Twin Crystal Moiré Metasurfaces for Crossing Flat‐Band Transport

To date, the flat bands in magic‐angle bilayer metasurfaces have been observed to remain inherently parallel and non‐crossing in naturally occurring 2D materials. This limitation arises from the inherent in‐plane symmetry of the lattice. Here, we introduce a twinning lattice in acoustic bilayer twisted moiré metasurfaces that is mirror symmetric with respect to the twin crystal boundary. This new degree of freedom modulates the asymmetric moiré dispersive behavior and helps to achieve distinct features that are not possible in conventional symmetric twisted moiré structures. By adjusting the twin crystal angle and bilayer twisting angle, it endows remarkably different dispersion characteristics separated by the twin boundary, such as dual hyperbolic, hyperbolic‐flat band combinations or dual flat bands. Crucially, this transition is governed by a nontrivial topological invariant, the number of anti‐crossing points N ACPS , which dictates the global phase classification. In the dual flat band regime, the intersection at the twin boundary enables low‐loss acoustic propagation with tunable directionality, serving as a distinct point for wave manipulation. By providing experimental validation of dispersion crossover at the twin crystal interface, this study paves the way for the design and development of twin acoustic moiré metasurfaces.