{"id":"8b284640-3fae-423c-847e-b5b82404bba0","arxiv_id":"2502.03452","paper_version":1,"verdict":"REJECT","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"high","formal_verification":"none","parameter_count":4,"one_line_summary":"Y3Co8Sn4 shows a sign-changing unconventional anomalous Hall effect, but the proposed Weyl-point mechanism is undercut by the paper's own inconsistent nonmagnetic-phase analysis.","lead":"A combined experimental and computational study of the polar magnet Y3Co8Sn4 reports an anomalous Hall effect that changes sign across the magnetic transition temperature. The paper attributes this to Weyl points and magnetic textures, but internal contradictions in the analysis leave the central claim unsupported.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The sign-changing UAHE is not established: it is the residual of a scaling subtraction whose key assumption, that the unconventional term vanishes above Hc, is untested and could itself produce the reported sign reversal.","rationale":"The reader's weakest_assumption correctly identifies the scaling subtraction as the most load-bearing point: the paper's headline observation is exactly the residual of that subtraction, so an untested assumption invalidates the central claim regardless of the DFT interpretation. The DFT result in the nonmagnetic phase, a nonzero AHC despite time-reversal symmetry, is a separate and equally fatal problem for the above-TC mechanism; the authors' own argument that random domains cancel the transverse conductivity contradicts the reported negative peak. The extraction issue is logically prior because it bears on whether the sign-changing UAHE exists at all. Since the authors provide no raw data or code and no independent check of the subtraction, and since the internal contradiction in the NM-phase mechanism remains, the REJECT verdict is unchanged.","tokens_in":12723,"tokens_out":5508,"duration_ms":53823,"concrete_test":"Re-analyze the raw ρxy(H) isotherms using several Hc choices (for example, Hc ± 20% and Hc ± 50%) and an alternative high-field background model that does not impose the form ρUA = γρxx²M, such as a polynomial in H and M. If the positive-to-negative peak of the extracted ρUA_xy does not persist with a stable sign, amplitude, and field window across these variations, the dual UAHE is an artifact of the subtraction.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central observation, the dual positive-to-negative UAHE, is not directly measured but is defined as a residual: ρxy = R0H + γρxx²M + ρUA_xy. The authors fit (ρxy/H) versus (ρxx²M/H) only in the field range above Hc, where they assume ρUA_xy = 0, and then subtract the fitted normal and conventional contributions from the measured ρxy at all fields. If ρUA_xy does not vanish above the chosen Hc, or if the scaling form RS = γρxx²M is not exact, the fitted coefficients are contaminated and the residual can produce a sign-changing component even if no physical UAHE exists. No independent determination of Hc or of the scaling form is provided; the same data are used both to define and to test the model. Above TC, M(H) is small and nearly linear, making the fit of (ρxy/H) versus (ρxx²M/H) extremely sensitive to noise, contact misalignment, and magnetoresistance. The claimed dual behavior therefore collapses if this subtraction is not validated.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript reports a combined transport, magnetization, ac-susceptibility, and density-functional-theory study of the hexagonal polar magnet Y3Co8Sn4 (space group P63mc). The authors claim an unconventional anomalous Hall effect (UAHE) whose sign is positive below the magnetic transition (TC ≈ 53 K) and negative above it, which they attribute to Weyl-point Berry curvature in the ferrimagnetic ground state and in the high-temperature nonmagnetic phase, with a possible contribution from topological spin textures inferred from field-dependent ac susceptibility. The DFT study identifies a planar ferrimagnetic (FiM2) ground state with 3.3 µB/f.u. and five pairs of near-Fermi Weyl points, giving an intrinsic anomalous Hall conductivity (AHC) of about 168 S/cm, compared with a measured value of about 160 S/cm at 30 K. The authors also report four Weyl pairs in