Researchers of China have fashioned scanning tunneling microscope tips from α-Bi 4 Br 4 single crystals, a novel approach enabling the detection of altermagnetism, a surprising form of magnetism lacking net magnetization, at the atomic level. The team led by CHEN Xianhui and WANG Zhenyu demonstrated the technique by observing two-dimensional d-wave altermagnetic order on the surface of KV 2 Se 2 O. This builds on their earlier observation of “robust, weakly coupled helical edge states” in α-Bi 4 I 4, repurposed to probe the spin-related electronic states of other materials. The findings published in Nature link local atomic structure to spin-dependent electronic behavior, revealing a real-space pattern consistent with checkerboard antiferromagnetism.
Topological α-Bi₄Br₄ Tips Resolve Atomic-Scale Spin Sensitivity
Focused-ion-beam nanofabrication enabled the creation of scanning tunneling microscope tips from α-Bi₄Br₄ single crystals, a process yielding probes capable of resolving individual atoms while preserving key spectral characteristics. These tips exploit helical edge states, previously observed by the USTC team in α-Bi₄I₄, to function as spin-sensitive sensors. Reversing the polarity of the sample bias alters the direction of electron flow into or out of the tip, directly influencing its ability to detect spin.
The team demonstrated the technique by imaging quasiparticle interference patterns on KV₂Se₂O, a material exhibiting two-dimensional d-wave altermagnetic order. This newly identified form of magnetism is unusual because it lacks net magnetization, a property not typically associated with magnetic materials.
Researchers observed a 90-degree rotation in the dominant direction of orthogonal scattering features when the bias polarity was reversed, a phenomenon absent when using conventional, nonmagnetic STM tips. The researchers explained that “When an edge channel dominates electron tunneling, reversing the sample-bias polarity changes the direction in which electrons enter or leave the tip and therefore can change its spin sensitivity.” Further experiments ruled out structural anisotropy, local strain, or orbital selectivity as alternative explanations for the observed behavior, strengthening the evidence for d-wave altermagnetic order in the KV₂Se₂O surface layer.
Theoretical analysis corroborated these findings, confirming the link between the observed interference patterns and the material’s magnetic properties. This work establishes topological edge states as functional components within atomic-scale instruments, opening avenues for exploring other two-dimensional magnets and moiré materials.
KV₂Se₂O Surface Reveals d-Wave Altermagnetic Order
KV₂Se₂O exhibits two-dimensional d-wave altermagnetic order, a surprising finding given that altermagnetism, unlike typical magnetism, lacks net magnetization. Researchers of China (USTC) and collaborating institutions detected this order on the material’s surface using a novel scanning tunneling microscope (STM) tip constructed from α-Bi₄Br₄.
Conventional, nonmagnetic STM tips failed to reproduce this reversal, strengthening the evidence for a unique magnetic arrangement within KV₂Se₂O. The researchers report that “Together with theoretical analysis, these observations support d-wave altermagnetic order in the surface layer of KV₂Se₂O.” This approach opens avenues for probing other two-dimensional magnets and moiré materials, potentially revealing previously hidden magnetic phenomena and advancing materials characterization at the nanoscale, suggesting a new path for building atomic-scale tools with tailored spin sensitivity.
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