Acoustic Quadrupolar Surface Topological Semimetals

Abstract

Quadrupolar topological semimetals with a bulk quadrupole moment have been intensively investigated recently. Of special interest is the quadrupolar surface topological semimetal, which has gapped bulk but surface Dirac nodes attached to each other by hinge states. However, this novel state remains to be observed in condensed matter. Here, we implement the quadrupolar surface topological semimetal in acoustic crystals. By tuning the symmetry of the structure, the hinge states at a specific hinge of the quadrupolar topological semimetal can be engineered, thus allowing the coexistence of different hinge dispersions in a single system. The surface Dirac nodes and hinge states are revealed by momentum-space spectroscopy and real-space visualization. Our findings may provide alternative opportunities to manipulate acoustic waves.