Double tunnel junctions have been used to investigate the properties of superconducting Sn films under quasiparticle injection. At high injection level,
the gap is discontinuously suppressed to zero from a finite value. This instability for ambient Ta> Tโข the k temperature of liquid helium, results from the film current exceeding its critical value. Below the instability the general behavior of the gap suppression is shown to be consistent with the modified heating model.
Recently, superconducting thin films under strong tunnel injection have been the subject of extensive investigations. 1"2 Among the problems of current interest are the nature of the instability associated with the transition of a superconducting thin film into the normal state when the strength of the driving force is sufficiently strong, and the energy spectra of the quasiparticles and phonons in the driven state. Two analytic models have been proposed to describe the energy spectra of the quasiparticles and phonons in the driven steady state of the thin film. In the/~* model, 3 the quasiparticle distribution is a Fermi function characterized by the ambient temperature Ta and an effective chemical potential/.~* greater than the equilibrium value. This model predicts a first-order superconducting-to-normal-state transition in the sense that the energy gap is discontinuous at the point of instability. When the quasiparticle density fluctuations are taken into account, there is an instability against the uniform quasiparticle distribution that leads to a state of mixed superconducting and normal regions. 4-6 In another model, the T* model, 7 the energy distribution of the quasiparticles is assumed to be a Fermi function with an elevated effective temperature,