Correlated pair-amplitude fluctuations in disordered BCS superconductors

TL;DR

This paper investigates how disorder affects superconductivity, revealing long-range pair-amplitude fluctuations that explain experimental observations and challenge traditional theories like Anderson's theorem.

Contribution

It introduces a new mechanism based on pair-amplitude fluctuations that accounts for superconductor-insulator transition at high disorder levels.

Findings

Long-range pair-amplitude correlations activate quasiparticle scattering.

Disorder reduces pair-amplitude and critical temperature.

The theory explains broadening of the coherence peak and gap suppression.

Abstract

More than fifty years old "Anderson's theorem" suggests disorder-independent pair-amplitude and critical temperature at low to moderate strength of disorders. In spite of the extension of this paradigm and several subsequent theoretical proposals, mechanism for the superconductor-insulator transition (SIT) at large disorder is still an open problem. The understanding of experimental data at intermediate disorder is also lacking. Here, we proceed beyond "Anderson's theorem" by calculating disorder-averaged two-point pair-amplitude fluctuations. We find this correlation to be finite and long-ranged, activating scattering of quasiparticles in the novel "Cooper" channel. Without invoking any further parameter, this mechanism quantitatively predicts several testable properties: reduction of pair-amplitude at zero temperature; reduction of temperature, T_\Delta, at which pair-amplitude vanishes; suppression and broadening of coherence peak; and decrease of hard-gap in quasiparticle density of states, with the increase of disorder. We interpret old and new data at intermediate disorder in light of our predictions. The robustness of this theory will hopefully provide insight to unravel the true mechanism for SIT.