REVIEW 2 major objections 5 minor 55 references
In Fe2Mo3O8, flipping ferroelectric polarization reverses both altermagnetic spin splitting and shift-current sign, while a-axis strain turns on a Kerr effect that symmetry forbids in the pristine crystal.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-15 03:14 UTC pith:CZKZWE7E
load-bearing objection Solid DFT map of polarization-reversed g-wave spin texture + shift-current sign flip in Fe2Mo3O8, with strain-tuned spectra and a-axis MOKE activation; residual Kerr under “symmetry-preserving” strains is a real but secondary control issue, not a load-bearing collapse. the 2 major comments →
Strain-Tunable Shift Current and Magneto-Optical Kerr Effect in Multiferroic Altermagnet Fe2Mo3O8
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In altermagnetic Fe2Mo3O8, ferroelectric polarization reversal comprehensively reconstructs the nonrelativistic spin-splitting texture and inverts the sign of the shift current, while a-axis uniaxial strain lowers crystalline symmetry and activates a finite magneto-optical Kerr effect that is forbidden by the pristine magnetic point group.
What carries the argument
The g-wave altermagnetic spin-splitting texture of compensated Fe moments (opposite-spin sublattices linked by rotation/mirror operations rather than inversion or pure translation) and the symmetry-allowed optical tensors (shift-current σzyy and the off-diagonal dielectric εxy that controls polar MOKE).
Load-bearing premise
The predicted sign flips, spectral peaks, and strain-activated Kerr signal rest on DFT+U bands and independent-particle optical tensors for the assumed altermagnetic ground state; if correlations, excitons, or a different magnetic order change those bands or allowed tensors, the responses can shift or vanish.
What would settle it
Measure the shift-current spectrum (especially σzyy) and polar Kerr rotation on Fe2Mo3O8 while electrically reversing polarization and applying controlled a-axis uniaxial strain; absence of sign reversal or of Kerr activation under the reported strain would falsify the central coupling claims.
If this is right
- Electrical polarization switching can reverse both the k-space spin texture and the polarity of second-order photocurrent without net magnetization.
- Biaxial or isotropic strain can substantially enhance and spectrally reshape the shift current by increasing joint density of states and redistributing k-space contributions.
- A-axis uniaxial strain is a practical symmetry-breaking handle that turns on polar MOKE (Kerr rotation of order a few degrees at +5% strain) while preserving altermagnetism.
- Fe2Mo3O8 becomes a concrete materials platform for devices that couple ferroelectric control, nonlinear photocurrent, and strain-tunable magneto-optics in a compensated magnet.
Where Pith is reading between the lines
- If the polarization-switch pathway remains accessible under the same strains that activate MOKE, a single device geometry could electrically reverse photocurrent polarity and mechanically gate Kerr readout.
- Similar polarization–spin-texture locking may appear in other A2Mo3O8 polar magnets, making the family a broader search space for electrically switchable altermagnetic optics.
- The reported residual Kerr signals under nominally symmetry-preserving strains warn that relaxation-induced symmetry lowering must be controlled experimentally before claiming pure strain-mode activation.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This first-principles DFT+U study examines polar multiferroic altermagnet Fe2Mo3O8 (space group P63mc). It reports a g-wave nonrelativistic spin-splitting texture that is reversed, together with the sign of the dominant shift-current component σzyy, upon ferroelectric polarization switching that interchanges tetrahedral and octahedral Fe sites while preserving compensated net magnetization. Strain screening shows that ab biaxial and abc shape-preserving strains enhance and spectrally reshape σzyy (peaks up to ~44 and ~52 μA/V²), correlated with JDOS and k-resolved redistributions, while a-axis uniaxial strain is argued to lower the magnetic point group from 6′mm′ to 2′, thereby activating a finite polar MOKE (Kerr rotation up to ~3.13° at +5%) that is symmetry-forbidden (εxy = 0) in the pristine phase.
