🔬 materials science

Real-space identification of distinct magnetic configurations in a candidate d-wave altermagnet

By employing spin-polarized scanning tunneling microscopy and magnetic-field-dependent quasiparticle interference imaging, this study resolves the magnetic origin of momentum-dependent spin splitting in the candidate d-wave altermagnet KV2Se2O by identifying coexisting C-type and G-type magnetic configurations and establishing a direct link between real-space magnetic order and momentum-space electronic signatures.

Original authors: Jin-Cheng Gu, Mingzhe Hu, Ziyin Song, Lihan Wang, Lihong Wang, Junming Zhang, Jiali Zhao, Hang Li, Shifeng Jin, Xin-Ding Zhang, Genfu Chen, Hongming Weng, Zhongxu Wei, Tian Qian

Published 2026-06-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Jin-Cheng Gu, Mingzhe Hu, Ziyin Song, Lihan Wang, Lihong Wang, Junming Zhang, Jiali Zhao, Hang Li, Shifeng Jin, Xin-Ding Zhang, Genfu Chen, Hongming Weng, Zhongxu Wei, Tian Qian

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Finding the "True" Magnetic Map

Imagine you are trying to figure out the layout of a city, but you can only see it from a satellite high in the sky. From up there, you see traffic patterns (electrons moving) that look like they are flowing in specific directions. You might guess the city is laid out in a specific way based on those traffic patterns.

However, this paper is about a special kind of "city" made of atoms called KV₂Se₂O. Scientists had been looking at this material from "space" (using a technique called ARPES) and saw traffic patterns that suggested it was a new type of magnetic material called an altermagnet.

An altermagnet is a bit of a paradox. It's like a crowd of people where half are wearing red shirts and half are wearing blue shirts. If you count the whole crowd, the colors cancel out (no net magnetism), but if you look at the movement of the red shirts versus the blue shirts, they move in very different, organized ways.

The Problem: The "satellite view" (momentum-space data) showed these organized movements, but it couldn't tell the scientists exactly how the red and blue shirts were arranged on the ground. Two different city layouts could produce the exact same traffic patterns from above. The scientists needed to get down to street level to see the real arrangement.

The Tool: The Magnetic Flashlight

To get a closer look, the researchers used a super-powerful microscope called Spin-Polarized Scanning Tunneling Microscopy (SP-STM).

Think of this microscope not just as a camera, but as a magnetic flashlight.

The tip of the microscope is like a tiny magnet.
When they shine this "light" on the material, it interacts differently with "red shirt" electrons than "blue shirt" electrons.
By flipping the magnet on their tip (like flipping a switch), they can see how the electrons react. If the reaction changes when they flip the switch, they know they are seeing the magnetic arrangement.

They also used a technique called Quasiparticle Interference (QPI). Imagine dropping a pebble in a pond; the ripples spread out and bounce off rocks. By looking at the pattern of these ripples (the interference), they can figure out where the "rocks" (magnetic atoms) are hiding.

The Discovery: Two Different Neighborhoods

When the researchers looked at the material atom-by-atom, they found something surprising. They discovered that the material wasn't just one uniform city; it had two different types of neighborhoods right next to each other.

The "C-Type" Neighborhood:
- Imagine a checkerboard where the red and blue squares alternate in one row, and the next row is exactly the same.
- In this arrangement, the magnetic pattern repeats perfectly as you go up through the layers of the material.
- This is the arrangement that scientists thought was an altermagnet.
The "G-Type" Neighborhood:
- Imagine the same checkerboard in the first row, but in the next row, the colors are flipped (where there was red, now there is blue).
- This is a more traditional antiferromagnet (like a standard magnet where neighbors cancel each other out).
- Crucially, from the "satellite view" (the top-down traffic patterns), this neighborhood looks almost identical to the C-Type one.

The "Step Edge" Test

How did they tell these two neighborhoods apart? They looked at the "stairs" between different layers of the material (called unit-cell step edges).

In the C-Type neighborhood: If you walk up a step to the next layer, the pattern of red and blue shirts looks exactly the same. The "magnetic flashlight" sees no change.
In the G-Type neighborhood: If you walk up a step, the pattern flips. The red becomes blue, and the blue becomes red. The "magnetic flashlight" sees a complete reversal.

By scanning across these steps, the researchers found both types of neighborhoods existing on the surface of the same material.

Why This Matters

The main takeaway is a warning for scientists studying these materials: You can't trust the "satellite view" alone.

Because the C-Type (altermagnet) and G-Type (conventional magnet) neighborhoods look so similar from a distance, previous studies might have been looking at the G-Type neighborhood but thinking it was the C-Type altermagnet.

This paper proves that to truly understand these magnetic materials, you need to combine the "satellite view" (momentum-space) with the "street view" (real-space). Only by seeing the actual arrangement of atoms can you know which type of magnetic order you are really dealing with.

