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

TL;DR

This paper develops a theoretical framework for ferroelectricity in organic conductors, explaining experimental observations of charge disproportionation, solitons, and optical properties through a combined Mott-Hubbard state and electron-phonon interactions.

Contribution

It introduces a comprehensive theory linking ferroelectric anomalies, charge disproportionation, and soliton excitations in (TMTTF)2X compounds, unifying optical and physical phenomena.

Findings

Identification of three types of solitons:  solitons,  solitons, and spin-charge solitons.

Explanation of optical gap and ferroelectric response via soliton pairs and collective modes.

Prediction of hidden ferroelectricity and charge disproportionation in related compounds.

Abstract

Recently the ferroelectric anomaly (Nad, Monceau, et al) followed by the charge disproportionation (Brown, et al) have been discovered in (TMTTF)2X compounds. The corresponding theory of the combined Mott-Hubbard state describes both effects by interference of the build-in nonequivalence of bonds and the spontaneous one of sites. The state gives rise to three types of solitons: \pi solitons (holons) are observed via the activation energy \Delta in the conductivity $G$; noninteger \alpha solitons (the FE domain walls) provide the frequency dispersion of the ferroelectric response; combined spin-charge solitons determine G(T) below subsequent structural transitions of the tetramerisation. The photoconductivity gap 2\Delta is determined by creations of soliton - antisoliton pairs. The optical edge lies well below, given by the collective ferroelectric mode which coexists with the combined electron-phonon resonance and the phonon antiresonance. The charge disproportionation and the ferroelectricity can exist hiddenly even in the Se subfamily giving rise to the unexplained yet low frequency optical peak, the enhanced pseudogap and traces of phonons activation.