Transport near the metal-insulator transition: Polypyrrole doped with PF6.

Abstract

Heavily doped polypyrrole-hexafluorophosphate, PPy(${\mathrm{PF}}_{6}$), undergoes a metal-insulator (M-I) transition at resistivity ratio ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$=\ensuremath{\rho}(1.4 K)/${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$(300 K)\ensuremath{\approxeq}10: for ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$10, the system is metallic with \ensuremath{\rho}(T) remaining finite as T\ensuremath{\rightarrow}0, whereas for ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$>10, the system is an insulator with \ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\infty} as T\ensuremath{\rightarrow}0. In the critical regime, \ensuremath{\rho}(T) shows a power-law temperature dependence, \ensuremath{\rho}(T)=${\mathit{T}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\beta}}}$, with 0.31. 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 ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$\ensuremath{\approxeq}2. At T=1.4 K, the interaction length ${\mathit{L}}_{\mathit{T}}$=(\ensuremath{\Elzxh}D/${\mathit{k}}_{\mathit{B}}$T${)}^{1/2}$\ensuremath{\approxeq}30 \AA{}.Since this is smaller than the inelastic-scattering length, ${\mathit{L}}_{\mathrm{in}\mathrm{\ensuremath{\approxeq}}}$300 \AA{}, the contribution to \ensuremath{\rho}(T) from the electron-electron interaction is dominant. Application of high pressure decreases ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$, induces the transition into the metallic regime, and enables fine tuning of the M-I transition. For samples close to the M-I transition, the thermoelectric power is proportional to the temperature in both the metallic and insulating regimes. The correlation length (${\mathit{L}}_{\mathit{c}}$) increases as the disorder, characterized by ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$, approaches the M-I transition from either side. The expected divergence in ${\mathit{L}}_{\mathit{c}}$ at the M-I transition is qualitatively consistent with the values for ${\mathit{L}}_{\mathit{c}}$ inferred from the extrapolated \ensuremath{\sigma}(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using ${\mathrm{\ensuremath{\rho}}}_{\mathit{r}}$ to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(${\mathrm{PF}}_{6}$) is developed.