Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Freitag
Näherung. Dies erlaubt die Berechnung von abgeschirmten Coulomb-
Matrixelementen und ist Grundlage für die Anwendung einer firstprinciples
”random phase approximation” (RPA) bzw. GW-Näherung.
Konkrete Rechnungen wurden für Li als denkbar einfachstes Metall
durchgeführt.
TT 32.5 Fr 11:15 H18
Spectral functions for systems with Hund’s rule correlated 5f
electrons — •Frank Pollmann and Gertrud Zwicknagl — Institut
für Mathematische Physik, TU-Braunschweig, Mendelssohnstraße
3, 38106 Braunschweig
We calculate the spectral functions for systems with 5f electrons by
performing exact diagonalization of small clusters and treating the intercluster
hopping as a perturbation. The perturbation is treated in the
formalism of the Strong Coupling Theory. In the calculated spectra well
defined quasiparticles and partial localization are observed.
The obtained results are compared with data from exact diagonalization
of finite clusters and measurements by angle resolved-photoemission
spectroscopy (ARPES).
TT 32.6 Fr 11:30 H18
The spin-state puzzle in cobaltates — •Z. Hu 1 , H. Wu 2 , D.
Madenci 1 , J. Baier 1 , T. Lorenz 1 , I. Bonn 3 , C. F e lser 3 , A.
Tanaka 4 , H.H. Hsieh 5 , H.-J. Lin 5 , C.T. C h en 5 , and L.H. Tjeng 1
— 1 II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77,
50937 Köln, Germany — 2 Max-Plank-Institut für Physik komplexer
Systeme, Nöthnitzer Str.38, 01187 Dresden, Germany — 3 Johannes
Gutenberg-Universität Mainz, Becher Wege 24, 55099 Mainz, Germany
472 — 4 Department of Materials Science, Faculty of Science, Hiroshima
University, Higaschi-Hiroschima, Japan — 5 Synchrotron Radiation Research
Center, No. 1 R&D Road VI, Science Based Industrial Park,
Hsinchu 300, Taiwan
We have carried out an experimental and theoretical study on the spinstate
of the Co 3+ ion in the layered Sr2CoO3Cl material. Using soft-x-ray
absorption spectroscopy at the Co L2,3 and O K edges, in combination
atomic multiplet cluster and LDA+U calculations, we found that the
Co 3+ ion is in the high spin-state, thereby falsifying all claims reported
so far in the literature which were based mostly on neutron, magnetic, or
crystallographic measurements. Detailed full-potential LDA+U calculations
reveal that the persistent high spin state and insulating behavior of
Sr2CoO3Cl are caused by the CoO5 pyramidal coordination and, particularly,
by the large plane corrugation of the basal CoO2 layer. Our finding
has far reaching implications for other layered cobalt oxides currently
under intense debat, e.g., RBaCo2O5+x, the layered double perovskites
showing metal-insulator transitions and colossal magneto-resistance.
TT 32.7 Fr 11:45 H18
Mott transition and suppression of orbital fluctuations in orthorhombic
3d 1 perovskites — •E. Pavarini 1 , S. Biermann 2,3 ,
A. Poteryaev 4 , A.I. Lichtenstein 4 , A. Georges 3 , and O.K.
Andersen 5 — 1 INFM and Dipartimento di Fisica ”A.Volta”, Pavia,
Italy — 2 Laboratoire de Physique des Solides, CNRS-UMR 8502, UPS
Bat. 510, 91405 Orsay France — 3 LPT-ENS CNRS-UMR 8549, 24 Rue
Lhomond, 75231 Paris Cedex 05, France — 4 NSRIM, UNSRIM, University
of Nijmegen, NL-6525 ED Nijmegen, The Netherlands — 5 Max-
Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569
Stuttgart, Germany
Using the first-principles downfolding technique, a low-energy Hamiltonian
is derived for several 3d 1 transition metal oxides. Electronic correlations
are included by combining this approach with an implementation
of dynamical mean-field theory appropriate for non-cubic systems. Good
agreement with photoemission data is obtained. The interplay of correlation
effects and structural distortions is found to strongly suppress orbital
fluctuations in both YTiO3 and LaTiO3 (in the latter less strongly), and
to favor the transition to the insulating state.
