Nonlinear conduction in composites above percolation threshold — beyond the backbone

We review here the response of mainly conductor-insulator mixtures just above the percolation threshold (pc) under dc bias. I–V characteristics changes from being linear to nonlinear as the applied bias is increased. Closer a sample is to pc, greater is the nonlinearity. Analysis of nonlinear data is made with emphasis on scaling properties. dI/dV is found to saturate at large bias at a level that depends on the linear conductance. A sample with a given p is found to possess two voltage scales — is Vo, the scale for onset of nonlinearity and other is Vs, the scale for saturation. The transport exponents in the saturated state are different from those in the linear region (V ≈ 0). Possible models are discussed. Finally, temperature-dependent nonlinearity is discussed to illustrate how the study of nonlinear conduction proves useful to unravel the primary role of the conductance in fixing various scales relevant to transport in disordered systems