NONLINEAR I-V-CHARACTERISTICS NEAR THE PERCOLATION-THRESHOLD

The I-V characteristics of a carbon-wax mixture slightly above the percolation threshold are found to be nonlinear. Generally for low (linear) -resistance samples, the I-V curves are smooth. The leading nonlinear term is quadratic, rather than cubic. For high-resistance samples, the onset of nonlinearity is marked by the appearance of resonance like structures in the R-V curves. The current I(c) at which nonlinearity starts scales with the linear conductance SIGMA-1 as I(c) approximately SIGMA-1x, with x almost-equal-to 1.4 in both cases. The second-order conductance, SIGMA-2, in the case of smooth curves, also scales with SIGMA-1 as SIGMA-2 approximately SIGMA-1y, with y almost-equal-to 0.59. Results are discussed within the framework of two previously suggested models. It is shown that none of these models can explain fully results obtained with the carbon-wax system.