Electronic modulation of vibrational frequencies: effect on the conductivity and thermopower of a narrow Hubbard band

Abstract

If the electronic bandwidth is small compared to intrasite vibrational frequencies, an electron stays at a site over several vibrational periods and has a chance to modulate the frequency. The effects of the resulting electron-vibration coupling on the properties of a narrow half-filled Hubbard band of electrons are studied. At low temperatures, the principal effect is the alteration of the Mott-Hubbard gap. The high-temperature conductivity falls of as 1/T². The coupling breaks the particle-hole symmetry of the half-filled band and gives rise to a nonzero thermopower. If additional electrons on a site cause vibrational softening, the thermpower increases from zero with increasing temperature, reaches a maximum, decreases, changes sign and saturates. The relevance of these results to the tetracyanoquinodimethane salts is discussed.