Muon spin rotation studies of the vortex matter in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

We report muon spin rotation studies of the magnetic field distribution n(B) in the vortex state of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. From our data on three sets of overdoped, nearly optimized and undoped 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 the intra-planar melting transition. The resulting flux line liquid phase persists over a sizeable temperature interval; before the inter-planar decoupling of the individual vortex lines takes place in a second step at the irreversibility line, resulting in pancake liquid phase. (author)