Linear and nonlinear optical characteristics of the Falicov-Kimball   model
  We calculate the linear and nonlinear optical properties of the Falicov-Kimball model for a mixed-valent system within the self-consistent mean-field approximation. Second-harmonic generation can only occur if the mixed-valent state has a built-in coherence between the itinerant d-electrons and the localized f-holes. By contrast, second-harmonic generation cannot occur for solutions of the model with f-site occupation as a good quantum number. As an experimental test of coherence in mixed-valent compounds we propose a measurement of the dynamic second-order susceptibility. 
T. Portengen (1), Th. Ostreich (1), L.J. Sham (1) ((1) University of   California, San Diego)