Anisotropic energy dissipation, flux flow and topological pinning in ceramic superconductors

Abstract
The dissipation in the high Tc ceramic superconductor La1.85Sr0.15CuO4- delta is interpreted in terms of interacting magnetic flux systems: vortices in the ceramic grains and fluxons between them. In zero field cooling experiments the dissipation shows a sin2 Theta dependence, where Theta is the angle between current and field. This result and the existence of an angle independent critical current are analyzed in the framework of a granular system where the dissipation is due to two contributions, one induced by movement of magnetic flux and the other associated to dissipation in the junctions.