a nonmagnetic calculation and an AHC of 154 S/cm, which they compare with the 60 K experimental value of 136 S/cm.","tokens_in":12895,"tokens_out":16007,"duration_ms":144253,"significance":"If the sign-changing dual UAHE were established experimentally, Y3Co8Sn4 would be a notable addition to the small family of compounds showing two-stage unconventional Hall behavior in a noncentrosymmetric magnet, with plausible relevance to spintronics. The DFT portion is a genuine strength: Table I documents relative energies and moments for all tested collinear configurations, Table II lists coordinates, types, energies, and chiralities of the Weyl points, and the computed intrinsic AHC (168 S/cm) and planar FiM2 ground state are consistent with the low-temperature magnetization data; these are concrete, falsifiable, first-principles predictions with no fitting to the transport data. However, the central transport claim is built entirely on an unvalidated subtraction residual, the nonmagnetic-phase AHC claim violates time-reversal symmetry, and the proposed high-temperature mechanism contradicts the reported observation; these problems are load-bearing and cannot be fixed by local revision. The transport analysis also contains no error bars anywhere.","major_comments":[{"comment":"The extraction of ρUAxy is self-referential: the linear fit of (ρxy/H) versus (ρxx²M/H) is restricted to fields above a critical field Hc, where ρUAxy is assumed to vanish, and the resulting R0 and γ are then subtracted from the full field range to define ρUAxy as a residual. The manuscript never states the values of Hc, the fitting windows, the number of points per fit, or any measure of fit quality, and no error bars are propagated into ρUAxy or the derived AHC. Above TC, M(H) is small and nearly linear, so ρxx²M/H is almost field-independent; in that regime the slope γ is ill-conditioned, and the reported negative peak in Fig. 2(f) is consistent with a subtraction artifact (for example, slight curvature in M(H) or magnetoresistance anisotropy). Because the dual positive-to-negative UAHE is the central claim and exists only as this residual, the claim is not established without independent validation (e.g., Hc determined from dM/dH or χ′(H), stability of the fit over varied windows, and a full-range global fit that includes a parameterized ρUAxy term).","section":"Results and Discussions, Fig. 2(c)–(f)"},{"comment":"The field range of the reported ρUAxy humps below 14 K coincides with the two anomalies in dM/dH at 60 mT and 0.3 T and with the peak/hump structure in χ′(H). Because the intrinsic AHE depends on the magnetization direction, a field-induced spin reorientation changes ρCAxy in a way that the smooth γρxx²M form cannot represent; the paper does not test whether the extracted ρUAxy is simply the deviation of the intrinsic AHE from linearity in M through these reorientations. This is a concrete, unaddressed alternative explanation for the low-temperature signal.","section":"Results and Discussions, Figs. 1(e)–(f) and 2(f)"},{"comment":"The calculated AHC of 154 S/cm at EF for the nonmagnetic phase is incompatible with time-reversal symmetry. In a spin-unpolarized, T-symmetric band structure, Ωn(−k) = −Ωn(k) and the occupation factor is even in k, so the Brillouin-zone integral defining σxy vanishes identically; the equal +/− chirality balance of the reported Weyl pairs, which the authors themselves connect to the Nielsen–Ninomiya theorem, likewise forces the net Berry-curvature contribution to zero. The claimed agreement between the computed NM AHC (154 S/cm) and the measured 60 K value (136 S/cm) therefore cannot be a physical comparison; either the NM calculation is not actually spin-unpolarized, or the reported AHC is a numerical artifact. The measured above-TC AHC is a field-induced quantity (time reversal broken by the applied field), so a zero-field T-symmetric calculation is not an appropriate reference in any case. This undermines the headline claim of a reasonably large AHC in both phases.","section":"Results and Discussions, nonmagnetic-phase DFT paragraph and Fig. S7"},{"comment":"The proposed explanation of the high-temperature behavior is internally inconsistent with the reported observation. The text states that within randomly