Higher-order Topological States in Acoustic Twisted Moiré Superlattices
Shi-Qiao Wu, Zhi‐Kang Lin, Bin Jiang, X. R. Zhou, Zhi Hong Hang, Bo Hou, Jian‐Hua Jiang
Abstract
Twisted moir\'e superlattices (TMSs) are emergent materials with exotic physical properties. Among their properties, higher-order topology is seldom realized and investigated in experiments. Here, we report on the experimental observation of a class of bilayer higher-order topological states in TMSs. We create the physical realization of acoustic TMSs using moir\'e twisting and ultrastrong interlayer couplings in bilayer honeycomb lattices of coupled acoustic cavities. Through such a design we reach an unexplored regime in TMSs that is not available in graphene TMSs and other solid-state TMS systems. We reveal that the ultrastrong interlayer couplings lead to a large acoustic band gap with unique higher-order band topology, which is characterized by unprecedented topological indices and layer-hybridized corner states. The higher-order topological edge and corner states are observed via acoustic pump-probe measurements, which show consistency with theory and simulations. Our study paves the way toward higher-order topological phenomena in TMSs.