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

Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a m etal-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 met allic 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 screen ed Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the cro ssover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopp ing is observed. In the metallic regime, the sign of the temperature c oefficient 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 le ngth, L(in) almost-equal-to 300 angstrom, the contribution to rho(T) f rom the electron-electron interaction is dominant. Application of high pressure decreases rho(r), induces the transition into the metallic r egime, and enables fine tuning of the M-I transition. For samples clos e to the M-I transition, the thermoelectric power is proportional to t he temperature in both the metallic and insulating regimes. The correl ation length (L(c)) increases as the disorder, characterized by rho(r) , approaches the M-I transition from either side. The expected diverge nce 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 metall ic regime and from analysis of the VRH magnetoresistance in the insula ting regime. Thus, by using rho(r) to characterize the magnitude of th e disorder, a complete and fully consistent picture of the M-I transit ion in PPy(PF6) is developed.