oriented short-range ferromagnetic domains the positive and negative chirality contributions cancel, 'explaining the lack of spontaneous component of UAHE in NM phase.' A cancellation mechanism predicts a vanishing net UAHE, yet Fig. 2(f) reports a prominent negative ρUAxy peak at 60, 70, and 100 K; the offered mechanism cannot produce the sign-changing behavior it is invoked to explain.","section":"Results and Discussions, paragraph beginning 'Such observation in the nonmagnetic phase can be understood'"}],"minor_comments":[{"comment":"The sentence 'the intercept of the linearly fitted (ρxy/H) vs (ρ2xxM/H) curves above the critical field (Hc) is nothing but γ' is incorrect as stated: the intercept of that plot is R0 and the slope is γ.","section":"Text near Fig. 2(c)"},{"comment":"The caption contains a typo: 'in-pane' should read 'in-plane'.","section":"Table I caption"},{"comment":"Reference [23] has a garbled author list; it should be formatted as 'M. Hirschberger, S. Kushwaha, Z. Wang, Q. Zhang, S. Liang, C. A. Belvin, B. A. Bernevig, R. J. Cava, N. P. Ong, Nat. Mater. 15, 1161 (2016)'.","section":"Reference [23]"},{"comment":"The Supplementary Material is not posted, yet key validation content (Fig. S3 for the TYJ/power-law analysis, Figs. S6–S7 for the nonmagnetic phase, and Tables ST1–ST2) is essential for checking the claims; the placeholder 'Supplementary URL to be added by journal (2025)' should be replaced before any further review.","section":"Supplementary Material notice"},{"comment":"The abstract attributes the UAHE partly to topological magnetic texture 'as inferred from the measured field-dependent ac susceptibility,' but the χ′(H) features vanish by 16 K (Fig. 1(f)) while the claimed UAHE persists to 100 K; the manuscript itself states that the mechanism above 14 K is different, so the abstract overstates the texture contribution.","section":"Abstract"},{"comment":"No error bars or measurement uncertainties are given for ρxy, ρUAxy, σCAxy, R0, γ, or the TYJ parameters; for a quantity defined as a subtraction residual this is not merely cosmetic and should be addressed in any revision.","section":"General transport analysis"},{"comment":"Quantitative reading of the key figures is difficult: the vertical offsets in Fig. 2(c) and the tiny insets (i)–(ii) of Fig. 2(f) make the temperature evolution of the anomaly hard to verify; enlarging the inset panels and labeling the offset values would help.","section":"Fig. 2(c) and Fig. 2(f)"}],"recommendation":"reject","confidential_remarks":"The DFT portion of this manuscript (FiM2 ground state, Weyl-node inventory, intrinsic AHC in the ordered phase) appears competent and could support a revised submission focused on the electronic structure and magnetism of Y3Co8Sn4. The transport-based central claim, however, rests on a circular and under-documented subtraction, and the nonmagnetic-phase AHC calculation contradicts time-reversal symmetry; in addition, the manuscript's own high-temperature mechanism paragraph predicts zero net UAHE, in direct conflict with its headline observation. I would want to see the raw ρxy isotherms, a documented Hc(T) criterion, error propagation, and a resolution of the NM-phase AHC discrepancy before considering this as a transport paper again. The paper fits the journal's materials-physics scope; the issue is the soundness of the central claim, not fit."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear colleague,\n\nWhat you should know first: this paper is an honest experimental report with a load-bearing interpretation that doesn't hold together. The authors find a Hall resistivity component in Y3Co8Sn4 that changes sign from positive below TC to negative above TC, and they attribute it to Weyl nodes. The observation itself, if real, is a genuine new data point for the UAHE literature. The DFT work is thorough on the magnetic side: they converge to a planar ferrimagnetic ground state, find five pairs of Weyl points near EF, and compute an intrinsic AHC of ~168 S/cm that matches the low-T experimental value. That part is credible.