Observation of multiple-gap states in tunnel-injected nonequilibrium superconducting tin films

We report the first observation of multiple-gap states in a tin film under high tunnel injection. The experiments are performed for both low- and high-voltage injection regimes. We observe the two-gap state for injection at the gap edge and the three-gap state for injection above the gap edge. The multiple-gap structure clearly appears for temperatures below $T_{\lambda }$, whereas it is considerably smeared or has disappeared above $T_{\lambda }$, indicating a significant dependence on phonon trapping. The observed phenomena are qualitatively similar to those for dirty aluminum films; they are interpreted by the model of diffusive instability accompanying abnormal quasiparticle flow from regions of low concentration to high concentration.