Critical Magnetic Field of Thin Superposed Films

Thin superposed films of two superconductors or of a superconductor and a normal metal are studied in a parallel magnetic field near a second-order transition. From the linearized integral equation for the position-dependent order parameter and the boundary conditions for the magnetic-field-dependent kernel at the interface, an implicit equation for the critical magnetic field is derived under the assumption that the order parameter has a different but constant value in each film. Near the transition temperature of the double film $T_{\mathrm{cns}}$ and at the absolute zero of temperature, the results are expressed in terms of the usual relevant parameters characterizing the two constituents of the compound film, and are compared with some recent experimental data.