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SYHS: Symposium Physics of van der Waals 2D Heterostructures
SYHS 1: Physics of van der Waals 2D Heterostructures
SYHS 1.1: Invited Talk
Friday, March 31, 2023, 09:30–10:00, HSZ 01
Novel moiré excitons and ultrafast optical dynamics in van der Waals 2D heterostructures — •Steven G. Louie — Physics Dept, University of California, Berkeley, and Lawrence Berkeley National Lab, Berkeley, CA, USA
Recent experiments have revealed signatures of novel exciton states and intriguing pump-probe optical responses in van der Waals 2D heterostructures. But their microscopic nature remains to be fully understood. We perform ab initio GW-Bethe Salpeter equation calculations and discover a rich diversity of excitonic states in these moiré systems. In rotationally aligned WSe2/WS2 heterobilayer, we find some excitons of a modulated Wannier character and others of a previously unidentified intralayer charge-transfer character. In 57.7∘ twisted bilayer WS2, we discover layer-hybridized excitons with in-plane charge transfer character. Many of the theoretical predictions are confirmed by experiments. Ultrafast optical dynamics in 2D heterostructures is also of fundamental and practical interest. We discover a new many-body mechanism for ultrafast photoexcited dynamics in pump-probe measurement of the optical response of transition metal dichalcogenide heterobilayers. We show that coupling between the intralayer and interlayer excitons dominates the measured ultrafast dynamics, conceptually different from a previously believed picture of single-particle charge transfer between layers. These studies are made possible with the development of two new methods that allow for the ab initio calculations of the excitonic physics and optical response of systems with thousands of atoms in the unit cell and in the time domain.