W. Koller

First- and second-order phase transitions in the Holstein-Hubbard model

We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant, the transition is to a Mott insulator; when the electron-phonon interaction dominates, the transition is to a localized bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double occupancy as functions of U and g.

Singular dynamics of underscreened magnetic impurity models

We give a comprehensive analysis of the singular dynamics and of the low-energy fixed point of one-channel impurity

Dynamic response functions for the Holstein-Hubbard model

s\text{\ensuremath{-}}d

Magnetic Field Effects on Quasiparticles in Strongly Correlated Local Systems

models with ferromagnetic and underscreened antiferromagnetic couplings. We use the numerical renormalization group (NRG) to perform calculations at

Polaronic Quasiparticles in a Strongly Correlated Electron Band

T=0

Magnetic polarons in the one-dimensional ferromagnetic Kondo model

. The spectral densities of the one-electron Greens functions and t-matrices are found to have very sharp cusps at the Fermi level

Effective spinless fermions in the strong-coupling Kondo model

(\ensuremath{\omega}=0)

A semi-continuous extended kinetic model

, but do not diverge. The approach of the Fermi level is governed by terms proportional to

Uniform hopping approach to the ferromagnetic Kondo model at finite temperature

1∕{\mathrm{ln}}^{2}(\ensuremath{\omega}∕{T}_{0})

Variational cluster perturbation theory for Bose-Hubbard models

as