Conductance anisotropy in granular high-Tc superconductors in a magnetic field

Abstract

Abstract The resistance of a granular high-T c superconductor cooled below the transition temperature T co and submitted to a magnetic field is found to depend on the angle between the external field H and the macroscopic current density 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 the intergrain regions strongly inhomogeneous. The probability for a weak link to be submitted to a local field of intensity H 1 is supposed to be dependent on the angle θ between the normal n to the weak link surface and the external field H This creates a structural anisotropy in the distribution of weak links undergoing the resistive transition under the effect of magnetic field and current. A theoretical expression is then obtained for the ratio η of the resistivity ρ ⊥in a direction perpendicular to the applied magnetic field H to the resistivity ρ‖ in a direction parallel to H. On the basis of the assumption that the resistive transition of the weak links depends only on the moduli of H i and j, the resistivity can be described as a tensor of order 2, completely determined by ρ ⊥ and ρ ‖. The theoretical predictions of the model are compared with the results of experimental measurements on a bulk specimen of granular superconductor.