Spin character of interlayer excitons in tungsten dichalcogenide heterostructures: GW-BSE calculations

Interlayer excitons (IXs) have become an ideal platform for studying exciton condensation, single-photon emission, and other quantum phenomena. Two-dimensional transition metal dichalcogenide (TMD) heterostructures, with type-II band alignment features, provide a simple framework for the formation of IXs. The intrinsic electric field of Janus TMDs can be introduced to tune the type-II band energies. In this paper, we perform GW-BSE calculations to explore how the Janus layers affect the interlayer excitations in WSSe/$\mathrm{W}{\mathrm{S}}_{2}$-based heterostructures by tuning the spin states. Our results reveal that the parallel-arranged intrinsic electric field structure with the S/Se interface mixes more spin-singlet state into the spin-triplet states, and hence, the lowest IX has a shorter radiative lifetime than other Janus WSSe@$\mathrm{W}{\mathrm{S}}_{2}$ heterostructures. We also find the S/Se interface makes the energy band more staggered and thus has no bound bright IXs. For S/S interface heterostructures in which the electric field points away from the interface, the strong band hybridization mixes 49% spin-singlet into the spin-triplet states, and therefore, the radiative lifetime of the lowest-energy bright IX is as short as ${10}^{\ensuremath{-}13}$ s at 0 K. Our explorations show that strong spin-orbit coupling plays a key role in the spin-singlet-triplet mixture in Janus WSSe structures by arranging the direction of the intrinsic electric field.