Researchers have achieved atomic-scale spin-resolved measurements in a layered material, KV2Se2O, using a novel spin-resolved scanning tunneling microscopy technique. The work uses helical edge modes of a higher-order topological insulator as a spin sensor to characterize altermagnetism, a state combining advantages of ferromagnetism and antiferromagnetism. Atomic-registered mapping reveals KV2Se2O’s checkerboard antiferromagnetic order alongside density-wave lattice modulation, while spectroscopic imaging directly visualizes d-wave spin splitting; critically, the team directly observed unidirectional, spin-polarized quasiparticle excitations from crystal symmetry-paired valleys. These experiments establish a foundation for exploring and using altermagnetism in 2D quantum materials.
Atomic-Scale Imaging Reveals KV2Se2O’s Checkerboard Antiferromagnetic Order
The visualization of this order required overcoming a significant hurdle in spin-resolved microscopy, specifically achieving both real and momentum space resolution simultaneously to fully characterize altermagnetism. The new microscopy technique revealed unidirectional, spin-polarized quasiparticle excitations originating from crystal symmetry-paired valleys around opposite magnetic sublattices, a unique spin response characteristic of altermagnetism. These excitations appear simultaneously in the X and Y valleys, a finding that demonstrates a symmetry-driven excitation pattern not previously observed in traditional magnets.
The ability to visualize these excitations relies on the topology-guaranteed spin filtering provided by the helical edge modes, ensuring reliable spin-resolved scanning tunneling microscopy and spectroscopic imaging. “By simultaneously measuring spin-resolved dual-space features with a single probe, our results unambiguously demonstrate a 2D altermagnetic order in KV2Se2O,” the paper states.
Traditional spin-polarized spectroscopic imaging with a scanning tunneling microscope presents challenges due to the sensitivity of spin polarization to the magnetic state of the tip apex. The team circumvented this limitation by employing the helical edge modes, which act as an intrinsic spin filter for quantum electronic tunneling. This approach ensures that electrons injected into or launched from the Bi4Br4 tip carry opposite spins, enforced by their helical nature.
The researchers demonstrated that spin-summed tunneling events show negligible differences in the observed quasiparticle interference intensities, indicating the distinct spin characteristics of the two sublattices. This finding provides direct evidence for the in-plane antiferromagnetic order within KV2Se2O. The experimental confirmation of the antiferromagnetic order, coupled with the visualization of the unique spin response, provides critical insight into the material’s magnetic properties and potential for future applications.
Helical Tunneling Enables Spin-Resolved Scanning Tunneling Microscopy
The ability to simultaneously map both real and momentum space at the atomic level has been achieved, overcoming a longstanding challenge in characterizing complex magnetic materials like KV2Se2O. This breakthrough stems from the use of helical edge modes within a higher-order topological insulator as an intrinsic spin filter during scanning tunneling microscopy, enabling spin-resolved measurements previously unattainable. This technique bypasses limitations inherent in traditional spin-polarized spectroscopic imaging, which relies on the often-unstable magnetic state of the STM tip itself.
Researchers demonstrated the versatility of this approach by switching the tip’s state between bulk-state and helical-edge-mode types in situ using electronic pulses, confirming the tunneling mechanism. The observed electronic structures of Bi4Br4 remained consistent during these transitions, further validating the role of helical edge modes in mediating the spin-resolved tunneling process. Visualizing the altermagnetic order within KV2Se2O required mapping magnetic contrast, achieved through analysis of the tunneling current at varying energies.
Spin-summed tunneling, using a conventional metallic tip, would equally image spin-polarized modulations along both the x and y axes. However, measurements using the helical-edge-mode tip revealed distinct, directional shifts in quasiparticle interference patterns, directly indicating opposite spin orientations for the Vx and Vy sublattices. These shifts were observed in magnetic contrast maps measured at both positive and negative energies, confirming the robustness of the spin-resolved signal.
The technique’s sensitivity extends to visualizing the spin-split band structure of KV2Se2O, a critical step in understanding its altermagnetic properties. This alignment provides strong evidence for the character of the altermagnetism, where the spin polarization is linked to the crystal symmetry. The ability to observe this symmetry-driven spin response in both real and momentum space represents an advancement in the field.
D-Wave Altermagnetism Defined by Dual-Space Feature
This observation confirms a complex interplay between magnetic and structural properties at the atomic level, previously inferred from global measurements but now resolved with unprecedented detail. The technique overcomes limitations of earlier studies hampered by unidentified magnetic structure, domain effects, and material imperfections that led to conflicting conclusions. The ability to distinguish spin polarization relies on a novel application of helical tunneling, where the direction of tunneling current selectively probes different spin-polarized states.
