Spin-triplet topological excitonic insulators in two-dimensional materials

Quantum spin -Hall insulators (QSHIs) possess nontrivial band topology. Using first -principles many -body perturbation theory (GW+Bathe-Salpeter equation), we show that excitonic insulators (EIs) can exist in QSHIs AsO and Mo2TiC2O2 with nonvanishing band gaps. Their single -particle topological properties can be described by the same low -energy model, and we show that the EI phase in this model is of spin -triplet type due to the nontrivial band geometry. The rotational symmetry is broken by the s -wave EI order parameter, and the anisotropic absorption together with possible lattice reconstruction can be used as signatures for this spin -triplet topological EI phase. Large amounts of spin -triplet excitons emerge spontaneously when an EI is achieved and exciton spin -Hall effect may be observed.