Critical Current Density and Critical Temperature Gradient in Thin Films of Type I Superconductors

Abstract

We have studied current induced and thermally induced flux motion through the flux flow resistivity and the Nernst effect, respectively, in superconducting films of lead, tin, and indium. The film thickness ranged between 0.5 and 11 μ. In addition to the flux flow resistivity and the transport entropy of a fluxoid, the critical current and the critical temperature gradient, at which current induced or thermally induced flux motion sets in, were determined. The critical current density and thereby the critical Lorentz force were found to increase strongly with decreasing film thickness. Further, with decreasing film thickness the critical Lorentz force became much larger than the critical thermal force. The experiments suggest that in a thin film the critical current is predominantly a surface current and that the critical surface currents contribute only very little to the Lorentz force on a fluxoid. Apparently, at and below its critical value the current flows in such a pattern that there is very little interaction with the fluxoids in the film.