PRESSURE AND MAGNETIC-FIELD DEPENDENCE OF THE LOW-TEMPERATURE RESISTIVITY OF PF6-DOPED POLYPYRROLE

The temperature coefficient of the resistivity (TCR) of doped polypyrrole changes sign below 25 K, from negative to positive, in samples with relatively weak disorder (as inferred from the resistivity ratio, rho(r) = rho(1.4 K)/rho(300 K)). We have studied the crossover from negative to positive TCR as a function of the disorder, and as a function of pressure and magnetic field. At ambient pressure, the sign change is systematically observed only for samples having rho(r) less-than-or-equal-to 2. For example, when rho(r) decreases from 1.97 to 1.33, the temperature (T1) defining the transition from negative to positive TCR increases from 7.5 to 24 K. Pressure enhances interchain transport and thereby decreases rho(r). For samples having rho(r) almost-equal-to 2-10, the TCR transition can be induced by applying high pressure. A magnetic field of 8 T suppresses the TCR transition. This anomalous temperature dependence of the low temperature electrical transport is discussed in terms of correlation corrections to the conductivity in the disordered metallic regime near the metal-insulator transition.