Reading Altermagnetic Domains with Photon Drag

arXiv:2610.10970 · cond-mat.mes-hall, cond-mat.mtrl-sci, cond-mat.str-el · Submitted 2026-10-07 · Read on arXiv

Listen

Radio episode about this paper

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Reading Altermagnetic Domains with Photon Drag".

Mira: The gist: Finite photon momentum allows second-order photocurrents in centrosymmetric space groups, where the conventional bulk photovoltaic effect is forbidden,

Kai: First, who's behind it and why it matters.

Title and authors: Kai: Let’s start by looking at the title and who wrote this paper, "Reading Altermagnetic Domains with Photon Drag". It sounds a bit like it's telling us exactly what we're going to learn from reading this thing.

Mira: It definitely suggests a direct measurement approach because they are using photon drag to read out the domains. That’s a very tangible way to link light interaction directly to the magnetic structure of the material.

Lev: So, when you look at Ali Akbar Ghorashi and Andrew M. Rappe as authors, what kind of expertise do we expect from them in this area?

Kai: Kai is an experimentalist who deals with quantum hardware and measurements, so we expect him to be focused on the physical setup—what was actually built and cooled—to ensure the results are measurable.

Mira: And Mira is a condensed matter theorist, so we're looking for her to be focused on making sure all those claims about symmetry and mechanisms hold up under the right theoretical assumptions.

Lev: From an error correction standpoint, we’re watching how this kind of measurement dependence affects what we could actually run on real quantum hardware later.

Kai: Right. So, it sounds like a good pairing for understanding both the physical realization and the underlying theory of this specific paper.

Mira: It is; they have to make sure the theoretical framework matches what Kai can physically measure when he’s looking at those angular harmonics in the data.

Lev: That matching process is where we often run into trouble, so I'm interested in how robust these domain identification methods are against experimental noise.

Kai: Well, the paper addresses that by showing how the rotation of ninety degrees allows them to isolate the domain-odd current, which helps distinguish it from what’s just a nonmagnetic background <ref:2610.10970#pg1,isolate the domain-odd current>.

Mira: And they build on that by showing how polarization dictates whether you see the shift term or those injection and principal-value terms dominating.

Lev: That’s interesting because it means we have different experimental paths—linear versus circular light—to look at the same fundamental physics of domain identification.

Kai: So, if you are setting up an experiment, this paper gives you two distinct ways to probe the domains using just a single sample and rotation.

Mira: It really moves the discussion away from needing external magnetic fields for every measurement, which is a big practical win for characterizing these complex materials.

Lev: That’s what I want to hear—a method that simplifies the experimental protocol significantly, making it more accessible than traditional methods.

The paper's summary: Kai: Now let's look at the actual summary of "Reading Altermagnetic Domains with Photon Drag". Essentially, they’re using light to probe altermagnets that have two degenerate Néel domains.

Mira: So the core finding is that finite photon momentum lets them get a second-order photocurrent where it normally wouldn't be seen in centrosymmetric space groups. That opens up a whole new avenue for study.

Lev: So, what does this mean for the material itself? Is it just confirming things we already knew about the magnetic order?

Kai: No, it goes deeper than that because they show that the angular dependence of this current reveals both the symmetry of the magnetic order and which Néel domain you are looking at.

Mira: It’s not just detecting magnetism; it’s identifying the specific way those two degenerate domains are connected—whether by translation or crystal rotation.

Lev: So, if they can tell them apart, that means we can finally characterize these altermagnets properly instead of just saying they have some kind of alternating spin splitting.

Kai: Exactly. They’ve shown that a ninety-degree rotation flips the sign of the order magnetic-state relation drag response, which is how you switch between those domains in their experiment <ref:2610.10970#pg1>.

Mira: And they break down that total current into two parts: one component reverses sign with the domain, and another part doesn't change its sign.

Lev: That decomposition is what allows them to determine the domain from a single measurement, which is a huge step forward for experimentalists.

Kai: So, if we take this paper as a whole, it’s about using photon drag to read out the Néel domain of an altermagnet without needing magnetic field switching.

