Various band structure excitations in twisted bilayer phononic graphene with layer-resolved measurement

Twistronics, an exciting upsurging field, has been generated by studying the twisted moiré bilayer graphene, where many phases of fundamental and practical interests are observed. Flat bands are formed in twisted bilayer graphene through intralayer and interlayer coupling at magic twisting angles. However, the coupling between the two layers with subtle interlayer interactions has not been directly observed experimentally. This is limited in reality due to the lack of non-invasive probes between layers of graphene, which is in the order of nanometers. Here, we introduce a phononic solution for direct measurement. PTBG is an analog to the electronic system. Due to its larger structural sizes, direct measurements of the excitations from each layer are possible, enabling in situ layer resolution of the special flat-band excitations. The layer-resolved phononic measurement, together with theoretical modeling, directly demonstrates that the interlayer coupling effect in various angled PTBG, among them, the magical angled twisted case is most significant. On the other hand, in large-angle twisted PTBG, the low-energy states are only mildly affected by the moiré effect and can be approximated by those from two decoupled Dirac cones from the individual phononic layers. This work opens new phononic approaches to investigate moiré physics.