Electronic transport in the metallic state of oriented poly(p-phenylenevinylene)

Abstract

The room-temperature electrical conductivity of tensile-drawn and oriented poly(p-phenylenevinylene), PPV, doped with sulfuric acid (${\mathrm{H}}_2$ ${\mathrm{SO}}_4$) is approximately ${10}^4$ S/cm for current along the draw direction; the anisotropy ${\mathrm{\sigma }}_{\mathrm{\parallel }}$/${\mathrm{\sigma }}_{\mathrm{\perp }}$≊100 where ${\mathrm{\sigma }}_{\mathrm{\parallel }}$ and ${\mathrm{\sigma }}_{\mathrm{\perp }}$ refer to the conductivity parallel to and perpendicular to the axis of orientation. The resistivity, ρ(T), is nearly temperature independent with a weak negative temperature coefficient, ${\mathrm{\rho }}_{\mathit{r}}$≡ρ(1.3 K)/ρ(200 K)≊1.07–1.3. A positive temperature coefficient (resistivity) appears below 20 K. The magnetoconductance (MC) is anisotropic and dependent on the direction of the applied magnetic field with respect to the chain axis. When the field is perpendicular to the chain axis, the MC is positive and nearly independent of temperature at low fields; at high fields, the MC gradually decreases as the temperature is lowered from 4.2 to 1.3 K. When the field is parallel to the chain axis, the MC is negative. The MC is nearly identical, however, when the current direction is parallel and perpendicular to the chain axis. The temperature dependence of the conductivity and the rich interplay of positive and negative MC arises from the importance of weak localization (WL) and electron-electron (e-e) interactions. Specifically, the anisotropy of the MC is shown to result from the anisotropy of the WL contribution. The contributions from WL and e-e interactions were verified from [Δσ(H,T)/${\mathit{T}}^{1/2}$] vs (H/T) plots. © 1996 The American Physical Society.