Generalization of the effective-medium approximation for hopping transportin amorphous materials

We present a new version of the effective-medium approximation (EMA) for ho pping in structurally disordered systems such as amorphous semiconductors o r impurity-bands of crystalline semiconductors. Tn contrast to previous the ories we are able to include the influence of closed loops in the path summ ation. These terms are responsible for the algebraic long-time tails of the velocity autocorrelation function which also show up as non-analytic terms in the low-frequency part of the AC conductivity. Tf the closed-loop contr ibutions are neglected, the two-site EMA of Gochanour et al. and Movaghar e t al. is re-obtained. We have tested the results of the present theory agai nst simulations for an r-hopping network.