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Plenarvorträge - DPG-Tagungen

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Tiefe Temperaturen Mittwoch<br />

is a consequence of the Lang-Firsov transformation, the combination of<br />

these elements enables us to perform efficient simulations for a wide range<br />

of temperature, phonon frequency and electron-phonon coupling in one<br />

to three dimensions, and on clusters large enough to avoid significant<br />

finite-size effects. The algorithm is employed to investigate the problem<br />

of small polaron formation in the Holstein model, and comparison with<br />

exact results and with existing work is made. Moreover, the new ideas<br />

presented here can also be applied to the many-electron case.<br />

TT 24.5 Mi 14:30 Poster A<br />

Variational cluster approach to extended Hubbard models at<br />

half filling — •Markus Aichhorn 1 , Hans Gerd Evertz 1 , Wolfgang<br />

von der Linden 1 , and Michael Potthoff 2 — 1 Institut für<br />

Theoretische Physik und Computational Physics, Technische Universität<br />

Graz, Petersgasse 16, A-8010 Graz, Austria — 2 Institut für Theoretische<br />

Physik und Astrophysik, Universität Würzburg, Am Hubland, D-97074<br />

Würzburg, Germany<br />

We present an extension of the recently proposed variational cluster<br />

perturbation theory to extended Hubbard models at half filling with<br />

repulsive nearest neighbor interaction. The method takes into account<br />

short-range correlations exactly by the exact treatment of clusters of finite<br />

size via exact diagonalization, whereas long-range order beyond the<br />

size of the clusters is treated on a mean-field level. We show that for one<br />

dimension results known from Quantum Monte Carlo and Density Matrix<br />

Renormalization Group can be reproduced with very good accuracy.<br />

Moreover we apply the method to the two-dimensional extended Hubbard<br />

model on a square lattice. Different from one dimension, we find a first order<br />

phase transition between spin density wave phase and charge density<br />

wave phase down to onsite interaction U = 3t. In addition, the singleparticle<br />

spectral function is calculated for both the one-dimensional and<br />

the two-dimensional system.<br />

TT 24.6 Mi 14:30 Poster A<br />

Influence of long-range Coulomb interaction and on-site<br />

Hubbard repulsion on the formation of d-wave Copper-pairing<br />

in high-Tc cuprates — •Dirk Manske 1 , Ernst Pashitskii 2 ,<br />

Vsevolod Pentegov 2 , and Ilya Eremin 1 — 1 Institut für Theoretische<br />

Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin<br />

— 2 Institute of Physics NAS of Ukraine, 46 Nauki str., Kiev, 03028,<br />

Ukraine<br />

We develop a diagram technique for the self-consistent treatment of<br />

the long-range Coulomb interaction and on-site Hubbard repulsion in<br />

the normal and superconducting state of high-Tc cuprates. The resultant<br />

analytical expression for the screened matrix elements taking into<br />

account long-range and on-site repulsion has been derived. In particular,<br />

it accounts for processes with and without spin-flip due to an exchange<br />

of spin and charge density fluctuations. Furthermore, we derive the expressions<br />

for the normal and anomalous self-energy parts near the superconducting<br />

transition temperature Tc that takes into account the vertex<br />

corrections including crossing diagrams. The contribution of the crossing<br />

parts is taken within the ladder approximation (similar to Fluctuations<br />

Exchange approximation) where the role of Hubbard on-site interaction<br />

is replaced by the Coulomb matrix element with a spin-flip averaged<br />

over the momentum. Finally, the developed scheme allows to analyze<br />

the formation of d-wave superconductivity and its stability in presence<br />

of the long-range Coulomb repulsion within a self-consistent anisotropic<br />

Eliashberg-like approach.<br />

TT 24.7 Mi 14:30 Poster A<br />

Electronic structure of quasi-one-dimensional metals: Theory vs<br />

Experiment — •Holger Benthien 1 , Eric Jeckelmann 2 , Florian<br />

Gebhard 1 , Michael Sing 3 , and Ralph Claessen 3 — 1 Fachbereich<br />

Physik, Universität Marburg, 35032 Marburg — 2 Institut für Physik,<br />

Universität Mainz, 55099 Mainz — 3 Experimental Physik II, Universität<br />

Augsburg, 86135 Augsburg<br />

We compare theoretical and experimental results for the electronic<br />

structure of quasi-one-dimensional correlated metals. On the one hand<br />

the one-particle Green’s function is calculated numerically in the onedimensional<br />

Hubbard model away from half filling using the dynamical<br />

density-matrix renormalization group method [1]. On the other hand the<br />

angle-resolved photoemission spectrum has been measured in the quasione-dimensional<br />

organic charge transfer salt TTF-TCNQ [2]. We show<br />

that the experimental spectrum at intermediate to high energy (up to<br />

about 1eV) can be explained consistently in the framework of the onedimensional<br />

