In many real insulator-conductor composites the ac data do not follow the scaling behavior predicted by percolation theory. To account for these discrepancies, we have incorporated quantum and/or hopping assisted tunneling into percolation theory. This model leads to a new "tunneling" scaling law for the complex conductivity which gives rise to an exponential variation of the dc conductivity, with concentration and a dispersion of the ac complex conductivity, with nonuniversal exponents close to the observed ones.

• Received 3 June 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2895