3DXYScaling of the Irreversibility Line ofYBa2Cu3O7Crystals

We find experimentally that the irreversibility line, H{sub irr}(T) , and the reversible magnetization of twinned YBa{sub 2}Cu {sub 3}O{sub 7} 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{sub irr}(T) of oxygen deficient YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} for 0{lt}{delta}{lt}0.6 , implies that H{sub irr}(T) is strongly influenced by critical fluctuations. It is possible that these values of H{sub irr}(T) and the recently discovered vortex melting line, H{sub m}(T) , both correspond to an upper critical field line which has been suppressed by thermodynamic fluctuations. {copyright} {ital 1997} {ital The American Physical Society}