Density-wave charge dynamics in conducting polypyrrole

Reflectance spectra of polypyrrole (PPy) doped with PF6, obtained at 250 K and below, show the formation of a peak in the joint density of states at 1 10 cm(-1) and an associated redistribution of oscillator strength. In addit ion, a Fano-like resonance is observed at 70 cm(-1). We discuss the results in terms of a transition to a density-wave ground state characterized by a n energy gap (2 Delta/hc approximate to 100 cm(-1)) and a pinned collective mode (at 70 cm(-1)). Despite the energy gap, PPy-PF6 remains "metallic" at low temperatures; i.e., the gap spans only a part of the Fermi surface.