Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
is a consequence of the Lang-Firsov transformation, the combination of
these elements enables us to perform efficient simulations for a wide range
of temperature, phonon frequency and electron-phonon coupling in one
to three dimensions, and on clusters large enough to avoid significant
finite-size effects. The algorithm is employed to investigate the problem
of small polaron formation in the Holstein model, and comparison with
exact results and with existing work is made. Moreover, the new ideas
presented here can also be applied to the many-electron case.
TT 24.5 Mi 14:30 Poster A
Variational cluster approach to extended Hubbard models at
half filling — •Markus Aichhorn 1 , Hans Gerd Evertz 1 , Wolfgang
von der Linden 1 , and Michael Potthoff 2 — 1 Institut für
Theoretische Physik und Computational Physics, Technische Universität
Graz, Petersgasse 16, A-8010 Graz, Austria — 2 Institut für Theoretische
Physik und Astrophysik, Universität Würzburg, Am Hubland, D-97074
Würzburg, Germany
We present an extension of the recently proposed variational cluster
perturbation theory to extended Hubbard models at half filling with
repulsive nearest neighbor interaction. The method takes into account
short-range correlations exactly by the exact treatment of clusters of finite
size via exact diagonalization, whereas long-range order beyond the
size of the clusters is treated on a mean-field level. We show that for one
dimension results known from Quantum Monte Carlo and Density Matrix
Renormalization Group can be reproduced with very good accuracy.
Moreover we apply the method to the two-dimensional extended Hubbard
model on a square lattice. Different from one dimension, we find a first order
phase transition between spin density wave phase and charge density
wave phase down to onsite interaction U = 3t. In addition, the singleparticle
spectral function is calculated for both the one-dimensional and
the two-dimensional system.
TT 24.6 Mi 14:30 Poster A
Influence of long-range Coulomb interaction and on-site
Hubbard repulsion on the formation of d-wave Copper-pairing
in high-Tc cuprates — •Dirk Manske 1 , Ernst Pashitskii 2 ,
Vsevolod Pentegov 2 , and Ilya Eremin 1 — 1 Institut für Theoretische
Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin
— 2 Institute of Physics NAS of Ukraine, 46 Nauki str., Kiev, 03028,
Ukraine
We develop a diagram technique for the self-consistent treatment of
the long-range Coulomb interaction and on-site Hubbard repulsion in
the normal and superconducting state of high-Tc cuprates. The resultant
analytical expression for the screened matrix elements taking into
account long-range and on-site repulsion has been derived. In particular,
it accounts for processes with and without spin-flip due to an exchange
of spin and charge density fluctuations. Furthermore, we derive the expressions
for the normal and anomalous self-energy parts near the superconducting
transition temperature Tc that takes into account the vertex
corrections including crossing diagrams. The contribution of the crossing
parts is taken within the ladder approximation (similar to Fluctuations
Exchange approximation) where the role of Hubbard on-site interaction
is replaced by the Coulomb matrix element with a spin-flip averaged
over the momentum. Finally, the developed scheme allows to analyze
the formation of d-wave superconductivity and its stability in presence
of the long-range Coulomb repulsion within a self-consistent anisotropic
Eliashberg-like approach.
TT 24.7 Mi 14:30 Poster A
Electronic structure of quasi-one-dimensional metals: Theory vs
Experiment — •Holger Benthien 1 , Eric Jeckelmann 2 , Florian
Gebhard 1 , Michael Sing 3 , and Ralph Claessen 3 — 1 Fachbereich
Physik, Universität Marburg, 35032 Marburg — 2 Institut für Physik,
Universität Mainz, 55099 Mainz — 3 Experimental Physik II, Universität
Augsburg, 86135 Augsburg
We compare theoretical and experimental results for the electronic
structure of quasi-one-dimensional correlated metals. On the one hand
the one-particle Green’s function is calculated numerically in the onedimensional
Hubbard model away from half filling using the dynamical
density-matrix renormalization group method [1]. On the other hand the
angle-resolved photoemission spectrum has been measured in the quasione-dimensional
organic charge transfer salt TTF-TCNQ [2]. We show
that the experimental spectrum at intermediate to high energy (up to
about 1eV) can be explained consistently in the framework of the onedimensional
Hubbard model. In particular, our results show clear evi-
dence for spin-charge separation in the excitation spectrum (and thus for
a non-Fermi liquid) up to the energy scale of the bandwidth.
