Crossover in Electrical Frequency Response through an Insulator-Metal Transition

The insulator-metal transition (IMT) for a model quasi-one-dimensional (quasi-1D) conducting polymer (polyaniline) is probed at room temperature (RT) over an unusually broad frequency range (2 meV–6 eV) and also via $T$-dependent dc conductivity ( ${\sigma }_{\mathrm{dc}}$). We determine that the IMT is not monotonic with increasing ${\sigma }_{\mathrm{dc}}\left(\mathrm{RT}\right)$. The RT far infrared scattering time ( $\tau$) becomes unusually long ( $\ge {10}^{-13\mathrm{}}\mathrm{s}$) as ${\sigma }_{\mathrm{dc}}\left(\mathrm{RT}\right)$ increases, even for samples on the insulating side of the IMT. We conclude that the IMT is due to percolation in the presence of inhomogeneous disorder and quasi-1D localization rather than 3D Anderson localization.