Perturbation theory of a superconducting 0 - π impurity quantum phase transition.

A single-level quantum dot with Coulomb repulsion attached to two superconducting leads is studied via the perturbation expansion in the interaction strength. We use the Nambu formalism and the standard many-body diagrammatic representation of the impurity Green functions to formulate the Matsubara self-consistent perturbation expansion. We show that at zero temperature second order of the expansion in its spin-symmetric version yields a nearly perfect agreement with the numerically exact calculations for the position of the 0 - π phase boundary at which the Andreev bound states reach the Fermi energy as well as for the values of single-particle quantities in the 0-phase.

Vertex corrections to the mean-field electrical conductivity in disordered electron systems.

The mean-field theory for noninteracting disordered electron systems is widely and successfully used to describe equilibrium properties of alloys over the whole range of disorder strengths. However, it fails to take into account the effects of quantum coherence and localizing backscattering effects when applied to transport phenomena. Vertex corrections due to multiple backscatterings may turn the electrical conductivity negative and make expansions around the mean field in strong disorder problematic.

Equilibrium state of the mean-field Potts glass.

We investigate the low-temperature equilibrium state of the p-state mean-field Potts glass. It is the presently believed that there is a temperature interval T(2) < T < T(c) within which the equilibrium state for p > p(*) approximately 2.82 is described by the cavity method corresponding to the first level of replica-symmetry breaking.

Integrability of the diffusion pole in the diagrammatic description of noninteracting electrons in a random potential.

We discuss restrictions on the existence of the diffusion pole in the translationally invariant diagrammatic treatment of disordered electron systems. We analyze Bethe-Salpeter equations for the two-particle vertex in the electron-hole and the electron-electron scattering channels and derive for systems with electron-hole symmetry a nonlinear integral equation that the two-particle irreducible vertices from both channels must obey. We use this equation and a parquet decomposition of the full vertex to set restrictions on an admissible form of the two-particle singularity induced by probability conservation.

Effect of Coulomb correlations on the electronic structure of PuCoGa(5).

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic structure over that given by the LSDA approach. At variance with the LSDA, the LSDA+U suggests "jj"-like coupling for the Pu 5f manifold.

Dynamical correlations in multiorbital Hubbard models: fluctuation exchange approximations.

We study the two-band degenerate Hubbard model using the fluctuation exchange approximation (FLEX) and compare the results with quantum Monte Carlo (QMC) calculations. Both the self-consistent and the non-self-consistent versions of the FLEX scheme are investigated. We find that, unlike in the one-band case, in the multiband case, good agreement with the quantum Monte Carlo results is obtained within the electron-electron T-matrix approximation using the full renormalization of the one-particle propagators.