Tuning through the critical regime of the metal-insulator transition in conducting polymers by pressure and magnetic field

Abstract

In the critical regime of the disorder-induced metal insulator (M-I) transition, the temperature dependence of conductivity follows a power law, σ(T) ∝ T-β, and the reduced activation energy function, W=−d(lnρ)/d(lnT), is temperature independent (W=β). We have observed transport in the critical regime for four conducting polymer systems: potassium doped polyacetylene (CH)_X, iodine doped polyacetylene, phosphorous hexafluoride (PF₆) doped polypyrrole (PPy) and camphor sulfonic acid (CSA) dope polyaniline (PANI). For both oriented polyacetylene doped with either potassium or iodine and PPy-PF₆, W is temperature independent in a wide range of temperature 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 behavior. Application of a magnetic field (8 Tesla) 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 M-I transition into the metallic or insulating regimes by pressure and magnetic field, respectively.