Excitonic insulating phase in monolayer transition-metal dichalcogenides: An ab initio study

Excitonic insulator (EI) is a novel phase of matter characterized by a cooperative phenomenon in which electron-hole excitations within a system exhibit lower energy states in comparison to the conventional band ground state. Through GW plus Bethe-Salpeter equation (GW-BSE) calculations, several two-dimensional (2D) semiconducting materials have been proposed as EI candidates, as the binding energy between the excited electron and hole is larger than the bandgap. Nevertheless, the theoretical understanding of the EI phase has primarily relied on simplified two-band models with typically simplified analytical electron-hole Coulomb interaction kernel, containing several adjustable parameters such as electron and hole effective mass, and 2D polarizability. For real 2D systems, such modeling may be inadequate for obtaining a quantitative understanding of the novel physics underlying the EI state, especially in providing accurate results for comparison with experiments. Here, we develop a parameter-free ab initio framework on top of GW and GW-BSE calculated quasiparticle self-energies and electron-hole kernel matrix elements, to investigate the spectroscopic properties in the EI phase. We discuss results of the temperature-dependent electronic properties (e.g., single-particle excitation energies) and explore several experimental signatures (e.g., local density of states) of EI phase in monolayer 1T'-MoS₂.