{"id":"5061c811-a1e9-4db3-a952-c2056faffca8","arxiv_id":"1908.00484","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In LaFe1-xCoxAsO, the nematic susceptibility shows a double-peak doping profile and its Curie-Weiss temperature crosses zero near optimal doping, suggesting a nematic quantum critical point.","lead":"By stretching tiny crystals of an iron-based superconductor and watching how their electrical resistance responds, the authors traced a quantity called electronic nematic susceptibility across a range of cobalt doping. They found two doping levels where this susceptibility is strongly enhanced, one near the edge of magnetic order and one under the superconducting dome, which they connect to a nematic quantum critical point.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The nQCP claim hinges on three-parameter Curie-Weiss fits with a free background n0; without raw data or the missing supplemental, the Tnem sign change may be a fitting artifact.","rationale":"I read the paper as reporting a systematic elastoresistivity study whose central quantitative claims are the sign change of Tnem at x≈0.06 and the double peak in C. Both are derived quantities: they are outputs of the Curie-Weiss fit, not direct observables. The raw n(T) data in Fig. 3 do show broad Curie-Weiss-like increases, and the authors' use of a 20 K fit-range variation shows awareness of the need for error estimates. That is real support. However, the dominant uncertainty in a three-parameter Curie-Weiss fit with a free offset is not the fit range but the physical value of n0; a T-independent background is assumed by Eq. (1), and any temperature dependence in n0 will bias Tnem and C. Without the missing supplemental and without reporting n0 values, the reader cannot check whether the zero crossing and double peak are artifacts. I therefore keep the reader's CONDITIONAL verdict, adding a concrete analysis condition. I do not regard this as a rejection: the measurements are systematic and the Curie-Weiss behavior is plausible, but the strongest claim is one extrapolation step beyond the data. The reader's concern about unpublished phase-diagram references [47,49] is legitimate but secondary: even with a perfect phase diagram, the extracted parameters would need to be robust, and that is where the argument is least secure.","tokens_in":11432,"tokens_out":7994,"duration_ms":95364,"concrete_test":"Re-fit all twelve datasets with n0 constrained to the high-temperature asymptotic value (or to an independently measured non-nematic background) rather than leaving it as a free parameter, and compare the resulting Tnem(x) and C(x) with Fig. 4; check specifically whether Tnem still crosses zero within uncertainties at x≈0.06 and whether the C enhancement at x≈0.04 and x≈0.06 persists. The authors should also report the n0 values and the fit-range sensitivity for each sample.","verdict_should_be":"UNCHANGED","load_bearing_attack":"In Eq. (1), n = n0 + C/(T - Tnem), the 'intrinsic piezoresistivity' n0 is not independently constrained in the reported analysis, and the paper does not state how n0 was fixed or list its fitted values. The parameters n0, C, and Tnem are strongly covariant: because the Curie-Weiss term changes slowly over the measured 100-300 K window, a modest adjustment of n0 can move the extrapolated Tnem by tens of kelvin. In particular, for overdoped samples an overestimated background can turn a positive Tnem into the negative value that is read as a sign change at x≈0.06. The double-peak evolution of C is extracted from the same three-parameter fits. The authors only report uncertainty from shifting the fit range by 20 K, and the Supplemental Material [55] that is supposed to document this procedure is not present. If the sign change or the C peaks disappear under a physically constrained treatment of n0, the central nQCP claim lacks empirical support.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript reports elastoresistivity measurements on LaFe1−xCoxAsO single crystals with cobalt content x from 0 to 0.075. For each doping level the nematic susceptibility ñ(T) in the tetragonal phase is fit to a Curie-Weiss form ñ = ñ0 + C/(T − Tnem). The authors find that Tnem decreases linearly with Co doping and changes sign near x ≈ 0.06, and that the Curie constant C and the magnitude of ñ show two enhancements, one near x ≈ 0.04 and one near optimal doping. They interpret the sign change and the enhancement at x ≈ 0.06 as evidence for a nematic quantum critical point beneath the superconducting dome, and attribute the x ≈ 0.04 peak to coupling to critical magnetic fluctuations.","tokens_in":11617,"tokens_out":6105,"duration_ms":62448,"significance":"If the central claims hold, LaFe1−xCoxAsO would be a particularly clean example of an iron-based superconductor in which magnetic and nematic quantum criticalities are separated in doping, strengthening the case that nematic fluctuations can promote superconductivity independently of magnetism. The paper's strengths are its systematic coverage of eleven doping levels with a standard elastoresistivity technique, the explicit use of nominal versus actual cobalt concentration, the comparison with Ba(Fe1−xCox)2As2 and FeSe1−xSx, and the falsifiable prediction of a similar double-peak profile in NaFeAs. However, the principal quantitative conclusions rest on three-parameter fits with an unconstrained