Phase transition in the vortex liquid and the critical endpoint in YBa₂Cu₃O_y

The vortex phase diagram of optimally doped untwinned YBa₂Cu₃O_y is studied. We find a first-order transition T_L(H) in the vortex liquid above the terminal point H_mcp (≃7 T) of both the vortex glass line T_g(H) and the field-driven disordering transition line H*(T). The obtained small entropy change (∼0.02 k_B/vortex/layer) and the critical endpoint H_cep (≃11 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., 6.90≤y≤6.92) 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.