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

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 [] 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 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.