Significance. If the predicted couplings hold, the work supplies a concrete multiferroic altermagnet platform in which electrical polarization, nonrelativistic spin texture, second-order photocurrent, and strain-activated magneto-optics can be jointly controlled—relevant to electrically readable altermagnetic spintronics and nonlinear optoelectronics. Strengths include explicit magnetic-point-group analysis of εxy, Wannier-interpolated shift-current spectra with JDOS and k-resolved maps, and a systematic strain survey that separates magnitude tuning from symmetry activation. The results are forward DFT predictions rather than parameter fits to the target observables, and they connect polarization switching, altermagnetic texture, and optical tensors in a single material.
major comments (2)
- Table 1 and the MOKE discussion (Results, Fig. 4 and surrounding text): the central claim that a-axis uniaxial strain uniquely activates finite polar MOKE by lowering MPG 6′mm′ → 2′ is undercut by residual θK Max = −0.85° already under +2% abc strain, which is introduced as a symmetry-preserving mode yet is reported with MPG 2′ after fully relativistic relaxation. The authors attribute residuals to SOC/relaxation artifacts and exclude them as non-robust channels, but the same relaxation protocol is used for the a-axis series that supplies the headline 3.13°. Without a controlled comparison that freezes ions to the intended high-symmetry cell (or quantifies εxy before vs after free relaxation for both modes), the numerical distinction between deliberate a-axis activation and uncontrolled symmetry lowering remains incomplete and load-bearing for the MOKE claim.
- Computational Methods and Results (ALM reference, Ueff = 4 eV, independent-particle shift current and dielectric tensors): all quantitative claims—sign reversal of σzyy, peak magnitudes ~23–52 μA/V², and Kerr angles up to ~3°—rest on a single DFT+U functional and the independent-particle approximation, with ALM taken as ground state from prior total-energy ranking. Band-edge positions, JDOS, and allowed tensor components can shift under different U, hybrid functionals, or excitonic corrections. At minimum, a short sensitivity check (e.g., Ueff variation or a hybrid single-point on key strained cells) is needed to establish that the qualitative polarization reversal and strain-activated MOKE survive beyond the chosen setup.
minor comments (5)
- Introduction and Results: several statements of prior work (polarization pathway, ALM vs FM/CFIM ranking) rely heavily on Ref. [32]; a brief self-contained summary of the energy ordering and switching barrier would help readers who do not have that paper at hand.
- Fig. 2(c) and Fig. 3(d–f): color scales and units for spin-splitting maps and k-resolved σzyy are not fully specified in the captions; adding them would improve reproducibility of the visual claims.
- Table 1: θK Max for pristine is left blank while residual values appear for abc; a consistent convention (0 or “—” with a footnote) would avoid ambiguity.
- Methods: the polar MOKE formula is standard, but the text should state explicitly whether local-field or scissors corrections are omitted, so that the independent-particle limitation is transparent.
- Typographical: “V ASP” spacing, “Ernerhof”, and occasional double spaces appear in the Methods and figure captions; a light copy-edit pass is warranted.
Circularity Check
No significant circularity: shift-current sign reversal, spin-texture reconstruction, and strain-activated MOKE are forward DFT+U/Wannier predictions, not forced by fitted parameters or self-citation chains.
full rationale
The paper’s load-bearing results are independent-particle evaluations of the shift-current tensor (Sipe–Shkrebtii via Wannier90) and the dielectric tensor (for polar Kerr rotation) on DFT+U (Ueff = 4 eV) band structures of Fe2Mo3O8 under polarization reversal and three strain modes. These are ordinary first-principles outputs; no free parameter is fitted to the target spectra or Kerr angles and then re-presented as a prediction. The choice of the altermagnetic reference state and the polarization-switching pathway are taken from an external prior calculation (Gao et al., Appl. Phys. Lett. 2025) whose author list does not overlap with the present work; that citation supplies a structural starting point, not a uniqueness theorem that forces the subsequent optical tensors. Overlapping-author citations (e.g., the medium-throughput optics framework) supply only methodological context and do not enter the numerical evaluation of σzyy or θK for this compound. Residual Kerr signals under nominally symmetry-preserving strains are acknowledged by the authors as relaxation artifacts and are explicitly excluded from the “activation” claim; that is a numerical-control issue, not a definitional circularity. Consequently the derivation chain does not reduce to its inputs by construction.
