Title: Imaging and Harnessing the Topological Boundary Modes

Speaker: Prof. Zhenyu Wang（University of Science and Technology of China）

Abstract: Topologically protected edge channels show prospects for quantum devices [1]. In this talk we provide evidence for robust topological helical edge states within a novel topology named three-dimensional (3D) QSHIs. Its topology originates solely from a nonzero spin Chern number for each plane of the crystal and is realized in both bulk α-Bi4I4 with trivial symmetry indicators and α-Bi₄Br₄ with inversion Z₄ index. We experimentally observe the edge modes at each type of monolayer and bilayer step of these materials by scanning tunneling microscopy (STM). These edges states are linearly dispersing protected by a large bulk gap spanning E_F, robust against disorders and vertical stacking [2]. Based on these findings, we fabricate a novel “topological” STM tip with these materials, and show how the tunneling of the helical edge state becomes a metrological resource for atomic-scale spin sensing owing to the time-reversal-symmetry paired spin-momentum locking. As a stringent benchmark, we apply this helical tunneling spin probe to the layered altermagnetic material KV₂Se₂O and directly image the defining alternating spin texture in both real and momentum space [3].

[1] S. Phark et al., Nano Futures 9, 032001(2025);

[2] S. Yu et al., Phys. Rev. X 14, 041048 (2024);

[3] Z. Wang et al., arXiv: 2512.23290 (2025)