TT 32.8 Fr 12:00 H18
Zero Temperature Quantum Monte Carlo for Dynamical Mean
Field Theory — •Martin Feldbacher 1 , Karsten Held 1 und Fakher
Assaad 2 — 1 Max-Planck-Institut für Festkörperforschung, Heisenbergstraße
1, D-70569 Stuttgart — 2 Universität Würzburg, Institut für
Theoretische Physik I, Am Hubland, 97074 Würzburg
In recent years there has been a revival of interest in Kondo-like physics,
in particular in quantum dot systems and in connection with the
dynamical mean field theory (DMFT). The numerical solution of the
underlying Anderson impurity models is, however, limited: In the Numerical
Renormalization Group treatment the effort grows exponentially
with the number of orbitals, allowing not more than two interacting orbitals;
the Hirsch-Fye Quantum Monte Carlo (QMC) algorithm on the
other hand scales like T −3 (T: temperature) and quickly becomes too expensive
in CPU time. This limitation is especially severe when DMFT is
used to model materials with strong electron correlations where, in order
to observe the physics of interest, low temperatures need to be achieved.
We propose a projective QMC algorithm for the Anderson impurity
model which converges rapidly to the ground state. With this new impurity
solver we study the Mott-Hubbard metal-insulator transition in
the Hubbard model, demonstrating that it gives reliable “T = 0” DMFT
results.
TT 32.9 Fr 12:15 H18
Energy of the Mott insulator - 10th order perturbation theory
extended to infinite order using QMC — •Nils Blümer 1 and Eva
Kalinowski 2 — 1 Institut für Physik, Johannes-Gutenberg-Universität,
55099 Mainz — 2 FB Physik, Philipps Universität, 35032 Marburg
Of fundamental interest in solid state theory is the correlation induced
Mott-Hubbard metal-insulator transition (MIT) between a paramagnetic
metal and a paramagnetic insulator. Much progress has been made within
the last decade by application of the nonperturbative dynamical meanfield
theory to the frustrated one-band Hubbard model with semielliptic
density of states. In particular, a coexistence region of metallic and insulating
solutions has been established at low temperatures.
Only recently, numerical energy estimates have been obtained with
high enough precision for reliably pinpointing the first order MIT line.
Even higher accuracy is reached in this work for the ground state insulator
using 10 th order perturbation theory (PT) and an improved quantum
Monte Carlo (QMC) scheme which treats the high-frequency part of the
self-energy exactly. Extrapolating the PT coefficients, we obtain a continuous
estimate of the energy of the insulating phase with precision
O(10 −5 ) throughout its stability region. Analyzing the critical behavior,
we determine the (lower) stability edge as Uc1 ≈ 1.19 ×bandwidth.
TT 32.10 Fr 12:30 H18
Charge ordering in quarter filled ladder systems coupled to the
lattice — •H.G. Evertz 1 , M. Aichhorn 1 , M. Hohenadler 1 , C.
Gabriel 1 , J. Spitaler 2 , C. Ambrosch-Draxl 2 , and E.Ya. Sherman
1 — 1 Theor. Physik, TU Graz, Austria — 2 Theor. Physik, KFU
Graz, Austria
Using DMRG, Exact Diagonalization, and QMC, we investigate charge
ordering in the presence of electron-phonon coupling for quarter-filled ladder
systems. Parameters of a corresponding extended Hubbard model are
obtained from first-principles band-structure calculations for NaV2O5.
We show that dynamical Holstein phonons, which are strong in
NaV2O5, together with the nearest-neighbor Coulomb repulsion V result
in a static zig-zag lattice distortion. Such a distortion, as also measured
in NaV2O5, causes the system to order already at small values of V . We
calculate dynamical susceptibilities, including spin and charge spectra
and examine the effective Heisenberg model along the ladder in the
ordered state. Energy and length of kink-like excitations of the ordered
state are determined.
On finite systems, charge ordering also takes place as an effective finite
temperature transition because of a rapidly growing correlation length.
In a grand canonical ensemble, the charge order manifests itself as a
plateau in the density as a function of chemical potential. In ongoing
calculations, we are investigating Raman spectra and the effects of interladder
coupling.
TT 32.11 Fr 12:45 H18
Finite-temperature analysis of 2D spin frustrated Vanadium
compounds — •Burkhard Schmidt 1 , Nic Shannon 1,2 , Karlo
Penc 3 , and Peter Thalmeier 1 — 1 MPI für Chemische Physik fester
Stoffe, 01187 Dresden, Germany — 2 SPEC, CEA Saclay, Orme des
Merisiers, F-91191 Gif sur Yvette CEDEX, France — 3 Research Institute
of Solid State Physics and Optics, H-1525 Budapest, P.O. Box 49,
Hungary
We present an analysis of the finite-temperature properties of the J1-J2
Heisenberg model on a square-lattice based on the exact diagonalisation
of 8, 16, and 20 site clusters. Our method provides a possibility to determine
the exchange constants of recent experimental realisations of the
model in a unique way. In particular, the two-dimensional compounds
Li2VO(Si,Ge)O4 and Pb2VO(PO4)2 will be discussed.