The Anisotropic Upper Critical Field of Single Crystal YBa₂Cu₃O_x

Abstract

We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa 2 Cu 3 O x . The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c -axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy H c2 ⊥ / H c2 // is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.