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REVIEW 4 major objections 5 minor 2 references

Altermagnetism-Induced Spin-resolved electronic structure in Janus FeX0.5Y0.5 Monolayers (X, Y = S, Se, Te)

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read This paper predicts that Janus FeX0.5Y0.5 monolayers (X,Y = S, Se, Te) are altermagnets, with spin splitting throughout the Brillouin zone, spin-orbit gaps up to 51.4 meV, $\mathbb{Z}_2 = 1$, and Néel temperatures up to 415 K.

desk verdict Solid new altermagnetic material family; the quantum spin Hall claim needs a symmetry fix before it stands. read the letter →

arxiv 2608.09054 v1 pith:SQXVY3VE submitted 2026-08-10 cond-mat.mtrl-sci

Mengyang Zhang , Jie Li , Yifei Chen , Shifang Li , Zhentao Fu , Jianxin Zhong This is my paper · ORCID
classification cond-mat.mtrl-sci
keywords altermagnetismtwo-dimensionalmaterialsJanusmonolayersiron-basedsuperconductorsspin-resolvedelectronicstructurevalleypolarizationquantumspinHallfirst-principlescalculations
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper argues that making a Janus monolayer from an iron-chalcogenide parent—different chalcogen atoms on the two faces—breaks the symmetry that keeps the two antiparallel Fe sublattices equivalent, and that this is enough to turn the material into an altermagnet. An altermagnet has zero net magnetization but spin-split bands at every momentum, so the predicted FeX0.5Y0.5 monolayers would offer spin-resolved electronic structure without the stray fields of a ferromagnet. If the calculations are right, the same materials also carry large spin-orbit-induced gaps (up to 51.4 meV), a nontrivial $\mathbb{Z}_2$ invariant, edge states, and magnetic order up to 415 K, placing them among the few 2D altermagnetic platforms with strong topological character. The paper further claims that in-plane strain tunes a valley polarization that, together with a shifted Fermi level, yields a controllable anomalous Hall response.

What carries the argument

The load-bearing object is the Janus FeX$_{0.5}$Y$_{0.5}$ monolayer built on the duplex checkerboard lattice: two antiparallel Fe sublattices plus nonmagnetic chalcogen sites, with one chalcogen species on top and a different one below. The identity that carries the argument is the anisotropy parameter $\Delta t = t_{2a} - t_{2b}$ in the Kondo-type model Hamiltonian, the difference between next-nearest-neighbor hoppings along the two in-plane directions mediated by the two chalcogens' $p_z$ orbitals. When $\Delta t = 0$ the model's bands stay spin-degenerate under $PT$ symmetry; when $\Delta t \neq 0$ the same model develops full-Brillouin-zone spin splitting, the signature of altermagnetism. In the DFT calculations the Janus geometry supplies $\Delta t$ through asymmetric superexchange and an out-of-plane dipole, and this symmetry reduction is what converts antiferromagnetism into altermagnetism and, with spin-orbit coupling, opens the topological gaps.

What would settle it

Recompute the Wannier charge centers or Wilson-loop spectrum with spin-orbit coupling using the full magnetic space group of the altermagnetic state, without assuming any effective spinless time-reversal symmetry; if the Wilson loop is not gapped or the invariant is not quantized to 1, the topological claim is refuted.

