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

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:msub><a:mi mathvariant="bold">S</a:mi><a:mi mathvariant="bold-italic">z</a:mi></a:msub></a:mrow></a:math> spin Chern number for each <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msub><e:mi mathvariant="bold">k</e:mi><e:mi mathvariant="bold-italic">z</e:mi></e:msub></e:mrow></e:math> plane of the crystal and is realized in bulk <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mi>α</i:mi><i:mtext>−</i:mtext><i:msub><i:mrow><i:mi>Bi</i:mi></i:mrow><i:mn>4</i:mn></i:msub><i:msub><i:mi mathvariant="normal">I</i:mi><i:mn>4</i:mn></i:msub></i:mrow></i:math> 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 <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:mi>α</l:mi><l:mtext>−</l:mtext><l:msub><l:mrow><l:mi>Bi</l:mi></l:mrow><l:mrow><l:mn>4</l:mn></l:mrow></l:msub><l:msub><l:mrow><l:mi mathvariant="normal">I</l:mi></l:mrow><l:mrow><l:mn>4</l:mn></l:mrow></l:msub></l:mrow></l:math> 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