Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
linearly to a bath of harmonic oscillators with power law spectral density.
We find that the NRG technique works well for this bosonic systems, despite
the fact that we have to limit the number of bosons on each site. For
the sub-Ohmic bath with exponents 0 < S < 1, we find clear evidence for
a line of continuous quantum phase transitions between localized and delocalized
phases. The line terminates in a Kosterlitz-Thouless transition
at S = 1. Further application to other bosonic impurity problems and
the combination with the dynamical mean-field theory for lattice bosonic
systems are to be explored.
TT 30.3 Do 14:30 Poster A
Numerical Renormalization Group for the Bose-Fermi Kondo
Model — •Tae-Hyoung Gimm and Ralf Bulla — Theoretische
Physik III, Universität Augsburg
Wilson’s numerical renormalization group (NRG) method is a powerful
tool for the investigations of quantum impurity systems. It was originally
developed for the Kondo problem to study the physics of a magnetic impurity
in a fermionic bath. Recently, the NRG was successfully applied
to a simple two state system with a bosonic bath, that is, the spin-boson
model. Here we consider the Ising Bose-Fermi Kondo model, which describes
a magnetic impurity simultaneously coupled to both a fermionic
and a bosonic bath. When the Kondo lattice system is analyzed within
an extended dynamical mean field theory, the Kondo lattice Hamiltonian
is mapped onto an effective single-site problem and represented by
the Bose-Fermi Kondo Hamiltonian. We present a progress report of our
NRG investigation of the Bose-Fermi Kondo model with Ising anisotropy
for the boson field.
TT 30.4 Do 14:30 Poster A
Magnetic response of the non-Fermi-liquid YbRh2Si2 — •O.
Stockert 1 , N. Bernhoeft 2 , M. Koza 3 , J. Ferstl 1 , A. Murani 3 ,
C. Geibel 1 , and F. Steglich 1 — 1 Max-Planck-Institut CPfS,
Nöthnitzer Straße 40, D-01187 Dresden — 2 CEA Grenoble, DRFMC,
F-38054 Grenoble, France — 3 Institut Laue-Langevin, BP 156, F-38042
Grenoble, France
Deviations from the usual Landau-Fermi-liquid theory are observed in
an increasing number of correlated electron system which are close to a
magnetic instability where a transition from a nonmagnetic ground state
to a magnetically ordered state occurs. YbRh2Si2 is the first Yb-based
compound which obeys non-Fermi-liquid behavior as seen in unusual
temperature dependences of thermodynamic and transport properties at
temperatures above the very low ordering temperature TN = 65mK. In
the vicinity of the magnetic instability in YbRh2Si2 one expects critical
spin fluctuations which are the origin of the non-Fermi-liquid behavior.
Inelastic neutron scattering on YbRh2Si2 powder has been performed to
investigate the magnetic response. The high energy response is dominated
by crystalline electric field excitations, found at ≈ 27, and 42meV.
In contrast, the low energy response is quasielastic and shows a critical
slowing down as the temperature is reduced. However, the quasielastic
response seems to be non-Lorentzian like, but could be fitted by a phenomenological
model for the susceptibility using a distribution of relaxation
rates with an upper, temperature independent bound and a lower
bound varying roughly linearly with T.
TT 30.5 Do 14:30 Poster A
Anderson localization in disordered electron-phonon systems —
•Andreas Alvermann, Franz X. Bronold, and Holger Fehske
— Institut für Physik, Ernst-Moritz-Arndt-Universität Greifswald, 17489
Greifswald
In order to study the competition between Anderson localization and
polaron formation we adopt the statistical dynamic mean field theory to a
generic model for a strongly coupled disordered electron-phonon system.
The localization properties of a single polaron are discussed in detail.
TT 30.6 Do 14:30 Poster A
Comparative numerical study of localization in disordered electron
systems — •Gerald Schubert 1 , Alexander Weiße 2 , and
Holger Fehske 1 — 1 Institut für Physik, Ernst-Moritz-Arndt Universität
Greifswald, 17487 Greifswald, Germany — 2 School of Physics, The
University of New South Wales, Sydney, NSW 2052, Australia
Taking into account that a proper description of disordered systems
should focus on distribution functions, the authors develop a powerful
numerical scheme for the determination of the probability distribution
of the local density of states (LDOS), which is based on a Chebyshev
expansion with kernel polynomial refinement and allows the study of
large finite clusters (up to 100 3 ). As the distribution of the LDOS shows
a significant change at the disorder induced delocalization-localization
transition, the so-called typical density of states, defined as the geometric
mean of the LDOS, emerges as a natural order parameter. Using this
method, we calculate the phase diagram for the three dimensional Anderson
model, treat correlated disorder in one dimension and consider
the quantum percolation problem on a simple cubic lattice.
