| ABSTRACT |
Superlattice, an engineered solid with external long-ranged periodic potential, can show unexpected physical properties far from its pristine characteristics1?3. Recent experiments on Mott insulating and superconducting twisted bilayer graphene4,5 reinvigorates the pursuits for emergent functionalities in superlattices by stacking two-dimensional (2D) crystals with rotational stacking faults or moir? superlattices6. One of the major problems in the structures is an inevitable lattice deformation owing to intrinsic softness of 2D crystals7?9. Although large scale reconstructions are identified within a narrow range of twist angles10?12, significant challenges are still posed to understand their local geometries as well as influences on physical properties in the entire range of possible twist angles. Here, we report that the local atomic structures of moir? superlattices made of single layers MoS2 and WSe2 experience a pair of torsional strains with opposite chirality within the moir? unit cell. Unlike typical effects of strains on 2D crystals13,14, the whirlpool-shaped periodic lattice distortions introduce characteristic fuzziness in the Raman scattering signals and universal red-shift to intralayer exciton energies for all twist angles. We demonstrate that both the modulations become stronger as the twist angle decreases and are turned off when the constituent layers are not tightly coupled, thus establishing an essential structure-property relationship for moir? superlattices. As the Raman shifts are accepted as important gauges on electronic, structural, and chemical properties of single-layered 2D crystals15, we expect that our current establishments will be useful in ascertaining local distortions as well as layer coupling strength in twisted bilayer systems. |
