The anisotropic nature of the superconducting properties of single crystal Y₁Ba₂Cu₃O_{7 - x}

A series of noncontact magnetic measurements on high-quality single crystals of Y1Ba2Cu3O7 - x have enabled us to demonstrate that the superconducting-state properties of Y1Ba2Cu3O7 - x are those of a conventional, anisotropic superconductor with the anisotropy being associated with the highly conducting CuO sheets in the Y1Ba2Cu3O7 - x crystal structure. The anisotropy in the superconducting state is reflected most strongly in the critical current anisotropy, which is as large as 20 to 1 at low temperatures and low fields and gets arbitrarily large at higher temperatures and higher fields. The upper critical field HC2 shows an anisotropy that varies from 6:1 to 10:1 in different crystals of high quality. Along the copper-oxygen sheets the upper critical field is enormous, extrapolating to ∼60 Tesla by 77 K and implying a low-temperature Ginzburg-Landau coherence length normal to the layers of 2 A ̊ - 4 A ̊. Strong upward curvature is evident in the HC2 data in both orientations of applied field. This curvature can be interpreted in terms of thermodynamic fluctuations, and, if this explanation is correct, the critical region is enormous, extending more than 25 K below TC. © 1988.