Melting of regular and decoupled vortex lattices in BSCCO crystals

The angular dependence of the first-order phase transition (FOT) in the vortex lattice in Bi₂Sr₂CaCu₂O₈ crystals was investigated by a low frequency AC shielding technique (with the AC field ∥ c), in which the static-field component parallel to c- (H_⊥) was varied with the in-plane field H_∥ held constant. The linear decrease of the FOT field H_⊥ ^FOT with increasing H_∥ ends at a temperature-dependent critical value of H_∥. A new transition, marked by the abrupt drop of the ab-plane shielding current, appears at this point. We draw a new phase diagram with H_∥ and H_⊥ field components as coordinates; this features at least two distinct regions in the vortex solid phase, that are determined by the different interplay between the pancake vortex- and Josephson vortex lattice.