Hubbard model. In particular, our results show clear evi-<br />

dence for spin-charge separation in the excitation spectrum (and thus for<br />

a non-Fermi liquid) up to the energy scale of the bandwidth.<br />

[1] E. Jeckelmann, Phys. Rev. B 66, 045114 (2002).<br />

[2] R. Claessen et al., Phys. Rev. Lett. 88, 096402 (2002).<br />

TT 24.8 Mi 14:30 Poster A<br />

Thermodynamic properties of ferromagnetic mixed-spin chain<br />

systems — •Noboru Fukushima 1 , Andreas Honecker 1 , Stefan<br />

Wessel 2 , and Wolfram Brenig 1 — 1 Institut für Theoretische Physik,<br />

Technische Universität Braunschweig, Germany — 2 Theoretische Physik,<br />

ETH Zürich, Switzerland<br />

Using a combination of high-temperature series expansion, exact diagonalization<br />

and quantum Monte Carlo, we perform a complementary<br />

analysis of the thermodynamic properties of one-dimensional mixed-spin<br />

systems with alternating magnetic moments. In particular, we focus on<br />

the difference between ferromagnetic (FM) and antiferromagnetic (AFM)<br />

exchange. Not only the peak structure of the specific heat is more pronounced<br />

in the FM case, but also the number of the peaks appears different.<br />

Namely, the FM case seems to have some additional energy scale(s).<br />

The origin of these differences will be discussed. In addition, the impact<br />

of interchain exchange will be also investigated.<br />

TT 24.9 Mi 14:30 Poster A<br />

Scaling of the conductance in interacting quantum wires —<br />

•Tilman Enss 1 , Sabine Andergassen 1 , Volker Meden 2 , Walter<br />

Metzner 1 , Ulrich Schollwöck 3 , and Kurt Schönhammer 2<br />

— 1 Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart<br />

— 2 Institut für Theoretische Physik, Universität Göttingen, D-37073<br />

Göttingen — 3 Sektion Physik, Universität München, Theresienstr. 37,<br />

D-80333 München<br />

The conductance through an interacting nanowire containing impurities<br />

and coupled to non-interacting leads is studied at zero and finite temperature.<br />

Our functional RG method starts from the microscopical model<br />

and arrives at an effective low-energy model correctly showing Luttinger<br />

liquid behavior. For a single impurity we observe one-parameter scaling<br />

of the conductance. Using several impurities, we study resonances of the<br />

conductance and temperature-dependent transport through a quantum<br />

dot.<br />

TT 24.10 Mi 14:30 Poster A<br />

Functional RG for Luttinger liquids with impurities — •Sabine<br />

Andergassen 1 , Tilman Enss 1 , Volker Meden 2 , Walter Metzner<br />

1 , Ulrich Schollwöck 3 , and Kurt Schönhammer 2 — 1 Max-<br />

Planck-Institut für Festkörperforschung, D-70569 Stuttgart — 2 Institut<br />

für Theoretische Physik, Universität Göttingen, D-37073 Göttingen —<br />

3 Sektion Physik, Universität München, D-80333 München<br />

Using a functional RG we compute the flow of the renormalized impurity<br />

potential for a single impurity in a Luttinger liquid over the entire<br />

energy range from the microscopic scale of a lattice-fermion model<br />

down to the low-energy limit. The non-perturbative method provides a<br />

complete real-space picture of the effective impurity potential. Further<br />

developments of the functional RG approach including 2-particle vertex<br />

renormalization considerably improve the quantitative accuracy of the<br />

results in particular at moderate interaction strenghts, which are compared<br />

to exact DMRG results. Focusing on the spectral properties of the<br />

single-particle excitations near an impurity or boundary and on the density<br />

profile, we confirm the universality of the open chain fixed point, but<br />

it turns out that very large systems are required to reach the asymptotic<br />

regime for realistic choices of the impurity and interaction parameters.<br />

TT 24.11 Mi 14:30 Poster A<br />

Coexistence of s-wave Superconductivity and Antiferromagnetism<br />

— •Martin Feldbacher 1 , Fakher Assaad 2 , Frederic<br />

Hébert 3 , and George Batrouni 3 — 1 Max-Planck-Institut für<br />

Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart —<br />

2 Universität Würzburg, Institut für Theoretische Physik I, Am Hubland,<br />

97074 Würzburg — 3 Institut Non-Linéaire de Nice, Université de<br />

Nice-Sophia Antipolis, France<br />

We study the phase diagram of a new model that exhibits a first order<br />

transition between s-wave superconducting and antiferromagnetic phases.<br />

The model, a generalized Hubbard model augmented with competing<br />

spin-spin and pair-pair interactions, was investigated using the projector<br />

Quantum Monte Carlo method. Upon varying the Hubbard U from<br />

attractive to repulsive we find a first order phase transition between superconducting<br />

and antiferromagnetic states. Phys. Rev. Lett., 91:056401,

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