\n\nThe soft spots are serious. First, the nonmagnetic-phase AHC calculation reports 154 S/cm at EF for a state with zero net moment. Under time-reversal symmetry, the intrinsic AHC must vanish. Either the calculation is actually spin-polarized (which would mean it is not the nonmagnetic phase) or there is a numerical error. The authors use this number to claim agreement with the 60 K data, so this is not peripheral.\n\nSecond, the explanation for the negative UAHE above TC is internally inconsistent. The text says random FM domains produce Weyl nodes with opposite chiralities in different domains, and that positive and negative contributions cancel, explaining the lack of spontaneous UAHE. But they claim to observe a field-induced negative UAHE in exactly that temperature range. How the field breaks the cancellation is never explained.\n\nThird, the extraction of ρUA_xy is a scaling subtraction: they fit (ρxy/H) vs (ρxx^2 M/H) above some Hc, assume the unconventional term vanishes there, and treat the residual as UAHE. This is a standard recipe, but here the fit is done where M is small and nearly linear, so the fitted coefficients are very sensitive to noise and to the choice of Hc. No error bars and no independent check of the assumption are given. The sign-changing signal could be an artifact.\n\nSo: worth a serious referee because the experimental phenomenon, if confirmed, is interesting and the DFT has solid parts. As written, the mechanism is not established and the nonmagnetic AHC result looks wrong. I would send it to review rather than desk-reject, with clear instructions to address the TRS issue and to validate the subtraction. My own verdict is skeptical until those are fixed.","headline":"Interesting experimental finding undermined by a broken nonmagnetic-phase AHC calculation and an unvalidated scaling subtraction.","tokens_in":13488,"tokens_out":3485,"would_cite":false,"duration_ms":32046,"reading_group":"maybe","serious_thinker":"no","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The hexagonal polar magnet Y3Co8Sn4 shows an unconventional anomalous Hall effect that switches from positive below its magnetic transition to negative above it, which the authors trace to Weyl points near the Fermi level.","keywords":["unconventional anomalous Hall effect","Weyl points","Berry curvature","topological spin texture","polar magnet","Y3Co8Sn4","anomalous Hall conductivity","ferrimagnetism"],"falsifier":"Measure the Hall resistivity of a single crystal of Y$_3$Co$_8$Sn$_4$ at temperatures above and below $T_C$ while tuning the field, and check whether the extracted $\\rho^{UA}_{xy}$ follows $\\gamma \\rho_{xx}^2 M$ across a wider field range; in particular, if $\\rho^{UA}_{xy}$ remains nonzero after full field polarization well beyond the critical field, the scaling assumption fails. Alternatively, perform Lorentz transmission electron microscopy or small-angle neutron scattering to see whether the topological spin texture presumed below $T_C$ actually exists and vanishes at the field where the anomaly disappears.","tokens_in":12451,"feed_emoji":"🧲","tokens_out":8441,"duration_ms":70368,"temperature":0.7,"pith_summary":"The paper reports that the noncentrosymmetric hexagonal magnet Y3Co8Sn4 shows an unconventional anomalous Hall effect (UAHE) whose sign is positive below the magnetic ordering temperature $T_C \\approx 53$ K and negative above it. This two-stage, sign-changing behavior is rare, and the authors argue that it arises from intrinsic Berry curvature produced by Weyl points lying near the Fermi level in both the low-temperature ferrimagnetic phase and the high-temperature nonmagnetic phase, with an additional contribution from a topological magnetic spin texture below $T_C$. The claim matters because it identifies a single material in which momentum-space topology and real-space magnetism cooperate to produce a transport signature that can be switched by temperature, and it points to the $R_3$Co$_8$Sn$_4$ polar family as a promising setting for spintronic and topological-transport studies. The paper supports the claim with magnetization, resistivity, Hall, and ac-susceptibility measurements together with density-functional calculations of Weyl nodes and anomalous Hall conductivity.","feed_headline":"A magnet's Hall signal flips sign across its magnetic transition","feed_subtitle":"Weyl points near the Fermi level explain a two-stage Hall signal, a first for a noncentrosymmetric