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 σσ but also weakens the disorder, which affects the VRH mechanism. This is revealed in the σσ vs temperature (T  ) characteristics σ∝exp⁡[−(T0/T)α]σ∝exp⁡[−(T0/T)α], where αα and T0T0 carry information on the underlying physics. Below ∼∼200 K, αα undergoes a significant disorder dependent change at ∼∼60% of the full doping: α=1/2α=1/2 for lower doping, whereas more-doped samples exhibit α=2/5α=2/5. From ∼∼200 K up to room temperature, most of the samples (i.e., those doped below 80%) exhibit α=1/4α=1/4 regardless of the corresponding αα 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=1/2,2/5,1/4α=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 T0T0 for a given α=1/2α=1/2 or 2/52/5 also follows the FTS predictions reasonably well. The appearance of α=1/4α=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.