Universality of ac conduction in anisotropic disordered systems: An effective-medium-approximation study

Anisotropic disordered systems are studied in this work within the random barrier model. In such systems the transition probabilities in different directions have different probability density functions. The frequency-dependent conductivity at low temperatures is obtained using an effective medium approximation. It is shown that the isotropic universal ac-conduction law, (u) over tilde=(sigma) over tilde ln (sigma) over tilde, is recovered if properly scaled conductivity ((sigma) over tilde) and frequency ((u) over tilde) variables are used.