Electronic Ferroelectricity in the Falicov-Kimball Model

I show that a spontaneous electric polarization is developed in the solution of the Falicov-Kimball model (FKM) by mapping the strong coupling limit of the model into an $xxz$ spin 1/2 model with a magnetic field. In this way, I can determine the phase diagram of the strongly interacting model that shows a transition to a mixed valence regime containing two phases: orbital ordering and Bose-Einstein condensation of excitons with a built in electric polarization. The results are extended to the intermeditate and weak coupling regimes.