Weyl points and Fermi arcs in a chiral phononic crystal

Topological semimetals are materials whose band structure contains touching points that are topologically nontrivial and can host quasiparticle excitations that behave as Dirac or Weyl fermions1-7. These so-called Weyl points not only exist in electronic systems, but can also be found in artificial periodic structures with classical waves, such as electromagnetic waves in photonic crystals8-11 and acoustic waves in phononic crystals12,13. Due to the lack of spin and a diffculty in breaking time-reversal symmetry for sound, however, topological acoustic materials cannot be achieved in the same way as electronic or optical systems. And despite many theoretical predictions12,13, experimentally realizingWeyl points in phononic crystals remains challenging. Here, we experimentally realizeWeyl points in a chiral phononic crystal system, and demonstrate surface states associated with the Weyl points that are topological in nature, and can host modes that propagate only in one direction. As with their photonic counterparts, chiral phononic crystals bring topologicalphysics to the macroscopic scale.