background term and on unpublished phase-diagram references, so the empirical support is not yet fully documented.","major_comments":[{"comment":"The background term n0 is a free parameter in the Curie-Weiss fit, but the manuscript does not report n0 values or describe any independent constraint on it. Because the Curie-Weiss term is a slowly varying function over the measured temperature window, the parameters n0, C, and Tnem are strongly covariant, and the extrapolated Tnem is sensitive to the chosen n0. The reported 20 K variation of the fit window does not probe this covariance. This issue is load-bearing for the central sign-change claim at x ≈ 0.06. Please provide the fitted n0 values, state how n0 was fixed, and demonstrate that the extracted Tnem(x) and C(x) are stable under physically reasonable n0 variations.","section":"Eq. (1) and the paragraph beginning \"We analyze the ñ(T) data\""},{"comment":"The phase-diagram placement of the underdoped peak relies on the TS, TN, and Tc lines taken from Refs [47,49], both of which are unpublished. The statement that Tnem ≈ 35 K at x ≈ 0.04 'roughly matches' TN is therefore not verifiable from the manuscript. If the actual TN(x) for the measured crystals differs from those unpublished lines, the assignment of the low-doping peak to magnetic-fluctuation-enhanced nematicity would need revision. Please either replace these with published phase-diagram data or include the supporting transition-line data for the measured crystals.","section":"Fig. 4 and the paragraph starting \"The information extracted from Fig. 3\""},{"comment":"The manuscript repeatedly refers to the Supplemental Material for the fit-window selection, the uncertainty estimate, and the ruling out of a nematic quantum critical point at x ≈ 0.04, but no supplemental file is included with the arXiv submission. Without this material, the central numerical claims (the Tnem sign change and the double-peak structure of C) cannot be independently assessed. Please provide the supplemental or incorporate the necessary information into the main text.","section":"Supplemental Material [55]"}],"minor_comments":[{"comment":"\"Sliver paint\" should be \"silver paint\" in the description of the electrical contacts.","section":"Experimental setup"},{"comment":"The phrase \"the report of newly recognized electronic orders beside magnetism in unconventional superconductors is infectious\" is unclear; consider rewording to something like \"has inspired renewed interest.\"","section":"Introduction"},{"comment":"\"All paring channels\" should be \"all pairing channels.\"","section":"Theoretical discussion paragraph"},{"comment":"The sentence \"The temperatures between which Curie-Weiss fitting was performed were determined by minimizing the systematic deviation of the Curie constant\" is vague; please define the quantity being minimized.","section":"Fig. 3 caption"},{"comment":"The color plot of the magnitude of ñ is not described in sufficient detail; please specify the plotted quantity, the color scale, and how the background contribution is treated in that plot.","section":"Fig. 4 caption"}],"recommendation":"major_revision","confidential_remarks":"To the editor: the load-bearing external inputs [47,49] are unpublished works from the same group; I recommend requesting that these data be made available or included as supporting material before final acceptance. The missing supplemental [55] should be required in the revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's the take: this is a genuinely useful dataset—the first systematic elastoresistivity sweep across LaFe1-xCoxAsO—and the double-peak in the Curie constant plus the sign change of T_nem are interesting enough to warrant a careful referee. But the central nQCP claim is not yet load-bearing. The authors fit a three-parameter Curie-Weiss form with a free background n0, and the paper never shows how n0 is constrained or what its fitted values are. With data over a roughly 100–300 K window, n0, C, and T_nem are strongly covariant; a modest change in the background could move the extrapolated T_nem by tens of kelvin and potentially erase the sign change at x≈0.06. The stress-test note has this right. The supplemental that supposedly describes the fitting procedure is absent, and two of the phase-diagram pins (refs 47 and 49) are unpublished same-group work. That is a lot of weight on unpublished context.\n\nCredit where due: the measurement itself looks careful—strain gauge, waiting at each strain step, twelve doping levels, and the Curie-Weiss form holds over a wide temperature range in every panel. The double-peak structure is visible in the contour plot, not just in fitted parameters. The authors also behave well: they call it a 'possible' nQCP, they do not claim to settle the leading-instability question, and they openly suggest the underdoped peak may be magnetic in origin. That is honest framing.\n\nThe soft spots are real but fixable. For the nQCP claim to stand, the authors need to show that the sign change is robust to plausible variations in n0—for example, by fixing n0 from high-temperature data or from an independent model, or by showing what an n0=0 fit looks like. They also need to make the phase-diagram data publicly available or rely on published sources, and the missing supplemental should be supplied. None of these is fatal; the qualitative double-peak is likely to survive scrutiny. But as it stands, the quantitative claim, especially the sign change, is a fit-dependent extrapolation.