In short: The material is a mix of two different magnetic patterns that look identical from far away but are completely different up close. The researchers used a magnetic microscope to prove that both patterns exist side-by-side, solving a mystery about what the material actually is.

Technical Summary: Real-space identification of distinct magnetic configurations in a candidate d-wave altermagnet

Problem Statement
Altermagnetism is a distinct magnetic order characterized by vanishing net magnetization coexisting with momentum-dependent spin-split electronic structures. While momentum-space signatures of altermagnetism have been observed in various materials via spin- and angle-resolved photoemission spectroscopy (SARPES), a critical ambiguity remains: similar spin-split electronic structures can arise from distinct real-space magnetic configurations. Specifically, in the candidate material KV $_2$ Se $_2$ O, the microscopic origin of the observed spin splitting has been controversial. First-principles calculations and SARPES data suggested a C-type magnetic configuration (consistent with d-wave altermagnetism), whereas neutron diffraction measurements indicated a G-type configuration (a conventional antiferromagnetic state). Both configurations share an identical checkerboard-like antiparallel spin texture within individual V $_2$ O layers, generating similar single-layer electronic structures and momentum-space signatures. However, their interlayer spin arrangements differ fundamentally, leading to distinct global electronic states. The inability to distinguish between these configurations using momentum-space data alone necessitates a direct real-space probe of the magnetic order.

Methodology
To resolve this ambiguity, the authors employed spin-polarized scanning tunneling microscopy (SP-STM) combined with magnetic-field-dependent quasiparticle interference (QPI) imaging. This approach allows for atomic-scale resolution of magnetic structures in real space.

Spin-Resolved QPI: Since KV $_2$ Se $_2$ O cleaves within the K layer (not the magnetic V $_2$ O layer), the authors utilized interband scattering between quasi-one-dimensional Fermi surface (FS) sheets. These sheets carry opposite spin polarizations. By using a magnetic Cr tip and reversing the external magnetic field, they modulated the tunneling probability into spin-polarized states.
Contrast Analysis: The technique relies on the fact that reversing the tip magnetization produces opposite intensity changes in scattering channels associated with different spin polarizations.
Interlayer Probing: The authors extended this analysis across unit-cell step edges. In a C-type configuration, the spin texture remains identical across layers, resulting in unchanged QPI contrast. In a G-type configuration, the spin texture reverses between layers, leading to a reversal of the field-dependent QPI contrast across the step edge.

Key Results

Identification of Scattering Channels: The study first established the QPI scattering vectors ( $q_1$ and $q_2$ ) corresponding to interband scattering between specific Fermi surface sheets ( $\alpha_{1,2}$ and $\beta_{1,2}$ ). Spin-unpolarized measurements confirmed that magnetic field reversal alone does not affect the QPI signal, establishing a baseline.
In-Plane Spin Texture: Spin-resolved QPI measurements revealed a pronounced anisotropic spin-dependent contrast. The difference maps ( $g_\uparrow - g_\downarrow$ ) showed positive contrast along the $a$ -axis and negative contrast along the $b$ -axis. This confirmed a checkerboard-like antiparallel spin texture within the V $_2$ O layer, where V sublattices associated with $a$ -axis chains are spin-up and those with $b$ -axis chains are spin-down.
Distinct Interlayer Configurations: By analyzing unit-cell step edges, the authors identified two distinct magnetic domains:
- C-type Configuration: On some step edges, the spin-dependent QPI contrast remained identical across the step, indicating that the spin texture does not change between adjacent layers.
- G-type Configuration: On other step edges, the spin-dependent QPI contrast reversed across the step, indicating a reversal of the spin texture between adjacent layers.
Coexistence: The simultaneous observation of both C-type and G-type configurations at the surface suggests a complex magnetic landscape arising from nearly degenerate magnetic states, consistent with the weak interlayer exchange coupling calculated for the material.

Significance and Claims
The paper establishes a direct connection between momentum-space spin splitting and real-space magnetic order, providing a framework for identifying the microscopic origin of spin-split electronic structures in altermagnetic materials. The primary contribution is demonstrating that momentum-dependent spin splitting alone is insufficient to uniquely identify altermagnetic order, as conventional antiferromagnetic configurations (G-type) can mimic the signatures of altermagnets (C-type) at the single-layer level.

The authors claim that their work resolves the controversy regarding the magnetic origin in KV $_2$ Se $_2$ O by revealing the coexistence of both C-type and G-type configurations. They posit that the realization of specific magnetic states in layered altermagnets may be influenced by subtle perturbations such as lattice distortion, strain, surface relaxation, or dimensional confinement. Consequently, the ability to manipulate these competing interlayer configurations could offer a route to control magnetic symmetry and associated electronic properties in layered altermagnetic materials. The study underscores the necessity of combining momentum-space and real-space probes to fully characterize candidate altermagnets.