Optical spectrum of ferrovalley materials: a case study of Janus H-VSSe

As opposed to the prototypical MoS2 with centroasymmetry, Janus ferrovalley materials such as H-VSSe is less symmetric with the mirror symmetry and time reversal symmetry broken, and hence possess spontaneous valley polarization and strong ferroelasticity. The optical transition is an important means to excite the valley carriers. We investigated the optical spectrum of H-VSSe by using the many-body perturbation-based GW approach and solving the Bethe-Salpeter equation (BSE) to include the electron-hole interactions. It is found that after the GW correction, the band gaps of the quasiparticle bands are much larger than those obtained by the normal density functional theory. The system is ferromagnetic and the valley gaps become non-degenerate due to spin-orbit coupling (SOC). The position of the lowest BSE peak is much lower than the quasiparticle band gap, indicative of a large excitonic effect. The peak is split into two peaks by the SOC. The binding energy difference between these two BSE peaks is about the same as the difference between the inequivalent valley gaps. Our results show that in Janus H-VSSe the two lowest exciton peaks are from the two inequivalent valleys with different gaps, in contrast to the A and B exciton peaks of MoS2 which are from the same valley.