High anisotropy and a dimensionality crossover in the irreversibility behavior of oxygen-deficient YBa_2Cu_3O_(7-y)

Abstract

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 YBa_2Cu_3O_(7-y), we show here that oxygen-deficient, YBa_2Cu_3O_(7-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 Tl_2Ba_2CaCu_2O_x and Bi_2Sr_2CaCu_2O_x, 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.