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arxiv: 2512.00737 · v2 · submitted 2025-11-30 · ❄️ cond-mat.str-el

Bi-altermagnetism unveiled by sublattice-specific circular dichroism in resonant inelastic X-ray scattering

G. Channagowdra , A. Singh , H. Y. Huang , M. Furo , Bin Gao , Pengcheng Dai , C.T. Chen , J. Kunes
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A. Fujimori S-W. Cheong A. Hariki D. J. Huang
This is my paper

Pith reviewed 2026-05-17 03:39 UTC · model grok-4.3

classification ❄️ cond-mat.str-el
keywords bi-altermagnetismRIXS circular dichroismFe2Mo3O8altermagnetmagnetic sublatticeszero net magnetizationmirror symmetry breaking
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The pith

Circular dichroism in resonant inelastic X-ray scattering detects bi-altermagnetism by revealing symmetry breaking on two distinct magnetic sublattices in Fe2Mo3O8.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper aims to show that Fe2Mo3O8 exhibits bi-altermagnetism, a form where two altermagnetic sublattices with different local environments break mirror symmetry while maintaining zero net magnetization. This is detected through the appearance of circular dichroism in RIXS at excitations specific to octahedral and tetrahedral iron sites. A reader would care because it offers a way to identify such magnetic orders in materials without net magnetism, which could be useful for applications requiring spin polarization without stray fields. The authors support this with experiments and model calculations that match the observed dichroism.

Core claim

Bi-altermagnetism in Fe2Mo3O8 consists of two altermagnetic sublattices—one with alternating quasi-octahedral Fe environments and the other with alternating tetrahedral Fe environments—whose combined order along the c axis breaks mirror symmetry, producing sublattice-specific circular dichroism in RIXS even in an achiral system with zero net magnetization. Model calculations with this order reproduce the CD signals.

What carries the argument

Sublattice-specific circular dichroism in RIXS, which acts as a probe of mirror-symmetry breaking associated with bi-altermagnetic order on both the octahedral and tetrahedral Fe sites.

If this is right

  • Bi-altermagnetic order can be confirmed in other zero-magnetization insulators using RIXS-CD.
  • The technique distinguishes magnetic behavior on different sublattices in complex magnetic structures.
  • Systems with multiple altermagnetic sublattices may show enhanced or unique symmetry-breaking effects.
  • Calculations based on c-axis bi-altermagnetic order provide a framework for predicting CD in similar materials.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Bi-altermagnetism might enable new spintronic devices that operate without external magnetic fields due to the combined properties of multiple sublattices.
  • This approach could be extended to probe altermagnetic orders in other correlated insulators or even under different experimental conditions like varying temperature or pressure.
  • Neighboring problems in condensed matter physics, such as understanding PT symmetry breaking in multi-sublattice systems, may benefit from similar dichroism measurements.

Load-bearing premise

The observed circular dichroism arises specifically from mirror-symmetry breaking due to bi-altermagnetic order on both sublattices rather than from other possible symmetry-lowering effects or experimental artifacts.

What would settle it

If experiments on Fe2Mo3O8 show no circular dichroism or if the dichroism does not match the predictions from the bi-altermagnetic model with order along the c axis, or if it appears uniformly across sites rather than sublattice-specific, the claim would be falsified.

Figures

Figures reproduced from arXiv: 2512.00737 by A. Fujimori, A. Hariki, A. Singh, Bin Gao, C.T. Chen, D. J. Huang, G. Channagowdra, H. Y. Huang, J. Kunes, M. Furo, Pengcheng Dai, S-W. Cheong.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: a shows the RIXS spectra of Fe2Mo3O8 with an incident photon energy tuned to the L3 XAS peak (706 eV) at momentum transfer Q = (0.05, 0, 0.65) in reciprocal lat￾tice units, with which all momentum transfers are expressed throughout the Article. The spectra exhibit characteristic fea￾tures labeled A, B, and C, which are well separated in energy. These features exhibit distinct Fe-site character. Feature B i… view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
read the original abstract

An altermagnet is a recently identified class of magnets that exhibit a zero net magnetic moment but break symmetry under the combined operations of parity and time reversal. It typically consists of two magnetic sites of opposite spins related by rotation within the unit cell. Here, we use circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) to identify a new form of altermagnetism, namely bi-altermagnetism, in the correlated insulator Fe2Mo3O8, which comprises two altermagnetic sublattices: one with alternating quasi-octahedral Fe environments and the other with alternating tetrahedral Fe environments. We experimentally revealed the emergence of CD in an achiral, zero-magnetization system, thereby probing mirror-symmetry breaking associated with altermagnetic order. Notably, the CD appeared at sublattice-specific excitations of the octahedral and tetrahedral sites, indicating symmetry breaking in both altermagnetic sublattices. Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD. Our findings provide compelling evidence for bi-altermagnetism in Fe2Mo3O8, and showcase the use of RIXS-CD as a probe of magnetic sublattices in systems with zero net magnetization.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper claims to identify a new form of altermagnetism termed bi-altermagnetism in the zero-net-magnetization correlated insulator Fe2Mo3O8. Using resonant inelastic X-ray scattering with circular dichroism (RIXS-CD), the authors report sublattice-specific CD signals at excitations associated with quasi-octahedral and tetrahedral Fe sites. They show that a model incorporating bi-altermagnetic order along the c-axis reproduces the observed CD, interpreting this as evidence of mirror-symmetry breaking on both magnetic sublattices and positioning RIXS-CD as a probe for such compensated magnetic order.

