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) t ransition, the temperature dependence of conductivity follows a power law, sigma(T) is-proportional-to T(beta), and the reduced activation e nergy function, W = -T{DELTA(ln rho)/DELTAT}, is temperature independe nt (W = beta). We have observed transport in the critical regime for f our conducting polymer systems: potassium-doped polyacetylene (K-(CH)x ), iodine-doped polyacetylene (I-(CH)x), phosphorous hexafluoride-dope d polypyrrole (PPy-PF6) and camphor sulfonic acid-doped polyaniline (P ANI-CSA). For oriented polyacetylene doped with either potassium or io dine and for PPy-PF6, W is temperature independent over a wide tempera ture range at ambient pressure; while, at high pressures (8-10 kbar), W has a positive temperature coefficient, indicating a pressure-induce d crossover to the metallic regime. The enhanced interchain transport at high pressures causes the crossover from the critical regime to met allic behaviour. Application of a magnetic field (8 T) leads to a nega tive temperature coefficient of W for K-(CH)x, PPy-PF6 and PANI-CSA, i ndicating a crossover from the critical regime to the insulating regim e. Magnetic field-induced localization causes the crossover from the c ritical regime to insulating behavior. Thus, the electrical properties of conducting polymers can be tuned through the disorder-induced crit ical regime of the M-I transition into the metallic or insulating regi mes by pressure and magnetic field, respectively.