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Extended reading notes

Core claim

The central discovery claimed is a mechanism, not just a set of materials: in a duplex checkerboard lattice, the anisotropic local environment of the two magnetic sublattices—created here by the Janus arrangement of S, Se, or Te on the two faces—changes the symmetry from $PT$-protected antiferromagnetism to $C_4\mathcal{T}$-symmetric altermagnetism. The band structures of FeSe$_{0.5}$Te$_{0.5}$, FeS$_{0.5}$Se$_{0.5}$, and FeS$_{0.5}$Te$_{0.5}$ monolayers show spin splitting across the whole Brillouin zone, dominated near the Fermi level by Fe $d$ states; the different superexchange through the two chalcogen species sets the anisotropy. With spin-orbit coupling the paper finds band gaps of 51.4 meV (FeSe$_{0.5}$Te$_{0.5}$), 38.3 meV (FeS$_{0.5}$Se$_{0.5}$), and 47.1 meV (FeS$_{0.5}$Te$_{0.5}$), a $\mathbb{Z}_2 = 1$ invariant from Wannier charge centers, and edge states in nanoribbon spectra that it attributes to time-reversal protection. Exchange parameters fitted to four magnetic configurations give Néel temperatures of 415 K, 335 K, and 360 K, and the valley polarization gap at the M point varies from about −40 meV to +25 meV under −2% to +2% in-plane strain.

Load-bearing premise

The topological conclusion assumes there is a symmetry, left unnamed in the paper, that makes $\mathbb{Z}_2$ well-defined in the magnetically ordered, spin-orbit-coupled state; if only ordinary time-reversal symmetry is present, the altermagnetic order breaks it and the computed $\mathbb{Z}_2 = 1$ and protected edge states would not stand.

Editorial extensions

If this is right

  • FeSe$_{0.5}$Te$_{0.5}$, FeS$_{0.5}$Se$_{0.5}$, and FeS$_{0.5}$Te$_{0.5}$ monolayers would be 2D altermagnets with spin-split bands across the entire Brillouin zone and no net magnetization, so they can supply spin-polarized carriers without ferromagnetic stray fields.
  • The 51.4 meV spin-orbit gap and $\mathbb{Z}_2 = 1$ would make FeSe$_{0.5}$Te$_{0.5}$ a quantum spin Hall insulator in monolayer form, with edge channels that are protected against nonmagnetic backscattering.
  • Néel temperatures of 335–415 K imply the altermagnetic order persists at room temperature, so devices built on these gaps and edge states would not need cryogenic magnetic order.
  • In-plane strain between −2% and +2% moves the valley polarization gap from about −40 meV to +25 meV, and shifting the Fermi level into one valley produces a net anomalous Hall conductivity without an external magnetic field.
  • Because the materials are derived from FeSe, FeTe, and FeS—parents of iron-based superconductors—the work points to a route for combining altermagnetic spin splitting with superconductivity in related compounds.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The design rule implied by the model—anisotropic next-nearest-neighbor hopping between antiparallel spin sublattices—should carry over to other antiferromagnetic monolayers with checkerboard order, so the same Janus asymmetric-capping trick can be tested on a broader family of parent superconductors and antiferromagnets.
  • If the $\mathbb{Z}_2$ classification survives a magnetic-space-group check, these chalcogenide monolayers become natural candidates for proximity-induced topological superconductivity: combining the altermagnetic order with the superconducting pairing of the FeSe/FeTe parent family could give Majorana-type states at edges or vortices, a step the paper does not itself take.
  • A direct experimental probe of the strain-valley coupling is to measure the anomalous Hall response while sweeping in-plane strain through the sign-change point of the valley polarization gap; the prediction is a sign reversal in Hall conductivity as strain crosses the zero-gap configuration.
  • Because the spin splitting does not rely on spin-orbit coupling, the same altermagnetic ordering could allow spin-polarized transport in the normal state while the spin-orbit gap provides a switchable topological state, two functionalities that could be addressed separately in one monolayer.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The manuscript predicts that Janus FeX0.5Y0.5 monolayers (X, Y = S, Se, Te), derived from iron-based superconductors, are two-dimensional altermagnets. Using a Kondo-type lattice model as a design guide and DFT+U calculations, the authors report spin-split bands throughout the Brillouin zone, SOC-induced gaps up to 51.4 meV, Z2 = 1 from Wannier charge centers, edge states in nanoribbons, Néel temperatures up to 415 K from Monte Carlo simulations of a fitted Heisenberg model, and strain-tunable valley polarization. They propose these materials as a new platform for spin-resolved electronics in a superconducting context.

