S. Ejima, T. Kaneko, Y. Ohta, H. Fehske
Using exact numerical techniques we investigate the nature of excitonic (electron-hole) bound states and the development of exciton coherence in the one-dimensional half-filled extended Falicov-Kimball model. The ground-state phase diagram of the model exhibits besides band insulator and staggered orbital ordered phases an excitonic insulator (EI) with power-law correlations. The criticality of the EI state shows up in the von Neuman entropy. The anomalous spectral function and condensation amplitude provide the binding energy and coherence length of the electron-hole pairs which, on their part, point towards a Coulomb interaction driven crossover from BCS-like electron-hole pairing fluctuations to tightly bound excitons. We show that while a mass imbalance between electrons and holes does not affect the location of the BCS-BEC crossover regime it favors staggered orbital ordering to the disadvantage of the EI.
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http://arxiv.org/abs/1308.0361
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