Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
TT 26.6 Do 10:45 H18
Optische Leitfähigkeit eines Hubbard-Rings mit Störstelle —
•Cosima Schuster und Philipp Brune — Institut für Physik, Universität
Augsburg, 86135 Augsburg
Wir untersuchen mittels exakter Diagonalisierung (Lanzcos-Verfahren)
die optische Leitfähigkeit eines Hubbard-Rings in Anwesenheit einer Potentialstörstelle.
Wir konzentrieren uns dabei auf den ersten angeregten,
stromtragenden Zustand, d.h. auf verdrehte Randbedingungen. In der
metallischen Phase liegt ein wesentlicher Teil des spektralen Gewichts
im Drude-Peak σ(ω) = Dcδ(ω) + σreg. Im Fall U = 0 lässt sich dieser
Drude-Peak in unseren Rechnungen bei ω = 0 darstellen, da hier auch
für endliche Kettenlängen der Sprung im chemischen Potential verschwindet.
Bei Addition von einer Störstelle ist dieser Sprung im chemischen
Potential endlich und damit der Haupt-Peak verschoben zu endlichen Frequenzen.
Für repulsive Wechselwirkung wird dieser störstelleninduzierte
Peak allerdings erst sichtbar, wenn eine gewisse Störstellenstärke, bei der
die Grösse des Sprungs im chemischen Potential mit und ohne Störstelle
übereinstimmt, überschritten wird. Wir können also in der optischen
Leitfähigkeit für endliche metallische Systeme anhand den Sprung im
chemischen Potential verfolgen.
TT 26.7 Do 11:00 H18
Electron-spin Vertex in the Two-dimensional One-band Hubbard
Model — •Zhongbing Huang, Werner Hanke, and Enrico
Arrigoni — Institut für Theoretische Physik, Universität Würzburg,
am Hubland, 97074 Würzburg, Germany
Using Quantum Monte Carlo techniques, we study the renormalization
of the electron-spin (el-sp) interaction or vertex due to Coulomb
correlations in a two-dimensional one-band Hubbard model with spinfluctuation
momentum transfer q = (π, π). In the physically relevant
strong-correlation regime, we find that the renormalized el-sp vertex decreases
monotonically and rather dramatically with increasing doping
from the underdoped to the overdoped region. In the underdoped region
the spin susceptibility increases very quickly with decreasing temperature,
leading to a substantial increase of the effective pairing interaction.
In the overdoped case, the temperature dependence of the effective
pairing interaction is rather weak. Our findings imply that antiferromagnetic
spin fluctuations play an important role in the transport and
superconducting properties of the underdoped high-temperature superconductors.
This will further be substantiated with similar calculations
for the electron-phonon vertex.
11:15 Pause
TT 26.8 Do 11:30 H18
Variational cluster extension approach to the doped Hubbard
model — •Björn Michaelis, Michael Potthoff, and Werner
Hanke — Institut für theoretische Physik und Astrophysik, Universität
Würzburg, Am Hubland, D-97074 Würzburg
Thermodynamic properties and single particle excitations of the oneband
Hubbard model are calculated in the one and two dimensional
case on the basis of the recently proposed selfenergy-functional theory
(”SFT”, Eur. Phys. J. B 32 (429) ,2003). We use exact diagonalization
to get the selfenergy of finite systems and then obtain the Greensfunction
of the infinite lattice by means of SFT. A particle reservoir is included and
determined selfconsistently to achieve continuous doping. Results about
the filling and bandwith controlled Mott metal-insulator transition are
presented.
TT 26.9 Do 11:45 H18
Gossamer metals — •Marcus Kollar — Institut für Theoretische
Physik, Universität Frankfurt, Robert-Mayer-Str. 8, 60054 Frankfurt/Main
Using Laughlin’s gossamer method we construct a class of extended
Hubbard models with long-range correlated hopping that have exact
metallic Gutzwiller ground states, with arbitrary particle density, noninteracting
dispersion, and lattice dimensionality [1]. The susceptibility
and magnetization curves are obtained, showing that the Pauli susceptibility
is enhanced by correlations. The elementary quasiparticle excitations
of this correlated “gossamer metal” are gapless, except for a halffilled
band, where a Mott metal-insulator transition of Brinkman-Rice
type occurs.
[1] M. Kollar, cond-mat/0308513.
TT 26.10 Do 12:00 H18
Contractor Renormalization Group Calculation for the
Hubbard model including Fermionic and Bosonic excitations.
