Conductance anisotropy in granular high-T-c superconductors in a magnetic field

The resistance of a granular high-T-c superconductor cooled below the trans ition temperature T-co and submitted to a magnetic held is found to depend on the angle between the external field H and the macroscopic current densi ty j. To explain this effect a simple model is proposed, in which the flux exclusion by the superconducting grains makes the local magnetic field in t he intergrain regions strongly inhomogeneous. The probability for a weak li nk to be submitted to a local field of intensity HI is supposed to be depen dent on the angle theta between the normal n to the weak link surface and t he external field H. This creates a structural anisotropy in the distributi on of weak links undergoing the resistive transition under the effect of ma gnetic field and current. A theoretical expression is then obtained for the ratio eta of the resistivity rho(perpendicular to) in a direction perpendi cular to the applied magnetic field H to the resistivity pit in a direction parallel to H. On the basis of the assumption that the resistive transitio n of the weak links depends only on the moduli of H-I and j, the resistivit y can be described as a tensor of order 2, completely determined by rho(par allel to) and rho(perpendicular to). The theoretical predictions of the mod el are compared with the results of experimental measurements on a bulk spe cimen of granular superconductor.