{"id":"1b555306-7b56-4336-8add-574abcdb92bb","arxiv_id":"2411.17493","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Anti-parallel stacking of van der Waals magnetic layers can produce coexisting sliding ferroelectricity and altermagnetism, as shown by first-principles calculations.","lead":"This paper predicts that stacking ordinary 2D magnetic layers in certain anti-parallel arrangements can create materials that are both ferroelectric and altermagnetic. If correct, this gives a practical path to electric-field control of spintronic properties in van der Waals materials.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central claim presupposes AFM interlayer coupling and antiparallel stacking for each candidate, but the paper neither reports AFM-versus-FM energy differences nor tests sensitivity to U or competing stackings.","rationale":"The reader's weakest assumption and my analysis coincide: the predicted altermagnetism depends on the interlayer AFM coupling and the accessibility of the antiparallel stacking. This is the most load-bearing point because the symmetry construction is internally consistent only after those two conditions are granted. The paper provides no quantitative check of either condition: no AFM-FM energy differences, no U-sensitivity tests, and no stacking-energy comparison. The same methodology gives FM for CrI3/CrBr3, which shows the framework does not automatically produce AFM coupling. The twisted-bilayer result also lacks a stated twist angle, making the moire part untestable. These are correctness risks, not disagreements with the community consensus. If the proposed AFM states prove robust under U and stacking tests, the central claim would be substantially supported; until then, the conditional verdict is appropriate.","tokens_in":7222,"tokens_out":4342,"duration_ms":44871,"concrete_test":"For NiCl2, VSe2, CrSe2, and MnBi2Te4 in the proposed antiparallel stacking, compute the total-energy difference between the AFM interlayer configuration and the FM interlayer configuration using U = 0, the nominal U, and nominal U + 1 eV, and using at least two vdW treatments (DFT-D3 and optB86b-vdW). If the AFM state is not the ground state across this range, or if the energy difference changes sign, the altermagnetic classification is not robust. Also relax each bulk from several alternative stackings to confirm that the antiparallel stacking is a local or global minimum, and for the twisted 2H VS2 bilayer specify the twist angle and repeat the band-structure calculation.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that these multilayers are altermagnetic requires two conditions: (i) the antiparallel stacking is the realized/stable phase, and (ii) the interlayer magnetic coupling is antiferromagnetic. The symmetry analysis is then plausible, but the magnetic ground state is the load-bearing assumption. The paper states, for example, 'According to our DFT calculations, NiCl2 monolayer is FM in the ground state, while for the anti-parallel stacking bulk phase ... the interlayer couplings are AFM', yet no AFM-FM energy difference is reported. For VSe2, CrSe2, and twisted 2H VS2 the same assertion is made without quantitative support. The Methods lists fixed U values from previous studies with no sensitivity test, and the paper itself notes that CrI3 and CrBr3, under the same formalism, come out FM. Layered magnets are known to have small interlayer exchange that can change sign with U, van der Waals functional, and stacking; a wrong sign removes the compensated spin sublattices and destroys the altermagnetic spin splitting. Additionally, aside from NiCl2, there is no evidence that the antiparallel stacking is the equilibrium or experimentally accessible stacking. For the twisted 2H VS2 bilayer, no twist angle is specified, so that result is also not reproducible. These issues directly undercut the 'ubiquitous' and 'pervasive' framing, while the individual symmetry arguments would remain valid if the AFM ground state were robustly established.