Indirect observation of the semiconductor-metal transition in AsF 5 -doped trans -polyacetylene

New experimental results on the temperature dependence of the electrical conductivity of ${\mathrm{AsF}}_{5}$-doped trans-polyacetylene clearly show a transition from a variable-range-hopping conduction mechanism characteristic of a three-dimensional amorphous semiconductor to a high conducting (metallic) state, as a function of the dopant concentration. The temperature behavior of the electrical conductivity for high doping levels has been well fitted with the use of Sheng's model of fluctuation-induced tunneling conduction. The accuracy of the measurements has been shown to be critical in order to be able to choose between those plausible models which are usually evoked. A general model involving both the intrinsic resistance of the fibrils and the resistance of the junctions between these fibrils is proposed. The relative magnitude of these resistances as a function of dopant concentration can explain the observation of a pure ${T}^{\ensuremath{-}\frac{1}{4}}$ law at low concentrations and the observation of a fluctuation-induced tunneling conduction mechanism at the highest doping levels.