Josephson behavior in large-cross-section bridges of type I superconductor

Current-induced breakdown of superconductivity has been investigated in superconducting indium bridges with large cross section (thickness 2 µm; width 15–200 µm) and 4–20 µm length. Under rf radiation, Josephson steps in the voltage-current characteristics have been observed in all In bridges less than about 80 µm wide. A model for explaining our results must combine the current-induced nucleation of trains of flux tubes, containing many flux quanta, with a single-quantum relaxation process. We propose a model according to which the nucleation of the flux tubes near the sample edge proceeds via single flux quanta. We have studied the variation of the amplitude of the Josephson steps with rf power, and reasonable agreement with the Bessel function behavior expected from the simple voltage-source model of a superconducting weak link has been found in some cases.Supported by a grant from the Deutsche Forschungsgemeinschaft.