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper proposes a stacking-engineering strategy to realize van der Waals altermagnetism combined with sliding or moiré ferroelectricity. Using DFT (GGA+U) and spin-space-group analysis, the authors report non-relativistic spin splittings in antiparallel-stacked bulk NiCl2, VSe2, CrSe2, and MnBi2Te4, as well as in a twisted 2H VS2 bilayer and an untwisted polar 2H VS2 bilayer. They also compute ferroelectric switching barriers and polarizations for the bulk examples. The central claim is that these systems form a 'ubiquitous' and 'pervasive' class of altermagnetic ferroelectrics whose spin splitting can be switched by interlayer sliding.","tokens_in":7536,"tokens_out":3849,"duration_ms":36392,"significance":"The idea of combining sliding ferroelectricity with altermagnetism in layered van der Waals magnets is original and timely, and the specific DFT band structures for the bulk antiparallel-stacked examples, together with the spin-space-group assignments, provide concrete evidence that the proposed symmetry mechanism works in those model systems. The paper also identifies a potentially useful 'pseudo-altermagnetic' bilayer regime with zero net magnetization and field-reversible spin splitting. However, the claims of universality and of moiré ferroelectricity go beyond the presented calculations, and the physical realization of the altermagnetic state depends on a magnetic ground-state assumption that is not quantitatively established.","major_comments":[{"comment":"The AFM interlayer coupling is a load-bearing assumption that is asserted but not demonstrated. The text states that NiCl2, VSe2, CrSe2, and MnBi2Te4 in antiparallel stacking are AFM-coupled, but no AFM-versus-FM total-energy differences, no exchange parameters, and no Hubbard-U or vdW-functional sensitivity tests are reported. Because the same DFT setup yields FM order for CrI3 and CrBr3 (as the authors note), the sign of the interlayer exchange is not established by the presented data. If the true ground state is FM, the compensated spin sublattices disappear and the reported spin splittings do not describe the equilibrium phase.","section":"Results and Discussion (Figs. 2-3) and Methods"},{"comment":"The twisted 2H VS2 bilayer result is not reproducible because no twist angle, moiré cell size, or relaxation procedure is specified. The band structure in Fig. 4(a) appears to come from a small cell, and the moiré ferroelectricity claim is supported only by a schematic domain cartoon. A specific commensurate twist angle and a calculation of the moiré polarization are required to substantiate the combination of moiré ferroelectricity with altermagnetism.","section":"Results and Discussion, Fig. 4(a)-(b)"},{"comment":"The terms 'ubiquitous' and 'pervasive' overstate the evidence. The paper presents four bulk examples and one untwisted bilayer with spin splitting, plus one incompletely specified twisted bilayer, but gives no general structural criterion or broad materials survey that would justify universality. The central claim should be reformulated as a design principle, or the authors should provide a systematic search over a wider family of layered magnets.","section":"Abstract and Conclusions"},{"comment":"The concept of 'pseudo-altermagnetism' is not clearly distinguished from ordinary spin splitting induced by the vertical polarization. The text claims non-relativistic spin splitting with zero net magnetization, but no spin-space-group analysis is given for the untwisted 2H VS2 bilayer. Without a symmetry classification, the reader cannot tell whether the effect is a compensated altermagnetic splitting or simply a Rashba-like splitting from the polar field; a definition and symmetry analysis should be added.","section":"Results and Discussion, Fig. 4(c)"}],"minor_comments":[{"comment":"The Hubbard U values are taken from previous studies without any discussion of how sensitive the AFM/FM ordering or the spin splitting is to these parameters; even a short qualitative statement would help the reader assess robustness.","section":"Methods"},{"comment":"The spin-resolved band structures do not indicate the spin-projection axis, the energy scale, or the magnitude of the spin splitting; adding axis labels and a marker for a representative splitting would improve interpretability.","section":"Figures 2-4"},{"comment":"The polarization of the untwisted 2H VS2 bilayer is reported as 0.71 pC/m, whereas bulk polarizations elsewhere are in μC/m²; please check the units and conversion factors and use a consistent notation.","section":"Fig. 4(c)"},{"comment":"References [23], [24], and [27] are cited as arXiv preprints; if published versions now exist (for example, in Physical Review journals), they should be updated.","section":"References"},{"comment":"The text uses 'unitcell' without a space in several places; this should be corrected to 'unit cell'.