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Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a metal-insulator (M-I) transition at resistivity ratio p, = p(1.4 K)/p"(300 K) = The effect of the partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the crossover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopping is observed. In the metallic regime, the sign of the temperature coeScient of 0 the resistivity changes at p, =2. At T=1. 4 K, the interaction length L&=(AD/k&T)' =30 A. Since this is smaller than the inelastic-scattering length, L;"=300 A, the contribution to p(T) from the electron-electron interaction is dominant. Application of high pressure decreases p" induces the transition into the metallic regime, and enables fine tuning of the M-I transition. For samples close to the M-I transition, the thermoelectric power is proportional to the temperature in both the metallic and insulating regimes. The correlation length (L, ) increases as the disorder, characterized by p", approaches the M-I transition from either side. The expected divergence in L, at the M-I transition is qualitatively consistent with the values for L, inferred from the extrapolated cr(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using p" to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(PF6) is developed. I. INTRODUCirONRecent studies of doped polypyrrole (PPy), polymerized electrochemically at relatively low temperatures ( -20'C to -30'C), have shown that the roomtemperature conductivity oar=200 -500 S/cm, increasing to approximately 1000 S/cm after tensile drawing. 'A positive temperature coefficient of the resistivity (TCR) was' reported for temperatures below T =10-20 K for PFs-doped PPy, PPy(PFs). ' To date, however, the physical aspects of these phenomena as related to the metallic nature of heavily doped conjugated polymers have not been clearly understood. Models suggested earlier, ' such as the electron-hopping (or tunneling) conduction, small-polaron tunneling, local superconductivity, etc. seem to be either inappropriate or incapable of describing the entire range for the data (i.e. , in both metallic and insulating samples.Many of the properties that characterize heavily doped conducting polymers, such as relatively high electrical conductivity, temperature independent magnetic susceptibility, linear temperature dependence of thermoelectric power, ' '" absorption throughout the infrared with no energy gap, ' etc. , suggest that the electronic structure is that of a metal. However, the disorder generated during synthesis and during the doping process plays a critical role; microscopic disorder and/or structurally amorphous regions can dominate the transport.%'e present the results of a systematic study of the transport properties of PPy(PFs) near the disorderinduced metal-to-insulator (M I) transition. The ex-tent of di...

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The metal-insulator (M-I) transition in conducting polymers is particularly interesting; critical behaviour has been observed over a relatively wide temperature range in a number of systems, including polyacetylene (CH) x , polypyrrole (PPy), poly(pphenylenevinylene) (PPV), and polyaniline (PANI). In each case, the metallic, critical, and insulating regimes near the M-I transition have been identified from plots of W = ( ln σ/ ln T ) versus T : in the metallic and insulating regimes W (T ) exhibits positive and negative temperature coefficients, respectively, while in the critical regime W (T ) is temperature independent. Thus, in the critical regime, the conductivity follows a power law, σ (T ) = AT β , where β is the critical exponent which is predicted by scaling theory to be between one-third and unity. In conducting polymers, the critical regime is easily tunable by varying the extent of disorder, or by applying external pressure and/or magnetic fields. The transitions from metallic to critical behaviour and from critical to insulating behaviour have been induced with a magnetic field, and that from insulating to critical and then to metallic behaviour with increasing external pressure. The detailed evolution of σ (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, β systematically increases from values less than one-third on the metallic side towards unity as the system is moved toward the insulating side.