Dual topological materials are unique topological phases that host coexisting surface states of different topological nature on the same or on different material facets. Here, we show that Bi₂TeI is a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band, which classify it as a weak topological insulator with metallic states on its ‘side’ surfaces. The mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. We investigated Bi₂TeI spectroscopically to show the existence of both two-dimensional Dirac surface states, which are susceptible to mirror symmetry breaking, and one-dimensional channels that reside along the step edges. Their mutual coexistence on the step edge, where both facets join, is facilitated by momentum and energy segregation. Our observation of a dual topological insulator should stimulate investigations of other dual topology classes with distinct surface manifestations coexisting at their boundaries.

双重拓扑材料是独特的拓扑阶段，在相同或不同的材料面上具有不同拓扑性质的共存表面状态。在这里，我们表明Bi ₂ TeI是双重拓扑绝缘体。它在体带的两个时间反转对称点处表现出带反转，将其归类为在其“侧面”表面具有金属态的弱拓扑绝缘体。晶体结构的镜像对称性同时将其分类为拓扑晶体绝缘体。我们调查了Bi ₂光谱分析表明存在二维的狄拉克表面状态和沿台阶边缘存在的一维通道，二维状态容易受到镜面对称性破坏。动量和能量隔离促进了它们在两个面都接合的台阶边缘上的共存。我们对双重拓扑绝缘子的观察应能激发对其他双重拓扑类别的研究，这些类别在其边界处共存有明显的表面表现。