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The manuscript addresses a timely and active topic, and the symmetry reasoning for bulk antiparallel-stacked systems is sound in itself. The missing ground-state energy comparisons and the unspecified twisted structure are, however, central to the paper's main claims, so I recommend major revision rather than rejection. The 'ubiquitous' framing should also be moderated unless a broader survey is added."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"What you should know first: this is a credible DFT-based proposal that antiparallel stacking of magnetic van der Waals layers yields coexisting sliding ferroelectricity and altermagnetism, with NiCl2, VSe2, CrSe2, and MnBi2Te4 as worked examples. The genuinely new piece is the explicit symmetry mechanism: in the bulk 1T stacked phases, the two spin sublattices are connected by a 180-degree rotation plus translation, which is a distinct route from the monolayer-only altermagnet proposals that dominate prior work. The paper also flags the possibility of combining altermagnetism with moire ferroelectricity in twisted bilayers, though that part is largely a proposal.\n\nWhat it does well: the symmetry analysis is careful. The authors use the spin-space-group formalism properly, identify the spin and magnetic space groups for NiCl2, and verify that band splitting appears exactly where the little-group criterion allows it. The ferroelectric switching pathways and barriers are computed, and the examples are experimentally synthesized materials, so the proposal is grounded. The observation that CrI3 and CrBr3 come out FM under the same treatment is an honest negative control. The citation pattern is fine—prior 2D altermagnet proposals are cited and the symmetry formalism is attributed correctly.\n\nWhere it gets soft: the 'ubiquitous' and 'pervasive' framing overreaches. Four examples, with no AFM-FM energy differences reported, no sensitivity tests for the Hubbard U, and no comparison against competing stackings, cannot support a generic claim. The stress-test concern lands here: the entire altermagnetism depends on the interlayer coupling being AFM and the antiparallel stacking being the stable phase. Layered magnets have small interlayer exchange that can flip sign with U or the vdW functional; the paper itself notes CrI3 and CrBr3 come out FM, so this is not a pedantic worry. The twisted 2H VS2 section is the thinnest: no twist angle is specified, and the moire ferroelectricity is illustrated with a cartoon rather than calculated. The term 'pseudo-altermagnetism' is introduced without a tight definition—it seems to mean zero net moment with spin-split bands from inequivalent layers, which is fine, but it needs sharper criteria if it is to be useful. No data deposited—'available upon request' is weak for a design paper.\n\nNone of this makes the core examples wrong. If the AFM ground states hold, the symmetry arguments and the direct DFT band splittings stand. The gap is between the results and the general claim.\n\nWho should read it: experimentalists working on 2D magnets and sliding ferroelectricity will find concrete candidate materials to test; theorists will appreciate the symmetry machinery. It deserves serious peer review. A referee should ask for the missing magnetic ground-state energies and U sensitivity, and for a real moire calculation in the twisted case. As it stands, this is a good idea with a too-broad title and a few missing control calculations.","headline":"Plausible stacking route to altermagnet-ferroelectric coexistence, but the 'ubiquitous' framing and missing magnetic ground-state checks mean the paper's strongest form overreaches; the specific bulk examples deserve a second look.","tokens_in":8093,"tokens_out":4273,"would_cite":false,"duration_ms":36735,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Anti-parallel stacking of magnetic van der Waals layers is predicted to make them simultaneously altermagnetic and sliding ferroelectric.","keywords":["altermagnetism","sliding ferroelectricity","moire ferroelectricity","van der Waals multilayers","non-relativistic spin splitting","pseudo-altermagnetism","antiferromagnetic interlayer coupling","first-principles calculations"],"falsifier":"A spin- and angle-resolved photoemission measurement on bulk antiparallel-stacked NiCl2, VSe2, CrSe2, or MnBi2Te4 looking along the predicted C-L path should detect non-relativistic spin-split bands; observing no splitting there, or finding a ferromagnetic ground state with net magnetization, would refute the central