Perpendicular upper critical field of superconducting<i>–</i>normal-metal multilayers

Abstract

An equation is derived for the upper critical field, ${H}_{c2}$(T), perpendicular to super- conductor--normal-metal (SN) multilayers. The theoretical ${H}_{c2}$(T) agrees with experimental data for Nb-Cu multilayers only if the mean free paths of both S and N metals are considerably smaller than the estimates obtained from longitudinal resistivity measurements. Positive curvature in ${H}_{c2}$(T) near the critical temperature is reproduced by the theory. Field-dependent corrections to the pair penetration depth into the N metal are shown to enhance ${H}_{c2}$(T).