DECOUPLING OF LAYERED SUPERCONDUCTING FILMS IN PARALLEL MAGNETIC-FIELD

The issue of the decoupling of extreme type-IT superconducting thin fi lms (lambda(L)-->infinity) with weakly Josephson-coupled layers in mag netic held parallel to the layers is considered via the corresponding frustrated XY model used to describe the mixed phase in the critical r egime. For the general case of arbitrary field orientations such that the perpendicular magnetic field component is larger than the decoupli ng crossover scale characteristic of layered superconductors, we obtai n independent parallel and perpendicular vortex lattices. Specializing to the double-layer case, we compute the parallel lower-critical fiel d with entropic effects included, and find that it vanishes exponentia lly as the temperature approaches the layer decoupling transition in z ero field. The parallel reversible magnetization is also calculated in this case, where we find that it shows a crossover phenomenon as a fu nction of parallel field in the intermediate regime of the mixed phase in lieu of a true layer-decoupling transition. It is argued that such is the case for any finite number of layers, since the isolated doubl e layer represents the weakest link.