Can slow recombination in ordered superconductors explain the excess quasiparticle population?

TL;DR

This study investigates whether slow quasiparticle recombination explains excess quasiparticles in superconductors, concluding that non-thermal sources are the primary cause rather than slow recombination.

Contribution

The paper provides experimental bounds on sub-gap states and demonstrates that slow recombination cannot account for quasiparticle excess in ordered superconductors.

Findings

Slow recombination is not predicted at observed quasiparticle densities.

Quasiparticle excess is mainly due to non-thermal generation sources.

Upper bounds on sub-gap states were established through tunnel junction measurements.

Abstract

An excess density of quasiparticles is widely observed in superconducting films. This excess causes performance degradation in a variety of superconducting devices, including decoherence in qubits. In this Letter, we evaluate the hypothesis of [1] that the quasiparticle excess is caused by anomalously slow recombination at low quasiparticle densities due to localization in sub-gap states. We probe the density of states in aluminum and niobium films using current-voltage measurements of tunnel junctions and extract upper bounds on the energy scales of the sub-gap states and gap smearing. With these parameters, we evaluate the recombination times predicted by [1] and find that slow recombination is not predicted to occur at observed quasiparticle densities in aluminumand niobium-based superconducting devices. These results suggest that the quasiparticle excess in ordered superconductors is primarily due to non-thermal sources of quasiparticle generation and not slow recombination.