Observation of Robust One-Dimensional Edge Channels in a Three-Dimensional Quantum Spin Hall Insulator

Topologically protected edge channels show prospects for spintronics devices. 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. We experimentally observe the related edge states at each type of monolayer and bilayer step of this material by scanning tunneling microscopy. 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.