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Atomic-scale spin sensing of a 2D d-wave altermagnet via helical tunneling
Nature
(2026) Cite this article
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Altermagnets simultaneously possess momentum-dependent nonrelativistic spin splitting and zero net magnetization (1-6). Their defining crystalline symmetry connects opposite-spin sublattices in real space and imposes alternating spin polarization on the symmetry-related electronic states in momentum space (1-3). Identifying such an order and its associated spin response therefore calls for atomic-scale spin-sensitive measurements in both real and momentum space, an experimental milestone yet to be achieved. Here we show that the helical edge modes of the higher-order topological insulator α-Bi4Br4 can serve as a spin-selective scanning tunnelling microscopy probe of the two-dimensional d-wave altermagnetic order in the surface layer of KV2Se2O. Atomically registered mapping reveals the spatial relationship between the two V sublattices and a density-wave lattice modulation, as well as real-space patterns consistent with the checkerboard antiferromagnetic order. Field-dependent spectroscopy further finds opposite energy shifts of the unidirectional quasiparticle excitations extending along two orthogonal crystal axes and a predominantly out-of-plane magnetic response. In momentum space, spin-sensitive quasiparticle interference imaging reveals a bias-reversing d-wave anisotropy between the symmetry-paired X and Y valleys. These results provide converging evidence for two-dimensional d-wave altermagnetism on the surface of KV2Se2O and demonstrate helical-edge-mode tunnelling as a route for atomic-scale spin sensing in quantum materials.
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These authors contributed equally: Zhuying Wang, Shuikang Yu, Xingkai Cheng, Xiaoyu Xiao
Department of Physics, University of Science and Technology of China, Hefei, Anhui, China
Zhuying Wang, Shuikang Yu, Wanru Ma, Yitian Ma, Feixiong Quan, Hongxi Song, Kunming Zhang, Yunmei Zhang, Qian Niu, Yang Gao, Zhenyu Wang & Xianhui Chen