Superconductivity at the three-dimensional Anderson metal-insulator transition

TL;DR

This paper investigates how superconductivity behaves near the Anderson metal-insulator transition in a disordered three-dimensional system, revealing multifractal spatial distributions and spectral correlations characteristic of the transition.

Contribution

It provides a detailed numerical analysis of the order parameter distribution, critical disorder level, and spectral properties at the Anderson transition in a superconductor.

Findings

Order parameter is enhanced near transition but drops sharply in insulator

Order parameter distribution becomes log-normal with multifractal features at transition

Spectral correlations at the transition resemble those in non-interacting disordered systems

Abstract

We study a disordered weakly-coupled superconductor around the Anderson transition by solving numerically the Bogoliubov-de Gennes (BdG) equations in a three dimensional lattice of size up to $20\times20\times20$ in the presence of a random potential. The spatial average of the order parameter is moderately enhanced as disorder approaches the transition but decreases sharply in the insulating region. The spatial distribution of the order parameter is sensitive to the disorder strength: for intermediate disorders below the transition, we already observe a highly asymmetric distribution with an exponential tail. Around the transition, it is well described by a log-normal distribution and a parabolic singularity spectrum. These features are typical of a multifractal measure. We determine quantitatively the critical disorder at which the insulator transition occurs by an analysis of level statistics in the spectral region that contributes to the formation of the order parameter. Interestingly, spectral correlations at the transition are similar to those found in non-interacting disordered systems at the Anderson transition. A percolation analysis suggests that the loss of phase coherence may occur around the critical disorder.