Researchers have fashioned a layered topological material into a scanning tunneling microscope (STM) tip that senses spin-related electronic states at atomic resolution.
The research team, led by CHEN Xianhui and WANG Zhenyu from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), together with XIANG Bin from USTC, LIU Junwei from the Hong Kong University of Science and Technology and other collaborators, used this probe to obtain converging evidence for two-dimensional d-wave altermagnetic order on the surface of KV2Se2O.
The study was published online in Nature on October 13 as an Accelerated Article Preview.
According to the researchers, the approach builds on the quantum spin Hall effect. Along an edge of a quantum spin Hall insulator, electrons traveling in opposite directions have opposite spin expectation values. Such one-dimensional "helical" edge channels have long been studied for their unusual electronic properties.
In earlier work, the USTC team observed robust, weakly coupled helical edge states at steps in the related material α-Bi4I4. Building on this finding, the researchers explored whether these topological edge channels could be used to probe other materials.
The researchers used focused-ion-beam nanofabrication to shape α-Bi4Br4 single crystals into STM tips. The fabricated tips can resolve individual atoms while retaining spectral features consistent with helical edge states. 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.
To test this principle, the researchers examined two established magnets. In Fe1+xTe, the magnetic-stripe contrast reversed with voltage polarity. In FeGe, the contrast between adjacent unit-cell layers changed markedly when an external magnetic field induced a spin-flop transition in the sample.
The research team then applied the probe to KV2Se2O, a candidate altermagnet. Altermagnets have no net magnetization, yet magnetic exchange can split electronic bands in a pattern that alternates across momentum space in accordance with their unique crystalline symmetries. Identifying this order requires linking local atomic structure to spin-dependent electronic behavior.
On the KV2Se2O surface, atomic imaging revealed a density-wave-related lattice modulation and different symmetry-related vanadium sites. Analysis of images acquired at opposite voltage polarities yielded a real-space pattern consistent with checkerboard antiferromagnetism.
Magnetic-field spectroscopy further showed that the energies of defect-associated excitations extending along two orthogonal crystal directions shifted in opposite ways, supporting opposite spin polarizations.
The researchers also imaged quasiparticle interference—the wave patterns produced when electrons scatter. With the α-Bi4Br4 tips, they observed that the dominant direction of two orthogonal scattering features rotated by 90 degrees when the bias polarity was reversed. Conventional nonmagnetic tips showed no comparable reversal.
Further control measurements indicated that structural anisotropy, local strain or orbital selectivity alone could not readily account for the observations.
Together with theoretical analysis, these observations support d-wave altermagnetic order in the surface layer of KV2Se2O. More broadly, the study demonstrates how topological edge states can become functional components of atomic-scale instruments, with potential applications to other two-dimensional and moiré materials.
