Resistive transition anisotropy in superconducting granular ceramics: Interplay of effective medium approximation and percolation

The resistive transition (formula presented) in granular high-(formula presented) superconductors is described by a combination of the effective-medium approximation for anisotropic mixtures with the percolation theory. We found a single parameter accounting for both the structural anisotropy as well as for the intrinsic resistivity anisotropy of a single grain. We obtained a direct (no fitting parameters) estimate of the superconducting volume fraction f as a function of temperature, as well as of the percolation threshold (formula presented) and of the zero-resisitivity threshold (formula presented) The model describes successfully the experimental (formula presented) dependence at different relative directions of the c axis, the current j and the magnetic field H in (formula presented) ceramics. We provided estimates for the value of the Josephson critical current between grains. The latter proves to be strongly suppressed even by weak magnetic fields (formula presented) This results in the appearance of the field-induced excess resistivity (formula presented) that obeys the dependence (formula presented) where (formula presented) is the characteristic field trapped by a single Josephson contact.