Diverse nature of excitonic states in transition metal dichalcogenide moiré superlattices*

Recent experimental measurements have demonstrated signatures of novel exciton states in the moiré superlattices of bilayer transition metal dichalcogenides. However, the microscopic nature and origin of these moiré excitons was not well understood, and previous studies relied often on empirically fit models that did not fully account for the electron and hole degrees of freedom of the exciton. We performed state-of-the-art first-principles GW-Bethe Salpeter equation calculations and discovered a rich diversity of excitonic states in large-area transition metal dichalcogenide moiré superlattices. These studies, which involve thousands of atoms in the reconstructed moiré unit-cell, are made possible by a novel computational approach we developed, the pristine unit-cell matrix projection (PUMP) method [1]. In rotationally aligned WSe₂/WS₂ moiré superlattice, we find some excitons of a modulated Wannier character and others of a previously unidentified intralayer charge-transfer character [1]. In 57.7° twisted bilayer WS₂, we discover layer-hybridized excitons with in-plane charge transfer character. These characteristics originate from the strong modulation of electron wavefunctions due to atomic reconstructions of the moiré superlattice. Due to the weaker binding and larger spatial extent of the in-plane charge-transfer excitons, they can be strongly modulated by external electrical field, doping charges, and substrate screening. Experimental reflection contrast [1], electron energy loss spectroscopy [2] and scanning tunneling spectroscopy confirm these predictions. 

[1] M. H. Naik, E. C. Regan, Z. Zhang, …, F. H. da Jornada, F. Wang and S. G. Louie, "Intralayer charge-transfer moiré excitons in van der Waals superlattices", Nature 609, 52–57 (2022)

[2] S. Susarla, M. H. Naik, …, F. H. da Jornada, S. G. Louie, P. Ercius and A. Raja, "Hyperspectral imaging of excitons within a moiré unit-cell with a sub-nanometer electron probe", arXiv:2207.13823 (2022)