Variable-range-hopping exponents 1/2, 2/5 and 1/4 in HCl-doped polyaniline pellets

We study the variable-range-hopping (VRH) charge transport in polyaniline pellets doped with HCl over a wide range. Increase of the doping not only enhances the electrical conductivity σ but also weakens the disorder, which affects the VRH mechanism. This is revealed in the σ vs temperature (T) characteristics σ ∝ exp ⁡ [ − ( T 0 / T ) α ] , where α and T 0 carry information on the underlying physics. Below ∼ 200 K, α undergoes a significant disorder dependent change at ∼ 60% of the full doping: α = 1 / 2 for lower doping, whereas more-doped samples exhibit α = 2 / 5 . From ∼ 200 K up to room temperature, most of the samples (i.e., those doped below 80%) exhibit α = 1 / 4 regardless of the corresponding α at low T. These results can be to a large extent explained by the VRH theory of Fogler et al. [M.M. Fogler, S. Teber, B.I. Shklovskii, Phys. Rev. B 69 (2004) 035413] (FTS). In particular, α = 1 / 2 , 2 / 5 , 1 / 4 are therein anticipated for three-dimensional transport together with crossovers between these values in response to changes in disorder and/or T. The doping (disorder) dependence of T 0 for a given α = 1 / 2 or 2 / 5 also follows the FTS predictions reasonably well. The appearance of α = 1 / 4 at high T can be understood from an analysis of energy scales that are relevant to VRH as described by the FTS model.