Frequency-dependent conductivity ofcis-polyacetylene

The frequency (f) and temperature (T) dependence of the conductivity (σ) of cis-(CH${)}_{\mathrm{x}}$ is reported. The data differ qualitatively from those of both trans-(CH${)}_{\mathrm{x}}$ and most insulating polymers, with ${\sigma }_{\mathrm{ac}}$ and ${\sigma }_{\mathrm{dc}\mathrm{\propto }\exp (\mathrm{-}{E}_a/k_B\mathit{T})}$, $E_a$=0.12 eV, and ${\sigma }_{\mathrm{ac}\mathrm{\propto }{f}^{1.0}}$. These unusual results imply thermal activation of holes from a defect level to the disordered band edge, and subsequent hopping among localized band-edge states. Comparison with earlier data suggests that the impurity level is the doubly positive bipolaron state of cis-(CH${)}_{\mathrm{x}}$.