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  • Letter
  • Open Access

Evaluating the contribution of slow recombination in localized subgap states to the excess quasiparticle population in ordered superconductors

Eva Gurra1,2,*, Douglas A. Bennett1, Shannon M. Duff1, Michael R. Vissers1, and Joel N. Ullom1,2,†

  • *Contact author: eva.gurra@colorado.edu
  • †Contact author: joel.ullom@nist.gov
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Phys. Rev. Research 8, L012053 – Published 5 March, 2026

DOI: https://doi.org/10.1103/v181-yd74

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 Bespalov et al. [Phys. Rev. Lett. 117, 117002 (2016)] that the quasiparticle excess is caused by anomalously slow recombination at low quasiparticle densities due to localization in subgap 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 subgap states and gap smearing. With these parameters, we evaluate the density-dependent recombination times calculated by Bespalov et al. and find that slow recombination is not predicted to occur at observed quasiparticle densities in aluminum- and niobium-based superconducting devices. These results suggest that the quasiparticle excess in ordered superconductors is primarily due to nonthermal sources of quasiparticle generation and not slow recombination.

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Phys. Rev. Research 8, L012053– Published 5 March, 2026
  • Received 7 July 2025
  • Accepted 13 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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