Muon spin rotation studies of the vortex matter in the high-T-c superconductor Bi2Sr2CaCu2O8+delta

We report muon spin rotation studies of the magnetic field distribution n(B ) in the vortex state of the high-temperature superconductor Bi2Sr2CaCu2O8delta. From our data on three sets of overdoped, nearly optimized and under doped single crystals we obtain evidence for a two-stage melting process of the vortex matter under equilibrium conditions. In the irreversible regime well below the irreversibility line we observe a transition in the vortex state which we associate with an intra-planar melting transition. The resul ting flux-line liquid phase persists over a sizeable temperature interval b efore the inter-planar decoupling of the individual vortex lines takes plac e in a second step at the irreversibility line, resulting in a pancake liqu id phase.