Limits for Metallic Conductivity in Conducting Polymers

The temperature ( $T$) dependent dc conductivity ( ${\sigma }_{\mathrm{DC}}$) (down to 20 mK) and dielectric function at optical frequencies (0.002–6 eV) and 6.5 GHz are used to probe the inhomogeneous disorder-driven insulator-metal transition in conducting polymers. A correlation between large low $T$ ${\sigma }_{\mathrm{DC}}$ and the presence to low $T$ of a small fraction of the carrier density delocalized with long transport times ( $>{10}^{-13\mathrm{}}\mathrm{s}$) indicates that metallic ${\sigma }_{\mathrm{DC}}$ is due to only a small fraction of the charge carriers. The achievable ${\sigma }_{\mathrm{DC}}$ for these systems when the entire charge carrier density participates is estimated to surpass that of copper.