THE INFLUENCE OF GRANULARITY ON DISSIPATION IN HIGH-TEMPERATURE SUPERCONDUCTOR

We have investigated AC losses in high temperature superconductors. Gr anular YBCO samples were prepared by the conventional solid state synt hesis process. Individual grains are anisotropic and these polycrystal line samples have randomly oriented c axes. Intrinsic and hysteretic l osses were investigated using a Quantum Design PPMS magnetometer. AC s usceptibility was measured as a function of magnetic field, frequency and temperature and field history. Three types of temperature dependen t loss peaks, T-p's, were observed. The three peak types (in chi '') a re attributed to intrinsic (London theory), intergranular Josephson ju nction, and intragranular pinning loss mechanisms, respectively, They are observed to change systematically as temperature and magnetic fiel d varies. We suggest physical models for the H, T and omega dependence of the loss peaks.