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

The transport properties of ${\mathrm{H}}_2$ ${\mathrm{SO}}_4$ -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, σ(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 (${\mathrm{\rho }}_{\mathrm{r}}$), where ${\mathrm{\rho }}_{\mathrm{r}}$=[ρ(1.3 K)/ρ(200 K)]. In the metallic regime (${\mathrm{\rho }}_{\mathrm{r}}$${\mathrm{\sigma }}_{\mathrm{\parallel }}$ (300 K)≅10 000 S/cm and ${\mathrm{\sigma }}_{\mathrm{\perp }}$ (300 K)≅100 S/cm; for T${\mathrm{T}}^{1\mathrm{/}2}$ dependence is observed for σ(T), and the MC shows positive and negative contributions at low and high fields, respectively. The positive contribution to the MC vanishes at the M-I transition boundary (${\mathrm{\rho }}_{\mathrm{r}}$≅2). The behaviors of σ(T)and the MC are consistent with the weak localization plus electron-electron interaction model. Very near the M-I transition, a field-induced transition from the metallic to the critical regime was observed {σ(T)∝${\mathrm{T}}^{0.1}$ at 8 T}. For samples in the critical regime with 4${\mathrm{\rho }}_{\mathrm{r}}$${\mathrm{T}}^{\mathrm{{\rm B}}}$ at low temperatures. In the insulating state (${\mathrm{\rho }}_{\mathrm{r}}$>50), ρ(T)∝exp(${\mathrm{T}}_0$/T${)}^{\mathrm{x}}$ 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.