Robust One-Dimensional Edge Channels Achieved in Three-Dimensional Quantum Spin Hall Insulator

The pursuit of topologically protected electronic states promises revolutionary advances in device technology, and scientists are actively exploring materials that host these states, including two-dimensional spin Hall insulators and higher-order topological insulators. Now, Shuikang Yu, Junze Deng, Wenhao Liu, and colleagues, working at institutions including Yunmei Zhang and Yiming Sun’s labs, present compelling evidence for topological edge channels within a newly identified three-dimensional quantum spin Hall insulator. Their research establishes bismuth monoiodide as the first material of its kind, demonstrating a unique topology arising from its spin characteristics, and importantly, goes beyond reliance on conventional symmetry indicators to define its topological state. By employing scanning tunneling microscopy, the team directly observes these robust edge channels, confirming their resistance to disruption from defects and their ability to remain separate even when layers of the material are stacked, paving the way for materials exhibiting nearly perfect spin conduction.

Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. The research team successfully grew single crystals of Bi4I4 using a chemical vapor transport reaction, carefully controlling temperature over two weeks and then annealing the crystals to improve their quality, resulting in needle-shaped crystals measuring approximately 8-10mm in length. To directly observe the topological edge states, the study pioneered in-situ scanning tunneling microscopy measurements performed at 80K under ultrahigh vacuum conditions, immediately after cleaving the Bi4I4 crystals, allowing examination of the electronic structure at monolayer and bilayer steps. The team revealed robust edge states, unaffected by defects, confirming their helical character through spectroscopic measurements analyzing the energy dispersion of these states, demonstrating two distinct channels at bilayer steps with no interaction gap. First-principles calculations demonstrated that the topology of Bi4I4 originates from a non-zero spin Chern number for each crystal plane, distinguishing it from previously known quantum spin Hall insulators, and confirming its classification as a 3D quantum spin Hall insulator with trivial symmetry indicators. These results establish Bi4I4 as a unique topological material, offering a pathway towards achieving nearly-quantized spin Hall conductivity and expanding the possibilities for novel spintronic devices.

Helical Edge States in Bismuth Iodide

Research on α-Bi4I4 confirms its status as a topological material, specifically a quantum spin Hall insulator, possessing conducting states on its surface and edges. The material exhibits helical edge states, spin-polarized conducting channels at the edges where the spin direction is linked to the electron’s momentum, making them resistant to backscattering. Scientists used scanning tunneling microscopy to directly observe these helical edge states at monolayer steps, boundaries between layers of the material, and found they persisted even at bilayer steps, displaying two distinct edge channels, further confirming the topological nature of the material. Calculations of symmetry-constrained nodal indicators confirm the presence of non-zero indicators, demonstrating the topological nature of the material. The team employed scanning tunneling microscopy to visualize the atomic structure and measure the local density of states to map out the edge states, using differential conductance measurements to identify the energy and location of these topological edge states. Theoretical calculations, based on density functional theory, were used to calculate the band structure, model the wavefunction of the edge states, and confirm the material’s topological nature through symmetry-constrained nodal calculations.

Bismuth Monoiodide, A 3D Quantum Spin Hall Insulator

This research definitively classifies bismuth monoiodide as the first three-dimensional quantum spin Hall insulator, extending beyond traditional symmetry-based definitions of topological materials. Scientists demonstrated this unique topology through detailed calculations and direct observation of helical edge states using scanning tunneling microscopy, confirming their existence at both monolayer and bilayer step edges. These edge states exhibited robustness, remaining uninterrupted by defects, a key characteristic of topologically protected conduction. The findings resolve a debate surrounding the topological nature of a specific phase of bismuth tetraiodide, confirming it also possesses the properties of a non-symmetry-indicated 3D quantum spin Hall insulator. The observed linear dispersion of edge modes and weak interlayer coupling within this material suggest it holds significant promise for achieving nearly-quantized spin Hall conductivity, a phenomenon with potential applications in spintronic devices. Further research is needed to fully explore the potential of these materials and optimize their properties for practical applications, and investigation into the interplay between stacking sequences and topological states could reveal new avenues for materials design.