Keldysh action for disordered superconductors

TL;DR

This paper develops a Keldysh functional integral approach to disordered superconductors, deriving an effective action and analyzing Coulomb interaction effects on conductance and Josephson coupling, revealing strong suppression at low temperatures.

Contribution

It introduces a microscopically derived Keldysh action for disordered superconductors that captures Coulomb interaction effects without using the replica trick.

Findings

Coulomb interaction suppresses subgap conductance and Josephson coupling at low temperatures.

The approach reproduces known Coulomb renormalization results.

Strong suppression due to zero-bias anomaly and disorder-enhanced repulsion.

Abstract

Keldysh representation of the functional integral for the interacting electron system with disorder is used to derive microscopically an effective action for dirty superconductors. In the most general case this action is a functional of the 8 x 8 matrix Q(t,t') which depends on two time variables, and on the fluctuating order parameter field and electric potential. We show that this approach reproduces, without the use of the replica trick, the well-known result for the Coulomb-induced renormalization of the electron-electron coupling constant in the Cooper channel. Turning to the new results, we calculate the effects of the Coulomb interaction upon: i) the subgap Andreev conductance between superconductor and 2D dirty normal metal, and ii) the Josephson proximity coupling between superconductive islands via such a metal. These quantities are shown to be strongly suppressed by the Coulomb interaction at sufficiently low temperatures due to both zero-bias anomaly in the density of states and disorder-enhanced repulsion in the Cooper channel.