Fluxon transition in layered superconductors and its role in dimensional crossover

In a layered superconductor, fluctuations of flux loops parallel to the layers result in destruction of the Josephson coupling between layers above a critical temperature T_f. When Fluctuations of point vortices in the layers are included, a 3-dimensional phase transition results at T_c<T_f. Study of this fluxon transition shows that variations of T_c as function of anisotropy can be much larger than those in an anisotropic XY model. This can account for the large variations of T_c in multilayers. Furthermore, the fluxon transition can be directly observed, corresponding to the onset of nonlinearity in the c axis I-V relation, as observed in Bi₂Sr₂CaCu₂O_8-y. This onset is at a higher temperature then the onset of nonlinearity in the ab plane.