Metal-insulator transition in oriented poly(p-phenylenevinylene)

The transport properties of H2SO4-doped, tensile drawn, and oriented poly(phenylenevinylene) have been studied in the metallic, critical, and insulating regimes of the disorder-induced metal-insulator transition (M-I) transition. The temperature dependence of the conductivity, sigma(T) and the magnetoconductance (MC) were investigated between room temperature and 1.3 K and in magnetic fields up to 8 T, in freshly doped samples and in samples during controlled dedoping (aging). A complete set of measurements were carried out on a single, fully doped sample that was followed during ageing from the metallic state through the critical regime into the insulting state. The transport properties are characterized as a function of the resistivity ratio (rho(r)), where rho(r) = [rho(1.3 K)/rho(200 K)]. In the metallic regime (rho(r) < 2), sigma(parallel to) (300 K) congruent to 10 000 S/cm and sigma(perpendicular to) (300 K) congruent to 100 S/cm; for T < 4 K, a T-1/2 dependence is observed for sigma(T), and the MC shows positive and negative contributions at low and high fields, respectively. The positive contribution to the MC vanishes at the M-I transition boundary (rho(r) congruent to 2). The behaviors of sigma(T) and the MC are consistent with the weak localization plus electron-electron interaction model. Very near the M-I transition, a field-induced transition from the metallic to the critical regime was observed {sigma(T) proportional to T-0.1 at 8 T}. For samples in the critical regime with 4 < rho(r) < 30, sigma(T) proportional to T-beta at low temperatures. In the insulating state (rho(r) > 50), rho(T) proportional to exp(T-0/T)(x) indicating variable-range-hopping transport. Although anisotropic, the field and temperature dependences of the transport are similar both parallel and perpendicular to the chain axis, implying that oriented conducting polymers are anisotropic three-dimensional conductors.