TRANSPORT NEAR THE METAL-INSULATOR-TRANSITION - POLYPYRROLE DOPED WITH PF(6)

Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a metal-insulator (M-I) transition at resistivity ratio rho(r) = rho(1.4 K)/rho(r)(300 K) almost-equal-to 10: for rho(r) < 10 the system is metallic with rho(T) remaining finite as T --> 0, whereas for rho(r) > 10, the system is an insulator with rho --> infinity as T --> 0. In the critical regime, rho(T) shows a power-law temperature dependence, rho(T) = T(-beta), with 0.3 < beta < 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 rho(r) almost-equal-to 2. At T = 1.4 K, the interaction length L(T) = (HBARD/k(B)T1/2 almost-equal-to 30 angstrom. Since this is smaller than the inelastic-scattering length, L(in) almost-equal-to 300 angstrom, the contribution to rho(T) from the electron-electron interaction is dominant. Application of high pressure decreases rho(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 (L(c)) increases as the disorder, characterized by rho(r), approaches the M-I transition from either side. The expected divergence in L(c) at the M-I transition is qualitatively consistent with the values for L(c) inferred from the extrapolated sigma(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using rho(r) to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(PF6) is developed.