CLUSTER APPROXIMATION IN THE THEORY OF THE AC HOPPING CONDUCTIVITY IN DISORDERED SYSTEMS .2. 3-DIMENSIONAL CASE

For pt.I see ibid., vol.12, p.2797 (1979). AC hopping conductivity calculations are performed using a method analogous to the one used in percolation theory for frequencies lower than those described by two-site models. An expression was obtained for sigma ( omega ) approximately omega f( omega tau c), f being a slowly decreasing function of omega tau c( tau c is a few orders of magnitude larger than the characteristic time in two-site models, tau 0 approximately 1/ nu ph). Over a wide frequency range sigma ( omega ) approximately omega s, with s<1, and s may depend on omega , temperature and concentration of localisation centres. Agreement between the theory and the available experimental data is discussed.