{"id":"bcaf81e7-24da-4f65-8c14-63a0dc0be627","arxiv_id":"1908.02790","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"The nematic order parameter changes sign between the Brillouin zone center and corner in both FeSe and BaFe2As2, with a common momentum dependence.","lead":"This paper used angle-resolved photoemission with strain tuning to measure the momentum-dependent nematic band splitting in two iron-based superconductors, FeSe and BaFe2As2. The authors find the same sign-changing momentum profile in both compounds, suggesting a shared microscopic mechanism.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Strain-induced nematic splitting in BaFe2As2 is treated as proportional to the equilibrium φ_nem(k) with a k-independent coefficient; a k-dependent electron-strain coupling would break the comparison with FeSe.","rationale":"Read in good faith, the paper makes a credible and interesting comparison. The FeSe detwinned data and the BaFe2As2 strain-tuned data independently show a sign change of ΔEnem near 0.3 Å^-1, and the authors are careful about SOC and orbital admixture. The weakest link is not the data analysis per se but the interpretive bridge from a strain-induced electronic response to the equilibrium nematic order parameter in BaFe2As2. A uniform strain field breaks the same symmetry, but the k-resolved response function can in principle have a different momentum structure than the spontaneous order parameter, especially with k-dependent electron-strain coupling. This is precisely the assumption the reader flagged. The proposed FeSe above-Tnem strain experiment would be a direct, same-material calibration of that assumption. Because the concern is real but addressable, and because the reader already assigned CONDITIONAL, the verdict should remain unchanged.","tokens_in":11873,"tokens_out":7325,"duration_ms":87607,"concrete_test":"Perform the same strain-tuned ARPES measurement on FeSe above T_nem (~100 K), using the piezoelectric device, and compare the normalized strain-induced splitting ΔEnem(k)/δ with the spontaneous splitting at 15 K in Fig. 4(a). If the two k-dependences coincide—including the zero-crossing position and the amplitude ratio between the BZ center and corner—the k-independent proportionality is supported. If they differ, the BaFe2As2 strain response cannot be taken as a direct measure of the equilibrium nematic order parameter, and the same-momentum-dependence claim needs revision.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central comparison in Fig. 4(a,b) requires that the strain-induced ΔEnem(k)/δ measured in BaFe2As2 at 160 K equals the spontaneous nematic splitting profile in FeSe up to a k-independent scale. That is an assumption, not a consequence of the cited Ginzburg-Landau argument (Ref. 28), because a uniform antisymmetric strain couples to the k-integrated B2g operator; the induced momentum-resolved splitting is a convolution of the nematic susceptibility χ(k,k') with the bare electron-strain coupling g(k'). Unless the response is dominated by a single collective mode with a k-independent projected coupling, the measured profile reflects g(k') and χ, not simply the equilibrium form factor f(k) of φ_nem(k)=φ0 f(k). Strain-modulated hopping/bond-order terms would naturally make g(k') momentum dependent. Ref. 28 is a bulk thermodynamic measurement and cannot certify k-independence. The manuscript acknowledges other caveats (different T/T_nem, orbital admixture) but does not test this one. If the zero crossing at |k|≈0.3 Å^-1 is robust only under this assumption, the claim that both compounds share the same φ_nem(k) is conditional.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports ARPES measurements of the momentum-dependent nematic band splitting in FeSe (T = 15 K, detwinned) and in BaFe2As2 (T = 160 K > T_nem, under tunable uniaxial strain). The authors define the nematic band splitting ΔE_nem(k) as half the difference between the binding-energy shifts along the two orthogonal in-plane directions and identify it with the momentum-resolved nematic order parameter φ_nem(k), accounting for spin-orbit-coupling effects near Γ. The central experimental result is that ΔE_nem(k) has the same k-dependence in both compounds, with a sign change between the zone center and the zone corner at |k| ≈ 0.3 Å^-1. On this basis the authors conclude that the same microscopic mechanism drives nematic order in Fe-based superconductors with and without magnetism.","tokens_in":12092,"tokens_out":9366,"duration_ms":105033,"significance":"If the comparison is valid, this is a significant experimental result: it extends the momentum-resolved characterization