Characteristic temperature dependence of resistivity in PPy(PF 6) near the metal-insulator transition

Abstract

We present a systematic method to characterize transport properties of doped conducting polymers near the metal-insulator ( M-I) transition in terms of the resistivity ratio ρ ≡ ρ(1.4K)/gr(300K). Heavily doped polypyrrole-hexafluorophosphate, PPy(PF 6), undergoes a M-I transition at ρ r ∼ 10. For ρ r < 10, the system is metallic with ρ remaning finite as T → 0, whereas for ϱ r > 10, the system is an insulator with ρ→ ∞ as T → 0. In the critical regime, ρ(T) shows a power law dependence, ρ(T) ∞ T-β, with 0.3 < β < 1. The effect of partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the sign of the temperature coefficient of resistivity changes at ρ r = 2. In the insulating regime, the crossover from Mott to Efros-Shklovskii variable range hopping (VRH) is observed. The correlation length (L c) inferred from the extrapolated (0) in the metallic regime and from analysis of the VRH in the insulating regime increases as ρ r approaches M-I transition from either side.