Low Temperature Magnetoresistance of Iodine-Doped Polyacetylene in Metal-Nonmetal Transition Region

Highly conductive heavily iodine-doped polyacetylene changes from metallic to nonmetallic by room-temperature aging. For this change, thermoelectric power varies little, suggesting that the transition is not ascribed to change in carrier concentration but to increase of the degrees of disorder. Corresponding to the transition, low temperature magnetoresistance changes from negative to positive on crossing the transition. The magnetoresistance in the metallic region is interpreted in terms of weak localization, while that in the nonmetallic state is explained by field effect to variable range hopping conduction among Anderson localization states. The transition to the nonmetallic state proceeds inhomogeneously.