Dirac semimetals, the materials featuring fourfold degenerate Dirac points, are critical states of topologically distinct phases. Such gapless topological states have been accomplished by a band-inversion mechanism, in which the Dirac points can be annihilated pairwise by perturbations without changing the symmetry of the system. Here, we report an experimental observation of Dirac points that are enforced completely by the crystal symmetry using a nonsymmorphic three-dimensional phononic crystal. Intriguingly, our Dirac phononic crystal hosts four spiral topological surface states, in which the surface states of opposite helicities intersect gaplessly along certain momentum lines, as confirmed by additional surface measurements. The novel Dirac system may release new opportunities for studying elusive (pseudo) and offer a unique prototype platform for acoustic applications.

狄拉克半金属是具有四倍简并狄拉克点的材料，是拓扑上不同相的临界状态。这种无间隙的拓扑状态是通过带反转机制实现的，其中狄拉克点可以通过扰动成对地消灭，而不会改变系统的对称性。在这里，我们报告了狄拉克点的实验观察，这些狄拉克点完全由使用非对称三维声子晶体的晶体对称性所强制。有趣的是，我们的狄拉克（Dirac）声子晶体具有四个螺旋拓扑表面状态，其中相对螺旋线的表面状态沿着某些动量线无间隙地相交，这已通过其他表面测量得以证实。