MAGNETORESISTANCE IN POLYANILINE NETWORKS NEAR THE PERCOLATION-THRESHOLD

The self-assembled network of conducting polyaniline (PANI), protonate d by camphor sulfonic acid (CSA), in a matrix of insulating polymethyl methacrylate (PMMA) has a remarkably low percolation threshold. The cr itical volume fraction (f(c)) of PANI-CSA phase segregated in PMMA is f(c) similar to 0.003 (0.3%). The conductivity at room temperature nea r the percolation threshold is quite high, 3 x 10(-3) S/cm. The system atic increase in the exponent of the temperature dependence of conduct ivity (gamma) results from transport on the fractal structure and to t he related superlocalization of the electronic wavefunctions with syst ematic variations in the fractal dimensionality upon dilution of the P ANI-CSA. Below the percolation threshold, the temperature: dependence of the resistivity is like that of granular metals with gamma = 0.5. T he positive magnetoresistance (MR) shows a maximum upon decreasing the volume fraction of PANI-CSA in agreement viith effective medium theor y. Analysis of the MR indicates that the localization length near the percolation threshold is approximately 25 Angstrom at 4.2 K.