Conductivity and magnetoconductance in iodine-doped polyacetylene

Abstract

Disorder-induced localization plays a major role in the metal-insulator (M-I) transition of doped conducting polymers. Thus the quantification of disorder is important to understand the transport properties of conducting polymers. A plot of the logarithmic derivative of conductivity ( W= (Δ In σ/Δ In T)) versus temperature is a simple and sensitive technique to characterize the extent of disorder, and to identify the metallic (M), critical (C) and insulating (I) regimes. A typical example is shown in the case of I-(CH) x samples. Another important parameter that determines the M-I transition in conducting polymers is the inter-chain interaction. Pressure and magnetic field can tune the relative position of the mobility edge with respect to the Fermi level. The localization-interaction model is observed to be satisfactory in the metallic regime. A power-law behavior of the temperature dependence of conductivity is observed in the critical regime. The positive temperature coefficient of resistivity (TCR), at low temperatures, in oriented I-(CH) x samples is sensitive to the direction of field with respect to the chain axis. The anisotropic magnetoconductance (MC) indicates the subtle interplay of weak localization and e-e interaction contributions, and the positive contribution to MC is less when the field is parallel to the chain axis. This indicates that the anisotropic MC probes the microscopic anisotropy in charge transport parameters. The temperature dependences of conductivity and magnetoresistance are important tools to study the M-I transition due to disorder, inter-chain interaction, etc., in conducting polymers.