Vortex Phase Transition and Oxygen Vacancy in YBa₂Cu₃O_y Single Crystals

Author： Nishizaki T. Shibata K. Maki M. Kobayashi N.

ISSN： 0022-2291

Source： Journal of Low Temperature Physics, Vol.131, Iss.5-6, 2003-06, pp. : 931-940

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Abstract

The atomic images of YBa₂Cu₃O_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 kB/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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