Superconducting ``metals'' and ``insulators''

TL;DR

This paper characterizes zero-temperature phases in disordered three-dimensional superconductors based on quasiparticle transport, identifying distinct superconducting 'metal' and 'insulator' phases and proposing experiments to observe a novel transition between them.

Contribution

It introduces a new classification of superconducting phases based on quasiparticle localization and suggests experimental methods to explore the transition between these phases.

Findings

Identification of two distinct superconducting phases: metal and insulator.

Proposal of experimental tuning to observe a superconductor metal-insulator transition.

Implications for understanding superconductor destruction at zero temperature.

Abstract

We propose a characterization of zero temperature phases in disordered superconductors on the basis of the nature of quasiparticle transport. In three dimensional systems, there are two distinct phases in close analogy to the distinction between normal metals and insulators: the superconducting "metal" with delocalized quasiparticle excitations and the superconducting "insulator" with localized quasiparticles. We describe experimental realizations of either phase, and study their general properties theoretically. We suggest experiments where it should be possible to tune from one superconducting phase to the other, thereby probing a novel "metal-insulator" transition inside a superconductor. We point out various implications of our results for the phase transitions where the superconductor is destroyed at zero temperature to form either a normal metal or a normal insulator.