Spin-orbit coupling, exchange, and magnetism in exciton physics of 2D semiconductors*

Interactions (e.g., spin-orbit coupling (SOC), electron-hole (e-h), magnetic ordering, etc.) often give rise to dramatic new features in the photophysics of 2D materials. With newly developed full-spinor ab initio GW & Bethe-Salpeter equation (GW-BSE) methods, we investigate the interplay among these interactions in 2D semiconductors. We discover that e-h exchange interaction in monolayer transition metal dichalcogenides mixes the well-known A and B excitons that heretofore were believed to be completely independent of each other, since A and B excitons had been viewed as derived from inter-band transitions between different pairs of spin-polarized bands [1]. In another study, we elucidate the physical origin of giant excitonic and magneto-optical responses in monolayer CrI₃, a prototypical 2D ferromagnetic semiconductor [2]. Our calculations demonstrate that the optical properties of ferromagnetic monolayer CrI₃ are dominated by exciton states that extend over several unit cells. By simulating a realistic experimental setup, we further predict a strong dependence of magneto-optical Kerr effect signals on excitation frequency and substrate configuration.

[1] L. Guo, M. Wu, T. Cao, D. M. Monahan, Y.-H. Lee, S. G. Louie, and G. R. Fleming, “Exchange-driven intravalley mixing of excitons in monolayer transition metal dichalcogenides” Nat. Phys. 15, 228 (2019).
[2] M. Wu, Z. Li, T. Cao, and S. G. Louie, “Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators,” Nat. Commun. 10, 2371 (2019).