Electrical anisotropy in high-T_c granular superconductors in a magnetic field

We propose an analytical model devoted to explain the anisotropy of the electrical resistance observed below the critical temperature in granular high-T_c superconductors submitted to a magnetic field H. Reported experimental results obtained on a YBCO sample show that the superconducting transition occurs in two stages, with a steep drop of the resistance at T_c and a subsequent, smoother decrease. In this second stage, the resistance versus temperature curve is strongly dependent not only on the field intensity, but also on the angle between H and the macroscopic current density j. We start from the assumption that the resistance below T_c is mainly due to the weak links between grains. In the model, weak links are thought of as flat surface elements separating adjacent grains. We calculate the probability for a weak link to undergo the transition to the resistive state as a function of the angle it makes with the external magnetic field H and the macroscopic current density j. In doing this, an important role is given to the strong nonuniformity of the local magnetic field within the specimen, due to the effect of the screening supercurrents flowing on the surface of the grains. Finally, we calculate the electrical resistance of the sample in the two cases H⊥j and H∥j. The predictions of this simple model turn out to be in reasonable agreement with reported experimental results obtained on a YBCO granular specimen.