Change in the order of the melting transition with oxygen content in YBa2Cu3O7-δ

The vortex phase transition was systematically studied in twin-free YBa₂Cu₃O_7-δ for various δ. For 7-δ>=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-δ<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-δ<6.89. Three-dimensional XY scaling satisfactorily fits the data at 1-7 T for 7-δ<6.94 and for 7-δ<6.89, but does not fit for 6.89<7-δ<6.94, where the CP passes through this field range.