The notion of higher-order topological insulators has endowed materials with topological states beyond the first order. Particularly, a three-dimensional (3D) higher-order topological insulator can host topologically protected 1D hinge states, referred to as the second-order topological insulator, or 0D corner states, referred to as the third-order topological insulator. Similarly, a 3D higher-order topological semimetal can be envisaged if it hosts states on the 1D hinges. Here we report the realization of a second-order topological Weyl semimetal in a 3D-printed acoustic crystal, which possesses Weyl points in 3D momentum space, 2D Fermi arc states on surfaces and 1D gapless states on hinges. Like the arc surface states, the hinge states also connect the projections of the Weyl points. Our experimental results evidence the existence of the higher-order topological semimetal, which may pave the way towards innovative acoustic devices.

高阶拓扑绝缘体的概念赋予了材料超越一阶的拓扑状态。特别是，三维（3D）高阶拓扑绝缘体可以承载拓扑保护的一维铰链态，称为二阶拓扑绝缘体，或零维角态，称为三阶拓扑绝缘体。类似地，如果 3D 高阶拓扑半金属承载 1D 铰链上的状态，则可以设想它。在这里，我们报告了在 3D 打印声学晶体中实现二阶拓扑外尔半金属，其在 3D 动量空间中具有外尔点，在表面上具有 2D 费米弧状态，在铰链上具有 1D 无间隙状态。与弧面状态一样，铰链状态也连接外尔点的投影。