Strain modulation of transport criticality in heterogeneous solids

A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponents. The lack of universality implies that critical indexes may depend on material properties such as the particular microstructure or the nature of the constituents, and that in principle they can be influenced by suitable applied perturbations leading to important informations about the origin of nonuniversality. Here we show that in RuO$_2$-glass composites the nonuniversal exponent can be modulated by an applied mechanical strain, signaled by a logarithmic divergence of the piezoresistive response at the percolation threshold. We interpret this phenomenon as being due to a tunneling-distance dependence of the transport exponent, supporting therefore a theory of transport nonuniversality proposed some years ago.