Critical voltage of a mesoscopic superconductor

We study the role of the quasiparticle distribution function f on the properties of a superconducting nanowire. We employ a numerical calculation based upon the Usadel equation. Going beyond linear response, we find a non-thermal distribution for f caused by an applied bias voltage. We demonstrate that the even part of f (the energy mode f_L) drives a first order transition from the superconducting state to the normal state irrespective of the current.