Significance. If substantiated, the prediction of a new family of 2D altermagnets with large spin splitting, sizable topological gaps, and high ordering temperatures would be a useful contribution to the altermagnetism and 2D spintronics literature. The paper's strengths include a systematic DFT workflow with phonon and AIMD stability checks, direct calculation of spin splitting and Berry curvature, and WCC-based Z2 evaluation. The central altermagnetism claim is based on standard DFT+U calculations and is largely independent of the interpretive model. However, the topological claims rest on an unidentified symmetry, and several load-bearing numerical inputs (U value, exchange model, superconducting relevance) are not critically examined.

major comments (4)
  1. [Sec. III, final paragraph; Fig. 4(d)] The manuscript claims Z2 = 1 and states that the edge states in Fig. 4(d) are 'protected by time-reversal symmetry,' but the altermagnetic ground state with spin-orbit coupling breaks ordinary spinful time-reversal symmetry. The text never identifies the antiunitary symmetry (e.g., C4zT) under which the Wannier charge center calculation is performed, nor does it verify that such a symmetry survives SOC in the Janus structure. Because the C4z rotation is broken in a nanoribbon geometry, the edge states in Fig. 4(d) are not automatically protected by a bulk invariant that relies on that fourfold rotation. The authors should specify the protecting symmetry, compute the Z2 invariant with respect to that symmetry, and test the edge states under symmetry-preserving perturbations; otherwise the 51.4 meV gap cannot be claimed as a quantum spin Hall gap.
  2. [Sec. II; Sec. III; Table I] The Hubbard U = 1.0 eV is a fixed input, yet the exchange couplings, Néel temperatures, and the SOC-induced gap are central quantitative claims. No U-dependence study is presented, so it is unclear whether the topological invariant, the 51.4 meV gap, and the high Néel temperatures are robust. A variation of U over a reasonable range (e.g., 0-3 eV) with reporting of the resulting band gap, Z2, and exchange parameters is needed to support the predictions.
  3. [Eq. (2) and Table I] The Heisenberg Hamiltonian in Eq. (2) is written with a single next-nearest-neighbor coupling J2, but Table I lists two distinct couplings J2a and J2b. The manuscript does not give the mapping between the fitted total energies and the anisotropic J2 terms, nor does it report the details of the four magnetic configurations used in the fitting. Without this information, the Monte Carlo Néel temperatures in Table I cannot be reproduced or assessed. The authors should present the full fitting equations and a validation of the fitted parameters.
  4. [Abstract and Conclusion] The manuscript repeatedly frames the results as 'spin-splitting electronic states in superconducting materials' and 'superconducting spintronics,' but no superconducting property of the Janus monolayers is computed or demonstrated. The parent compounds are superconducting, but whether the proposed Janus monolayers retain superconductivity is not established. The authors should either add calculations or explicitly soften the superconducting claims so that the paper's conclusions match what is actually shown.
minor comments (5)
  1. [Abstract] The phrase 'spin-splittingelectronic states' contains a typo and should be 'spin-splitting electronic states.'
  2. [Eq. (1)] Equation (1) is garbled in the manuscript, with subscript and symbol corruption (e.g., 't",' 'cos&!"' and missing matrix elements). This makes the model Hamiltonian impossible to read; the equation should be typeset correctly.
  3. [Fig. 1 caption and text] The text refers to Fig. 1(b) and Fig. 1(d) for the isotropic and anisotropic Lieb-lattice bands, but the caption only describes panels (a)-(c), and panel (c) is described as the anisotropic case. The panel references should be made consistent.
  4. [Sec. III] The phrase 'small different from those of FeSe, FeTe and FeS' should read 'slightly different from those of FeSe, FeTe, and FeS.'
  5. [Sec. III] The notation 'C4T-symmetry' is used without a precise definition of the operator action on spin and spatial coordinates; a short symmetry-group statement would help the reader understand the claimed altermagnetic classification.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: DFT and Wannier results are self-contained; the one self-citation [40] is motivational, not load-bearing, and the questionable TRS statement is a correctness issue, not circularity.

