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

Abstract

Recently the ferroelectric anomaly (Nad, Monceau, et al) followed by the charge disproportionation (Brown, et al) have been discovered in (T M T T F ) 2 X 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: π− solitons (holons) are observed via the activation energy ∆ in the conductivity G; noninteger α− 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∆ 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.

Theoretical Framework for Quasi-One Dimensional Systems

“…Note that because of the anions other transitions can exist such as a ferroelectric transition. 122,123 These results are in good agreement with other measurements. The measured values of K F are con-sistent with the photoemission data.…”

“…In the TMTTF family, dimerization is larger, and it is unclear there which process is dominant. Note that because of the anions other transitions can exist such as a ferroelectric transition. , …”

Conduction anisotropy and Hall effect in the organic conductor(TMTTF)2AsF6: Evidence for Luttinger liquid behavior and charge ordering

“…It was also suggested that the FE transition is triggered by the uniform shift of ions yielding a macroscopic FE polarization which is gigantically amplified by the charge disproportionation at the molecular stack. 35 The fact that the increase in ∆ is anisotropic or somehow higher along the b ′ and c ⋆ axes than along the a axis may further support this suggestion.…”

Structural and Electronic Properties of (TMTTF)2X Salts with Tetrahedral Anions

“…The Curie-behavior has drawn a lot of attention [84,85,[87][88][89][90] because it is characteristic for ferroelectricity. Its origin is considered to be mainly electronic, and thus referred to as "electronic ferroelectricity" [33,[91][92][93]. Nevertheless, the observation of a hysteresis in the electric polarization P(E) is impeded by the high conductivity of the (TMTTF) 2 X salts compared with canonical ferroelectrics.…”