Anisotropy of the electrical resistance in high-T_c granular superconductors under magnetic field

The electrical resistance of granular high-T_c superconductors submitted to a magnetic field and crossed by a supercritical current strongly depends on the angle between magnetic field and current density. In monocrystalline samples, this effect may be simply explained in terms of the angle-dependent coupling between fluxoids and current. In the case of polycrystalline superconductors, an alternative explanation is needed to take into account the effect of the granular structure. In this paper, we propose a quantitative model in which the resistance is mainly determined by the resistive transition of weak links. The electrical anisotropy is related to the inhomogeneous distribution of local magnetic field intensities and local current densities in the weak links, due to the magnetic screening effect of the superconducting grains and to the angular distribution of weak links surfaces. The theoretical results of this model are compared to the experimental ones obtained on a YBCO specimen under different physical conditions.