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

The transport properties of H₂ SO₄ -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 (ρᵣ), where ρᵣ=[{ρ}(1.3 K)/{ρ}(200 K)]. In the metallic regime (ρᵣ2), σ_∥ (300 K){}10 000 S/cm and σ_⊥ (300 K){}100 S/cm; for T4 K, a T1/2 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 (ρᵣ{}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){∝}T0.1 at 8 T}. For samples in the critical regime with 4ρᵣ30, {σ}(T){∝}T^B at low temperatures. In the insulating state (ρᵣ>50), {ρ}(T){∝}exp(T₀/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.