Theory of 2D Materials: Excitons, Valley-Spin Physics, and Magnetism.*

Recent advances in the experimental and theoretical studies of atomically thin two-dimensional (2D) materials have opened up opportunities in exploring new phenomena and properties as well as related applications absent in conventional bulk materials. In the first part of my talk, we will present theoretical studies on the optical responses of monolayer transition metal dichalcogenides. By ab initio GW-BSE calculations, we demonstrate a previously unrecognized valley-spin character of bright excitons, which leads to interesting ultrafast phenomena in monolayer transition metal dichalcogenides [1]. We then discuss the theory of dark excitons and trions, and how they could be brightened by applying an external magnetic field [2] or using other approaches. In the second part of my talk, we will discuss theoretical studies of magnetism in 2D materials. We discuss the physical origins and control of ferromagnetism in materials with different chemical species and structural configurations. We further connect our theoretical discoveries to experimental results and explore their potential applications.

[1] L. Guo*, M. Wu*, T. Cao*, D. M. Monahan*, Y.-H. Lee, S. G. Louie, and G. R. Fleming, Nature Phys., in press (2018).
[2] X.-X. Zhang, T. Cao, Z. Lu, Y.-C. Lin, F. Zhang, Y. Wang, Z. Li, J. C. Hone, J. A. Robinson, D. Smirnov, S. G. Louie, and T. F. Heinz, Nature Nanotech. 12, 883–888 (2017).