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 temperaure dependence of conductivity follows a power l aw, sigma(T) proportional to T--beta, and the reduced activation energ y function, W=-d(ln rho)/d(lnT), is temperature independent (W=beta). We have observed transport in the critical regime for four conducting polymer systems: potassium doped polyacetylene (CH)(X), iodine eloped polyacetylene, phosphorous hexafluoride (PF6) doped polypyrrole (PPy) and camphor sulfonic acid (CSA) doped polyaniline (PANI). For both ori ented polyacetylene doped with either potassium or iodine and PPy-PF6, W is temperature independent in a wide range of temperature at ambien t pressure; while at high pressures (8-10 kbar), W has a positive temp erature coefficient indicating a pressure-induced crossover to the met allic regime. The enhanced interchain transport at high pressures caus es the crossover from the critical regime to metallic behavior. Applic ation of a magnetic field (8 Tesla) leads to a negative temperature co efficient of W for K-(CH)(X), PPy-PF6 and PANI-CSA, indicating a cross over 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 po lymers can be tuned through the disorder-induced critical regime of M- I transition into the metallic or insulating regimes by pressure and m agnetic field, respectively.