Perpendicular upper critical field of a proximity-coupled superconducting film

The temperature dependence of the perpendicular upper critical field Bc2⊥ of a single superconducting Nb film (S) sandwiched between insulating (I) and/or normal-metal layers (N=Cu) is investigated. For the ISI configuration, Bc2⊥ exhibits the usual linear T-dependence near the transition temperature Tc in contrast to the NSI and NSN configurations where a positive curvature of Bc2⊥(T) is observed near Tc. This demonstrates the influence of the different boundary conditions on the Bc2⊥(T) behavior of a single S film in contact with N or I. Deviations from the linear T-dependence are thus attributed to the proximity effect due to the presence of an N–S boundary.