Anisotropy and structural-defect contributions to percolative conduction in granular copper oxide superconductors

A well-defined temperature-independent slope of the current-voltage characteristics (CVC) is a common feature of granular high-temperature superconductors in the paracoherent state (grains superconducting; intergranular regions normal). By analyzing the contributions of anisotropy and structural defects to percolative conduction processes both in the normal state and in the paracoherent state, we quantitatively account for the observed CVC slopes using only the normal-state resistivity values. In particular, from effective-medium theory it is found that the CVC slope of nontextured granular YBa2CU3O7-delta (YBCO) should be equal to approximately one-third of the normal-state resistivity extrapolated to zero temperature. Simultaneous measurements of the CVC and normal-state resistivity on a batch of granular YBCO samples are also presented that verify our predictions with no free parameters.