Topologically protected edge channels show prospects for quantum devices. They have been found experimentally in two-dimensional quantum spin Hall insulators (QSHIs), weak topological insulators, and higher-order topological insulators, but the number of materials realizing these topologies is still quite limited. Here, we provide evidence for topological edge states within a novel topology named three-dimensional QSHIs. Its topology originates solely from a nonzero Sz spin Chern number for each kz plane of the crystal and is realized in bulk α−Bi4I4 with trivial symmetry indicators, as we show by density-functional-theory calculations. We experimentally observe the related edge states at each type of monolayer and bilayer step of this material by scanning tunneling microscopy. Consistently, the edge states are neither interrupted nor backscattered by defects at the step edges corroborating their helical character as expected from the nontrivial topology. Furthermore, two individual edge channels are directly observed at bilayer steps without visible interaction gap opening, demonstrating the robustness of these edge modes against vertical stacking. Our results establish α−Bi4I4 as the first material realization of a 3D QSHI whose definition goes beyond the scope of topological symmetry indicators, and provide a pathway for realizing nearly quantized spin Hall conductivity per unit cell in a bulk crystal. Published by the American Physical Society 2024

中文翻译：

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三维量子自旋霍尔绝缘体中鲁棒一维边缘通道的观察


拓扑保护的边缘通道显示了量子设备的前景。它们已在二维量子自旋霍尔绝缘体 （QSHI）、弱拓扑绝缘体和高阶拓扑绝缘体中实验发现，但实现这些拓扑的材料数量仍然相当有限。在这里，我们为一种名为三维 QSHIs 的新型拓扑结构中的拓扑边缘状态提供了证据。它的拓扑结构完全源自晶体每个 kz 平面的非零 Sz 自旋陈数，并以块状 α−Bi4I4 实现，具有微不足道的对称指示符，正如我们通过密度泛函理论计算所示。我们通过扫描隧道显微镜实验观察这种材料每种类型的单层和双层步骤的相关边缘状态。始终如一地，边缘状态既不会被阶梯边缘的缺陷打断，也不会被反向散射，这证实了它们在非平凡拓扑中预期的螺旋特性。此外，在双层步骤中直接观察到两个单独的边缘通道，没有可见的交互间隙打开，证明了这些边缘模式对垂直堆叠的稳健性。我们的结果将 α−Bi4I4 确立为 3D QSHI 的第一个材料实现，其定义超出了拓扑对称指标的范围，并为在块晶体中实现每个晶胞的近乎量子化的自旋霍尔电导率提供了一条途径。 美国物理学会 2024 年出版