Mira: And they connect that back to polarization effects by showing how the microscopic mechanism changes depending on whether you use linear or circular light.

Lev: It’s like they're giving us a complete picture, from the input light properties all the way down to the magnetic order itself.

The paper's improvements: Kai: So what are the actual improvements this paper suggests? It seems like they’re proposing ways to make this measurement more robust and easier to use.

Mira: They suggest that by looking at the angular harmonics, specifically 4n versus 4n+two you can separate the domain-even and domain-odd parts of the current <ref:2610.10970#pg1,the domain-even and domain-odd parts of the current>.

Lev: So, if I’m setting up a setup, I need to know which harmonic channel corresponds to which part of the magnetic state.

Kai: Right. For tetragonal d-wave altermagnets, they specify that the domain-even part appears in harmonics 4n and the odd part in 4n+two <ref:2610.10970#pg1,For tetragonal $d$-wave altermagnets>.

Mira: This is a very specific piece of information because it links the symmetry representation directly to the harmonic channels you are measuring.

Lev: So if we want to classify a material, we look at which nulls show up in those harmonics and which ones don't, based on this paper.

Kai: And they also point out that mirror symmetries fix the angular phase of each harmonic, which helps them distinguish between different orders like dx two-y squared, dxy, and g-wave <ref:2610.10970#pg1,mirror symmetries fix the angular phase of each harmonic>.

Mira: And they specifically note that for the tetragonal A2g g-wave state, it follows a different symmetry requirement with a specific form factor.

Lev: So if we want to rule out a candidate magnetic order, we can check if its predicted null points align with what the current actually shows in these harmonic channels.

Kai: It’s also important to note that the paper emphasizes that a symmetry-enforced zero is useful for ruling out candidate orders because it violates expected patterns.

Mira: And they show how the microscopic mechanism changes sign based on polarization, which gives you an extra handle for independent domain determination using only linear or circular light.

Lev: So the improvement isn't just in getting a better signal, but in getting a way to classify different orders using these harmonic signatures and nulls.

Conclusion: Kai: Okay, to wrap up on "Reading Altermagnetic Domains with Photon Drag", the main implication is that we now have a method for reading out the Néel domain of an altermagnet without magnetic field switching.

Mira: So, this means we can do this characterization using just optical measurements and crystal rotation as the key tool.

Lev: For me, it’s about making it feasible to move these types of studies from complex setups into something that can actually be tested on more accessible experimental platforms.

Kai: Exactly. It gives us a route to all the optical magnetic readouts of altermagnets, and this paper provides a strategy for reading those domains without relying on external magnetic fields.

Mira: And they conclude that while the altermagnetic spin splitting doesn't need spin-orbit coupling to exist, SOC is essential to observe that domain-odd charge current at all.

Lev: So even though SOC isn’t strictly required for the splitting itself, you still need it for that specific charge current signal to appear.

Kai: That’s the final piece of the puzzle: if you don't have SOC, every domain-odd charge current simply vanishes.

Mira: And so, this paper provides a comprehensive strategy for reading Néel domains without magnetic field switching using photon drag measurements on altermagnets.

Lev: So we’ve covered how to use crystal rotation and polarization to isolate domain-odd signals and classify the magnetic orders based on their harmonic nulls.

Kai: That’s what we've been covering with this paper, "Reading Altermagnetic Domains with Photon Drag".

Department of Chemistry, University of Pennsylvania

cond-mat.mes-hall, cond-mat.mtrl-sci, cond-mat.str-el

Submitted: 2026-10-07

Updated: 2026-10-07

Comments: Submitted, 2 Figures

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 75/100

The gist: The gist: Finite photon momentum allows second-order photocurrents in centrosymmetric space groups, where the conventional bulk photovoltaic effect is forbidden, enabling the study of altermagnets

Key concepts

Altermagnets
These are collinear magnets where two spin sublattices are linked by crystal rotation instead of translation or inversion. They have alternating spin splitting in the Brillouin zone but no net magnetization, making them difficult to distinguish between their two Néel domains.
Photon Drag Mechanism
This mechanism uses the finite momentum of light to bypass the centrosymmetry rule that usually forbids second-order photocurrents. Rotating the sample relative to the light beam effectively switches which Néel domain is being probed, allowing for domain identification.
Domain Parity and Angular Harmonics
The resulting current's angular dependence is split into even and odd parts, each carried by different harmonics. For tetragonal magnets, a 90-degree rotation shifts the even part to one set of harmonics (4n) and the odd part to another (4n+2), helping to map out the magnetic structure.

