Theory of electronic ferroelectricity

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