Possible origin of the smaller-than-universal percolation-conductivity exponent in the continuum

For quite a few systems in the continuum, such as carbon nanotube polymer composites and segregated composites, percolation electrical conductivity exponents that are much smaller than the universal value have been reported. This is unexpected in view of the classical lattice percolation theory. Here we provide a simple general phenomenological model that accounts for such observations within the framework of universality. We suggest that these small value exponents are due to the interplay between the connectivity and the structural variations that follow the increase of the fractional volume content of the conducting phase.