Critical Current and Field in Nonideal Superconductors

Abstract

The upper critical fields of the V-Ti system were measured at 4.2 deg K and agree with Gorkov-Shapoval's expression for the upper critical field of superconducting alloys. The critical current of samples of V with varying Ti content and also of Nb--25% Zr, all of rectangular crosssection (0.008 x 0.025 in.), was measured as a function of the magnetic field H, applied parallel to the width of the strip. When the critical current I is plotted in units of the Silsbee current I/sub c/ (based on the thermodynamic critical field H/sub c/), versus the applied magnetic field measured in units of the Gorkov-Shapoval upper critical field (H/sub 2/), the experimental data for all the samples tested fall on a single curve. This critical curve is deduced from the free energy of a laminar mixed state model such that the applied current flows mainly within a penetration depth along the width of the strip (end effects are neglected). The critical curve for almost the whole range of applied fields is is shownto be the same as j/sub a/ j/sub s/ = j/sub c/, where j/sub a/, and j/sub s/ are the maxim um local applied and supercurrent densities, respectively, and j/submore » c/ is the thermrodynamic critical current density (4 pi H/sub c/c/ lambda ), where lambda is the penetration depth. (auth)« less