Axiom & Free-Parameter Ledger
free parameters (3)
- Ueff on Fe d orbitals =
4 eV
- Strain amplitude range and selected ratios =
−5% to +5%; highlights +5% a, +4% ab, +2% abc
- k-mesh and energy cutoff choices =
500 eV; 5×5×3, 10×10×6, 100×100×100
axioms (6)
- domain assumption GGA-PBE + Dudarev DFT+U adequately describes Fe2Mo3O8 electronic structure and magnetic order for optical tensor trends.
- domain assumption Independent-particle approximation (Sipe–Shkrebtii) captures the dominant shift-current and dielectric response without excitonic or local-field corrections.
- domain assumption The altermagnetic configuration is the relevant magnetic ground state (energetically favored over FM and CFIM in the pristine structure).
- standard math Magnetic point-group operations (e.g., 2′001) force εxy → −εxy hence εxy = 0 in 6′mm′, and lowering to 2′ allows finite MOKE.
- domain assumption Weak type-I multiferroic coupling preserves local Fe moment directions under polarization switching, so only spin channels interchange.
- domain assumption Maximally localized Wannier functions from the DFT bands faithfully interpolate interband matrix elements for dense-mesh shift-current integration.
read the original abstract
Altermagnetism has recently emerged as a compelling frontier in spintronics, seamlessly merging the agile tunability of ferromagnets with the hallmark merits of antiferromagnets. As a prototypical polar multiferroic featuring distinctive altermagnetism, Fe2Mo3O8 hosts an ideal playground for exploring the intricate interplay among ferroelectric polarization, altermagnetic order, and spin-dependent responses. Here, employing first-principles calculations, we systematically investigate the coupling among polarization, spin splitting, shift current, and magneto-optical responses in Fe2Mo3O8. Our findings reveal that switching the ferroelectric polarization not only inverts the sign of the shift current but also comprehensively reshapes the momentum-space spin-splitting texture. Furthermore, the shift current and magneto-optical spectrum exhibits strong tunability under mechanical strain. Remarkably, the application of a-axis uniaxial strain breaks the crystalline symmetry, thereby activating a finite magneto-optical Kerr effect that is otherwise forbidden in the pristine phase.
Figures
Reference graph
Works this paper leans on
-
[1]
Krempaský et al., Altermagnetic Lifting of Kramers Spin Degeneracy, Nature 626, 517 (2024)
J. Krempaský et al., Altermagnetic Lifting of Kramers Spin Degeneracy, Nature 626, 517 (2024)
2024
-
[2]
C. Song, H. Bai, Z. Zhou, L. Han, H. Reichlova, J. H. Dil, J. Liu, X. Chen, and F. Pan, Altermagnets as a new class of functional materials, Nat Rev Mater 10, 473 (2025)
2025
-
[3]
X. Chen, Y . Liu, P. Liu, Y . Yu, J. Ren, J. Li, A. Zhang, and Q. Liu, Unconventional Magnons in Collinear Magnets Dictated by Spin Space Groups, Nature 640, 349 (2025)
2025
-
[4]
Šmejkal, J
L. Šmejkal, J. Sinova, and T. Jungwirth, Beyond Conventional Ferromagnetism and Antiferromagnetism: A Phase with Nonrelativistic Spin and Crystal Rotation Symmetry, Phys. Rev. X 12, 031042 (2022)
