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 Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ crystals was investigated by a low frequency AC shielding technique (with the AC field $\parallel c$), in which the static-field component parallel to $c$- ($H_{\perp }$) was varied with the in-plane field $H_{\parallel }$ held constant. The linear decrease of the FOT field $H_{\perp }^{FOT}$ with increasing $H_{\parallel }$ ends at a temperature–dependent critical value of $H_{\parallel }$. 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 $H_{\parallel }$ and $H_{\perp }$ 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.