Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
rochlore lattices, Annalen der Physik 11, 892 (2002)
TT 15.6 Di 15:45 H19
Periodic Anderson Model in the Charge Transfer Regime —
•Konrad Matho — CRTBT CNRS, 38042 GRENOBLE CEDEX 9,
FRANCE
We have proposed a fast track to calculate correlated fermionic
Green functions in momentum representation as Padé approximants,
with the aim of data analysis in ARPES experiments
(J.Electr.Spectr.Rel.Phen.117,13 (2001)). A stringent test is the comparison
to microscopic results beyond the one band Hubbard model. Therefore,
we have implemented spectra of the periodic Anderson model. In
this talk, we focus on the generics of charge transfer phenomena in a
range of metallic densities above and below one hole per site and discuss
the following properties: (i) Position of the chemical potential (ii)
Position and width of charge gap and Hubbard satellites (iii) Existence
of quasiparticle bands and their dependence on charge transfer and hybridisation
strength (iv) Differences in the spectra upon electron doping
versus hole doping (v) Spin correlations of the Zhang-Rice type.
TT 15.7 Di 16:00 H19
Continuous Unitary Transformations: Dynamic Structure Factors
at Zero and Finite Temperatures — •Christian Knetter 1 ,
Götz S. Uhrig 2 , and Hans Kroha 1 — 1 Physikalisches Institut, Universität
Bonn, Nussallee 12, D-53115 Bonn — 2 Institut für Theoretische
Physik, Universität Köln, Zülpicher Str. 77, D-50937 Köln
Continuous Unitary Transformations (CUTs) have been successfully
used to quantitatively calculate the zero temperature spectral densities
of various experimentally relevant observables for spin systems in one and
two dimensions. Here we present results for the dynamic structure factor
of the two-dimensional S = 1/2 Shastry-Sutherland model. Our theoretical
findings are in excellent agreement with recently obtained inelastic
neutron scattering data.
The CUTs are constructed such that the resulting effective Hamiltonians
conserve suitably defined quasi-particles. This property is exploited
to extend the method to finite temperatures. We present results for the
temperature dependent spectral densities of a one-dimensional model including
two-particle interactions.
TT 15.8 Di 16:15 H19
Well-defined quasiparticles in interacting metallic grains —
•Dominique Gobert, Ulrich Schollwöck, and Jan von Delft
— CeNS und Sektion Physik, LMU München
We analyze spectral functions of the reduced BCS Hamiltionian and of
the “universal Hamiltonian” of mesoscopic systems with large dimensionless
conductance. We show that an important class of spectral functions is
dominated by one single eigenstate only, which implies an infinite lifetime
of the quasiparticles. Furthermore, this property allows us to calculate
zero-temperature spectral functions with high accuracy using the densitymatrix
renormalization group. We illustrate the use of this method by
calculating various physically relevant quantities.
16:30 Pause
TT 16 Korrelierte Elektronen: Quantenstörstellen, Kondo-Physik
Zeit: Dienstag 16:45–19:00 Raum: H19
TT 16.1 Di 16:45 H19
Flowing uphill: From single- to multi-channel Kondo effect in
quantum dots due to weak charging energies in the leads —
•Serge Florens 1 and Achim Rosch 2 — 1 Institut für Theorie der
Kondensierten Materie,Universität Karlsruhe, Postfach 6980, 76128 Karlsruhe
— 2 Institut für Theoretische Physik, Zülpicher Str.77, D-50937
Köln
We have studied a model for quantum dots which takes into account
Coulomb blockade in the electrodes, a setup proposed recently by Oreg
and Goldhaber-Gordon as a way to observe exotic transport in nanostructures.
Even if the charging energy in the leads Ec is much smaller than
the Kondo temperature TK, we find surprisingly that the long-ranged interactions
destabilize the single-channel Kondo effect and induce a flow
towards a multi-channel Kondo fixed point associated with a rise of the
impurity entropy with decreasing temperature. Such an “uphill flow”
implies a negative impurity specific heat, in contrast to all systems with
local interactions. An exact solution found for a large number of channels
allows us to capture this physics and to predict transport properties
through the device.
