Perpendicular upper critical field of superconducting–normal-metal multilayers

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).