of nematicity from FeSe to a magnetic Fe-pnictide, provides a direct constraint that rules out purely on-site ferro-orbital order, and introduces a piezoelectric strain platform for ARPES that will be useful beyond this specific pair of compounds. The paper's strengths are the controlled detwinning/strain protocol, the direct extraction of band dispersions without fitting a model, and the explicit treatment of the distinction between ΔE_nem(k) and φ_nem(k) near Γ. The principal weakness is that the central FeSe/BaFe2As2 comparison rests on an assumption of momentum-independent proportionality between the strain-induced splitting and the equilibrium order parameter, which is not directly established by the data or by the cited bulk thermodynamic reference.","major_comments":[{"comment":"The central comparison assumes that the strain-induced splitting ΔE_nem(k) = [ΔE_B(k_y) - ΔE_B(k_x)]/2 measured in BaFe2As2 at 160 K is proportional to the equilibrium nematic order parameter φ_nem(k) with a k-independent coefficient. The linear coupling of a uniform antisymmetric strain to the B2g order parameter (Ref. 28) does not by itself guarantee this: the induced momentum-resolved splitting is in general a convolution of the bare electron-strain coupling g(k') with the nematic susceptibility χ(k,k'), and a k-dependent g(k') (for instance from strain-modulated hopping or bond-order terms) would imprint its own structure on the measured profile. Ref. 28 is a bulk thermodynamic measurement and cannot certify k-independence of g(k'). Because the zero crossing at |k| ≈ 0.3 Å^-1 and the claimed form-factor identity between the two compounds are read directly from this strain-induced profile, this assumption is load-bearing. Please provide a microscopic estimate or symmetry argument for g(k), perform a control measurement (for example on a second compound or with a different strain geometry), or explicitly reframe the conclusion as applying to the strain-induced susceptibility response and discuss how a k-dependent g could shift the crossing.","section":"Fig. 4(a,b) and text near Fig. 3"},{"comment":"The manuscript acknowledges that the band assignment and the resulting splitting beyond |k| = 0.8 Å^-1 are debated in the literature and excludes that region from the analysis, yet the abstract and conclusions state a sign change 'between the BZ center and the BZ corner.' The measured zero crossing at |k| ≈ 0.3 Å^-1 lies outside the excluded region, so the sign-change claim is not invalidated by this exclusion. However, the statement that '|φ_nem| is approximately twice as large at the BZ corner' and the assertion that the functional form is 'the same' in both compounds rely on data near the edge of, or extrapolating beyond, the included window. Please either restrict the claims to the measured k-range or provide a quantitative analysis of the band-crossing region showing that the form-factor and magnitude statements are insensitive to the band-assignment ambiguity.","section":"Page 3, text near 'We disregard the region beyond k = -0.8 Å^-1' and Fig. 4(a)"},{"comment":"The comparison of normalized magnitudes between FeSe and BaFe2As2 via ΔE_nem/δ assumes that the proportionality between the applied/measured strain δ and the induced band splitting is the same in both materials, or at least that residual differences do not affect the order-of-magnitude statement. Since the electron-strain coupling and the nematic susceptibility are material-specific, the claim that the two compounds have 'a similar strength of the nematic order' is not established to the same standard as the sign-change claim. This does not affect the central sign-change result, but the magnitude comparison should be softened or supported by an explicit discussion of the material-dependent couplings.","section":"Fig. 4(a,b), comparison of ΔE_nem/δ"}],"minor_comments":[{"comment":"The name 'R. Fernendes' appears to be a typo and should read 'R. Fernandes.'","section":"Acknowledgements, page 7"},{"comment":"Only 'representative error bars' are shown; please state how the errors were estimated and, if possible, show error bars for all points or a typical error envelope, since the comparison of the shapes of the two curves is central to the paper.","section":"Fig. 4(a,b)"},{"comment":"Please clarify whether Δl/l is the total length change or the antisymmetric strain component; for uniaxial stress the antisymmetric strain involves the Poisson ratio, so this normalization could change the magnitudes in Fig. 4(b) by an O(1) factor.","section":"Page 4, near 'we estimate δ = Δl/2l = 0.08%'"},{"comment":"The axis label 'X/Y -0.5 Γ' is ambiguous; a reader