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

The vortex phase transition was systematically studied in twin-free YBa2Cu3O7-delta for various delta. For 7-delta greater than or equal to 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-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-delta <6.89. Three-dimensional XY scaling satisfactorily fits the data at 1-7 T for 7-delta<6.94 and for 7-delta <6.89, but does not fit for 6.89<7-delta<6.94, where the CP passes through this field range. [S0163-1829(99)14633-2].