Phonon-induced nonmetal-metal transition of a doped polyaniline

We report the crossover for the microwave dielectric constant of camphor-sulfonic-acid-doped polyaniline prepared in m-cresol from very large room-temperature negative values to large positive ones with decreasing temperature below Tₛ{~}20 K. We attribute this change to the transition from a phonon-controlled metallic state to a nonmetallic state with localized carriers and hopping transport below Tₛ. Electronically, one-dimensional chains that couple the three-dimensional crystalline regions are likely central for understanding the origin of this transition as weak localization. At TTₛ we also observed an increase in the thermoelectric power and a linear decrease in the reduced activation energy for dc conductivity.