Crossover in Electrical Frequency Response through an Insulator-Metal Transition

Abstract

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 ( ${\ensuremath{\sigma}}_{\mathrm{dc}}$). We determine that the IMT is not monotonic with increasing ${\ensuremath{\sigma}}_{\mathrm{dc}}(\mathrm{RT})$. The RT far infrared scattering time ( $\ensuremath{\tau}$) becomes unusually long ( $\ensuremath{\ge}{10}^{\ensuremath{-}13}\mathrm{s}$) as ${\ensuremath{\sigma}}_{\mathrm{dc}}(\mathrm{RT})$ 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.