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Excitons, which are composite boson quasi-particles composed of boundelectrons and holes, have many fascinating properties and great potential inpractical applications. Though experimental studies on exciton dynamics arewell-developed, the ab initio simulation ones still remain vacant until twoyears ago. Here, we apply the density functional theory (DFT) and many-bodyperturbation theory (MBPT) on 2D MoSi$_2$N$_4$ to study its exciton-relatedphysics and non-adiabatic ultrafast exciton dynamics theoretically andnumerically for the first time. We calculate the photoluminescence (PL) spectrawith final states as bright excitons, yet lots of them are contributed by thedark ones, and the results match the experimental ones perfectly. We also studythe dark-exciton-involved processes, which were barely studied in the past butdominate in many physical processes, and obtain several main results like: (i)High scattering rates over the whole Brillouin zone (BZ) within the order ofmagnitude from 10$^{-2}$ fs$^{-1}$ to 10$^{-1}$ fs$^{-1}$; (ii) Thoroughanalysis for the dynamics of the dark excitons at $\Lambda$ valley, which havenegative effective mass and the highest scattering rate among several excitonstates; (iii) Simulate the time-resolved evolution of the excitons afterphoto-excitation with the real-time Boltzmann transport equation (rt-BTE)techniques, in which process excitons at K/K' valley play an important role;(iv) Exciton dynamics with spin-valley locking at K/K' valley are alsodiscussed here; (v) A new approach is proposed for modulating the non-adiabaticeffects for excitons, accompanied by a chiral phonon absorption/emission, bytuning the chirality of the external circularly polarized light. All theresults show that the 2D material MoSi$_2$N$_4$ is an ideal platform to studythe exciton-involved physics and has great application value.

A comprehensive study of complex non-adiabatic exciton dynamics in MoSi$_2$N$_4$