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.
@article{arxiv.2507.03217,
title = {Can slow recombination in ordered superconductors explain the excess quasiparticle population?},
author = {Eva Gurra and Douglas A. Bennett and Shannon M. Duff and Michael R. Vissers and Joel N. Ullom},
journal= {arXiv preprint arXiv:2507.03217},
year = {2025}
}