Summary
A Nature study used helical edge modes as a spin-selective scanning tunnelling microscopy probe to map signatures of d-wave altermagnetism on the surface of KV₂Se₂O. The researchers report evidence from both real-space and momentum-space measurements.
Researchers report evidence of two-dimensional d-wave altermagnetism on the surface of KV₂Se₂O, using helical edge modes from the higher-order topological insulator α-Bi₄Br₄ to make scanning tunnelling microscopy (STM) spin-selective. The study, published in Nature on 13 October 2026, combines atomic-scale maps with measurements of electronic states in momentum space.
Altermagnets have zero net magnetization but can still show spin-split electronic states whose spin polarization varies with momentum. This combination makes them different from ferromagnets, where a net magnetic moment is present, and creates a measurement challenge: evidence for their order must connect the arrangement of atoms and spins in real space with the spin-dependent electronic structure in momentum space.
Using helical modes to sense spin
STM measures the tunnelling current between a probe and a material to reveal local structure and electronic properties. In this study, the researchers used helical edge modes of α-Bi₄Br₄ as a spin-selective probe. Helical modes link an electron’s spin with its direction of motion, providing a way to distinguish spin-dependent tunnelling signals.
The atomically registered maps showed how the material’s two vanadium (V) sublattices relate to a density-wave lattice modulation. The researchers also observed real-space patterns consistent with checkerboard antiferromagnetic order. These measurements help relate the atomic arrangement to the proposed magnetic pattern, rather than relying on an electronic signal alone.
Field-dependent spectroscopy found opposite energy shifts in unidirectional quasiparticle excitations extending along two perpendicular crystal axes. The measured magnetic response was predominantly out of plane. The paper reports these observations as part of the evidence for the surface’s magnetic order.
A matching signature in momentum space
The team also used spin-sensitive quasiparticle interference imaging. Quasiparticle interference patterns arise when electronic states scatter and interfere; their structure can be used to infer properties of electronic states across momentum space. The measurements showed a d-wave anisotropy between the symmetry-related X and Y valleys that reversed with the measurement bias.
Taken together, the spatial mapping, field-dependent spectroscopy and momentum-space response provide converging evidence for two-dimensional d-wave altermagnetism in the surface layer of KV₂Se₂O, the authors conclude. The work also demonstrates a measurement approach for studying spin order at atomic scale across both real and momentum space. The result is specific to this material’s surface; the study does not establish that the same signatures occur in other altermagnets.
The Nature page identifies the paper as peer-reviewed accepted research and notes that this early version may receive further edits before the final Version of Record.