2022
-
[5]
L.-D. Yuan, A. B. Georgescu, and J. M. Rondinelli, Nonrelativistic Spin Splitting at the Brillouin Zone Center in Compensated Magnets, Phys. Rev. Lett. 133, 216701 (2024)
2024
-
[6]
L.-D. Yuan, X. Zhang, C. M. Acosta, and A. Zunger, Uncovering Spin-Orbit Coupling- Independent Hidden Spin Polarization of Energy Bands in Antiferromagnets, Nat Commun 14, 5301 (2023)
2023
-
[7]
L. Bai, W. Feng, S. Liu, L. Šmejkal, Y . Mokrousov, and Y . Yao, Altermagnetism: Exploring New Frontiers in Magnetism and Spintronics, Advanced Functional Materials 34, 2409327 (2024)
2024
-
[8]
S. S. Fender, O. Gonzalez, and D. K. Bediako, Altermagnetism: A Chemical Perspective, J. Am. Chem. Soc. 147, 2257 (2025)
2025
-
[9]
Šmejkal, J
L. Šmejkal, J. Sinova, and T. Jungwirth, Emerging Research Landscape of Altermagnetism, Phys. Rev. X 12, 040501 (2022)
2022
-
[10]
X. Zhou, W. Feng, R.-W. Zhang, L. Šmejkal, J. Sinova, Y . Mokrousov, and Y . Yao, Crystal Thermal Transport in Altermagnetic RuO2, Phys. Rev. Lett. 132, 056701 (2024)
2024
-
[11]
Feng et al., An Anomalous Hall Effect in Altermagnetic Ruthenium Dioxide, Nat Electron 5, 735 (2022)
Z. Feng et al., An Anomalous Hall Effect in Altermagnetic Ruthenium Dioxide, Nat Electron 5, 735 (2022)
2022
-
[12]
R. D. Gonzalez Betancourt et al., Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor, Phys. Rev. Lett. 130, 036702 (2023)
2023
-
[13]
Šmejkal, A
L. Šmejkal, A. B. Hellenes, R. González-Hernández, J. Sinova, and T. Jungwirth, Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling, Phys. Rev. X 12, 011028 (2022)
2022
-
[14]
Hoyer, R
R. Hoyer, R. Jaeschke-Ubiergo, K.-H. Ahn, L. Šmejkal, and A. Mook, Spontaneous Crystal Thermal Hall Effect in Insulating Altermagnets, Phys. Rev. B 111, L020412 (2025)
2025
-
[15]
Iguchi, H
S. Iguchi, H. Kobayashi, Y . Ikemoto, T. Furukawa, H. Itoh, S. Iwai, T. Moriwaki, and T. Sasaki, Magneto-Optical Spectra of an Organic Antiferromagnet as a Candidate for an Altermagnet, Phys. Rev. Res. 7, 033026 (2025)
2025
-
[16]
Jiang, U
X. Jiang, U. Jeong, S. Sato, D. Shin, K. Yabana, B. Yan, and N. Park, Nonlinear Photocurrent as a Hallmark of Altermagnet, ACS Nano 19, 23620 (2025)
2025
-
[17]
F. Li, B. Zhao, V . Chaudhary, S. Wang, C. Shen, H. Wang, and H. Zhang, Medium- Throughput Evaluation of Transport and Optical Responses in Altermagnets, arXiv:2604.17071
-
[18]
L. Wei, F. Li, Y . Liu, H. Zhang, J. Luo, and Z. Sun, Shift current anomalous photovoltaics in a double perovskite ferroelectric, Proceedings of the National Academy of Sciences 123, e2518381123 (2026)
2026
-
[19]
Huang, C
Y . Huang, C. Hua, R. Xu, J. Liu, Y . Zheng, and Y . Lu, Spin Inversion Enforced by Crystal Symmetry in Ferroelastic Altermagnets, Phys. Rev. Lett. 135, 266701 (2025)
2025
-
[20]
J. D. Cao, K. S. Denisov, Y . Liu, and I. Žutić, Symmetry Classification for Alternating Excitons in Two-Dimensional Altermagnets, Phys. Rev. Lett. 135, 266703 (2025)
2025
-
[21]
Yan and M
Y . Yan and M. Wu, Ionic Sliding Ferroelectricity in Layered Ion Conductors, Phys. Rev. Lett. 135, 236801 (2025)
