Change in the order of the melting transition with oxygen content inYBa2Cu3O7−δ

Abstract

The vortex phase transition was systematically studied in twin-free ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for various \ensuremath{\delta}. For $7\ensuremath{-}\ensuremath{\delta}>~6.94,$ the first order transition was evidenced by an abrupt jump and a hysteresis of magnetization in the field-cooled (FC) cooling-warming cycles, which indicates the coexistence of vortex crystallites and liquid. For $7\ensuremath{-}\ensuremath{\delta}<6.89$ the jump was not sharp and FC magnetization was reversible, suggesting a second order transition. The first and the second order transitions merge at a critical point (CP) which goes to zero field for $7\ensuremath{-}\ensuremath{\delta}<6.89.$ Three-dimensional XY scaling satisfactorily fits the data at 1--7 T for $7\ensuremath{-}\ensuremath{\delta}<6.94$ and for $7\ensuremath{-}\ensuremath{\delta}<6.89,$ but does not fit for $6.89<7\ensuremath{-}\ensuremath{\delta}<6.94,$ where the CP passes through this field range.