Hopping conductivity of a nearly 1D fractal: A model for conducting polymers

We suggest treating a conducting network of oriented polymer chains as an anisotropic fractal whose dimensionality D=1+epsilon is close to 1. Percolation on such a fractal is studied within the real space renormalization group of Migdal and Kadanoff. We find that the threshold value and all the critical exponents are strongly nonanalytic functions of epsilon as epsilon-->0, e.g., the critical exponent of conductivity is epsilon(-2)exp(-1-1/epsilon). The distribution function for conductivity of finite samples at the percolation threshold is established. It is shown that the central body of the distribution is given by a universal scaling function and only the low-conductivity tail of distribution remains epsilon dependent. Variable range hopping conductivity in the polymer network is studied: both de conductivity and ac conductivity in the multiple hopping regime are found to obey a quasi-one-dimensional Mott law. The present results are consistent with electrical properties of poorly conducting polymers. [S0163-1829(98)04841-3].