Reflectance spectra of polypyrrole (PPy) doped with ${\mathrm{PF}}_6,$ obtained at 250 K and below, show the formation of a peak in the joint density of states at 110 ${\mathrm{cm}}^{\mathrm{-}1}$ and an associated redistribution of oscillator strength. In addition, a Fano-like resonance is observed at 70 ${\mathrm{cm}}^{\mathrm{-}1}$. We discuss the results in terms of a transition to a density-wave ground state characterized by an energy gap $(2\mathrm{}\Delta /hc\approx 100{\mathrm{cm}}^{\mathrm{-}1})$ and a pinned collective mode (at 70 ${\mathrm{cm}}^{\mathrm{-}1}$). Despite the energy gap, ${\mathrm{P}\mathrm{P}\mathrm{y}-\mathrm{P}\mathrm{F}}_6$ remains “metallic” at low temperatures; i.e., the gap spans only a part of the Fermi surface.

• Received 23 August 1999

DOI:https://doi.org/10.1103/PhysRevB.61.1635