Gapless inhomogeneous superfluid phase with spin-dependent disorder

TL;DR

This paper demonstrates that spin-dependent disorder in superfluids induces a gapless phase with inhomogeneous pairing, extending the superfluid's resilience to disorder beyond traditional predictions, and characterizes the nature of low-energy excitations.

Contribution

It reveals the existence of a gapless superfluid phase caused by spin-dependent disorder and shows its persistence at higher disorder levels due to inhomogeneous pairing, using Bogoliubov-de Gennes theory.

Findings

Identification of a distinct gapless superfluid phase due to spin-dependent disorder

Inhomogeneous pairing extends superfluid stability against disorder

Low-energy states are localized where pairing vanishes

Abstract

We show that the presence of a spin-dependent random potential in a superconductor or a superfluid atomic gas leads to distinct transitions at which the energy gap and average order parameter vanish, generating an intermediate gapless superfluid phase, in marked contrast to the case of spin-symmetric randomness where no such gapless superfluid phase is seen. By allowing the pairing amplitude to become inhomogeneous, the gapless superconducting phase persists to considerably higher disorder compared with the prediction of Abrikosov-Gorkov. The low-lying excited states are located predominantly in regions where the pairing amplitude vanishes and coexist with the superfluid regions with a finite pairing. Our results are based on inhomogeneous Bogoliubov-de Gennes mean field theory for a two dimensional attractive Hubbard model with spin-dependent disorder.