Josephson-coupled layered superconductors with two order parameters. I. Upper critical magnetic field

Abstract

By using the Gor'kov-Nambu microscopic theory, an expression for the free-energy functional is obtained for Josephson-coupled layered superconductors with two order parameters corresponding to intra- and interlayer Cooper pairs. The existence of two order parameters in a system leads to the appearance of new terms in the free-energy functional known as the 'Josephson' terms and the Lifshitz invariant. Of these, the Josephson terms characterize the tunnelling of both intra- and interlayer Cooper pairs between neighbouring superconducting sheets. The upper critical field, Hc2, is studied for the superconductors under investigation. It is found that a magnetic field parallel to the superconducting layers does not destroy the superconductivity for temperatures T<T*, where T* depends on the resonance integral between two neighbouring superconducting layers. This effect gives rise to a positive curvature in the temperature dependence of Hc2/sup ///.