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

We report high-precision reflectance, R(ω), measurements on films of conducting polypyrrole oriented by tensile drawing. The results obtained from Kramers-Kronig analysis of R(ω) demonstrate that the optical conductivity, {σ}({ω}), becomes frequency-independent below approximately 250 cm^-1 and that there is a zero crossing in the dielectric function, {ε}({ω}), to negative values below 20 cm^-1. Since {σ}({ω}) extrapolates to the dc value and since {ε}({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.