Excitonic Order and Superconductivity in the Two-Orbital Hubbard Model: Variational Cluster Approach

Abstract

Using the variational cluster approach based on the self-energy functional theory, we study the possible occurrence of excitonic order and superconductivity in the two-orbital Hubbard model with intra-and inter-orbital Coulomb interactions. It is known that an antiferromagnetic Mott insulator state appears in the regime of strong intra-orbital interaction, a band insulator state appears in the regime of strong interorbital interaction, and an excitonic insulator state appears between them. In addition to these states, we find that the s ± -wave superconducting state appears in the small-correlation regime, and the d x 2 −y 2wave superconducting state appears on the boundary of the antiferromagnetic Mott insulator state. We calculate the single-particle spectral function of the model and compare the band gap formation due to the superconducting and excitonic orders.

Photoinduced electron-electron pairing in the extended Falicov-Kimball model

“…Moreover, the EFKM considered in this paper is the spinless model. The realistic models for possible excitonic materials should have the spin degrees of freedom, and thus our theory has to be extended to a spinful model such as the two-band Hubbard model [78][79][80][81]. Furthermore, the importance of the electron-phonon coupling has been pointed out in the excitonic candidate materials TiSe 2 and Ta 2 NiSe 5 [82][83][84].…”