Arnd Hübsch 1,2 — 1 Institut für Theoretische Physik,
Technische Universität Dresden, D-01062 Dresden — 2 Department of
Physics, University of California, Davis, CA 95616, USA
A recently developed projector-based renormalization method (PRM)
for many-particle Hamiltonians is applied to the periodic Anderson model
with the aim to describe heavy Fermion behavior. In this method highenergetic
excitation operators instead of high energetic states are eliminated.
We arrive at an effective Hamiltonian for a quasi-free system which
consists of two heavy-quasiparticle bands. In contrast to the usual slave
boson mean-field (SB) treatment one of the bands describes a subsystem
of renormalized but still correlated f electrons whereas the second band
stands for renormalized conduction electrons. The resulting renormalization
equations for the parameters of the Hamiltonian are valid for large
as well as small degeneracy νf of the angular momentum. An expansion
in 1/νf is avoided. Within an additional approximation which adapts the
idea of a fixed renormalized f level ˜εf, we obtain coupled equations for ˜εf
and the averaged f occupation 〈nf 〉. These equations resemble to a certain
extent those of the SB theory. In particular, for large νf the results
for the PRM and the SB approach agree quite well whereas considerable
differences are found for small νf.
TT 7.3 Mo 15:00 H19
Theory for Coexistence of Superconductivity and A-phase in
CeCu2Si2 — •Matthias Neef and Gertrud Zwicknagl — Institut
für Mathematische Physik, TU Braunschweig, Mendelsonstr.3, 38106
Braunschweig
We study the interplay between superconductivity and spin density
waves in CeCu2Si2. We adopt realistic quasiparticle bands as calculated
by means of Renormalized band method. The experimental data concerning
the magnetic structure are confirmed. In addition we investigate
the superconductivity states, which can either coexist with or expel the
A-phase. Results are reported for magnetic susceptibility and thermodynamic
properties. The phase diagram is discussed.
TT 7.4 Mo 15:15 H19
Heavy fermion properties in filled skutterudites — •Tanja
Westerkamp and Gertrud Zwicknagl — Institut für Mathematische
Physik, TU Braunschweig, Mendelssohnstr.3, 38106 Braunschweig
We calculate the temperature-dependence of the effective mass of the
heavy quasiparticles in the filled skutterudites. The latter is assumed
to arise from the low lying Γ1 − Γ5 exitations in the CEF level scheme
suggested by Aoki et al.[1] and Tayama et al.[2].
In addition, we discuss the behavior of the quadropole susceptibility.
[1] Y.Aoki, T.Namiki, S.Ohsaki, S.R.Saha, H.Sugawara, H.Sato 2002
cond-mat/0206193
[2] T.Tayama, T.Sakakibara, H.Sugawara, Y.Aoki, and H.Sato 2003
cond-mat/0303542
15:30 Pause
TT 7.5 Mo 15:45 H19
Microwave Conductivity Spectra of Heavy-Fermion UPd2Al3
— •Marc Scheffler 1 , Martin Dressel 1 , M. Jourdan 2 , and H.
Adrian 2 — 1 1. Physikalisches Institut, Universität Stuttgart, 70550
Stuttgart — 2 Institut für Physik, Johannes Gutenberg Universität, 55099
Mainz
UPd2Al3 is a well-known heavy-fermion system that undergoes antiferromagnetic
and superconducting phase transitions at TN = 14 K and
Tc = 2 K, respectively. Previous optical spectroscopy studies [1] of the
heavy-fermion nature of this system down to frequencies of about 1 cm −1
(= 30 GHz) showed the influence of heavy-fermion as well as magnetic
characteristics and in particular revealed a well-pronounced pseudogap
at low frequencies (below 3 cm −1 ) that was attributed to magnetic correlations.
Thus the optical conductivity at even lower frequencies is of
notable interest because there the Drude roll-off (the high-frequency characteristic
of a metal) remained hidden at extremely low frequencies.
Using a recently developed cryogenic broadband microwave spectrometer
employing the Corbino geometry we have studied the complex optical
conductivity of UPd2Al3 thin films in the frequency range from 45 MHz
to 20 GHz at temperatures from 1.65 K to 300 K. We present spectra
revealing the emergence of a strong Drude-like conductivity roll-off at
frequencies of only a few GHz as the temperature is decreased (in agreement
with the increase of dc conductivity) and discuss the microwave
conductivity of UPd2Al3 in the framework of the optical properties determined
by previous experiments.
[1] M. Dressel et al., Phys. Rev. Lett. 88, 186404 (2002)
TT 7.6 Mo 16:00 H19
Quantum Critical Behavior in Yb1−xLaxRh2Si2 Studied by
Low Temperature Resistivity and Specific Heat — •Franziska
Weickert, Teodora Rus, Julia Ferstl, Philipp Gegenwart,
Christoph Geibel, and Frank Steglich — Max-Planck-Institute
for Chemical Physics of Solids, Nöthnitzer Str.40, 01187 Dresden
The Heavy Fermion System YbRh2Si2 is a very suitable compound
to study non Fermi liquid properties near a Quantum Critical Point
(QCP). At zero magnetic field very weak antiferromagnetic order occurs
at 70 mK. A tiny 0.15% volume expansion due to a substitution of
nominally 5% of Si by larger Ge reduces TN to 20 mK. Difficulties in
sample preperation prevent a complete suppression of TN by Ge-doping.
An alternative and successful route to tune the system through
the zero-field QCP is the partial substitution of Yb by larger La in
Yb1−xLaxRh2Si2.
Here we report low temperature resistivity and specific heat measurements
on x = 0.05 and 0.1 single crystals, which show the complete
suppression of AF order.
TT 7.7 Mo 16:15 H19
YbIr2Si2− A new Yb−based Heavy Fermion System situated
close to Quantum Critical Point — •Z. Hossain, C. Geibel,
H. Jeevan, F. Weickert, P. Gegenwart, T. Lühmann, and F.
Steglich — Max Planck Institute for Chemical Physics of Solids, 01187
Dresden, Germany
While YbRh2Si2 is presently subject of thorough investigation moti-