arXiv subjects

Johannes Bauer

Publications and source records attributed to Johannes Bauer.

22 records · Page 2Linked to original sources

arXiv2010

Competition between antiferromagnetic and charge order in the Hubbard-Holstein model

We study the competition between an instantaneous local Coulomb repulsion and a boson mediated retarded attraction, as described by the Hubbard-Holstein model. Restricting to the case of half filling, the ground-state phase diagram and the transitions from antiferromagnetically ordered states to charge ordered states are analyzed. The calculations are based on the model in large dimensions, so that dynamical mean field theory can be applied, and the associated impurity problem is solved using the numerical renormalization group method. The transition is found to occur when electron-electron coupling strength $U$ and the induced interaction $λ$ due to electron-phonon coupling approximately coincide, $U\simeq λ$. We find a continuous transition for small coupling and large $ω_0$, and a discontinuous one for large coupling and/or small $ω_0$. We present results for the order parameters, the static expectation values for the electrons and phonons, and the corresponding spectral functions. They illustrate the different types of behavior to be seen near the transitions. Additionally, the quasiparticle properties are calculated in the normal state, which leads to a consistent interpretation of the low energy excitations.

cond-mat.str-el↗

arXiv2010

Competing interactions and symmetry breaking in the Hubbard-Holstein model

Competing interactions are often responsible for intriguing phase diagrams in correlated electron systems. Here we analyze the competition of instantaneous short range Coulomb interaction $U$ with the retarded electron-electron interaction induced by an electron-phonon coupling $g$ as described by the Hubbard-Holstein model. The ground state phase diagram of this model in the limit of large dimensions at half filling is established. The study is based on dynamical mean field theory combined with the numerical renormalization group. Depending on $U$, $g$, and the phonon frequency $ω_0$, the ground state is antiferromagnetically (AFM) or charge ordered (CO). We find quantum phase transitions from the AFM to CO state to occur when $U-λ\simeq 0$, where $λ$ characterizes the phonon induced effective attraction. The transition is continuous for small couplings and large phonon frequencies $ω_0$ and becomes discontinuous for large couplings and small values of $ω_0$. We comment on the possible relevance of this work for Ba${}_{1-x}$K${}_x$BiO${}_3$.

cond-mat.str-el↗

arXiv2010

Numerical Renormalization Group Study of Probability Distributions for Local Fluctuations in the Anderson-Holstein and Holstein-Hubbard Models

We show that information on the probability density of local fluctuations can be obtained from a numerical renormalisation group calculation of a reduced density matrix. We apply this approach to the Anderson-Holstein impurity model to calculate the ground state probability density $ρ(x)$ for the displacement $x$ of the local oscillator. From this density we can deduce an effective local potential for the oscillator and compare its form with that obtained from a semiclassical approximation as a function of the coupling strength. The method is extended to infinite dimensional Holstein-Hubbard model using dynamical mean field theory. We use this approach to compare the probability densities for the displacement of the local oscillator in the normal, antiferromagnetic and charge ordered phases.

cond-mat.str-el↗

arXiv2009

Quasiparticle properties of strongly correlated electron systems with itinerant metamagnetic behavior

A brief account of the zero temperature magnetic response of a system of strongly correlated electrons in strong magnetic field is given in terms of its quasiparticle properties. The scenario is based on the paramagnetic phase of the half-filled Hubbard model, and the calculations are carried out with the dynamical mean field theory (DMFT) together with the numerical renormalization group (NRG). As well known, in a certain parameter regime one finds a magnetic susceptibility which increases with the field strength. Here, we analyze this metamagnetic response based on Fermi liquid parameters, which can be calculated within the DMFT-NRG procedure. The results indicate that the metamagnetic response can be driven by field-induced effective mass enhancement. However, also the contribution due to quasiparticle interactions can play a significant role. We put our results in context with experimental studies of itinerant metamagnetic materials.

cond-mat.str-el↗