A comprehensive study of complex non-adiabatic exciton dynamics in MoSi2N4

Excitons, which are composite boson quasi-particles composed of bound electrons and holes, have many fascinating properties and great potential in practical applications. Though experimental studies on exciton dynamics are well-developed, the ab initio simulation ones still remain vacant until two years ago. Here, we apply the density functional theory (DFT) and many-body perturbation theory (MBPT) on 2D MoSi2N4 to study its exciton-related physics and non-adiabatic ultrafast exciton dynamics theoretically and numerically for the first time. Due to its wide band gap, large exciton binding energy and similar 2D hexagonal crystal structure to transition-metal dichalcogenides (TMDs), we expect MoSi2N4 to have distinguished excitonic properties. We calculate the photoluminescence (PL) spectra with final states as bright excitons, yet lots of them are contributed by the dark ones, and the results match the experimental ones perfectly. We also study the dark-exciton-involved processes, which were barely studied in the past but dominate in many physical processes, and obtain several main results like: (i) High scattering rates over the whole Brillouin zone (BZ) within the order of magnitude from 10(-2) fs(-1) to 10(1) fs(-1); (ii) Thorough analysis for the dynamics of the dark excitons at lambda valley, which have negative effective mass and the highest scattering rate among several exciton states; (iii) Simulate the time-resolved evolution of the excitons after photo-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 also discussed here; (v) A new approach is proposed for modulating the non-adiabatic effects for excitons, accompanied by a chiral phonon absorption/emission, by tuning the chirality of the external circularly polarized light. All the results show that the 2D material MoSi2N4 is an ideal platform to study the exciton-involved physics and has great application value.