may not immediately see that the Γ–X and Γ–Y dispersions are plotted on the same negative-k axis. Please make the legend and axis annotation self-explanatory.","section":"Fig. 1(f) and Fig. 4(a)"}],"recommendation":"major_revision","confidential_remarks":"The sign-change result is visually compelling and the experimental approach is well suited to the question. My main reservation is the untested assumption of a k-independent electron-strain coupling in the BaFe2As2 comparison; I would not reject on this basis, but the authors should be asked to address it explicitly and to calibrate the strength of the conclusions accordingly. The paper is likely to be publishable after a revision that either supports the assumption or narrows the claims."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a solid experimental paper that does something genuinely new—strain-tuned ARPES on BaFe2As2 above Tnem—and reports a momentum-resolved nematic splitting that looks like the one in FeSe, sign change and all. The comparison is suggestive and probably important, but I wouldn't call the universality claim airtight.\n\nWhat's actually new is the BaFe2As2 experiment. Using a piezoelectric strain device, they extract the antisymmetric response of the hole band along kx and ky, define ΔEnem(k), and show it changes sign around |k|≈0.3 Å⁻¹, similar to what they find in detwinned FeSe at 15 K. The care in separating antisymmetric from symmetric strain responses, and the explicit discussion of spin-orbit coupling near Γ, are good. The FeSe data are consistent with earlier work (Ref 14) and extend it, though the novelty there is limited.\n\nSoft spots, in order of severity. First, the mapping from strain-induced splitting in BaFe2As2 to the equilibrium order parameter φ_nem(k) assumes the response is proportional to the same form factor f(k) with a k-independent constant. The Ginzburg-Landau argument in Ref 28 is a bulk thermodynamic result; it does not show that the coupling of uniform antisymmetric strain to each momentum state is the same. If the electron-strain coupling g(k) is momentum dependent, or the nematic susceptibility is not factorizable into a single mode, then ΔEnem(k)/δ mixes in other form factors. The paper does not address this. I don't think this kills the sign-change claim—a dominant d-wave-like nematic mode would preserve the zero crossing—but it makes the quantitative comparison with FeSe conditional.\n\nSecond, the comparison is qualitative. Figure 4 plots both data sets with representative error bars and uses a dashed guide to the eye. There's no fit or statistical procedure that quantifies \"same momentum dependence.\" That's a fair criticism a referee should raise.\n\nThird, the FeSe data stop at |k|≈0.8 Å⁻¹ because of the dxy/dxz crossing, and they exclude the debated region near the corner. The sign change at 0.3 is fine, but the claim about the \"BZ corner\" is somewhat overreaching since they don't actually reach the corner in FeSe.\n\nThe citation pattern looks clean. Ref 28 is appropriately cited for the bulk GL coupling, but it cannot carry the k-independence weight the paper places on it.\n\nWho's this for: anyone working on nematic order in iron-based superconductors. It's a useful data point and will likely be cited as evidence for a common nematic mechanism. I'd send it to a serious referee, and I'd ask that referee to push on the strain-coupling assumption and the lack of a quantitative comparison.","headline":"First strain-tuned ARPES on BaFe2As2 measures a momentum-resolved nematic splitting with a sign change, and the comparison with FeSe is suggestive—but the universality claim leans on an untested k-independence assumption.","tokens_in":12677,"tokens_out":5612,"would_cite":true,"duration_ms":59977,"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":"The nematic order parameter has the same momentum dependence in FeSe and BaFe2As2, with a sign change between the Brillouin-zone center and the corner, implying a common microscopic mechanism for nematicity in iron-based superconductors.","keywords":["nematic order parameter","iron-based superconductors","FeSe","BaFe2As2","angle-resolved photoemission spectroscopy","uniaxial strain","band splitting","momentum dependence"],"falsifier":"Apply the same strain-tunable ARPES measurement to BaFe2As2 at several strain magnitudes and check whether $\\Delta E_{\\rm nem}(k)/\\delta$ is independent of $\\delta$ for every momentum; any deviation would show that the proportionality is not momentum-independent. Alternatively, measure a fully detwinned BaFe2As2 crystal below $T_{\\rm nem}$ and verify that the sign-change pattern in the spontaneous