Effect of anisotropy on conductivity and magnetoconductance in heavily doped polyacetylene

The conductivity of iodine doped Shirakawa polyacetylene films with various stretching ratio (ll0 = 1∼6) was measured as a function of temperature, pressure and magnetic field. Unoriented samples show a power-law temperature dependence of the conductivity, with the characteristic resistivity ratio ρI ≡ ρ(1.4K)/ρ(300K) ∼ 80, and a positive magnetoreisitance at low temperature. Although the conductivity anisotropy (σ∥σ⊥) increases with stretching up to σ∥σ⊥ ∼ 110 for ll0 = 6, the temperature dependence of the conductivity (ρr = 3.0 ± 0.3) for stretched sample is nearly independent on the direction and the ratio of stretching. The negative magnetoresistance for the stretched sample indicates that weak localization contribution to the conductivity is dominant at low temperature. The anisotropy of magnetoconductance is close to the conductivity anisotropy implying that the bulk anisotropy originates from partially aligned fibrillar morphology. Application of high pressure decreases ρr and changes the dominant dephasing mechanism of weak localization from the inelastic electron-electron scattering in the dirty limit to that in clean limit.