Phase coherence and spectral functions in the two-dimensional excitonic systems

Abstract

a b s t r a c tThe nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose-Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered.

Influence of magnetic field on the electronic ferroelectricity in the extended Falicov-Kimball model

“…This effort was motivated by physical, but also practical reasons, as the existence of ferroelectrics based on the purely electronic mechanism would provide a new set of physical properties and technological applications, as for example, the possibilities of controlling optical properties of materials with magnetic fields. Since the Falicov-Kimball model (FKM) is probably the simplest model of electronic phase transitions in rare-earth and transition-metal compounds [9], it was natural to test the idea of electronic ferroelectricity [10][11][12][13][14][15][16][17][18][19][20][21], which is directly related with formation of excitonic insulator [22][23][24][25][26][27][28][29][30][31][32][33][34][35], just on this model and its extensions.…”

Density of states in the bilayer graphene with the excitonic pairing interaction

“…More recently, it has been shown [19,20] that the single-particle coherent density of states (DOS) in the usual two-dimensional (2D) [19] and three-dimensional (3D) [20] semiconducting systems at the zero temperature limit is always finite, reflecting with the excitonic condensate regime in these systems. For the 3D semiconducting systems, the coherent DOS spectra survive also for the higher temperatures [20].…”

“…Moreover, it has been shown that there exists a critical value of the interlayer hopping amplitude γ 1 [15], which provides an interesting energy cutoff, below which the electron-hole correlations do not drive the system towards the CSB excitonic transition. More recently, it has been shown [19,20] that the single-particle coherent density of states (DOS) in the usual two-dimensional (2D) [19] and three-dimensional (3D) [20] semiconducting systems at the zero temperature limit is always finite, reflecting with the excitonic condensate regime in these systems. For the 3D semiconducting systems, the coherent DOS spectra survive also for the higher temperatures [20].…”