PHASE-TRANSITIONS IN LAYERED SUPERCONDUCTORS

A system of superconducting layers with Josephson coupling J between them is studied. When the in-layer penetration depth lambda(e) is larger than the spacing d between layers, as in CuO2-based superconductors, there is a single three-dimensional transition temperature T(c). The ratio T(c)/tau, where tau = phi-0(2)/4-pi(2)lambda(e), is found to vary from approximately 1/8 to approximately 1, being near 1/8 when J/T(c) is exponentially small. When lambda(e) less-than-or-similar-to d a two-dimensional (2D) behavior is possible; in particular, bulk superconductors separated by 2D junctions exhibit a 2D transition, below the bulk transition, in which the junctions become ordered. This 2D behavior is due to the gauge coupling and is absent in an XY model where lambda(e) --> infinity.