Abstract
A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponent , in disagreement with the standard theory of percolation which predicts for all three-dimensional systems. Various models have been proposed in order to explain the origin of such universality breakdown. Among them, the tunneling-percolation model calls into play tunneling processes between conducting particles which, under some general circumstances, could lead to transport exponents dependent of the mean tunneling distance . The validity of such theory could be tested by changing the parameter by means of an applied mechanical strain. We have applied this idea to universal and nonuniversal -glass composites. We show that when the measured piezoresistive response , i.e., the relative change of resistivity under applied strain, diverges logarithmically at the percolation threshold, while for , does not show an appreciable dependence upon the volume fraction. These results are consistent with a mean tunneling dependence of the nonuniversal transport exponent as predicted by the tunneling-percolation model. The experimental results are compared with analytical and numerical calculations on a random-resistor network model of tunneling percolation.
5 More- Received 25 July 2004
DOI:https://doi.org/10.1103/PhysRevB.71.064201
©2005 American Physical Society