Theory for Decoupling in High-T_c Superconductors from an Analysis of   the Layered XY Model with Frustration
  The nature of decoupling in the mixed phase of extremely type-II layered superconductors is studied theoretically through a duality transformation of the layered XY model with frustration. In the limit of weak coupling, we generally find that the Josephson effect is absent if and only if the phase correlations within isolated layers are short range. In the case specific to uniform frustration, we notably identify a decoupled pancake vortex liquid phase that is bounded by first-order and second-order decoupling lines in the magnetic field vs. temperature plane. These transitions potentially account for the flux-lattice melting and for the flux-lattice depinning that is observed in clean high-temperature superconductors. 
J. P. Rodriguez