full rationale

The only potential circularity is the appeal to [40] in Sec. III: 'As in our previous work [40], the crystal environment provided by nonmagnetic sites is necessary for the presence of altermagnetism, the nonmagnetic sites are therefore also included in our DCB-lattice and a Kondo-type model was further constructed.' This is a self-citation by overlapping authors, but it is not load-bearing: the Kondo-type model is written out explicitly in this paper (Eq. 1), and the spin splitting is shown to follow from the anisotropic next-nearest-neighbor hopping term (Δt ≠ 0) within the paper itself. The central claims—spin splitting, SOC-induced gaps, Z2 = 1 from Wannier charge centers, and Néel temperatures—are computed from DFT, Wannier functions, and Monte Carlo simulations. The exchange parameters J1, J2a, J2b entering the Heisenberg model are fitted to DFT total energies of four magnetic states, not to the reported Néel temperatures; thus TN is an output of the model rather than a fitted target. Similarly, the valley polarization and its strain dependence are direct DFT outputs. The statement that edge states are 'protected by time-reversal symmetry' is physically questionable for an altermagnet with SOC, since ordinary spinful time-reversal is broken, and the symmetry underlying the Z2 computation is not stated; however, this is a rigor/correctness concern, not a circular reduction of the prediction to its inputs. Overall, the derivation chain is self-contained against first-principles data, so circularity is minimal.

Assumptions & free parameters 5 free parameters · 4 assumptions · 0 invented entities

The central numbers inherit the uncertainty of a single DFT+U setup and a truncated Heisenberg model. U = 1.0 eV fixes correlation strength; the exchange couplings J1, J2a, J2b are fitted from four magnetic configurations of a 3x3 supercell; the Monte Carlo uses S = 3/2; and the topological claims assume a symmetry classification that is not stated. The superconducting framing adds an extra assumption because no superconducting property is computed. These are the main things a reader pays for upstream.

free parameters (5)
  • Hubbard U on Fe 3d = 1.0 eV
    Applied in DFT+U; sets the strength of d-electron correlation and, through it, exchange splittings, band gaps, and fitted J values. Not fitted in this paper; taken from prior practice [39].
  • Fe local moment S in Heisenberg model = 3/2
    Chosen for Monte Carlo simulations of Neel temperature (Eq. 2); no DFT-derived moment or temperature dependence is given.
  • Exchange couplings J1, J2a, J2b for FeSe0.5Te0.5 = -43.17 meV, -52.09 meV, -62.61 meV
    Fitted to total energies of four magnetic configurations in a 3x3 supercell (Table I, Fig. S5-S7); these couplings produce TN = 415 K.
  • Exchange couplings J1, J2a, J2b for FeS0.5Te0.5 = -61.96 meV, -35.86 meV, -75.14 meV
    Same fitting procedure; these couplings produce TN = 335 K.
  • Exchange couplings J1, J2a, J2b for FeS0.5Se0.5 = -86.25 meV, -46.55 meV, -79.66 meV
    Same fitting procedure; these couplings produce TN = 360 K.
assumptions (4)
  • domain assumption PBE+U with U = 1.0 eV is quantitatively adequate for Fe d-electron magnetism in these monolayers.
    Used for all band structures, magnetic energies, and gaps (Methodology, p. 3-4); no U-dependence or hybrid-functional cross-check is reported.
  • domain assumption The classical Heisenberg Hamiltonian with only J1 and J2 couplings and S = 3/2 captures the magnetic thermodynamics.
    Eq. (2) and Table I; Neel temperatures and exchange parameters depend on this truncation.
  • ad hoc to paper Janus FeX0.5Y0.5 monolayers inherit or retain superconductivity from their parent iron-based superconductors.
    The abstract and conclusion frame the work as spin-resolved states in superconducting systems, but no superconducting order parameter is computed.
  • ad hoc to paper A topological Z2 invariant and time-reversal-protected edge states are well defined in the altermagnetic ground state with spin-orbit coupling.
    Final paragraph of Results and Fig. 4(d); ordinary TRS is broken by magnetic order, so an explicit symmetry such as a spin-group or compensated symmetry is required but not given.

