JOSEPHSON-COUPLED LAYERED SUPERCONDUCTORS WITH 2 ORDER PARAMETERS .1.UPPER CRITICAL MAGNETIC-FIELD

By using the Gor'kov-Nambu microscopic theory, an expression for the f ree-energy functional is obtained for Josephson-coupled layered superc onductors with two order parameters corresponding to intra- and interl ayer Cooper pairs. The existence of two order parameters in a system l eads to the appearance of new terms in the free-energy functional know n as the 'Josephson' terms and the Lifshitz invariant. Of these, the J osephson terms characterize the tunnelling of both intra- and interlay er Cooper pairs between neighbouring superconducting sheets. The upper critical field, H(c2), is studied for the superconductors under inves tigation. It is found that a magnetic field parallel to the supercondu cting layers does not destroy the superconductivity for temperatures T < T, where T* depends on the resonance integral between two neighbou ring superconducting layers. This effect gives rise to a positive curv ature in the temperature dependence of H(c2)parallel-to.