Bound state formation and nature of the excitonic insulator phase in the extended Falicov-Kimball model

Abstract

Motivated by the possibility of pressure-induced exciton condensation in intermediate-valence Tm[Se,Te] compounds we study the Falicov-Kimball model extended by a finite f-hole valence bandwidth. Calculating the Frenkel-type exciton propagator we obtain excitonic bound states above a characteristic value of the local interband Coulomb attraction. Depending on the system parameters coherence between c- and f-states may be established at low temperatures, leading to an excitonic insulator phase. We find strong evidence that the excitonic insulator typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate (BEC) of preformed excitons (strong-coupling regime), which points towards a BCS-BEC transition scenario as Coulomb correlations increase.

4 pages, 2 figures, completely revised version

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• Evidence for an excitonic insulator phase in 1T-TiSe${}_2$
• On the possibility of an excitonic insulator at the semiconductor-semimetal transition
• Crystallization in two-component Coulomb systems
• Weak-coupling Treatment of Electronic (Anti-)Ferroelectricity in the Extended Falicov-Kimball Model
• Hartree-Fock study of electronic ferroelectricity in the Falicov-Kimball model with $f$-$f$ hopping
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• Unveiling Excitonic Insulator Signatures in Ta${}_2$NiSe${}_5$