Phase Diagrams of the Excitonic Insulator State: Analyzing the Excitonic Susceptibility

In this paper, the formation of the excitonic insulator state in the rare-earth chalcogenides hasbeen investigated through the extended Falicov-Kimball model. Adapting the unrestricted Hartree-Fockapproximation, we have derived a set of explicitly self-consistent equations determining expectationvalues and the excitonic susceptibility in the system. Analyzing the excitonic susceptibility, we haveestablished phase diagrams of the excitonic insulator state depending on the model parameters. The phasestructures confirmed the excitonic insulator state is found at low temperature and between two criticalvalues of the Coulomb interaction. The effect of the external pressure on the formation of the excitonicinsulator state is also shown.