Theory of Electronic Ferroelectricity

TL;DR

This paper develops a theoretical framework for electronic ferroelectricity in the Falicov-Kimball model, predicting optical properties and symmetry breaking effects due to Coulomb interactions, with proposed experimental tests.

Contribution

It introduces a mean-field theory explaining electronic ferroelectricity and nonlinear optical phenomena arising from Coulomb-induced coherence.

Findings

Prediction of electronic ferroelectricity in the model

Identification of ferroelectric resonance phenomena

Proposal of experimental measurements for validation

Abstract

We present a theory of the linear and nonlinear optical characteristics of the insulating phase of the Falicov-Kimball model within the self-consistent mean-field approximation. The Coulomb attraction between the itinerant d-electrons and the localized f-holes gives rise to a built-in coherence between the d and f-states, which breaks the inversion symmetry of the underlying crystal, leading to: (1) electronic ferroelectricity, (2) ferroelectric resonance, and (3) a nonvanishing susceptibility for second-harmonic generation. As experimental tests of such a built-in coherence in mixed-valent compounds we propose measurements of the static dielectric constant, the microwave absorption spectrum, and the dynamic second-order susceptibility.