Probing Phase Coherence Via Density of States for Strongly Correlated Excitons

Abstract

We present the calculation of the coherent spectral functions and density of states (DOS) for excitonic systems in the frame of the three-dimensional extended Falicov–Kimball model. Using gage-invariant U(1) transformation to the usual fermions, we represent the electron operator as a fermion attached to the U(1) phase-flux tube. The emergent bosonic gage field, related to the phase variables, is crucial for the Bose–Einstein condensation (BEC) of excitons. Employing the path-integral formalism, we manipulate the bosonic and fermionic degrees of freedom to obtain the effective actions related to fermionic and bosonic sectors. Considering the normal and anomalous excitonic Green functions, we calculate the spectral functions, which have the forms of convolutions in the reciprocal space between bosonic and fermionic counterparts. For the fermionic incoherent part of the DOS, we have found the strong evidence of the hybridization gap in DOS spectra. Furthermore, considering Bogoliubov coherence mechanism, we calculate the coherent DOS spectra. For the coherent normal fermionic DOS, there is no hybridization gap found in the system due to strong coherence effects and phase stiffness. The similar behavior is observed also for the condensate part of the anomalous excitonic DOS spectra. We show that for small values of the Coulomb interaction the fermionic DOS exhibits a Bardeen–Cooper–Schrieffer (BCS)-like double-peak structure. In the BEC region of the BCS–BEC crossover, the double-peak structure disappears totally for both: coherent and incoherent DOS spectra. We discuss also the temperature dependence of DOS functions.