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 = T{ Δ(ln ϱ)/ ΔT}, is temperature independent ( W = β). We have observed transport in the critical regime for four conducting polymer systems: potassium-doped polyacetylene (C-CH) x ), iodine-doped polyacetylene (C-(CH) x ), phosphorous hexafluoride-doped polypyrrole (PPy-PF 6) and camphor sulfonic acid-doped polyaniline (PANI-CSA). For oriented polyacetylene doped with either potassium or iodine and for PPy-PF 6, 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-PF 6 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.