Terminology

Summary

The gist: Finite photon momentum allows second-order photocurrents in centrosymmetric space groups, where the conventional bulk photovoltaic effect is forbidden, enabling the study of altermagnets and revealing their magnetic order and Néel domain through angular dependence.

Introduction to Altermagnets

Altermagnets are defined as collinear magnets where the two spin sublattices are connected by crystal rotation rather than translation or inversion As a result, they combine alternating spin splitting in the Brillouin zone with vanishing net magnetization. This momentum-dependent spin splitting gives rise to various electronic phenomena, including anomalous transport and unconventional superconductivity. Distinguishing the two Néel domains of an altermagnet remains challenging because they are degenerate and do not carry a net moment.

Photon Drag Mechanism

The study demonstrates that photon drag overcomes both challenges by utilizing the finite momentum of light to remove the centrosymmetry restriction on the second-order photocurrent. The key ingredient for domain identification is crystal rotation, where a 90◦ rotation reverses the sign of the order magnetic-state relation drag response. This rotation redirects the photon momentum, meaning rotating the sample relative to the beam is equivalent to switching the domain.

Domain Parity and Angular Harmonics

The angular dependence of the current is separated into domain-even and domain-odd parts, which are carried by different angular harmonics. For tetragonal d-wave altermagnets, a 90◦ rotation results in the domain-even part appearing in harmonics 4n and the odd part in 4n + 2. The microscopic mechanism whose contribution changes sign depends on polarization: the resonant shift current for linearly polarized light, and the injection current together with two principal-value contributions for circularly polarized light.

Domain Readout and Comparison

The separation of domain-odd and domain-even currents allows the domain to be determined from a single sample, with linear polarization yielding independent measurements via the shift current versus circular polarization yielding independent measurements via injection, Fermi-surface, and contact principal-value terms. The directions along which the current vanishes differ between dx2−y2, dxy, and g-wave order. Mirror symmetries fix the angular phase of each harmonic and distinguish dx2−y2, dxy, and g-wave order. Conventional collinear antiferromagnets show no domain-odd photon-drag current, while tetragonal ferromagnets share the same harmonics for both parts.

Microscopic Mechanisms and Model

The microscopic origin of the currents is unpacked into four mechanism-resolved contributions: shift, injection, Fermi-surface (FS), and contact principal-value (bPV). The shift and injection components are strictly resonant, whereas the FS and bPV contributions are dispersive. The exact transition energy at finite q∥ is given by a formula involving the band splitting terms.

Symmetry Constraints and Order Identification

The specific magnetic order is pinned down by the combination of harmonic channels, phase shifts, and symmetry nodes where the current vanishes. The B1g (dx2−y2) and B2g (dxy) representations exchange domains under C4, but their mirror eigenvalues are inverted. The tetragonal A2g g-wave state follows a different symmetry requirement, possessing a form factor of gg(k) = sin kx sin ky(cos kx−cos ky).

Experimental Implementation

The analysis assumes an in-plane optical field, and the in-plane photon momentum q∥a = 0.2 serves as a finite-q∥ benchmark. This provides a strategy for reading Néel domains without magnetic field switching. The results provide a route to all optical magnetic readouts of altermagnets.

Conclusion on SOC Dependence

While the altermagnetic spin splitting does not require spin-orbit coupling (SOC), SOC is essential to observe the domain-odd charge current. The exact transition energy at finite q∥ is a sum of the two gaps plus a tilt from d0 for interband transitions, while for same-band transitions it is a difference that vanishes as q∥ → 0. Every domain-odd charge current therefore vanishes without SOC. The results provide a strategy for reading Néel domains without magnetic field switching.

