Nernst Effect and Flux Flow in Superconductors. I. Niobium

Flux flow induced by a temperature gradient and by an electrical current has been studied in foils of high-purity niobium with a thickness from 11 to 18 μ. From the data, the transport entropy $S_{\varphi }$ associated with a fluxoid was estimated. For magnetic fields below about 1000 G, the order of magnitude of $S_{\varphi }$ is similar to that of theoretical estimates for an isolated vortex. The critical temperature gradient was found to decrease with increasing magnetic field. The critical current was larger by 1-2 orders of magnitude than the value expected from the critical temperature gradient, assuming that the critical current contributes fully to the Lorentz force on a fluxoid. The experiments suggest that the critical current flows predominantly along the surface of the specimens in such a pattern that there is very little interaction with the flux lines and correspondingly only a small contribution to the Lorentz force. Using Nernst probes at more than one location of the same specimen, details of the flux flow caused by the temperature gradient could be determined. Besides the transverse Nernst voltages, relatively large longitudinal voltages were detected. These longitudinal voltages have opposite sign on both sides of the specimen and are associated with fluxoids entering or leaving the specimen through the sides in transverse direction.