— •Sascha Brehm 1 , Enrico Arrigoni 1,2 , and Werner Hanke 1
— 1 Institut für Theoretische Physik und Astrophysik, Universität
Würzburg, Am Hubland, D-97074 Würzburg, Germany — 2 Institut
für Theoretische Physik, Technische Universität Graz, Petersgasse 16,
A-8010 Graz, Austria
We apply the contractor renormalization group technique (CORE) to
extract a low-energy effective Hamiltonian for the 2-D Hubbard model.
In contrast to earlier studies1 we explicitly incorporate both bosonic
and fermionic low-energy excitations. It is shown that one basic effect
of the latter is to effectively enhance the hopping probability of the
charge-(pair-)excitations with increasing Hubbard U interactions. The
low-energy Hamiltonian is then solved with slave-boson techniques.
[1] E.Altman and A.Auerbach, Phys.Rev.B 65, 104508 (2002)
TT 26.11 Do 12:15 H18
First-order Mott transition at finite doping and temperature
— •Theo Costi, Achim Rosch, Matthias Vojta, and Peter
Woelfle — Institut fuer Theorie der Kondensierten Materie, Universitaet
Karlsruhe, 76128 Karlsruhe
The dynamical mean field theory in combination with the numerical
renormalization group is used to compute the phase diagram of the Hubbard
model away from half-filling in the (T,U) plane. We find coexistance
of two solutions with the same doping at finite temperature, as for half
filling. We also discuss the effect of a small doping on the single-particle
spectra of a Mott insulator.
TT 26.12 Do 12:30 H18
Anomalous self-energy and pseudogap formation in the 2D
Hubbard model — •Andrey Katanin 1,2,3 and Arno Kampf 1
— 1 Institut fuer Physik, Theoretische Physik III, Elektronische
Korrelationen und Magnetismus, Universitaet Augsburg, 86135
Augsburg, Germany — 2 Max Planck Institute for Solid State Research,
Heisenbergstrasse 1, D-70569 Stuttgart, Germany — 3 Institute of Metal
Physics, 620219 Ekaterinburg, Russia
We calculate [1] the self-energy in the weak-coupling regime of the t-t ′
Hubbard model using the functional RG approach. The flow of the vertices
is truncated at one-loop order. At this order and at van Hove (vH)
band fillings the quasiparticle concept is not valid at kF = (π, 0) due to
either thermal excitations of electrons at the vH singularities (vHs) or
pseudogap formation. At low temperature the quasiparticle weight along
the Fermi surface continuously vanishes from a finite value at the zone
diagonal towards the (π, 0) point. Away from vH band fillings the quasiparticle
peak is formed inside a pseudogap of size ∆, and within a finite
frequency window |ω| ≪ ∆ around the Fermi energy the electronic selfenergy
has the conventional Fermi-liquid characteristics. With increasing
separation between Fermi level and vHs the spectral anomalies gradually
disappear.
[1] A. A. Katanin and A. P. Kampf, cond-mat/0310112.
TT 26.13 Do 12:45 H18
Low-energy physics of cuprates beyond the standard pdσ-model
— •S.-L. Drechsler 1 , J. Málek 1 , H. Rosner 2 , A.S. Moskvin 3 ,
H. Eschrig 1 , M. Knupfer 1 und A. Kordyuk 1 — 1 IFW-Dresden,
P.O. Box 270116, D-01171 Dresden, Germany — 2 MPI-CPfS Dresden,
Germany — 3 Ural St.ate University, 620083 Ekaterinburg, Russia
The role of orbitals beyond the Cu 3dO2p set employed in the widely
used standard pdσ extended Hubbard model is considered with respect
to the low-energy physics of several cuprates and different experiments.
Special attention is paid to the theoretical description of the loss function
and optical conductivity data of the 1D compound Sr2CuO3 and the 2D
system Sr2CuO2Cl2 with respect to excited O 2π states. Consequences
for the orbital character of doped holes are considered. Effects of Cu 4s
orbitals and states derived from cations outside the CuO2 planes are discussed
with respect to the observed bi-layer splitting in Bi2212 [1], the
Néel state ordering in CaCuO2, and NMR data of La2CuO4 [2].
[1] A.A. Kordyuk et al., cond-mat/0311137
[2] E.P. Stoll et al., Int. J. of Mod. Phys. 17, 3329 (2003).