Multi-condensate states in BCS superconductors

TL;DR

This paper explores how out-of-equilibrium BCS superconductors can develop multiple energy gaps due to quantum instabilities and inelastic collisions, revealing complex multi-condensate states.

Contribution

It introduces a unified approach using Richardson's exact solution to demonstrate the emergence of multi-gap states in non-equilibrium superconductors.

Findings

Superconductors can develop more than one energy gap out of equilibrium.

Quantum instabilities lead to oscillations in the pairing amplitude.

Inelastic collisions are crucial for resolving instabilities and stabilizing multi-gap states.

Abstract

A BCS (Bardeen-Cooper-Schrieffer) superconductor, which is placed out of equilibrium, can develop quantum instabilities, which manifest themselves in oscillations of the superconductor's order parameter (pairing amplitude $\Delta$). These instabilities are a manifestations of the Cooper instability. Inelastic collisions are essential in resolving those instabilities. Incorporating the quantum instabilities and collisions in a unified approach based on Richardson's exact solution of the pairing Hamiltonian, we find that a BCS superconductor may end up in a state in which the spectrum has more than one gap.