TUNING THROUGH THE CRITICAL REGIME OF THE METAL-INSULATOR-TRANSITION IN CONDUCTING POLYMERS BY PRESSURE AND MAGNETIC-FIELD

In the critical regime of the disorder-induced metal-insulator (M-I) transition, the temperature dependence of conductivity follows a power law, sigma(T) is-proportional-to T(beta), and the reduced activation energy function, W = -T{DELTA(ln rho)/DELTAT}, is temperature independent (W = beta). We have observed transport in the critical regime for four conducting polymer systems: potassium-doped polyacetylene (K-(CH)x), iodine-doped polyacetylene (I-(CH)x), phosphorous hexafluoride-doped polypyrrole (PPy-PF6) and camphor sulfonic acid-doped polyaniline (PANI-CSA). For oriented polyacetylene doped with either potassium or iodine and for PPy-PF6, W is temperature independent over a wide temperature range at ambient pressure; while, at high pressures (8-10 kbar), W has a positive temperature coefficient, indicating a pressure-induced crossover to the metallic regime. The enhanced interchain transport at high pressures causes the crossover from the critical regime to metallic behaviour. Application of a magnetic field (8 T) leads to a negative temperature coefficient of W for K-(CH)x, PPy-PF6 and PANI-CSA, indicating a crossover from the critical regime to the insulating regime. Magnetic field-induced localization causes the crossover from the critical regime to insulating behavior. Thus, the electrical properties of conducting polymers can be tuned through the disorder-induced critical regime of the M-I transition into the metallic or insulating regimes by pressure and magnetic field, respectively.