H. Monien

Spin gaps and spin dynamics in La2-xSrxCuO4 and YBa2Cu3O7- delta.

A magnetic susceptibility which decreases with decreasing temperature is observed in all CuO 2 based superconductors with less than optimal doping. We propose that in La 2-x Sr x CuO 4 this is due to spin density wave ordering which is prevented by the low spatial dimensionality, while in YBa 2 Cu 3 O 6.6 it is due to the interplay between antiferromagnetic fluctuations within a plane and singlet pairing of electrons between nearest neighbor planes

One-dimensional Bose-Hubbard model with nearest-neighbor interaction

We study the one-dimensional Bose-Hubbard model using the Density-Matrix Renormalization Group (DMRG). For the cases of on-site interactions and additional nearest-neighbor interactions the phase boundaries of the Mott-insulators and charge density wave phases are determined. We find a direct phase transition between the charge density wave phase and the superfluid phase, and no supersolid or normal phases. In the presence of nearest-neighbor interaction the charge density wave phase is completely surrounded by a region in which the effective interactions in the superfluid phase are repulsive. It is known from Luttinger liquid theory that a single impurity causes the system to be insulating if the effective interactions are repulsive, and that an even bigger region of the superfluid phase is driven into a Bose-glass phase by any finite quenched disorder. We determine the boundaries of both regions in the phase diagram. The ac-conductivity in the superfluid phase in the attractive and the repulsive region is calculated, and a big superfluid stiffness is found in the attractive as well as the repulsive region.

Phases of the one-dimensional Bose-Hubbard model

The zero-temperature phase diagram of the one-dimensional Bose-Hubbard model with nearest-neighbor interaction is investigated using the density-matrix renormalization group. Recently normal phases without long-range order have been conjectured between the charge-density wave phase and the superfluid phase in one-dimensional bosonic systems without disorder. Our calculations demonstrate that there is no intermediate phase in the one-dimensional Bose-Hubbard model but a simultaneous vanishing of crystalline order and appearance of superfluid order. The complete phase diagrams with and without nearest-neighbor interaction are obtained. Both phase diagrams show reentrance from the superfluid phase to the insulator phase.

Phase diagram of the Bose-Hubbard model

The first reliable analytic calculation of the phase diagram of the Bose gas on a d-dimensional lattice with on-site repulsion is presented. In one dimension, the analytic calculation is in excellent agreement with the numerical Monte Carlo results. In higher dimensions, the deviations from the Monte Carlo calculations are larger, but the correct shape of the Mott-insulator lobes is still obtained. Explicit expressions for the energy of the Mott and the defect phase are given in a strong-coupling expansion.

DYNAMICS AND THERMODYNAMICS OF THE BOSE-HUBBARD MODEL

We report results from a systematic analytic strong-coupling expansion of the Bose-Hubbard model in one- and two-spatial dimensions. We obtain numerically exact results for the dispersion of single-particle and single-hole excitations in the Mott insulator. The boundary of the Mott phase can be determined with previously unattainable accuracy in one and two dimensions. In one dimension, we observe the occurrence of reentrant behavior from the compressible to the insulating phase in a region close to the critical point, which was conjectured in earlier work. Our calculation can be used as a benchmark for the development of numerical techniques for strongly correlated systems.

Efficient Perturbation Theory for Quantum Lattice Models

We present a novel approach to long-range correlations beyond dynamical mean-field theory, through a ladder approximation to dual fermions. The new technique is applied to the two-dimensional Hubbard model. We demonstrate that the transformed perturbation series for the nonlocal dual fermions has superior convergence properties over standard diagrammatic techniques. The critical Néel temperature of the mean-field solution is suppressed in the ladder approximation, in accordance with quantum Monte Carlo results. An illustration of how the approach captures and allows us to distinguish short- and long-range correlations is given.

Kinetics of quark-gluon plasmas☆

Abstract We discuss the basic physics of quark-gluon plasmas — including static screening, plasma oscillations, and dynamical screening of transverse interactions in elementary scattering processes — and briefly describe further conceptual problems such as the nature of the quasiparticle spectrum and the long wavelength elementary excitations.

Fictive Impurity Models: an Alternative Formulation of the Cluster Dynamical Mean Field Method

(Dated: February 2, 2008)‘Cluster’ extensions of the dynamical mean field method to include longer range correlations arediscussed. It is argued that the clusters arising in these methods are naturally interpreted not asactual subunits of a physical lattice but as algorithms for computing coefficients in an orthogonalfunction expansion of the momentum dependence of the electronic self-energy. The difficulties withcausality which have been found to plague cluster dynamical mean field methods are shown to berelated to the ‘ringing’ phenomenon familiar from Fourier analysis. The analogy is used to motivateproposals for simple filtering methods to circumvent them. The formalism is tested by comparisonto low order perturbative calculations and self consistent solutions.I. INTRODUCTION

Renormalization of the spin-Peierls transition due to phonon dynamics

We report results from a systematic strong-coupling expansion of a spin-½ Heisenberg chain coupled to Einstein phonons and a frustrating next-nearest-neighbor spin interaction. It is not obvious which interaction dominates in the regime of small coupling constants. In the non-adiabatic regime (Ω ≈ J) this model is used to describe the zero-temperature properties of CuGeO3. The linked cluster expansion allows the determination of observables in the thermodynamic limit preserving the full lattice dynamics without a truncation of the phononic Hilbert space. We show that the spin-phonon coupling leads to a renormalization of the elementary triplet dispersion. Surprisingly, in the non-adiabatic regime a substantial renormalization of the spin gap only sets in at much larger couplings than those proposed for CuGeO3. The ground-state magnetic correlations are found to be hardly affected by the spin-phonon coupling, but dominated by the frustrating magnetic interaction in the parameter regime relevant for CuGeO3.

Transverse interactions and transport in quark-gluon and QED plasmas

Abstract We review how inclusion of Landau damping of virtual exchanged quanta in the transverse interactions of particles in relativistic plasmas leads to finite rates of transport processes. We also summarize results of recent Boltzmann kinetic equation calculations, to leading order in the coupling strength, of the transport coefficients of relativistic plasmas.