[1] E. Jeckelmann, Phys. Rev. B 66, 045114 (2002).
[2] R. Claessen et al., Phys. Rev. Lett. 88, 096402 (2002).
TT 24.8 Mi 14:30 Poster A
Thermodynamic properties of ferromagnetic mixed-spin chain
systems — •Noboru Fukushima 1 , Andreas Honecker 1 , Stefan
Wessel 2 , and Wolfram Brenig 1 — 1 Institut für Theoretische Physik,
Technische Universität Braunschweig, Germany — 2 Theoretische Physik,
ETH Zürich, Switzerland
Using a combination of high-temperature series expansion, exact diagonalization
and quantum Monte Carlo, we perform a complementary
analysis of the thermodynamic properties of one-dimensional mixed-spin
systems with alternating magnetic moments. In particular, we focus on
the difference between ferromagnetic (FM) and antiferromagnetic (AFM)
exchange. Not only the peak structure of the specific heat is more pronounced
in the FM case, but also the number of the peaks appears different.
Namely, the FM case seems to have some additional energy scale(s).
The origin of these differences will be discussed. In addition, the impact
of interchain exchange will be also investigated.
TT 24.9 Mi 14:30 Poster A
Scaling of the conductance in interacting quantum wires —
•Tilman Enss 1 , Sabine Andergassen 1 , Volker Meden 2 , Walter
Metzner 1 , Ulrich Schollwöck 3 , and Kurt Schönhammer 2
— 1 Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart
— 2 Institut für Theoretische Physik, Universität Göttingen, D-37073
Göttingen — 3 Sektion Physik, Universität München, Theresienstr. 37,
D-80333 München
The conductance through an interacting nanowire containing impurities
and coupled to non-interacting leads is studied at zero and finite temperature.
Our functional RG method starts from the microscopical model
and arrives at an effective low-energy model correctly showing Luttinger
liquid behavior. For a single impurity we observe one-parameter scaling
of the conductance. Using several impurities, we study resonances of the
conductance and temperature-dependent transport through a quantum
dot.
TT 24.10 Mi 14:30 Poster A
Functional RG for Luttinger liquids with impurities — •Sabine
Andergassen 1 , Tilman Enss 1 , Volker Meden 2 , Walter Metzner
1 , Ulrich Schollwöck 3 , and Kurt Schönhammer 2 — 1 Max-
Planck-Institut für Festkörperforschung, D-70569 Stuttgart — 2 Institut
für Theoretische Physik, Universität Göttingen, D-37073 Göttingen —
3 Sektion Physik, Universität München, D-80333 München
Using a functional RG we compute the flow of the renormalized impurity
potential for a single impurity in a Luttinger liquid over the entire
energy range from the microscopic scale of a lattice-fermion model
down to the low-energy limit. The non-perturbative method provides a
complete real-space picture of the effective impurity potential. Further
developments of the functional RG approach including 2-particle vertex
renormalization considerably improve the quantitative accuracy of the
results in particular at moderate interaction strenghts, which are compared
to exact DMRG results. Focusing on the spectral properties of the
single-particle excitations near an impurity or boundary and on the density
profile, we confirm the universality of the open chain fixed point, but
it turns out that very large systems are required to reach the asymptotic
regime for realistic choices of the impurity and interaction parameters.
TT 24.11 Mi 14:30 Poster A
Coexistence of s-wave Superconductivity and Antiferromagnetism
— •Martin Feldbacher 1 , Fakher Assaad 2 , Frederic
Hébert 3 , and George Batrouni 3 — 1 Max-Planck-Institut für
Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart —
2 Universität Würzburg, Institut für Theoretische Physik I, Am Hubland,
97074 Würzburg — 3 Institut Non-Linéaire de Nice, Université de
Nice-Sophia Antipolis, France
We study the phase diagram of a new model that exhibits a first order
transition between s-wave superconducting and antiferromagnetic phases.
The model, a generalized Hubbard model augmented with competing
spin-spin and pair-pair interactions, was investigated using the projector
Quantum Monte Carlo method. Upon varying the Hubbard U from
attractive to repulsive we find a first order phase transition between superconducting
and antiferromagnetic states. Phys. Rev. Lett., 91:056401,