Probing phase coherence via density of states for strongly correlated excitons

TL;DR

This paper calculates the density of states and spectral functions for excitonic systems using a gauge-invariant approach, revealing hybridization gaps and BCS-BEC crossover effects in the spectral features.

Contribution

It introduces a gauge-invariant formalism for excitonic systems and analyzes the spectral functions and DOS, highlighting the effects of coherence and phase stiffness on the hybridization gap.

Findings

Hybridization gap observed in incoherent DOS spectra.

No hybridization gap in coherent DOS due to phase coherence.

BCS-like double-peak structure in fermionic DOS at small Coulomb interactions.

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. By using gauge-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 gauge 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 effets 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, fermionic DOS exhibits a Bardeen-Cooper-Schriffer (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, temperature dependence of DOS functions.