Perpendicular upper critical field of superconducting–normal-metal multilayers

An equation is derived for the upper critical field, Hc2(T), perpendicular to super- conductor--normal-metal (SN) multilayers. The theoretical Hc2(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 Hc2(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 Hc2(T).