Imaging moire flat bands in three-dimensional reconstructed WSe2/WS2 superlattices

Scanning tunnelling spectroscopy and ab initio simulations reveal buckling reconstruction and in-plane strain redistribution in WSe2/WS2 moire heterostructures. Moire superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moire flat bands and strong, long-range Coulomb interactions(1-9). However, microscopic knowledge of the atomically reconstructed moire superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moire phenomena. Here we quantitatively study the moire flat bands in three-dimensional (3D) reconstructed WSe2/WS2 moire superlattices by comparing scanning tunnelling spectroscopy (STS) of high-quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moire superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe2/WS2 moire heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moire flat band at the valence band edge of the heterostructure. A series of moire flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Our results reveal that the strain redistribution and 3D buckling in TMD heterostructures dominate the effective moire potential and the corresponding moire flat bands at the Brillouin zone K points.

Automated summary

Scanning tunnelling spectroscopy and ab initio simulations have revealed a strong 3D buckling reconstruction and large in-plane strain redistribution in WSe2/WS2 moire heterostructures, leading to remarkably narrow and highly localized K-point moire flat bands. These findings contradicted previous simplified theoretical models but were in quantitative agreement with ab initio simulations capturing the full 3D structural reconstruction, highlighting the dominance of strain redistribution and 3D buckling in determining the effective moire potential in TMD heterostructures.