Observation of a two-dimensional topological metal in acoustic metamaterials

A two-dimensional topological metal with anti-helical-like edge states has been predicted recently but has not been confirmed experimentally. In this paper, we report an experimental realization of this topological metal in acoustic metamaterial by introducing a time-reversal symmetry protected square lattice. The edge states appearing in gapless bulk bands are observed by measuring the projected dispersions and acoustic pressure field distributions. Moreover, these edge states propagate in the same direction when simultaneously exciting two sources with a fixed phase difference. Interestingly, by simply changing the coupling tubes, we realized the transformation of an acoustic topological metal to a topological insulator. Our work not only pushes forward the studies of topological metals but also inspires the design of multifunctional acoustic devices.