Nernst Effect and Flux Flow in Superconductors. III. Films of Tin and Indium

Flux motion induced by a temperature gradient and by an electrical current has been studied through the Nernst effect and the flux-flow resistivity in superconducting films of tin and indium. The film thickness ranged between 1 and 11 μm for tin and between 0.5 and 5 μm for indium. The data were taken at 2.0 K as a function of magnetic field and film thickness. In addition to the transport entropy of a fluxoid and the flux-flow resistivity, the critical thermal force and the critical Lorentz force, at which thermally induced and current-induced flux motion sets in, were determined. The transport entropy per unit length per unit flux, $\frac{S_{\varphi }}{\varphi }$, plotted versus film thickness was found to reach a maximum in Sn at about 7 μm and in In at about 3.5 μm. Around this maximum, $\frac{S_{\varphi }}{\varphi }$ was larger by a factor of 1.4 in Sn and 1.6 in In than the value calculated from the difference in entropy density of normal and superconducting material. With increasing film thickness, the critical Lorentz force decreased, while the critical thermal force increased. The discrepancy between the critical Lorentz force and the critical thermal force and its dependence on film thickness suggests that in the films the critical current flows along special channels at the surface for which the interaction with the fluxoids is strongly reduced. The results on Sn and In are consistent with the earlier results on Pb.