References

[8] Y. Fang, J. Cano, and S. A. A. Ghorashi, Quantum geometry induced nonlinear transport in altermagnets, Phys Rev Lett 133 106701 (2024).

[9] X. Jiang, S. A. A. Ghorashi, D. Lu, and J. Cano, Altermagnetism induced surface chern insulator (2025), arXiv:2510.15357 [cond-mat.mes-hall].

[10] R.-M Fernandes, V.-S de Carvalho, T Birol, and R G Pereira, Topological transition from nodal to nodeless zeeman splitting in altermagnets (2023), arXiv:2307.12380 [cond-mat.mes-hall].

[11] A Hadjipaschalis, S. A. A. Ghorashi, and J Cano, Majoranas with a twist: Tunable majorana zero modes in altermagnetic heterostructures (2025), arXiv:2507.00119 [cond-mat.mes-hall].

[12] P.-H Fu, S Mondal, J.-F Liu, and J Cayao, Lightinduced floquet spin-triplet cooper pairs in unconventional magnets, SciPost Physics 20 059 (2026).

[13] P.-H Fu, S Mondal, J.-F Liu, Y Tanaka, and J Cayao, Floquet engineering spin triplet states in unconventional magnets Phys Rev Lett 136 066703 (2026).

[14] Y Fukaya, B Lu, K Yada, Y Tanaka, and J Cayao, Superconducting phenomena in systems with unconventional magnets Journal of Physics: Condensed Matter 37 313003 (2025).

[15] P.-H Fu and S. A. A. Ghorashi, Complete magnetic hierarchy in bichromatically driven unconventional magnets (2026), arXiv:2610.03461 [cond-mat.mes-hall].

[16] J Ding et al., Large band splitting in g-wave altermagnet CrSb, Phys Rev Lett 133 206401 (2024).

[17] S Reimers, L Odenbreit, L Smejkal, V N Strocov, P Constantinou et al., Direct observation of altermagnetic band splitting in crsb thin films Nature Communications 15 2116 (2024).

[18] G Yang, Z Li, S Yang, J Li et al., Three-dimensional mapping of the altermagnetic spin splitting in crsb Nature Communications 16 1442 (2025).

[19] J Krempasky, L Smejkal, S D’souza et al., Altermagnetic lifting of kramers spin degeneracy Nature 626 517 (2024).

[20] Y.-P Zhu, X Chen, X.-R Liu et al., Observation of plaid-like spin splitting in a noncoplanar antiferromagnet Nature 626 523 (2024).

[31] Y.-M Xie and N Nagaosa, Photon-drag photovoltaic effects and quantum geometric nature Proc Natl Acad Sci USA 122 e24242941 (e) [cond-mat.mes-hall].

[30] L.

Improvements for AI systems

  1. Bold domain identification from photon drag measurements: The improved system can directly determine whether an altermagnet is in one of its two N´eel domains by analyzing angular harmonics, as The key ingredient is the crystal rotation that defines altermagnetism. This allows for a domain-readout scheme formulated in Eq. (3) to be implemented without needing magnetic field switching.

  2. Polarization-dependent mechanism isolation: The system can distinguish between microscopic origins of the current by polarization, as "the microscopic mechanism whose contribution changes sign depends on the polarization: the resonant shift current for linearly polarized light, and the injection current together with the two principal-value contributions for circularly polarized light." This enables independent domain determination using only linear or circular light.

  3. Magnetic order classification: The system can classify specific magnetic orders by examining their unique angular signatures, as the directions along which the current vanishes differ between dx2−y2, dxy, and g-wave order. This provides a method to distinguish between different altermagnetic representations based on harmonic channels and nulls.

  4. Symmetry enforcement for material screening: The system can identify material constraints by recognizing symmetry-enforced zeros, noting that a symmetry-enforced zero is primarily useful for ruling out candidate orders. This allows the system to efficiently eliminate magnetic models that violate observed null points in the current response.

  5. Finite momentum current vertex modeling: The improved AI can accurately model the finite-q response by using the exact lattice Ward identity [32] and accounting for straight-bond Peierls phases to generate all necessary vertices, providing a precise description of how magnetic order enters the response.

Sources

Related papers