The localization-interaction model applied to the direct-current conductivity of metallic conducting polymers

The low-temperature DC-transport properties of doped conducting polymers on the metallic side of the metal-insulator transition are analysed with modern localization-interaction theories. The DC conductivity at the lowest temperatures is governed by the 3D electron-electron interaction formula sigma = sigma(0) + mT(1/2), while the magnetoconductance is an interplay between electron-electron interaction and weak-localization contributions. The transition from negative to positive temperature coefficient of resistivity, below 20 K, can be explained by the sign change in the interaction coefficient m. On the basis of experimental data it is shown that the resistivity ratio rho(r) (rho(r) = rho(T congruent to 1 K)/rho(300 K)) is a controlling factor in determining the sign and magnitude of these effects. Furthermore, the normalization of relevant coefficients (e.g. m) with sigma(0) gives the result that the relative size of the effects is independent of the degree of chain orientation, and therefore the conductivity. However, it is found that the coefficient m dresses from negative to positive values at different values of rho(r), for oriented and non-oriented conducting polymers. The positive magnetoconductance stemming from weak localization is substantial and very anisotropic in highly oriented conducting polymers. It is shown that this positive magnetoconductance exhibits a maximum as a function of rho(r), in the vicinity of the metal-insulator transition. These results are compared with transport in other types of disordered metal.