Crossover to negative dielectric response in the low-frequency spectra of metallic polymers

We report high-precision reflectance, $R\left(\omega \right),$ measurements on films of conducting polypyrrole oriented by tensile drawing. The results obtained from Kramers-Kronig analysis of $R\left(\omega \right)$ demonstrate that the optical conductivity, σ(ω), becomes frequency-independent below approximately 250 ${\mathrm{cm}}^{\mathrm{-}1}$ and that there is a zero crossing in the dielectric function, ɛ(ω), to negative values below 20 ${\mathrm{cm}}^{\mathrm{-}1}$. Since σ(ω) extrapolates to the dc value and since ɛ(ω) is in excellent agreement with the data obtained from terahertz and microwave measurements, the controversy in the dielectric response of the metal physics of conducting polymers is finally resolved.