Transport near the metal-insulator transition: Polypyrrole doped withPF6

Heavily doped polypyrrole-hexafluorophosphate, PPy(${\mathrm{PF}}_6$), undergoes a metal-insulator (M-I) transition at resistivity ratio ${\mathrm{\rho }}_{\mathit{r}}$=ρ(1.4 K)/${\mathrm{\rho }}_{\mathit{r}}$(300 K)≊10: for ${\mathrm{\rho }}_{\mathit{r}}$<10, the system is metallic with ρ(T) remaining finite as T→0, whereas for ${\mathrm{\rho }}_{\mathit{r}}$>10, the system is an insulator with ρ→∞ as T→0. In the critical regime, ρ(T) shows a power-law temperature dependence, ρ(T)=${\mathit{T}}^{\mathrm{-}\mathrm{\beta }}$, with 0.3<β<1. The effect of the partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the crossover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopping is observed. In the metallic regime, the sign of the temperature coefficient of the resistivity changes at ${\mathrm{\rho }}_{\mathit{r}}$≊2. At T=1.4 K, the interaction length ${\mathit{L}}_{\mathit{T}}$=(ħD/${\mathit{k}}_{\mathit{B}}$T${)}^{1/2}$≊30 Å.