Significance. If the central claim is substantiated, the work is significant for introducing bi-altermagnetism as a distinct realization involving two altermagnetic sublattices and for demonstrating RIXS-CD as a sublattice-resolved probe in achiral, zero-moment systems. This could advance experimental characterization of altermagnets and correlated insulators, with potential implications for symmetry-breaking studies in compensated magnets.

major comments (2)
  1. [Abstract] Abstract: The claim that 'Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD' is presented without quantitative metrics of agreement (e.g., chi-squared, residual analysis), error bars on the experimental CD data, or details on data exclusion criteria. This information is load-bearing for assessing whether the reproduction is robust or could arise from other symmetry-lowering mechanisms.
  2. [Results/Discussion] Results/Discussion (modeling section): No comparisons are provided to alternative models, such as different magnetic configurations preserving zero net moment, structural distortions, or experimental geometry artifacts that could generate similar CD at the reported excitations. Without ruling these out, agreement with the bi-altermagnetic model alone does not establish uniqueness for the mirror-symmetry breaking interpretation on both sublattices.
minor comments (2)
  1. [Abstract] Abstract: The introduction of 'bi-altermagnetism' would benefit from a concise inline definition or reference to distinguish it from standard altermagnetism before describing the two sublattices.
  2. [Figures] Figure captions (assumed from typical structure): Ensure all RIXS spectra and CD plots include explicit labels for octahedral vs. tetrahedral site contributions and polarization geometries to improve clarity for readers.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for providing constructive feedback. We address each of the major comments below and have revised the manuscript accordingly to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The claim that 'Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD' is presented without quantitative metrics of agreement (e.g., chi-squared, residual analysis), error bars on the experimental CD data, or details on data exclusion criteria. This information is load-bearing for assessing whether the reproduction is robust or could arise from other symmetry-lowering mechanisms.

    Authors: We agree that providing quantitative metrics and error bars would improve the robustness assessment. In the revised manuscript, we will add error bars to the experimental circular dichroism data and include a quantitative comparison, such as a chi-squared value or residual analysis, between the experimental and calculated spectra. Additionally, we will provide details on the data exclusion criteria in the methods section. These additions will help confirm that the agreement is not coincidental. revision: yes

  2. Referee: [Results/Discussion] Results/Discussion (modeling section): No comparisons are provided to alternative models, such as different magnetic configurations preserving zero net moment, structural distortions, or experimental geometry artifacts that could generate similar CD at the reported excitations. Without ruling these out, agreement with the bi-altermagnetic model alone does not establish uniqueness for the mirror-symmetry breaking interpretation on both sublattices.

    Authors: We have examined alternative explanations and find that they do not account for the observed sublattice-specific CD. For instance, structural distortions without magnetic order would not produce the energy-dependent CD signals tied to the Fe d-d excitations. Similarly, other zero-net-moment configurations lack the mirror symmetry breaking on both sublattices required to match the CD signs at octahedral and tetrahedral sites. To make this explicit, we will add comparisons to these alternative models in the revised manuscript, demonstrating that the bi-altermagnetic order provides the best and unique fit to the data. revision: yes

Circularity Check

0 steps flagged

Experimental observation of CD reproduced by bi-altermagnetic model; no derivation reduces to fitted input by construction

full rationale

The paper's central claim rests on experimental detection of sublattice-specific circular dichroism in an achiral zero-magnetization system, followed by a model calculation that reproduces the observed CD. No equations or steps are presented in which a prediction is defined in terms of the target quantity itself, nor is a uniqueness theorem imported from self-citation to force the interpretation. The modeling step is described only as successful reproduction rather than a parameter-free first-principles derivation or a fit that is then relabeled as a prediction. This places the work in the normal non-circular range (0-2) for an experimental study whose load-bearing evidence is the raw CD signal rather than a closed mathematical loop.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available; therefore free parameters, axioms, and invented entities cannot be enumerated from the full text. The bi-altermagnetic model is invoked but its explicit assumptions and parameters are not stated here.

pith-pipeline@v0.9.0 · 5582 in / 1185 out tokens · 39529 ms · 2026-05-17T03:39:27.578749+00:00 · methodology

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Reference graph

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