TT 30.7 Do 14:30 Poster A
The extended-Harris criterion and scale-free disorder in the
3D Anderson model of localization — •Macleans L. Ndawana 1 ,
Rudolf A. Römer 1 , and Michael Schreiber 2 — 1 University of
Warwick, Department of Physics and Center for Scientific Computing,
CV4 7AL, Coventry, United Kingdom — 2 Institut für Physik, Technische
Universität, D-09107 Chemnitz
We investigate the 3D Anderson model of localization [1] in the presence
of scale-free [2] diagonal disorder via a modified transfer matrix
method. The diagonal disorder is characterized by a correlation function
that decays as r −α for separation r in the limit r → ∞ and α is
the correlation-strength exponent. The extended-Harris criterion [3] is
investigated by analyzing the dependence of the correlation-length critical
exponent ν on α. We observe that for fixed W, there is a critical
value αc below which, i.e. for α < αc, the universality class of the system
changes. For α > αc, ν remains unchanged. This value of αc separates
two regions, a region where correlations in disorder are relevant (α < αc)
and a region where they are irrelevant (α > αc). Furthermore, we observe
that at E = 0, ν is independent of α. This might suggest that the
metal-insulator transition at E = 0 exhibits strong universality.
[1] P. W. Anderson, Phys. Rev. 109, 1492 (1958)
[2] H. A. Makse, et. al Phys. Rev. B 53, 5443 (1996)
[3] A. Weinrib and B. I. Halperin, Phys. Rev. B 27, 413 (1983)
TT 30.8 Do 14:30 Poster A
Mott Transition on the Surface of 1T-TaSe2 studied by Angle-
Resolved Photoemission — •Markus Hoinkis, Hidenori Fujiwara,
Benjamin Schmid, Matthias Klemm, David Schrupp,
Jörg Schäfer, Michael Sing, and Ralph Claessen — Institut
für Physik, Universität Augsburg, 86135 Augsburg
The layered transition metal dichalcogenide 1T-TaSe2 is known to be
in a charge density wave phase below T1 = 475K. Recently, an additional
phase transition occurring only at the surface has been discovered
at T2 = 260K [1]. Below this temperature the density of states is strongly
depressed at the Fermi energy. The authors of [1] have interpreted this
as a bandwidth-controlled Mott transition: The temperature-dependent
modulation of the atomic positions leads to a smaller bandwidth W of
the Ta 5d-Band. While this modification of the ratio U/W (U is the onsite
Coulomb interaction) does not suffice to trigger a Mott transition
in the bulk, the effective bandwidth at the surface could be sufficiently
small for this effect.
By angle-resolved photoemission we have studied the spectral evolution
through the transition, with particular focus on the identification
and dispersion of the quasiparticle and lower Hubbard band peaks.
[1] L. Perfetti et al., Phys. Rev. Lett. 90, 166401 (2003)
TT 30.9 Do 14:30 Poster A
Characterisation of the Polaron Energy Spectrum in the Molecular
Holstein Model — •Daniela Schneider and Karl-Heinz
Höck — Theoretische Physik II, Institut für Physik, Universität Augsburg,
D-86135 Augsburg
The polaron energy spectrum of the molecular Holstein and E × β
Jahn-Teller model includes some frequently discussed aspects, which are
related to the crossover from itinerant to localized polarons. We studied
in particular the crossing and anticrossing effect of the energy branches.
We are also interested in the consequence of electron-phonon-coupling
on various physical quantities such as the absorption spectrum and optical
conductivity. The associated calculations are based on a closed form
for the matrix elements. Considerations from a group theoretical and
symmetry point of view are used to classify the eigenstates and give information
about both the possible transitions in the energy spectrum and
the correct analytical treatment of the model. The results are discussed
and compared with those of a numerical recurrence method.
This work is supported by DFG (SFB 484).