Vortex Phase Transition and Oxygen Vacancy in YBa2Cu3Oy Single Crystals

The atomic images of YBa2Cu3Oy (YBCO, y≃6.94) single crystals are measured by low-temperature scanning tunneling microscopy (LT-STM). We find that the oxygen vacancies form clusters along the b-axis in the CuO chain layer. Near the clusters, the strong atomic distortion is observed along the a-axis. The vortex phase diagram is very sensitive to the amount of oxygen vacancies in the region between optimal and overdoped regions. For optimally doped untwinned YBCO (y≃6.92), we find a first-order transition TL(H) in the vortex liquid above the terminal point Hmcp (≃7 T) of both the vortex glass line Tg (H) and the field-driven disordering transition line H*(T). The obtained small entropy change (∼0.02 kB/vortex/layer) and the critical endpoint Hcep (≃11 T) of TL(H) indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below Hmcp, the vortex liquid phase shows the first-order melting transition into the Bragg glass phase.