Role of Spatial Amplitude Fluctuations in Highly Disordered s-Wave Superconductors

TL;DR

This paper investigates how strong non-magnetic impurities cause spatial fluctuations in the pairing amplitude of 2D s-wave superconductors, revealing the formation of superconducting islands and a transition to a non-superconducting state.

Contribution

It provides a detailed analysis of disorder effects beyond the weak regime, highlighting the emergence of inhomogeneous superconducting regions and their impact on spectral and superfluid properties.

Findings

Local pairing amplitude becomes broadly distributed with increasing disorder.

The spectral gap persists despite high disorder due to superconducting islands.

Superfluid density significantly decreases at high disorder levels.

Abstract

The effect of non-magnetic impurities on 2D s-wave superconductors is studied beyond the weak disorder regime. Within the Bogoliubov-de Gennes (BdG) framework, the local pairing amplitude develops a broad distribution with significant weight near zero with increasing disorder. Surprisingly, the density of states continues to show a finite spectral gap. The persistence of the spectral gap at large disorder is shown to arise from the break up of the system into superconducting "islands". Superfluid density and off-diagonal correlations show a substantial reduction at high disorder.A simple analysis of phase fluctuations in the highly inhomogeneous BdG state is shown to lead to a transition to a non-superconducting state.