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arXiv · 0912.2854

On the existence of the excitonic insulator phase in the extended Falicov-Kimball model: an SO(2)-invariant slave-boson approach

B. Zenker·D. Ihle·F. X. Bronold·H. Fehske
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Abstract

We re-examine the three-dimensional spinless Falicov-Kimball model with dispersive $f$ electrons at half-filling, addressing the dispute about the formation of an excitonic condensate, which is closely related to the problem of electronic ferroelectricity. To this end, we work out a slave-boson functional integral representation of the suchlike extended Falicov-Kimball model that preserves the $SO(2)\otimes U(1)^{\otimes 2}$ invariance of the action. We find a spontaneous pairing of $c$ electrons with $f$ holes, building an excitonic insulator state at low temperatures, also for the case of initially non-degenerate orbitals. This is in contrast to recent predictions of scalar slave-boson mean-field theory but corroborates previous Hartree-Fock and RPA results. Our more precise treatment of correlation effects, however, leads to a substantial reduction of the critical temperature. The different behavior of the partial densities of states in the weak and strong inter-orbital Coulomb interaction regimes supports a BCS-BEC transition scenario.

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B. Zenker, D. Ihle, F. X. Bronold, H. Fehske. 2010-04-09. On the existence of the excitonic insulator phase in the extended Falicov-Kimball model: an SO(2)-invariant slave-boson approach. https://doi.org/10.1103/physrevb.81.115122

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