Nernst Effect and Flux Flow in Superconductors. II. Lead Films

We have studied thermally induced and current-induced flux motion through the Nernst effect and the flux-flow resistivity, respectively, in superconducting lead films. The film thickness ranged between 1 and 7 μ. The addition to the transport entropy of a fluxoid and the flux-flow resistivity, the critical temperature gradient and the critical current, at which thermally induced or current-induced flux motion sets in, were determined. At 375 Oe and 4.2°K, the transport entropy per unit flux $\frac{S_{\varphi }}{\varphi }$ was found to increase with increasing film thickness up to a thickness of about 3 μ. For thicker films, $\frac{S_{\varphi }}{\varphi }$ decreased with increasing film thickness. For a film thickness near 3 μ, $\frac{S_{\varphi }}{\varphi }$ was close to the value expected for a large flux bundle from the difference in entropy density of normal and superconducting material. The critical current density and thereby the critical Lorentz force per unit length of fluxoid were found to increase strongly with decreasing film thickness, whereas the critical thermal force varied little with film thickness. The critical apparent Lorentz force was larger than the critical thermal force by 2-3 orders of magnitude, the discrepancy increasing with decreasing film thickness. These results suggest that in a thin film the critical current flows predominantly along the surface in such a pattern that there is very little interaction with the fluxoids in the film. Apparently, at and below the critical current, the electrical current flow in a thin film is such that it contributes only very little to the Lorentz force on the fluxoids.