VORTEX AND FLUXON PHASE-TRANSITIONS IN LAYERED SUPERCONDUCTORS

The system of superconducting layers with Josephson coupling J is studied. The ordering temperature T(f) for the fluxon loops parallel to the layers is shown to determine the transition temperature T(c) of the system, except when J is exponentially small; in the latter case T(c) drops towards the J = 0 vortex transition, accounting for data on YBa2CU3O7/PrBa2CU3O7 superlattices. A magnetic field parallel to the layers produces a sequence of phases in which the induced fluxon lines are l layers apart. For l > 8 and J not too small, phases with two-dimensional fluxon correlations occur near T(c); this accounts for the observed V approximately I(a)(T) current-voltage relation in high-T(c) superconductors.