Thermally and optically induced interlayer decoupling in twisted-bilayer Moiré system

The Moiré twisted-bilayer systems have attracted much attention as platforms for exhibiting novel physics not seen in conventional bulk crystals. Determining crystal structures of twisted-bilayer systems, including out-of-plane interlayer distance, is crucial because the electronic and optical properties of these systems are strongly affected. However, the conventional X-ray diffraction technique suffers from the small (~um) sample size and atomic thickness.

In this study, we investigated the thermally and optically induced change in the interlayer distance of bilayer WSe₂ with a twist angle of 0.3°. A newly developed automated dark-field electron tomography technique enables us to determine the interlayer distance quantitatively. We found the interlayer distance of the natural and twisted bilayer systems to be elongated compared to the bulk sample. In addition, thermally induced change in the interlayer distance is much larger than in bulk. We further investigated the optically induced crystal structure dynamics in the picosecond range by ultrafast electron microscopy, revealing non-thermal change in interlayer distance. In the presentation, I would like to discuss the origin of thermally/optically induced change in the interlayer distance by comparing the results on twisted and non-twisted systems.