engleski

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

We present a study of 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 $\sigma$ but also weakens the disorder, which affects the VRH mechanism. This is revealed in the $\sigma$ vs temperature ($T$) characteristics $\sigma \propto \exp [ - (T_0/T)^{; ; ; ; \alpha}; ; ; ; ]$, where $\alpha$ and $T_0$ carry information on the underlying physics. Below $T \sim 200$ K, $\alpha$ undergoes a significant disorder-dependent change at $\sim 60$ \% of the full doping: $\alpha = 1/2$ for lower doping, whereas more-doped samples exhibit $\alpha = 2/5$. From $T \sim 200$ K up to room temperature, most of the samples (i.e., those doped below 80 \%) exhibit $\alpha = 1/4$ regardless of the corresponding $\alpha$ at low $T$. These results can be to a large extent explained by the VRH theory of Fogler, Teber and Shklovskii (FTS) [Phys. Rev. B {; ; ; ; \bf 69}; ; ; ; (2004), p. 035413]. In particular, $\alpha = 1/2, 2/5, 1/4$ are therein predicted for three dimensions, 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 $\alpha=1/2$ or $2/5$ also follows the FTS predictions reasonably well. The appearance of $\alpha=1/4$ at high $T$ can be understood from an analysis of the energy scales relevant to VRH as described by the FTS model.

polymers; organic compounds; physical properties of polymers

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