Anomalous Anderson transition in carbonized ion-implanted polymer p-phenylenebenzobisoxazole

We report a charge transport study which revealed an unusual insulator-metal transition in ion implanted polymer p-phenylenebenzobisoxazole. Upon ion implantation, a carbonized layer forms on him sample surface and becomes increasingly conductive with increasing ion implantation dosage. A drastic change in the temperature dependence of conductivity sigma(T) with increasing ion dosage is observed at this transition. The row dosage samples (greater than or equal to 7X10(16) ions/cm(2)) have a low temperature insulating conductivity: sigma(T) similar to exp[-(T-0/T)(gamma)], where gamma was obtained at 0.74 increasing to 1/4 with increasing dosage. To explain the unusual value gamma = 0.74, we extend Mott's variable range hopping model by including in the consideration of an energy dependence of density of states near the Fermi level. This model also explains a temperature dependent conductivity near the insulator-metal transition in these low dosage dielectric samples. Ou the other hand, the high dosage samples(greater than or equal to 10(17) ions/cm(2)) show a semimetallic conductivity: sigma(T) = sigma(0) + Delta sigma(T), where Delta sigma(T) is due to electron-electron interaction and weak localization effects with the latter undergoing a dimensional crossover from three dimensions to two dimensions below similar to 40-50 K as reported earlier.