Nonlinear ac response near percolation threshold and three-variable scaling

We report on measurements of complex ac conductance of a random system of carbon-wax mixtures using both frequency (f) and amplitude (V) as measuring probes. The latter gives rise to nonlinear effects which necessitate modification of the usual two-variable scaling law. We propose a three-variable scaling form that reproduces our key experimental results summarized in the form of a R-f-V diagram (R is the inverse of the real part of the ac conductance). The results are also discussed in the context of some existing models.