STRETCHING-INDUCED CROSSOVER TO THE METALLIC STATE IN IODINE-DOPED POLYACETYLENE SAMPLES

We report the coexistence of metallic and non-metallic states within d ifferent portions of samples of heavily iodine-doped stretched polyace tylene based upon the measurements of a huge negative dielectric const ant at microwave frequency (epsilon(mw)) for the center portions and a relatively small and positive epsilon(mw) for the neck and end portio ns. A Drude model in the low frequency limit is applied to estimate a transport time and a plasma frequency for the metallic portions of the samples. A modified Drude model including one-dimensional localizatio n effects is used to account for the temperature dependence of the mic rowave dielectric constant and conductivity. For the non-metallic port ions, an interrupted metallic strand model, which represents the mater ial as a bundle of metallic wires or polymer chains interrupted by ins ulating defects, is applied to analyze the localization behavior probe d by the positive dielectric constant. The variation of dielectric res ponse of samples stretched before doping and with aging after doping p oints to the key role of inhomogeneous disorder in charge transport fo r these fibrillar materials with incomplete crystallinity.