Upper Critical Field Measurements on a Single Crystal YBa₂Cu₃Ox up to 38 T

The anisotropic upper critical fields of a single crystal of orthorhombic YBa 2 Cu 3 O x have been measured in pulsed high magnetic fields up to 38 T. The critical fields B c2 defined by the onset of the resistance for parallel and perpendicular directions to the c -axis show a linear temperature dependence above 10 T with the slopes |d B c2 // /d T | = 1.1 T/K and |d B c2 ⊥ /d T | = 3.9 T/K, whereas the B c2 vs T curves show positive curvatures in the lower fields. At liquid nitrogen temperature, the zero resistance state cannot be destroyed by the field of 38 T and B c2 ⊥ is estimated as 45 T.