TT 16.2 Di 17:00 H19
Quantum Phase Transitions in the Fermi-Bose-Kondo and
Pseudogap Kondo Models — •Matthias Vojta, Lars Fritz,
and Marijana Kircan — Institut für Theorie der Kondensierten
Materie,Universität Karlsruhe, D-76128 Karlsruhe
We discuss aspects of zero-temperature phase transitions in variations
of the Kondo model, where fermionic Kondo screening can be suppressed
due to the coupling to a bosonic bath or due to a gap in the fermionic
density of states. In particular, we identify the lower and upper critical
“dimensions” for the pseudogap Kondo model and present the field theories
describing the critical behavior. We determine a number of universal
properties near the impurity phase transitions and discuss applications
to magnetic impurities in strongly correlated systems and to scenarios of
local criticality in heavy-fermion compounds.
TT 16.3 Di 17:15 H19
Quantum Phase Transitions in Models of Magnetic Impurities
— •Hyun Jung Lee 1 , Ralf Bulla 1 , Ning-Hua Tong 1 , and
Matthias Vojta 2 — 1 Theoretische Physik III, Universität Augsburg —
2 Institut für Theorie der Kondensierten Materie, Universität Karlsruhe
The numerical renormalization group method is used to investigate
zero temperature phase transitions in single-impurity models, where an
impurity couples to a non-trivial fermionic bath (Pseudogap Anderson
model) or to a (sub)ohmic bosonic bath (Spin-Boson Model). In both
cases, zero temperature phase transitions occur between two different
phases whose fixed points can be built up of non-interacting singleparticle
states. However the quantum critical point cannot be described
by non-interacting fermionic or bosonic excitation. To understand the
quantum critical behavior, we study the character of the different fixed
points as well as thermodynamic quantities such as entropy and specific
heat.
TT 16.4 Di 17:30 H19
Lattice path integral approach to the Kondo problem —
•Michael Bortz 1 und Andreas Klümper 2 — 1 Universität
Dortmund, Fachbereich Physik, Lehrstuhl T1, 44221 Dortmund —
2 Bergische Universität Wuppertal, Fachbereich Physik, 42097 Wuppertal
We propose a lattice regularization of the one-dimensional isotropic single
channel spin-1/2 Kondo model, including the host and the impurity.
From symmetry arguments, effective models for the more general anisotropic
m-channel spin-S case are given. By application of the Quantum
Transfer Matrix formalism, we obtain the free energy and other thermodynamic
equilibrium response functions exactly by a set of max(2S, m)+1
non-linear integral equations. These are studied analytically in certain
limiting ranges of the external parameters (temperature and magnetic
field) and numerically over wide parameter ranges. Both high- and lowtemperature
scales (Wilson ratios) are calculated; especially, the ratios
relating the regimes of high- and low-temperatures are derived for the
first time in the framework of an exact solution.
TT 16.5 Di 17:45 H19
Dynamics of a 3-level quantum impurity with orbital degeneracy
in a fermionic bath — •Michael Arnold 1 , Johann Kroha 1 ,
and Alfred Zawadowski 2 — 1 Physikalisches Institut, Universität
Bonn — 2 Department of Physics, TU Budapest
We consider a model of an atom moving in a mexican hat-like potential
with 3-fold rotational symmetry, corresponding to a small atom localized
in the interstitial space of, e.g., an fcc lattice. The atom is coupled to the
conduction electron sea via angular momentum scattering. The local impurity
spectrum consists of the ground state and two Kramers degenerate
excited states. We investigate the possibility of 2-channel Kondo correlations
developing in the Kramers doublet in an intermediate energy region,
in competition to the relaxation to the ground state. The two channels