Vortex and fluxon phase transitions in layered superconductors

The system of superconducting layers with Josephson coupling J is studied. The ordering temperature Tf for the fluxon loops parallel to the layers is shown to determine the transition temperature Tc of the system, except when J is exponentially small; in the latter case Tc 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 nd J not too small, phases with two-dimensional fluxon correlations occur near Tc; this accounts for the observed Vla(T) current-voltage relation in high-Tc superconductors.