order matches the strained-paramagnetic response; a mismatch would falsify the common-momentum-dependence claim.","tokens_in":11704,"feed_emoji":"🌀","tokens_out":11065,"duration_ms":94972,"temperature":0.7,"pith_summary":"This paper uses angle-resolved photoemission (ARPES) on detwinned, strained crystals to measure how the electronic nematic band splitting varies with momentum in two iron-based superconductors, FeSe and BaFe2As2. The authors find that the nematic order parameter $\\varphi_{\\rm nem}(k_x,k_y)$ has the same momentum profile in both materials, including a sign change between the Brillouin-zone center and the corner near $|k| = 0.3\\,\\text{Å}^{-1}$. This matters because FeSe orders nematically without magnetism, while BaFe2As2 has a spin-density wave, so a common momentum dependence would indicate that a single microscopic mechanism drives nematic order across the whole family. The result also rules out pure on-site ferro-orbital order and places strong constraints on candidate theories.","feed_headline":"Two iron superconductors share one nematic order shape","feed_subtitle":"Strain-tuned ARPES finds the same sign-changing momentum profile in FeSe and BaFe2As2, hinting at a shared mechanism.","key_machinery":"The central object is the momentum-resolved nematic order parameter $\\varphi_{\\rm nem}(k)$, defined experimentally as the difference in binding energy of the $d_{xz}$/$d_{yz}$ hole bands along two orthogonal momentum directions, $\\Delta E_{\\rm nem}(k) = E(k_x)-E(k_y)$. The load-bearing relation is the linear Ginzburg-Landau coupling between antisymmetric (B$_{2g}$) strain and the nematic order parameter, which makes it legitimate to measure the strain-induced splitting in the paramagnetic state of BaFe2As2 above $T_{\\rm nem}$ and compare it, normalized by orthorhombic distortion $\\delta$, with the spontaneous splitting in FeSe. The sign change in $\\Delta E_{\\rm nem}(k)$ is extracted from the middle hole band, whose pure orbital character at high-symmetry points guarantees that the band shift is proportional to $\\varphi_{\\rm nem}(k)$ there, and spin-orbit coupling is handled separately because it zeros $\\Delta E_{\\rm nem}$ at $\\Gamma$ while $\\varphi_{\\rm nem}$ stays nonzero.","core_discovery":"The central discovery is that the momentum dependence of the nematic order parameter is not material-specific: $\\varphi_{\\rm nem}(k_x,k_y) = \\varphi_0 f(k)$ has the same functional form $f(k)$ in FeSe and BaFe2As2, with a sign change between the center and the corner of the Brillouin zone. In FeSe, the spontaneous splitting below $T_{\\rm nem}$ is measured on a detwinned crystal; in BaFe2As2 above $T_{\\rm nem}$, tunable uniaxial strain induces a band splitting whose antisymmetric B$_{2g}$ part $\\Delta E_{\\rm nem}(k) = [\\Delta E_B(k_y) - \\Delta E_B(k_x)]/2$ is the strain-proportional equivalent. Away from the zone center, where spin-orbit coupling does not mix the $d_{xz}$ and $d_{yz}$ bands, $\\Delta E_{\\rm nem}(k)$ directly equals $\\varphi_{\\rm nem}(k)$. The sign change occurs near $|k| = 0.3\\,\\text{Å}^{-1}$, and $|\\varphi_{\\rm nem}|$ is about twice as large at the Brillouin-zone corner as at the center, where $\\varphi_{\\rm nem}(\\Gamma) \\approx 17$ meV for FeSe after accounting for spin-orbit coupling.","pith_inferences":["The same strain-tunable ARPES approach could be applied to other iron-based superconductors (e.g., LiFeAs or Co-doped BaFe2As2) to test whether the sign-changing momentum profile is truly universal; a deviation in any of them would bound the universality claim.","If the nematic order parameter's momentum dependence is universal, it likely imprints on the superconducting gap through orbital or spin-fluctuation pairing, so measuring the gap's sign structure would provide an independent cross-check.","The analysis excludes the region beyond the $d_{xz}$–$d_{xy}$ crossing near the Brillouin-zone corner, so a future measurement with different photon energies or higher resolution could reveal whether the sign change persists into that region or is modified by band hybridisation."],"forward_implications":["FeSe and BaFe2As2 share the same microscopic driver of nematic order, meaning theories must explain both materials with a single mechanism.","Pure on-site ferro-orbital order is excluded for both systems, since it cannot produce the observed sign change.","Candidate models (bond-orbital order, Pomeranchuk instabilities, orbital-selective spin fluctuations, frustrated magnetism, and spin-driven Ising-nematic order) must now reproduce the sign change near $|k| \\approx 0.3\\,\\text{Å}^{-1}$.","The comparable magnitude of the normalized