Phase transition in the vortex liquid and the critical endpoint in (formula presented)

The vortex phase diagram of optimally doped untwinned (formula presented) is studied. We find a first-order transition (formula presented) in the vortex liquid above the terminal point (formula presented) (formula presented) of both the vortex glass line (formula presented) and the field-driven disordering transition line (formula presented) The obtained small entropy change (formula presented) (formula presented) and the critical endpoint (formula presented) (formula presented) of the (formula presented) line indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below (formula presented) the vortex liquid phase shows the first-order melting transition into the Bragg glass phase. We also study the oxygen content y dependence of the vortex phase diagram and find that the vortex slush regime is located in the borderline (i.e., (formula presented) below which the vortex lattice melting transition disappears. The result indicates that the point disorder with the intermediate strength plays an important role in the vortex slush regime.