MODEL FOR LAYERED SUPERCONDUCTORS - FROM JOSEPHSON COUPLING TO 3-DIMENSIONAL BEHAVIOR

We present a model for layered superconductors, where the order parame ter between the layers is nonzero, due to proximity effects. In the li mit of decoupled layers, this model gives all the terms of the usual L awrence-Doniach free energy, as well as new terms arising from the exi stence of an order parameter between the layers. The corresponding Jos ephson coupling is found to be field dependent. The upper critical fie ld normal to the layers has positive curvature as a function of T, and decreases with increasing interlayer distance, contrary to the predic tions of the Lawrence-Doniach model. In addition, the anisotropy deduc ed from the critical fields H(c2) is markedly different from the low-f ield anisotropy.