PHONON-INDUCED NONMETAL-METAL TRANSITION OF A DOPED POLYANILINE

We report the crossover for the microwave dielectric constant of camph or-sulfonic-acid-doped polyaniline prepared in m-cresol from very larg e room-temperature negative values to large positive ones with decreas ing temperature below T-s similar to 20 K. We attribute this change to the transition from a phonon-controlled metallic state to a nonmetall ic state with localized carriers and hopping transport below T-s. Elec tronically, one-dimensional chains that couple the three-dimensional c rystalline regions are likely central for understanding the origin of this transition as weak localization. At T<T-s we also observed an inc rease in the thermoelectric power and a linear decrease in the reduced activation energy for dc conductivity.