CHARACTERISTIC TEMPERATURE-DEPENDENCE OF RESISTIVITY IN PPY(PF6) NEARTHE METAL-INSULATOR-TRANSITION

We present a systematic method to characterize transport properties of doped conducting polymers near the metal-insulator (M-I) transition i n terms of the resistivity ratio rho(r) = rho(1.4)/rho(300K). Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a M-I trans ition at rho(r) similar to 10. For rho(r) < 10, the system is metallic with rho remaining finite as T --> 0, whereas for rho(r) > 10, the sy stem is an insulator with rho --> infinity as T --> 0. In the critical regime, rho(T) shows a power law dependence, rho(T) proportional to T --beta with 0.3 < beta < 1. The effect of partially screened Coulomb i nteraction is substantial at low temperatures for samples on both side s of the M-I transition. In the insulating regime, the sign of the tem perature coefficient of resistivity changes at rho(r) = 2. In the insu lating regime, the crossover from Mott to Efros-Shldovskii variable ra nge hopping (VRH) is observed. The correlation length (L(c)) inferred from the extrapolated (sic)(0) in the metallic regime and from analysi s of the VRH in the insulating regime increases as rho(r) approaches M -I transition from either side.