3D XY scaling of the irreversibility line of YBa2Cu3O7 crystals

We find experimentally that the irreversibility line, H-irr(T), and the reversible magnetization of twinned YBa2Cu3O7 crystals both show 3D XY scaling, which suggests a common origin for both quantities. Analysis of this result and of a correlation between the condensation energy and H-irr(T) of oxygen deficient YBa2CU3O7-delta for 0 < delta < 0.6, implies that H-irr(T) is strongly influenced by critical fluctuations. It is possible that these values of H-irr(T) and the recently discovered vortex melting line, H-m(T), both correspond to an upper critical field line which has been suppressed by thermodynamic fluctuations.