This alignment demonstrates that the two vanadium sublattices, linked by mirror symmetry within the selenium-oxygen plane, exhibit opposing spin orientations. The symmetry allows for spin splitting in the electronic structure even without spin-orbit coupling, resulting in a nonzero spin-conserved current due to anisotropic spin-polarized conductivity. The observed symmetry-related local spin density in real space, coupled with alternating spin-splitting between paired k-points in momentum space, is a defining characteristic of altermagnetism. This discovery in a 2D material holds promise for developing few-layer spintronic devices based on altermagnetism, potentially enabling new physics and applications.
Topology-Guaranteed Spin Filtering via Higher-Order Topological Insulators
The ability to reliably measure spin at the atomic scale now benefits from a new approach using the inherent properties of higher-order topological insulators. Rather than relying on magnetically functionalized tips susceptible to demagnetization, this work demonstrates spin filtering directly through the helical edge modes present in the material Bi4Br4. The foundation of this technique lies in the unique behavior of electrons traveling through the helical edge modes.
This contrasts with traditional spin-sensitive tips that require external magnetic fields and careful training to maintain magnetization. The researchers highlight that this new method circumvents these challenges, offering a more stable and dependable platform for exploring complex quantum materials. The observed spin polarization manifests as opposite signs along the x and y axes, providing direct evidence of a character to the spin polarization. The experiments establish a solid basis for the exploration and use of altermagnetism in layered materials and further facilitate access to atomic-scale spin sensing and manipulating of 2D quantum materials.
Bi4Br4 Tip Functionalization with Helical Edge Modes
This precise control stems from the material’s helical edge modes, electronic states confined to the crystal edges, and their inherent property of carrying spin tied to their direction of travel. The resulting anisotropy in spin modulation between the x and y directions provides a distinct signature for altermagnetic spin splitting, a phenomenon where magnetism arises from the arrangement of electrons rather than their intrinsic magnetic moment.
The ability to switch between bulk-state and helical-edge-mode tips in situ, within the microscope itself, through the application of electronic pulses represents an advancement in spin-sensitive microscopy. This dynamic control allows researchers to directly probe the spin characteristics of materials without the need for complex tip preparation or magnetization preservation techniques.
This finding supports the claim that the tunneling is genuinely governed by these helical edge modes, rather than other surface effects. “At opposite biases, the ‘helical tunneling’ of the edge modes selects quasiparticles with opposite spins, resulting in highly anisotropic signatures between the x and y directions,” the study reports, highlighting the key mechanism behind the observed spin sensitivity. The presence of helical edge modes is also confirmed on different types of bilayer steps, despite variations in their intensity, demonstrating the robustness of this phenomenon.
Researchers believe this technique will become a primary method for investigating atomic-scale spin textures in complex two-dimensional magnets and moiré materials. This approach promises to provide new insights into the behavior of quantum materials and accelerate the development of advanced spintronic devices.
Nonrelativistic Spin Splitting in Altermagnets vs. Ferromagnets
This observation establishes a foundation for understanding the interplay between magnetism and lattice structure at the atomic level, critical for designing novel quantum materials. Simultaneously, spin-resolved spectroscopic imaging visualizes d-wave spin splitting within the material’s band structure, a key characteristic of altermagnetism and a departure from traditional magnetic behavior. This simultaneous excitation from opposing valleys demonstrates a symmetry-driven phenomenon not typically observed in conventional magnets, and it provides a direct visualization of the spin-momentum locking inherent to altermagnetic materials.
The ability to observe these excitations is enabled by a topology-guaranteed spin filter, an important component in characterizing the material’s unique spin properties. Conventional strategies for manipulating spin rely on either relativistic spin splitting due to spin-orbit coupling or nonrelativistic Zeeman splitting in ferromagnets, but altermagnets offer a distinct advantage.
These materials induce spin splitting through exchange interactions, potentially reaching electronvolt scales even without spin-orbit coupling, making them promising candidates for next-generation information and quantum devices. The defining feature of altermagnetism is a specific crystal symmetry that connects opposite-spin sublattices, enforcing a spin-momentum locking in momentum space. However, definitive experimental confirmation of altermagnetism has been challenging due to factors like unidentified magnetic structure, multiple magnetic domains, lattice distortions and variations in material composition.
Previous studies relied on global measurements, which could be obscured by these complexities and led to conflicting conclusions. The new technique overcomes these limitations by providing atomic-scale spatial resolution, allowing researchers to directly observe the d-wave spin splitting and confirm the material’s altermagnetic character. The layered structure of KV2Se2O features a two-dimensional Lieb lattice composed of vanadium and oxygen atoms, forming a Néel antiferromagnetic order.
More importantly, this anisotropy appears in the spin polarization electronic modulations for Vx and Vy and rotates 𝜋𝜋 2 / when the bias polarity reverses, which is a “smoking gun” proof for d-wave altermagnetic spin splitting. This technique not only confirms the presence of altermagnetism in KV2Se2O but also establishes a solid basis for exploring and using this phenomenon in other layered materials, and further facilitates access to atomic-scale spin sensing and manipulating of 2D quantum materials.
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