LAYERED SUPERCONDUCTORS .1. VORTEX AND FLUXON PHASE-TRANSITIONS

A system of superconducting layers with spacing d, in-layer penetration depth lambda(e) and Josephson coupling between neighboring layers J is studied. When J=0 the system exhibits a two-dimensional (2D) phase transition of vortex unbinding at a temperature T(v). When lambda(e) less than or similar to d a finite-size transition at T(v)eff > T(v) distinguishes this system from an XY model. When J not-equal 0, but vortices are neglected, Josephson fluxon loops lead to a distinct phase transition at T(f) > T(v) in which a significant second nearest-layer coupling is generated. Competing vortices and fluxon loops lead to a three-dimensional phase transition at T(c), where T(v) < T(c) < T(f). For CuO2-based superconductors (lambda(e) >> d) T(c) is near T(f) if T(c) greater than or similar to J >> T(c)exp(-E(c)'/T(c)), where E(c)' is a renormalized vortex-core energy; T(c) drops to T(v) as J is decreased, accounting for data on YBa2Cu3O7/PrBa2Cu3O7 superlattices.