Abstract
The atomic images of YBa 2 Cu 3 O y (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 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 T L (H) 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.
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