HIGH ANISOTROPY AND A DIMENSIONALITY CROSSOVER IN THE IRREVERSIBILITY BEHAVIOR OF OXYGEN-DEFICIENT YBA2CU3O7-Y

The width in temperature of the reversible, lossy state of high-temperature superconductors (HTS's) in a magnetic field H depends on the degree of anisotropy. Compared to the parent compound YBa2Cu3O7, we show here that oxygen-deficient, YBa2Cu3O7-y single crystals, in which T(c) was varied from 10 to 55 K, are much more anisotropic and that the occurrence of the reversible, lossy state for the H parallel-to c axis is consistent with a crossover from three-dimensional (3D) vortex lines to 2D vortices, as recently proposed for the other highly anisotropic HTS's. These results, together with those from Tl2Ba2CaCu2Ox and Bi2Sr2CaCu2Ox, indicate universal behavior of almost isolated Cu-O bilayer units, albeit with different doping levels, and which display magnetic reversibility controlled by the residual weak interplanar coupling.