Low-energy excitonic condensate excitations in semimetal and semiconducting materials

Abstract

The low-energy excitation properties of the excitonic condensate in the semimetal and semiconducting materials are inspected in the extended Falicov-Kimball model. In the framework of the unrestricted Hartree-Fock approximation, we have released a set of self-consistent equations so the excitonic condensate order parameter and then the optical conductivity and the dynamical excitonic susceptibility function are evaluated. We find that the optical conductivity becomes significant and shows a sharp peak at a frequency that is twice as of the excitonic condensate order parameter once the system settles in the excitonic condensate. In the meanwhile, out of the excitonic condensation state, the dynamical excitonic susceptibility function in the semimetal state releases a dominant peak at low frequency if Coulomb interaction is sufficiently large. This character indicates the possibility of the resonance bound excitonic condensate even in high-temperature normal semimetals. The resonance is dramatically reinforced inside of the semiconducting state and that specifies the stability of the bound excitonic coherent state of the preformed excitons in the semiconducting materials.