Proximity Effect on Flux Pinning Strength in Superconducting Nb-Ti with Thin α-Ti Ribbons

Abstract

It has been found by Lee et al. that the critical temperature reduces monotonically, while the critical current density first increases and is followed by a decrease, according as the diameter of superconducting multifilamentary Nb-Ti wire is decreased. This behavior is mainly caused by the proximity effect between superconducting matrix and normal α -Ti ribbons that becomes more remarkable as the thicknesses of two regions become smaller. The critical temperature and the elementary pinning strength of α -Ti ribbons are theoretically estimated by solving the phenomenological Ginzburg-Landau equations for multilayered structure with superconducting and normal layers. The critical current density is calculated from a statistic summation of the elementary pinning forces. The obtained critical temperature decreases monotonically, while the critical current density increases, with decreasing thicknesses of the two layers. The decrease in J_C observed by Lee et al. is considered to result from constriction of superconducting layers by heavy cold work.