magnet.","key_machinery":"The argument is carried by the decomposition of the measured Hall resistivity into ordinary, conventional anomalous, and unconventional components, using the scaling form $\\rho^{UA}_{xy} = \\gamma \\rho_{xx}^2 M$ for the unconventional part. The reciprocal-space machinery consists of Weyl points (band degeneracies near the Fermi level that act as sources and sinks of Berry curvature) identified by ab initio band-structure searches in both the FiM2 ferrimagnetic and nonmagnetic phases, with the intrinsic anomalous Hall conductivity computed by integrating Berry curvature over the Brillouin zone. The TYJ scaling analysis of the conventional anomalous Hall conductivity is used to establish that the intrinsic Karplus-Luttinger mechanism dominates, and the field dependence of ac susceptibility is used as evidence for the topological spin texture that contributes below $T_C$.","core_discovery":"The central discovery is that Y$_3$Co$_8$Sn$_4$ exhibits an unconventional anomalous Hall resistivity component $\\rho^{UA}_{xy}$ that is positive below $T_C$ and negative above $T_C$, with the magnitude evolving in opposite ways in the two temperature ranges. The authors attribute the effect to reciprocal-space topology: density-functional calculations find five pairs of Weyl points close to the Fermi level in the stable planar ferrimagnetic (FiM2) state below $T_C$ and four pairs in the nonmagnetic phase above $T_C$, and the computed anomalous Hall conductivities (about 168 S/cm and 154 S/cm respectively) agree with measured values. Field-dependent ac susceptibility shows peaks suggestive of topologically nontrivial spin textures, so a real-space contribution is also invoked. On the paper's account, this is the first observation of the two-stage sign-changing UAHE in a noncentrosymmetric magnet, and the mechanism differs between the magnetically ordered and nominally nonmagnetic regimes.","pith_inferences":["If the Weyl-point energy positions control the sign of $\\rho^{UA}_{xy}$, then chemical substitution or pressure that shifts the Fermi level should flip or tune the sign-change temperature; this is a testable consequence the paper does not pursue.","Direct imaging of the spin texture (for instance by Lorentz microscopy or small-angle neutron scattering) could separate the real-space and reciprocal-space contributions, since the momentum-space part should survive even where no texture is detected.","The resemblance to EuCd$_2$As$_2$'s two-stage UAHE suggests the sign-changing behavior may be a general property of polar magnets with Weyl nodes, not an accident of this one compound.","Because the extraction assumes $\\rho^{UA}_{xy} = \\gamma \\rho_{xx}^2 M$ vanishes above a critical field, independent verification of that scaling on a single crystal would substantially strengthen the claim."],"forward_implications":["Y$_3$Co$_8$Sn$_4$ is a rare single material whose unconventional Hall signal changes sign across its magnetic transition, implying that the dominant topological mechanism switches with temperature.","The computed Weyl points near the Fermi level in both phases make the polar magnet family $R_3$Co$_8$Sn$_4$ a platform for studying Weyl-mediated transport without requiring an applied field.","The intrinsic Berry-curvature contribution to the anomalous Hall effect is substantial even above $T_C$, where only short-range ferromagnetic correlations survive.","The sign and magnitude of the UAHE could serve as a sensitive probe of field-induced changes in magnetic topology in this compound."],"supporting_citations":[{"why":"Supplies the benchmark magnetic Weyl semimetal in which intrinsic Berry-curvature AHE is large, used as the comparison for the observed and computed AHC.","marker":"[4]"},{"why":"Reports the topological Hall signature in MnSi that defines the anomalous Hall anomaly now associated with topological spin textures.","marker":"[6]"},{"why":"Provides the earlier structural and magnetic characterization of Y3Co8Sn4 that the paper's crystal structure and incommensurate-order assignments build on.","marker":"[32]"},{"why":"Earlier report of the easy ab-plane and magnetic behavior in this material family that the ferrimagnetic FiM2 ground state is checked against.","marker":"[34]"},{"why":"Documents