Acoustic semimetal with Weyl points and surface states

Abstract: Two-dimensional topological edge states, immunizing against defects and disorders, have greatly revolutionized our scientific cognition on propagation and scattering of acoustic waves. Recently, the similar states have been predicted in three-dimensional acoustic systems consisting of chiral coupling resonance cavities. However, the direct observation of topological surface propagation, especially in a 3D acoustic system with simple scatterers but not complicatedly coupling resonances, has still not been exploited. Here, we design and fabricate a 3D acoustic semimetal composed of rotationally stacked rods. This semimetal not only produces a linear Weyl degeneracy with a charge of 1, but also yields a double Weyl point with a charge of 2. In its nontrivial gaps, we discover the surface states associated with these Weyl points and experimentally demonstrate the topologically protected one-way propagation of acoustic waves. Due to its good fabricability and topological non-reciprocity, this acoustic semimetal provides an excellent platform for the integration of various acoustic devices, from a macroscopic scale to a nanoscale.