The influence of granularity on dissipation in high temperature superconductor

We have investigated AC losses in high temperature superconductors. Granular YBCO samples were prepared by the conventional solid state synthesis process. Individual grains are anisotropic and these polycrystalline samples have randomly oriented c axes. Intrinsic and hysteretic losses were investigated using a Quantum Design PPMS magnetometer. AC susceptibility was measured as a function of magnetic field, frequency and temperature and field history. Three types of temperature dependent loss peaks, T/sub p/'s, were observed. The three peak types (in /spl chi/") are attributed to intrinsic (London theory), intergranular Josephson junction, and intragranular pinning loss mechanisms, respectively. They are observed to change systematically as temperature and magnetic field varies. We suggest physical models for the H, T and /spl omega/ dependence of the loss peaks.