Anisotropic nature of high-temperature superconductivity in single-crystal Y1Ba2Cu3O7-x

We report the first contact-free measurements of the upper critical fields, H_c2(T), of single-crystal Y₁Ba₂Cu₃O_7-x. In contrast to resistive measurements reported by others, we find that the anisotropy near T_c is temperature independent in agreement with the expectations of anisotropic Ginzburg-Landau theory. Estimates of the anisotropic Ginzburg-Landau and London parameters are reported. These indicate that despite the large anisotropy in H_c2 (5:1), the inferred low-temperature interplanar coherence length (ξ_z=7 A&#778) remains larger than the Cu-O layer spacing of 3.9 A&#778. The superconductivity in Y₁Ba₂Cu₃O_7-x thus remains fundamentally three dimensional in nature for a substantial temperature range below T_c.