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Cite this review

Pith. "Pith review of Altermagnetism-Induced Spin-resolved electronic structure in Janus FeX0.5Y0.5 Monolayers (X, Y = S, Se, Te)." pith.science (2026). https://pith.science/paper/SQXVY3VE

@misc{pith2026260809054,
  author       = {Pith},
  title        = {Pith review of: Altermagnetism-Induced Spin-resolved electronic structure in Janus FeX0.5Y0.5 Monolayers (X, Y = S, Se, Te)},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SQXVY3VE}},
  note         = {Machine review of arXiv:2608.09054}
}
read the original abstract

Realizing the spin-resolved electronic properties in superconducting materials stands as a critical frontier, offering both novel fundamental physics and potential for dissipationless spin-based devices. Here, we predict a series of Janus FeX0.5Y0.5 monolayers derived from iron-based superconductors (e.g., FeSe, FeTe, and FeS) by using Kondo-type model and first-principles calculations. These Janus structures exhibit significant spin-splittingelectronic states, large topological band gaps (51.4 meV) and high N\'eel temperatures (415 K). We further reveal that valley polarization can be effectively tuned via applied in-plane strain and the resulting valley-polarized anomalous Hall conductivity can be manipulated by shifting the Fermi level. Our work suggests a new strategy based on altermagnetism for engineering spin-splitting states in superconducting systems and inspires further exploration of superconducting spintronics.

Figures

Figures reproduced from arXiv: 2608.09054 by the authors.

Figure 1
Figure 1. (a) Schematics of 2D isotropic Lieb-lattice (red and blue spheres represent two antiparallel magnetic sublattices, green and gray spheres represent the nonmagnetic sites). (b) The corresponding band structure from the Kondo-type model with 𝑡! = 1, 𝑡" = 0.2 and ∆%! = 0. (c) The case of ∆%! = 0.3. Here 𝑡!, 𝑡", and ∆%! are in units of 𝑡!. Guided by the above model analysis, symmetry engineering is adopted to introduce … view at source ↗
Figure 2
Figure 2. (a) Structure of the optimized monolayer Janus FeSe [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 4
Figure 4. (a), (b) The band structures of FeSe0.5Te0.5 with the spin-orbit coupling (SOC) included. (b) The corresponding zoom-in band structures near the Fermi level around the M point. (c) The Berry curvature distributions in the Brillouin zone of FeSe0.5Te0.5. (d) The corresponding one-dimensional band structures. These sizable gaps are a highly desirable feature for topological materials (see [PITH_FULL_IMAGE:figures/ful… view at source ↗

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Works this paper leans on

2 extracted references · 2 canonical work pages

  1. [19]

    Robinson, J

    J. Robinson, J. Witt, and M. Blamire,Controlled injection of spin-triplet supercurrents into a strong ferromagnet, Science 329, 59 (2010). [20] D. Wang, H. Wang, L. Liu, J. Zhang, and H. Zhang,Electric-field-induced switchable two-dimensional altermagnets, Nano Lett. 25, 498 (2024). [21] J. A. Ouassou, A. Brataas, and J. Linder,dc Josephson effect in alte...

    work page 2010

  2. [40]

    J. Li, S. Li, M. Zhang, P. Zhou, J. Zhong, and R. Wu, arXiv:2606.02152 (2026). [41] D. S. Antonenko, R. M. Fernandes, and J. W. Venderbos,Mirror Chern bands and Weyl nodal loops in altermagnets, Phys. Rev. Lett. 134, 096703 (2025). [42] W. Sun, H. Ye, L. Liang, N. Ding, S. Dong, and S. S. Wang,Stacking-dependent ferroicity of a reversed bilayer: Altermagn...

    work page Pith review arXiv 2026

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Reviewed August 14, 2026 · model on record in the stance chip above.