THEORY OF ELECTRONIC FERROELECTRICITY

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