Current-induced breakdown of superconductivity in constricted type I superconducting films

In simultaneous magneto-optical and electrical measurements on constricted type I superconducting films step structure in the current-voltage characteristic was correlated with the nucleation of individual flux-tube trains. A central feature of this experiment is the development of a sample geometry with which the breakdown behavior can be restricted to a single, localized flux-tube train running perpendicular to the transport current. Apparently, the step structure in the I--V characteristic results from a Gibbs free-energy barrier against the nucleation of flux-tube trains. The experiments are performed with films of lead and indium. From the I--V characteristics of the indium constrictions it is concluded that the flux-tube nucleation rate can vary substantially with applied current beyond a voltage step at temperatures near T/sub c/