Higher-order Topological States in Acoustic Twisted Moiré Superlattices

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.