\n\nWho is this for? People working on nematicity in iron-based superconductors. It deserves a serious referee, and I would send it out with specific instructions to check the fitting degeneracy and the unpublished references. If those get addressed, this becomes a solid contribution.","headline":"A genuinely useful doping-series elastoresistivity dataset with an interesting double-peak/sign-change story, but the nQCP claim rests on free-background fits and unpublished phase-diagram pins and needs a closer look before it becomes load-bearing.","tokens_in":12251,"tokens_out":2450,"would_cite":false,"duration_ms":26858,"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":"In LaFe1-xCoxAsO, the nematic temperature crosses zero at optimal doping and the susceptibility amplitude peaks twice, indicating a nematic quantum critical point under the superconducting dome.","keywords":["nematic susceptibility","elastoresistivity","iron-based superconductors","LaFe1-xCoxAsO","nematic quantum critical point","Curie-Weiss behavior","double-peak feature","unconventional superconductivity"],"falsifier":"Measure the doping and temperature dependence of the magnetic fluctuation intensity (via inelastic neutron scattering or 75As NMR spin-lattice relaxation) on the same LaFe1-xCoxAsO crystals used for elastoresistivity. The double-peak scenario requires that the $x \\approx 0.04$ susceptibility peak coincides with the magnetic endpoint and that magnetic fluctuations are suppressed at $x \\approx 0.06$; a sample where the $x \\approx 0.06$ peak survives without suppression of magnetic fluctuations, or where $T_{\\mathrm{nem}}$ does not cross zero at the optimal doping, would falsify the nQCP claim.","tokens_in":11158,"feed_emoji":"⚛️","tokens_out":13624,"duration_ms":117684,"temperature":0.7,"pith_summary":"This paper tries to establish that in the iron-based superconductor LaFe$_{1-x}$Co$_x$AsO, electronic nematic fluctuations—rotational-symmetry-breaking charge fluctuations—follow Curie-Weiss behavior throughout the tetragonal phase and that their characteristic temperature $T_{\\mathrm{nem}}$ is driven to zero exactly where superconductivity is strongest. If true, nematic fluctuations, rather than magnetic fluctuations, could be the pairing force at optimal doping, because static magnetic order disappears before superconductivity appears. The paper also finds a second enhancement of the nematic susceptibility near the endpoint of antiferromagnetic order, suggesting two separate sources of nematicity: magnetic-fluctuation-driven behavior at low doping and a nearly pure nematic quantum critical point under the superconducting dome. A reader should care because this material separates the structural and magnetic transition lines, making it a direct test bed for the relationship between nematicity and superconductivity.","feed_headline":"Nematic temperature crosses zero at optimal cobalt doping","feed_subtitle":"The Curie-Weiss fit puts a nematic quantum critical point beneath the superconducting dome in LaFe1-xCoxAsO.","key_machinery":"The central object is the elastoresistivity response $\\tilde{n} = -(\\delta\\eta/\\delta\\varepsilon_x)$ at zero applied strain, measured by gluing a thin oriented crystal to a piezoelectric actuator and detecting the fractional resistivity change under controlled strain. This quantity probes the electronic nematic susceptibility; fitting it to a Curie-Weiss form with an offset $\\tilde{n}_0$ yields the mean-field nematic temperature $T_{\\mathrm{nem}}$ and the Curie constant $C$, which measures how strongly the susceptibility diverges. The doping and temperature dependence of these two fitted parameters carries the entire argument: the sign change of $T_{\\mathrm{nem}}$ locates the proposed quantum critical point, and the doping profile of $C$ reveals the double-peak structure.","core_discovery":"On the paper's own terms: elastoresistivity measurements on eleven LaFe$_{1-x}$Co$_x$AsO crystals show that the strain-induced resistivity anisotropy, a gauge of the electronic nematic susceptibility $\\tilde{n}$, diverges as $\\tilde{n} = \\tilde{n}_0 + C/(T - T_{\\mathrm{nem}})$ for every doping level studied. The fitted nematic temperature $T_{\\mathrm{nem}}$ falls linearly with cobalt content and crosses zero around $x \\approx 0.06$, the doping where $T_c$ is maximal; the authors read this as evidence for a nematic quantum critical point beneath the superconducting dome. Separately, the Curie constant $C$—the amplitude of the divergence—shows a double-peak doping profile: one peak near $x \\approx 0.04$ at the disappearance of antiferromagnetic order, and a second enhancement at optimal doping. The authors attribute the underdoped peak to coupling between nematic and critical magnetic fluctuations, and the optimal-doping peak to a primary nematic instability; the comparable sizes of the two peaks indicate similar strengths of the elasto-electronic and elasto-magnetic couplings.","pith_inferences":["If the proposed nematic quantum critical point is genuine, non-Fermi-liquid transport (for example a resistivity that varies linearly in temperature) should be observable in a fan around $x \\approx 0.06$; the paper does not report such a test.","The comparable Curie constants at the two peaks imply