ANISOTROPIC UPPER CRITICAL MAGNETIC FIELD IN SINGLE CRYSTAL Ba2YCu3O7 - y.

The upper critical magnetic field H//c//2 of Ba//2YCu//3O//7// minus //y has been measured using single crystals with applied fields up to 26 T. The results exhibit strong anisotropy with regard to the c-axis and the basal plane. The slope of temperature dependence of H//c//2 along the c-axis is minus 0. 56 T/K, while that along the basal plane is minus 3. 3 to minus 2. 6 T/K. H//c//2(0) for the c-axis and basal plane are estimated to be 35 T and 160 to 210 T, respectively, using the Werthamer-Helfand-Hohenberg theory. From these values, the coherence length along the c-axis is estimated to be 5. 1 Angstrom, and that in the basal plane is 31 Angstrom. The former value is comparable to the lattice constant c.