Effects of Oxygen Stoichiometry on Structure And Properties in Ba//2ycu//3O//X. .

Results of a series of studies on the oxygen stoichiometry in Ba//2YCu//3O//x, 6. 0 less than equivalent to x less than equivalent to 7. 0, are reported. The structure at x equals 7 contains layers of corner-shared, square pyramidal copper and chains of square planar copper. Oxygen is removed from the chains leaving two-fold coordinate copper at x equals 6. Superconductivity is optimal for x approximately equals 7 and for x equals 6 the material is a semiconductor. A valence description ascribes Cu** plus **2 to the Cu in the layers for all x with the Cu in the chains going from Cu** plus **3 at x equals 7 to Cu** plus **1 at x equals 6. A model for twinning based on oxygen defects similar to those in Ba//2YCu//3O//6 is discussed

Sharp Angular Sensitivity of Pinning Due to Twin Boundaries in Ba 2ycu3o7

We have measured the pinning force due to twin boundaries by measuring the magnetic torque produced when a field is applied perpendicular to the c axis and rotated with respect to the ab axes. At 76 K, the torque increases roughly fourfold, when the field is within 2°of the parallel to the twin boundaries. This peak is absent at 27 K. Irradiation with 5×101 6 cm-2 3.5 MeV protons increases the overall critical current and eliminates the peaks observed at 76 K