similar ac-susceptibility and Hall signatures in skyrmion/anti-skyrmion systems, used to argue for topological spin texture in Y3Co8Sn4.","marker":"[41]"},{"why":"Introduces the TYJ scaling relation used here to separate intrinsic and extrinsic contributions to the conventional anomalous Hall effect.","marker":"[48]"},{"why":"Establishes the gamma-rho-xx-squared-M scaling form for the topological Hall resistivity that the subtraction procedure relies on.","marker":"[55]"},{"why":"Provides the no-go theorem used to check that the Weyl-point chiralities sum to zero in the nonmagnetic phase.","marker":"[68]"}],"fun_headline_variants":["Weyl points flip Hall sign across transition in polar magnet","Hall signal reverses at magnetic transition in noncentrosymmetric magnet","Two-stage Hall anomaly emerges from Weyl topology in a polar magnet","First sign-flipping anomalous Hall effect seen in Weyl polar magnet"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The reported sign-changing unconventional Hall signal rests on assuming that the extra Hall term follows the magnetic-scattering form $\\rho^{UA}_{xy} = \\gamma \\rho_{xx}^2 M$ and disappears above a critical field; if that form is wrong or the term persists, the sign change could be an artifact of the subtraction.","fun_headline_variants_meta":{"raw":{"variants":["Weyl points flip Hall sign across transition in polar magnet","Hall signal reverses at magnetic transition in noncentrosymmetric magnet","Two-stage Hall anomaly emerges from Weyl topology in a polar magnet","First sign-flipping anomalous Hall effect seen in Weyl polar magnet"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000977,"raw_usage":{"total_tokens":4174,"prompt_tokens":996,"completion_tokens":3178,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":612,"completion_tokens_details":{"reasoning_tokens":3105}},"tokens_in":612,"tokens_out":3178,"duration_ms":22038,"temperature":1.0,"reasoning_tokens":3105,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-09T04:41:05.472163+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the Hall resistivity of a single crystal of Y$_3$Co$_8$Sn$_4$ at temperatures above and below $T_C$ while tuning the field, and check whether the extracted $\\rho^{UA}_{xy}$ follows $\\gamma \\rho_{xx}^2 M$ across a wider field range; in particular, if $\\rho^{UA}_{xy}$ remains nonzero after full field polarization well beyond the critical field, the scaling assumption fails. Alternatively, perform Lorentz transmission electron microscopy or small-angle neutron scattering to see whether the topological spin texture presumed below $T_C$ actually exists and vanishes at the field where the anomaly disappears.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the benchmark magnetic Weyl semimetal in which intrinsic Berry-curvature AHE is large, used as the comparison for the observed and computed AHC."},{"cited_title":"Neubauer, C","cited_arxiv_id":null,"evidence_quote":"Reports the topological Hall signature in MnSi that defines the anomalous Hall anomaly now associated with topological spin textures."},{"cited_title":"Canepa, M","cited_arxiv_id":null,"evidence_quote":"Provides the earlier structural and magnetic characterization of Y3Co8Sn4 that the paper's crystal structure and incommensurate-order assignments build on."},{"cited_title":"Takagi, J","cited_arxiv_id":null,"evidence_quote":"Earlier report of the easy ab-plane and magnetic behavior in this material family that the ferrimagnetic FiM2 ground state is checked against."},{"cited_title":"Kurumaji, T","cited_arxiv_id":null,"evidence_quote":"Documents similar ac-susceptibility and Hall signatures in skyrmion/anti-skyrmion systems, used to argue for topological spin texture in Y3Co8Sn4."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Introduces the TYJ scaling relation used here to separate intrinsic and extrinsic contributions to the conventional anomalous Hall effect."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the gamma-rho-xx-squared-M scaling form for the topological Hall resistivity that the subtraction procedure relies on."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the no-go theorem used to check that the Weyl-point chiralities sum to zero in the nonmagnetic phase."}],"review_version":1}