2025
-
[22]
X. Yang, X. Zhou, J. Shi, S. Qian, X. Wang, W. Wang, and Y . Li, Magnetic-Field and Strain Engineering of Modulated Transverse Transport in Altermagnetic Topological Materials, Phys. Rev. B 112, 214418 (2025)
2025
-
[23]
Li, A.-D
J.-Y . Li, A.-D. Fan, Y .-K. Wang, Y . Zhang, and S. Li, Strain-induced valley polarization, topological states, and piezomagnetism in two-dimensional altermagnetic V2Te2O, V2STeO, V2SSeO, and V2S2O, Applied Physics Letters 125, 222404 (2024)
2024
-
[24]
Z. Zhou, X. Cheng, M. Hu, R. Chu, H. Bai, L. Han, J. Liu, F. Pan, and C. Song, Manipulation of the altermagnetic order in CrSb via crystal symmetry, Nature 638, 645 (2025)
2025
-
[25]
Liang, F
T. Liang, F. Kong, J. Zhao, and X. Jiang, Strain-dependent nonlinear variation of the anomalous Hall effect in RuO2, Phys. Rev. B 112, 224318 (2025)
2025
-
[26]
Karetta, X
B. Karetta, X. H. Verbeek, R. Jaeschke-Ubiergo, L. Šmejkal, and J. Sinova, Strain- controlled g - to d -wave transition in altermagnetic CrSb, Phys. Rev. B 112, 094454 (2025)
2025
-
[27]
Fan, Y .-K
A.-D. Fan, Y .-K. Wang, J.-Y . Li, and S. Li, Valley-dependent electronic properties in two-dimensional altermagnetic iron-based transition metal chalcogenides, Phys. Rev. B 112, 235135 (2025)
2025
-
[28]
Y . Wang, G. L. Pascut, B. Gao, T. A. Tyson, K. Haule, V . Kiryukhin, and S.-W. Cheong, Unveiling hidden ferrimagnetism and giant magnetoelectricity in polar magnet Fe2Mo3O8, Sci Rep 5, 12268 (2015)
2015
-
[29]
Li et al., Oxygen-Self-Supply Synthesis of Two-Dimensional Fe2Mo3O8 Semiconductor Single Crystal With Colossal Ferroelectric Polarization, Advanced Materials, e18468 (n.d.)
X. Li et al., Oxygen-Self-Supply Synthesis of Two-Dimensional Fe2Mo3O8 Semiconductor Single Crystal With Colossal Ferroelectric Polarization, Advanced Materials, e18468 (n.d.)
-
[30]
J. Dong, K. Wu, M. Zhu, F. Zheng, X. Li, and J. Zhang, Nonrelativistic spin-splitting multiferroic antiferromagnets and compensated ferrimagnet with zero net magnetization, Phys. Rev. B 112, 024425 (2025)
2025
-
[31]
K. V . Vasin, Optical magnetoelectric effect in the polar honeycomb antiferromagnet Fe2Mo3O8, Phys. Rev. B 110, (2024)
2024
-
[32]
Y . Gao, S. Li, and M. Wu, Ferroelectric control of antiferromagnetism via coordination swapping in A2Mo3O8 (Aௗ=ௗMn, Fe, Co), Appl. Phys. Lett. 126, 252901 (2025)
2025
-
[33]
Kresse and J
G. Kresse and J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B 47, 558 (1993)
1993
-
[34]
Kresse and J
G. Kresse and J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal– amorphous-semiconductor transition in germanium, Phys. Rev. B 49, 14251 (1994)
1994
-
[35]
Kresse and J
G. Kresse and J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Computational Materials Science 6, 15 (1996)
1996
-
[36]
Kresse and J
G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B 54, 11169 (1996)
1996
-
[37]
P. E. Blöchl, Projector augmented-wave method, Phys. Rev. B 50, 17953 (1994)
1994
-
[38]
J. P. Perdew, K. Burke, and M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77, 3865 (1996)
