Current Transitions in Superconductive Tin Films

Except at temperatures just below the critical temperature the change of resistance of a superconducting film if the current through it is increased slowly takes place by a discontinuous transition, exhibits hysteresis, and is dominated by Joule heating effects. By applying current in short pulses, it has been possible to obtain isothermal transitions in films deposited on a flat surface. These transitions are smooth, do not exhibit hysteresis, and have a shape independent of film resistance. The critical current $I$ in films of thickness about twice the penetration depth is proportional to film width and can be related to the critical field of the film $H_C$, and the bulk critical field $H_{\mathrm{CB}}$, by the relation $I{H}_C=\mathrm{const}{H_{\mathrm{CB}}}^2$. It is also shown that by calculating the conditions of thermal equilibrium the dc transition curves with Joule heating can be derived from the isothermal transition results.