Theory of the ferroelectric phase in organic conductors: From physics of solitons to optics

Abstract
New phenomenona in (TMTTF) ₂X compounds unify an unusual variety of concepts: ferroelectricity of good conductors, structural instability towards Mott-Hubbard state, Wigner crystallization in a dense electronic system, ordered 4k _F density wave, richness of physics of solitons, interplay of structural and electronic symmetries. The ferroelectric state gives rise to three types of solitons: - solitons (holons) are observed via the activation energy in conductivity  G; noninteger - solitons (ferroelectric domain walls) provide the low frequency dispersion; topologically coupled combined spin-charge solitons determine G(T) below a subsequent structural transition of the tetramerisation. The photoconductivity gap is given by creations of soliton - antisoliton pairs. The lower optical absorption comes from a collective electronic mode which, for the FE case, becomes the electron-phonon resonance combined with the Fano antiresonance. The ferroelectric soft mode evolves from the overdamped response at T ₀. The reduced plasma frequency signifies the slowing down of electrons by acoustic phonons. The effects should exist hiddenly even in the Se subfamily giving rise to the low frequency optical peak, the enhanced pseudogap and traces of phonons' optical activation. Another, interchain, type of the charge disproportionation known in the relaxed (TMTSF) ₂ClO ₄ is still waiting for attention, possibly being hiddenly present in other superconducting cases. Key words. Ferroelectricity, charge disproportionation, solitons, optical absorption, conductivity, permittivity.