THE TEMPERATURE-DEPENDENCE OF THE CONDUCTIVITY IN THE CRITICAL REGIMEOF THE METAL-INSULATOR-TRANSITION IN CONDUCTING POLYMERS

The metal-insulator (M-I) transition in conducting polymers is particu larly interesting; critical behaviour has been observed over a relativ ely wide temperature range in a number of systems, including polyacety lene (CH)(x), polypyrrole (PPy), poly(p-phenylenevinylene) (PPV), and polyaniline (PANI). In each case, the metallic, critical, and insulati ng regimes near the M-I transition have been identified from plots of W = (Delta ln sigma/Delta ln T) versus T: in the metallic and insulati ng regimes W(T) exhibits positive and negative temperature coefficient s, respectively, while in the critical regime W(T) is temperature inde pendent. Thus, in the critical regime, the conductivity follows a powe r law, sigma(T) = AT(beta), where beta is the critical exponent which is predicted by scaling theory to be between one-third and unity. In c onducting polymers, the critical regime is easily tunable by varying t he extent of disorder, or by applying external pressure and/or magneti c fields. The transitions from metallic to critical behaviour and from critical to insulating behaviour have been induced with a magnetic fi eld, and that from insulating to critical and then to metallic behavio ur with increasing external pressure. The detailed evolution of sigma( T) in the critical regime at low temperatures can be observed in W(T) plots as the system is changed from metal to insulator. Although W(T) is temperature independent for a wide range of temperatures below 50 K , beta systematically increases from values less than one-third on the metallic side towards unity as the system is moved toward the insulat ing side.