Anderson Model in a Superconductor: $\Phi$-Derivable Theory

TL;DR

This paper develops a novel $\

Contribution

It introduces a $\

Findings

First generalization of U-perturbation theory to superconductors

Extends validity beyond Hartree-Fock approximation

Provides a conserving $\

Abstract

We introduce a new $\Phi $-derivable approach for the Anderson impurity model in a BCS superconductor. The regime of validity of this conserving theory extends well beyond that of the Hartree-Fock approximation. This is the first generalization of the U-perturbation theory to encompass a superconductor.