nematic susceptibility $\\Delta E_{\\rm nem}/\\delta$ indicates similar nematic coupling strengths in the two compounds.","The sign-changing order parameter creates a d-wave-like Fermi-surface distortion that could be reflected in superconducting pairing anisotropies."],"supporting_citations":[{"why":"It supplies the linear Ginzburg-Landau coupling of antisymmetric strain to the B2g nematic order parameter, the basis for extracting $\\Delta E_{\\rm nem}$ in strained BaFe2As2.","marker":"[28]"},{"why":"It provides the detwinned FeSe Fermi-surface measurement that first showed a sign change between hole and electron bands, which this paper extends to the full momentum dependence.","marker":"[14]"},{"why":"It reports a strong momentum dependence of the nematic band splitting in FeSe thin films, supporting the interpretation of $\\Delta E_{\\rm nem}$ as the order parameter profile.","marker":"[13]"},{"why":"It describes the piezoelectric strain device used to apply tunable uniaxial pressure in-situ during the ARPES measurements.","marker":"[37]"},{"why":"It demonstrates complete detwinning of FeSe using a BaFe2As2 substrate clamp, the method used for the FeSe measurement.","marker":"[36]"},{"why":"It gives the spin-orbit coupling form explaining why $\\Delta E_{\\rm nem}(\\Gamma)=0$ while $\\varphi_{\\rm nem}(\\Gamma)$ stays nonzero, allowing the extraction of $\\varphi_{\\rm nem}(\\Gamma) \\approx 17$ meV.","marker":"[44]"}],"fun_headline_variants":["Nematic order flips sign identically in FeSe and BaFe2As2","FeSe and BaFe2As2 share one nematic momentum profile","Common nematic order shape found in two iron superconductors","Strain-tuned ARPES reveals same nematic order in FeSe and BaFe2As2","Iron superconductors unify on nematic order sign change"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The comparison rests on the assumption that the strain-induced band splitting in BaFe2As2 is proportional to the equilibrium nematic order parameter with a momentum-independent proportionality constant; if the strain coupling is momentum-dependent, the comparison with the spontaneous FeSe splitting is invalid and the claimed universality would collapse.","fun_headline_variants_meta":{"raw":{"variants":["Nematic order flips sign identically in FeSe and BaFe2As2","FeSe and BaFe2As2 share one nematic momentum profile","Common nematic order shape found in two iron superconductors","Strain-tuned ARPES reveals same nematic order in FeSe and BaFe2As2","Iron superconductors unify on nematic order sign change"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000336,"raw_usage":{"total_tokens":1868,"prompt_tokens":961,"completion_tokens":907,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":577,"completion_tokens_details":{"reasoning_tokens":807}},"tokens_in":577,"tokens_out":907,"duration_ms":8302,"temperature":1.0,"reasoning_tokens":807,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T14:33:27.212403+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Apply the same strain-tunable ARPES measurement to BaFe2As2 at several strain magnitudes and check whether $\\Delta E_{\\rm nem}(k)/\\delta$ is independent of $\\delta$ for every momentum; any deviation would show that the proportionality is not momentum-independent. Alternatively, measure a fully detwinned BaFe2As2 crystal below $T_{\\rm nem}$ and verify that the sign-change pattern in the spontaneous order matches the strained-paramagnetic response; a mismatch would falsify the common-momentum-dependence claim.","supporting_citations":[{"cited_title":"Kuo and I","cited_arxiv_id":null,"evidence_quote":"It supplies the linear Ginzburg-Landau coupling of antisymmetric strain to the B2g nematic order parameter, the basis for extracting $\\Delta E_{\\rm nem}$ in strained BaFe2As2."},{"cited_title":"Zhang, M","cited_arxiv_id":null,"evidence_quote":"It reports a strong momentum dependence of the nematic band splitting in FeSe thin films, supporting the interpretation of $\\Delta E_{\\rm nem}$ as the order parameter profile."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It describes the piezoelectric strain device used to apply tunable uniaxial pressure in-situ during the ARPES measurements."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It demonstrates complete detwinning of FeSe using a BaFe2As2 substrate clamp, the method used for the FeSe measurement."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It gives the spin-orbit coupling form explaining why $\\Delta E_{\\rm nem}(\\Gamma)=0$ while $\\varphi_{\\rm nem}(\\Gamma)$ stays nonzero, allowing the extraction of $\\varphi_{\\rm nem}(\\Gamma) \\approx 17$ meV."}],"review_version":1}