Nonlinear ac resistivity in s-wave and d-wave disordered granular superconductors.

We model s-wave and d-wave disordered granular superconductors with a three-dimensional lattice of randomly distributed Josephson junctions. The nonlinear ac resistivity rho(2) of these systems was calculated using Langevin dynamical equations. The current amplitude dependence of rho(2) at the peak position is found to be a power law characterized by exponent alpha, which is not universal but depends on the self-inductance and current regimes. In the weak current regime alpha is independent of the self-inductance and alpha = 0.5+/-0.1 for both s- and d-wave materials. In accord with experiments, we find alpha approximately 1 for some interval of inductance in the strong current regime.