Excess quasiparticles and their dynamics in the presence of subgap states

TL;DR

This paper investigates how subgap states caused by material inhomogeneities affect quasiparticle dynamics in superconductors, revealing that localized and mobile quasiparticle recombination can limit quasiparticle density, with implications for superconducting devices.

Contribution

It demonstrates that recombination between localized and mobile quasiparticles can effectively limit quasiparticle density despite spatial separation.

Findings

Localized quasiparticles can be efficiently recombined with mobile ones.

Subgap states do not significantly increase quasiparticle density in aluminum devices.

Recombination dynamics are crucial for superconducting device performance.

Abstract

Material inhomogeneities in a superconductor generically lead to broadening of the density of states and to subgap states. The latter are associated with spatial fluctuations of the gap in which quasiparticles can be trapped. Recombination between such localized quasiparticles is hindered by their spatial separation and hence their density could be higher than expectations based on the recombination between mobile quasiparticles. We show here that the recombination between localized and mobile excitations can be efficient at limiting the quasiparticle density. We comment on the significance of our findings for devices such as superconducting resonators and qubits. We find that for typical aluminum devices, the subgap states do not significantly influence the quasiparticle density.