Straight nodal lines and waterslide surface states observed in acoustic metacrystals

Recently, topological semimetals featured with symmetry-protected nodal line degeneracies in three-dimensional momentum space have attracted great attention. In this work, we have constructed a three-dimensional acoustic metacrystal that hosts topological nodal lines pinning to the edges of a Brillouin zone. Markedly different from the nodal rings or chains observed previously, here the nodal lines are geometrically straight and fully stabilized by the crystal symmetry, which are highly unique and favored for detection. In addition to the symmetry-enforced nodal lines identified by transmission measurements, exotic waterslidelike surface states have been unveiled through scanning surface fields. Excellent agreements are found between our experiments and simulations. Our study may provoke new possibilities for controlling sound, such as realizing unusual sound radiation and scattering.