claim.","tokens_in":7003,"feed_emoji":"🧲","tokens_out":9147,"duration_ms":83861,"temperature":0.7,"pith_summary":"This paper argues that stacking magnetic van der Waals monolayers in anti-parallel configurations can make them altermagnetic while also giving them sliding ferroelectricity. The authors show by density-functional calculations that in bulk 1T NiCl2, VSe2, CrSe2, and MnBi2Te4 the interlayer antiferromagnetic coupling combined with inversion-symmetry-breaking stacking produces non-relativistic spin splitting with zero net magnetization. Twisted bilayers such as 2H VS2 are predicted to combine altermagnetism with moire ferroelectricity, and untwisted bilayers can show pseudo-altermagnetism whose spin splitting reverses when the polarization is switched by sliding. If correct, the work turns stacking into a general switch to create electric-field-controllable altermagnets.","feed_headline":"Magnetic stacks turn altermagnetic and ferroelectric at once","feed_subtitle":"First-principles calculations find this pairing is widespread, and sliding can flip the spin splitting.","key_machinery":"The central mechanism is the anti-parallel stacking of 1T magnetic MX2 layers, which breaks inversion symmetry and produces a vertical electric polarization switchable by interlayer sliding. In the periodic bulk, the vertical translation restores equivalence between adjacent layers, and the two spin sublattices are connected by the screw operation $A = \\{6^1_{001} \\mid \\tau(0,0,0.5)\\}$ (a 60-degree rotation about the c-axis combined with a half-cell translation), so the spin space group belongs to type SST-4B and non-relativistic spin splitting is allowed away from symmetry-protected k-paths. For twisted bilayers, a rotation operation connects the sublattices, and for untwisted bilayers the inversion-breaking polarization itself lifts spin degeneracy even though the layers are inequivalent. The paper uses spin space group classification, DFT band structures, nudged-elastic-band pathways for switching barriers, and Berry-phase polarization calculations to establish these predictions.","core_discovery":"The central claim is that altermagnetism, a collinear compensated magnetic order with non-relativistic spin splitting, can be engineered by the same anti-parallel stacking that creates sliding ferroelectricity. In the bulk 1T phases, each ferromagnetic layer couples antiferromagnetically to its neighbor, and the two opposing spin sublattices are connected by a rotation-plus-translation operation rather than by inversion, satisfying the symmetry rule for altermagnetism; the spin splitting appears along directions such as C-L while remaining symmetry-protected along high-symmetry paths. In twisted bilayers the rotation operation makes the two layers equivalent, giving altermagnetism combined with moire ferroelectric domains. For some untwisted bilayers the vertical polarization makes the layers inequivalent, yet the net magnetization is still zero and the spin splitting is coupled to sliding ferroelectricity, a state the authors call pseudo-altermagnetism.","pith_inferences":["The symmetry argument implies a wider design rule: any 1T magnetic MX2 monolayer with ferromagnetic intralayer order and antiferromagnetic interlayer coupling in an anti-parallel stack should be altermagnetic, so the candidate list can be extended beyond the compounds calculated here.","If the predicted switching survives in devices, the low sliding barriers (tens of meV per cell) would allow nonvolatile electric-field reversal of the spin-splitting sign at speeds and endurance comparable to state-of-the-art sliding ferroelectrics, something the paper notes but does not simulate.","A direct transport calculation of spin-polarized current in the pseudo-altermagnetic bilayer would test whether pseudo order behaves like true altermagnetism for practical spintronics; this is not reported in the paper.","The moire ferroelectric-altermagnetic bilayer suggests that twist angle could be used to pattern altermagnetic domains, and that local probes of the domain walls might reveal spin-split edge states; these are consequences the paper leaves implicit."],"forward_implications":["Bulk antiparallel-stacked 1T NiCl2, VSe2, CrSe2, and MnBi2Te4 are predicted to be simultaneous sliding ferroelectrics and altermagnets, with spin splitting along the C-L and related paths.","Ferroelectric sliding should reverse the sign of the non-relativistic spin splitting in untwisted bilayer 2H VS2, giving electric-field control of spin channels without moving