1996
-
[39]
Reschke, A
S. Reschke, A. A. Tsirlin, N. Khan, L. Prodan, V . Tsurkan, I. Kézsmárki, and J. Deisenhofer, Structure, phonons, and orbital degrees of freedom in Fe2Mo3O8, Phys. Rev. B 102, 094307 (2020)
2020
-
[40]
Pizzi et al., Wannier90 as a community code: new features and applications, J
G. Pizzi et al., Wannier90 as a community code: new features and applications, J. Phys.: Condens. Matter 32, 165902 (2020)
2020
-
[41]
Ibañez-Azpiroz, S
J. Ibañez-Azpiroz, S. S. Tsirkin, and I. Souza, Ab initio calculation of the shift photocurrent by Wannier interpolation, Phys. Rev. B 97, 245143 (2018)
2018
-
[42]
J. E. Sipe and A. I. Shkrebtii, Second-order optical response in semiconductors, Phys. Rev. B 61, 5337 (2000)
2000
-
[43]
Molina-Sánchez, G
A. Molina-Sánchez, G. Catarina, D. Sangalli, and J. Fernández-Rossier, Magneto- optical response of chromium trihalide monolayers: chemical trends, J. Mater. Chem. C 8, 8856 (2020)
2020
-
[44]
M. Wu, Z. Li, S. Louie. Optical and Magneto-Optical Properties of Ferromagnetic Monolayer: A First-Principles and plus Bethe-Salpeter Equation Study, PhysRevMaterials.6.014008
-
[45]
M. Wu, Z. Li, T. Cao, and S. G. Louie, Physical origin of giant excitonic and magneto- optical responses in two-dimensional ferromagnetic insulators, Nat Commun 10, 2371 (2019)
2019
-
[46]
Ideue, T
T. Ideue, T. Kurumaji, S. Ishiwata, and Y . Tokura, Giant thermal Hall effect in multiferroics, Nature Mater 16, 797 (2017)
2017
-
[47]
Chang, Y
Y . Chang, Y . Weng, Y . Xie, B. You, J. Wang, L. Li, J.-M. Liu, S. Dong, and C. Lu, Colossal Linear Magnetoelectricity in Polar Magnet Fe2Mo3O8, Phys. Rev. Lett. 131, 136701 (2023)
2023
-
[48]
Kurumaji, S
T. Kurumaji, S. Ishiwata, and Y . Tokura. Doping-Tunable Ferrimagnetic Phase with Large Linear Magnetoelectric Effect in a Polar Magnet, PhysRevX.5.031034
-
[49]
M. Gu, Y . Liu, H. Zhu, K. Yananose, X. Chen, Y . Hu, A. Stroppa, and Q. Liu, Ferroelectric Switchable Altermagnetism, Phys. Rev. Lett. 134, 106802 (2025)
2025
-
[50]
Y . Chen, X. Liu, H.-Z. Lu, and X. C. Xie, Electrical Switching of Altermagnetism, Phys. Rev. Lett. 135, 016701 (2025)
2025
-
[51]
J. Wang, J. B. Neaton, H. Zheng, V . Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V . V aithyanathan, D. G. Schlom, and R. Ramesh. Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures. Science 299, 1719 (2003)
2003
-
[52]
B. B. Van Aken, T. T. M. Palstra, A. Filippetti, and N. A. Spaldin, The origin of ferroelectricity in magnetoelectric YMnO3, Nature Mater 3, 164 (2004)
2004
-
[53]
N. T. Kaner et al., Enhanced Shift Currents in Monolayer 2D GeS and SnS by Strain- Induced Band Gap Engineering, ACS Omega 5, 17207 (2020)
2020
-
[54]
H.-G. Min, C. Lyi, and Y . Kim, Tunable bulk photovoltaic effect in strained γ -GeSe, Phys. Rev. B 106, 205153 (2022)
2022
-
[55]
Ye et al., Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides, Nat Commun 14, 5911 (2023)
L. Ye et al., Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides, Nat Commun 14, 5911 (2023)
2023
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.