Twist‐Induced All‐Flat‐Band Higher‐Order Acoustic Topological Insulator

Twisted Moiré superlattices (TMSs) exhibit unique geometric periodicity and interlayer Bragg scattering effects, which have been widely exploited for flat-band engineering. However, to date, no report has directly addressed the mechanism of achieving all-flat-band topology with the intrinsic eigenmodes of Moiré periodicity. In this work, by introducing periodic strong localized resonances into the superlattices, a novel acoustic flat-band higher-order topological insulator is proposed. Based on the standing-wave effect of multipolar Mie resonances, varied-frequency edge states and high-quality (high-Q) corner states are observed in the topological all-flat-bandgaps. More interestingly, owing to the operational simplicity of acoustic TMS, it can be readily stacked into a 3D configuration, yielding out-of-phase topological hinge states. This study unveils a simple strategy to construct acoustic all-flat-band topological insulators, providing new potential for applications such as energy harvesting and sensing.