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Based on the SO(2)-invariant slave-boson scheme, the static charge, orbital,and excitonic susceptibilities in the extended Falicov-Kimball model arecalculated. Analyzing the phase without long-range order we find instabilitiestowards charge order, orbital order, and the excitonic insulator (EI) phase.The instability towards the EI is in agreement with the saddle-point phasediagram. We also evaluate the dynamic excitonic susceptibility, which allowsthe investigation of uncondensed excitons. We find qualitatively differentfeatures of the exciton dispersion at the semimetal-EI and at thesemiconductor-EI transition supporting a crossover scenario between a BCS-typeelectron-hole condensation and a Bose-Einstein condensation of preformed boundelectron-hole pairs.

Slave-boson field fluctuation approach to the extended Falicov-Kimball model: charge, orbital, and excitonic susceptibilities