Phase transition in the vortex liquid and the critical endpoint in YBa2Cu3Oy -: art. no. 214518

The vortex phase diagram of optimally doped untwinned YBa2Cu3Oy is studied. We find a first-order transition T-L(H) in the vortex liquid above the terminal point H-mcp (similar or equal to7 T) of both the vortex glass line T-g(H) and the field-driven disordering transition line H*(T). The obtained small entropy change (similar to0.02 k(B)/vortex/layer) and the critical endpoint H-cep (similar or equal to11 T) of the T-L(H) line indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below H-mcp, 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., 6.90less than or equal toyless than or equal to6.92) 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.