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

The anisotropic upper critical fields of a single crystal of orthorhombic YBa2Cu3Ox have been measured in pulsed high magnetic fields up to 38 T. The critical fields Bc2 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 |dBc2///dT| = 1.1 T/K and |dBc2⊥/dT| = 3.9 T/K, whereas the Bc2 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 Bc2⊥ is estimated as 45 T.