comparable magnetic and nematic coupling strengths; measuring the magnetic susceptibility independently and comparing its doping profile would show whether the underdoped peak tracks magnetism while the optimal-doping peak does not.","A uniaxial-pressure experiment that tunes $T_{\\mathrm{nem}}$ through zero while tracking $T_c$ would directly test whether superconductivity is enhanced exactly at the nematic critical point, separating the nQCP scenario from a mere coincidence of doping levels.","The double-peak interpretation would be strengthened if the same crystals used for elastoresistivity had their own structural, magnetic, and superconducting transitions measured directly, rather than relying on a separately determined phase diagram."],"forward_implications":["If the sign change of $T_{\\mathrm{nem}}$ at $x \\approx 0.06$ is real, a nematic quantum critical point sits directly beneath the superconducting dome, making nematic fluctuations a plausible pairing mediator in this compound.","Because static antiferromagnetic order is fully suppressed before superconductivity appears, magnetic fluctuations near the magnetic critical point cannot by themselves explain the optimal-doping enhancement; nematicity must contribute independently.","The underdoped susceptibility peak near $x \\approx 0.04$ implies that nematic fluctuations can be amplified by proximity to magnetic order even in a material where the structural and magnetic transitions are well separated.","The similarity between the nematic susceptibility profile and the superconducting dome suggests that $T_c$ should track the nematic susceptibility under tuning, a direct test of the proposed connection.","The authors expect a similar double-peak nematic susceptibility profile in the NaFeAs family, where separate magnetic and nematic quantum critical points have been reported."],"supporting_citations":[{"why":"Supplies the elastoresistivity method and the definition of the probe used here to measure the nematic susceptibility.","marker":"[24]"},{"why":"Provides the Curie-Weiss fitting procedure and the claim of ubiquitous nematic quantum criticality that the optimal-doping result is compared against.","marker":"[26]"},{"why":"Shows a nematic quantum critical point without static magnetism in FeSe1-xSx, the reference case for interpreting the LaFe1-xCoxAsO data.","marker":"[27]"},{"why":"Documents the single-crystal growth method for LaFeAsO and the parent compound's structural transition temperature.","marker":"[46]"},{"why":"Unpublished NMR/NQR work that supplies evidence of nanoscale competing charge environments and part of the phase diagram.","marker":"[47]"},{"why":"Gives the doping-dependent structural transition temperature and related nematicity data used for comparison.","marker":"[48]"},{"why":"Unpublished thermodynamic study that supplies the doping dependence of TS, TN, and Tc on which the placement of the susceptibility peaks relies.","marker":"[49]"}],"fun_headline_variants":[],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the phase diagram used to identify where the peaks sit—the structural, magnetic, and superconducting transition lines taken from references [47–49], two of them unpublished—is correct for the exact crystals measured.","fun_headline_variants_meta":{"error":"Client error '402 Payment Required' for url 'https://api.deepseek.com/chat/completions'\nFor more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/402"},"cache_creation_input_tokens":0},"created_at":"2026-08-14T15:52:48.435082+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the doping and temperature dependence of the magnetic fluctuation intensity (via inelastic neutron scattering or 75As NMR spin-lattice relaxation) on the same LaFe1-xCoxAsO crystals used for elastoresistivity. The double-peak scenario requires that the $x \\approx 0.04$ susceptibility peak coincides with the magnetic endpoint and that magnetic fluctuations are suppressed at $x \\approx 0.06$; a sample where the $x \\approx 0.06$ peak survives without suppression of magnetic fluctuations, or where $T_{\\mathrm{nem}}$ does not cross zero at the optimal doping, would falsify the nQCP claim.","supporting_citations":[{"cited_title":"Kuo, J.-H","cited_arxiv_id":null,"evidence_quote":"Provides the Curie-Weiss fitting procedure and the claim of ubiquitous nematic quantum criticality that the optimal-doping result is compared against."},{"cited_title":"Hosoi, K","cited_arxiv_id":null,"evidence_quote":"Shows a nematic quantum critical point without static magnetism in FeSe1-xSx, the reference case for interpreting the LaFe1-xCoxAsO data."},{"cited_title":"Kappenberger, S","cited_arxiv_id":null,"evidence_quote":"Documents the single-crystal growth method for LaFeAsO and the parent compound's structural transition temperature."},{"cited_title":"Lepucki, H.-J","cited_arxiv_id":null,"evidence_quote":"Unpublished NMR/NQR work that supplies evidence of nanoscale competing charge environments and part of the phase diagram."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Gives the doping-dependent structural transition temperature and related nematicity data used for comparison."},{"cited_title":"Scaravaggi, A","cited_arxiv_id":null,"evidence_quote":"Unpublished thermodynamic study that supplies the doping dependence of TS, TN, and Tc on which the placement of the susceptibility peaks relies."}],"review_version":1}