magnetization.","Twisted magnetic bilayers such as 2H VS2 combine moire ferroelectricity with altermagnetism, producing spin-split bands in a ferroelectric domain superlattice.","The design rule is generic, so many van der Waals magnetic multilayers beyond the calculated examples are expected to show this coexistence.","The coexistence offers a crystal platform where spin splitting and electric polarization are coupled, potentially useful for electrically switchable spin transport."],"supporting_citations":[{"why":"Introduces the sliding ferroelectricity concept in stacked bilayers that the paper generalizes to altermagnetism.","marker":"[1]"},{"why":"Extends sliding ferroelectricity to moire systems and provides the ferroelectric domain picture used for twisted bilayers.","marker":"[2]"},{"why":"Supplies the elementary symmetry rules for identifying altermagnets that the stacking design must satisfy.","marker":"[30]"},{"why":"Provides the spin space group framework used to classify the magnetic configurations.","marker":"[24]"},{"why":"Defines the SST-4B altermagnetic type assigned to the anti-parallel stacked bulk phases.","marker":"[59]"},{"why":"Reports experimental synthesis of bulk NiCl2 used as the main case study.","marker":"[58]"},{"why":"Provides the experimental ferromagnetic 1T phase of VSe2 monolayers used as a candidate.","marker":"[60]"},{"why":"Supplies the Hubbard U value used in the NiCl2 calculations.","marker":"[67]"},{"why":"Supplies the U and J parameters used for MnBi2Te4.","marker":"[68]"},{"why":"Supplies the U, J, and van der Waals functional used for CrSe2.","marker":"[69]"}],"fun_headline_variants":["Stacking turns 2D magnets altermagnetic and ferroelectric","Sliding ferroelectricity pairs with altermagnetism","Moire stacks give altermagnetism plus ferroelectric domains","Anti-parallel stacks create ubiquitous altermagnetism","Twist and slide: altermagnetism meets ferroelectricity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The calculations assume the anti-parallel stacking configurations are the stable or experimentally accessible phases and that the adopted Hubbard U values capture antiferromagnetic interlayer coupling; if the real stacking or magnetic ground state differs, the predicted spin splitting will not appear.","fun_headline_variants_meta":{"raw":{"variants":["Stacking turns 2D magnets altermagnetic and ferroelectric","Sliding ferroelectricity pairs with altermagnetism","Moire stacks give altermagnetism plus ferroelectric domains","Anti-parallel stacks create ubiquitous altermagnetism","Twist and slide: altermagnetism meets ferroelectricity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000196,"raw_usage":{"total_tokens":1342,"prompt_tokens":905,"completion_tokens":437,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":521,"completion_tokens_details":{"reasoning_tokens":352}},"tokens_in":521,"tokens_out":437,"duration_ms":4390,"temperature":1.0,"reasoning_tokens":352,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T12:02:37.019206+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A spin- and angle-resolved photoemission measurement on bulk antiparallel-stacked NiCl2, VSe2, CrSe2, or MnBi2Te4 looking along the predicted C-L path should detect non-relativistic spin-split bands; observing no splitting there, or finding a ferromagnetic ground state with net magnetization, would refute the central claim.","supporting_citations":[{"cited_title":"Šmejkal, J","cited_arxiv_id":null,"evidence_quote":"Supplies the elementary symmetry rules for identifying altermagnets that the stacking design must satisfy."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Defines the SST-4B altermagnetic type assigned to the anti-parallel stacked bulk phases."},{"cited_title":"Aguirre et al., Adv Mater 36, e2402723 (2024)","cited_arxiv_id":null,"evidence_quote":"Reports experimental synthesis of bulk NiCl2 used as the main case study."},{"cited_title":"Bonilla et al., Nat Nanotechnol 13, 289 (2018)","cited_arxiv_id":null,"evidence_quote":"Provides the experimental ferromagnetic 1T phase of VSe2 monolayers used as a candidate."},{"cited_title":"Ghojavand, M","cited_arxiv_id":null,"evidence_quote":"Supplies the Hubbard U value used in the NiCl2 calculations."},{"cited_title":"Liu et al., Nano Lett 24, 16 (2024)","cited_arxiv_id":null,"evidence_quote":"Supplies the U and J parameters used for MnBi2Te4."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the U, J, and van der Waals functional used for CrSe2."}],"review_version":1}