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

The temperature coefficient of the resistivity (TCR) of doped polypyrr ole changes sign below 25 K, from negative to positive, in samples wit h relatively weak disorder (as inferred from the resistivity ratio, rh o(r) = rho(1.4 K)/rho(300 K)). We have studied the crossover from nega tive to positive TCR as a function of the disorder, and as a function of pressure and magnetic field. At ambient pressure, the sign change i s 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 an d 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 m agnetic field of 8 T suppresses the TCR transition. This anomalous tem perature dependence of the low temperature electrical transport is dis cussed in terms of correlation corrections to the conductivity in the disordered metallic regime near the metal-insulator transition.