Anomalous temperature dependence of the quasiparticle-injection-induced voltage onset in superconducting lead films

The large quasiparticle injection through a tunnel barrier is studied, in particular when the bias tunnel voltage is close to the gap edge. It is shown that the quasiparticle-injection-induced voltage threshold in a superconducting lead film has an anomalous temperature dependence above T. The effect cannot be accounted for by simple heating. Below T, a first-order transition of the lead film is observed. A possible explanation based on a nonequilibrium model is given.