{"id":"23964bb5-e601-4de8-a5ec-df4ed0b8f52f","arxiv_id":"2412.18787","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Nematic fluctuations, not magnetic ones, appear to produce non-Fermi liquid transport and enhanced effective mass in FeSe_xTe_{1-x} thin films near x=0.45.","lead":"Thin films of FeSe_xTe_{1-x} show strange metal behavior, with electrical resistance growing linearly with temperature and thermoelectricity growing logarithmically, strongest at the composition where a nematic electronic order would vanish at zero temperature. The results point to fluctuations of broken rotational symmetry, not magnetism, as the driver of non-Fermi liquid transport and heavy-mass superconductivity.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central claim rests on identifying x=0.45 as the nematic QCP in the MBE films, a value imported from bulk crystals without direct film measurement.","rationale":"I read the paper in good faith and find a coherent experimental study with multiple independent transport signatures (T-linear resistivity, log S/T, Pauli-limited Hc2) that together strengthen the case for NFL behavior. The film characterization (c lattice constant, parent-compound anomalies, Kondo-like resistivity) is careful, and the qualitative trends in Fig. 4 are consistent with the authors' narrative. However, the central claim that the NFL transport is driven specifically by nematic fluctuations at a QCP at x=0.45 depends on the composition of the nematic QCP in these particular thin films. That value is taken from bulk crystals (ref 64), and the paper provides no direct evidence for the nematic transition or its endpoint in the films at intermediate x. Since the argument is essentially the spatial coincidence of a transport peak with an assumed QCP, a shift in the film QCP would undermine the main conclusion. This is the same concern the reader identified as the weakest assumption. A direct elastoresistance or anisotropy measurement on the films would settle it. I therefore see no reason to change the conditional verdict.","tokens_in":18825,"tokens_out":8487,"duration_ms":73444,"concrete_test":"Measure the in-plane resistivity anisotropy or the elastoresistance coefficient as a function of temperature for MBE-grown FST thin films on the same substrates (STO and CdTe) with identical Te-annealing and capping, across the composition range 0.3≤x≤0.6. Extract Tnem(x) from the onset of anisotropy or the divergence of the elastoresistance, and determine the composition where Tnem extrapolates to zero. If this extrapolated QCP differs from x=0.45 by more than the composition step size of about 0.05, the central coincidence between the |A_S| peak and the nematic QCP is falsified. Repeating each composition on at least two films would also provide the error bars needed to establish the sharpness of the peak.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central argument is a coincidence argument: the peak of |A_S| in Fig. 4(b) and the region where α>1 both align with x=0.45, which is taken from bulk single-crystal Tnem data (ref 64). For the MBE films, there is no direct measurement of the nematic transition for intermediate x; the only film evidence is the FeSe endpoint anomaly in Fig. S2. The films are 40 nm thick, grown on STO or CdTe, Te-annealed, and capped with FeTe2, so substrate strain, finite thickness, and annealing could shift or broaden the QCP. The lattice-constant match (Fig. S1) is encouraging but does not guarantee that the electronic nematic QCP is at the same x. If the true film QCP were at x≈0.40 or x≈0.50, the observed pronounced peak in |A_S| would no longer coincide with the QCP, and the attribution of the NFL transport to nematic fluctuations would lose its primary support. A shift of about 0.05-0.1 in x is plausible given typical strain effects in chalcogenide films. The authors themselves note the wide NFL range 0.06≤x≤0.45 and discuss extended quantum criticality, which makes the precise boundary x=0.45 even more dependent on the assumed QCP.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports a systematic transport study of MBE-grown FeSe_xTe_{1-x} thin films with x from 0 to 0.72. It identifies non-Fermi liquid (NFL) behavior—T-linear resistivity and logarithmic divergence of S/T—over 0.06 ≤ x ≤ 0.45, with the logarithmic slope |AS| peaking at x = 0.45, which the authors equate with the nematic quantum critical point using bulk single-crystal data. Upper critical field measurements fitted with the WHH formula yield a Maki parameter α exceeding unity in the same composition range, which is interpreted as a crossover from orbital- to Pauli-limited pair breaking and as evidence for strong mass enhancement.","tokens_in":19065,"tokens_out":7693,"duration_ms":65263,"significance":"If the identification of x=0.45 as the nematic QCP in the films is correct, the paper provides one of the cleanest demonstrations that nematic fluctuations alone can produce NFL transport and a large Maki parameter, with no competing magnetic order. The strength of the manuscript is the comprehensive experimental dataset: transport and thermoelectric measurements across a wide composition range, high-field Hc2 data up to 60 T, and the use of an MBE platform that avoids the phase separation issue of bulk crystals. The internal consistency of the raw data is good, and the WHH fitting procedure is described in detail in the supplement. The main weakness is that the central attribution to nematic quantum criticality depends on an unmeasured bulk-derived phase boundary and on fits whose uncertainties and fitting ranges are not reported.","major_comments":[{"comment":"The central claim that |AS| peaks at the nematic QCP x=0.45 relies on a phase boundary adopted from bulk single-crystal resistivity data (ref [64]) and on endpoint anomalies in the films (Fig. S2 for FeSe and FeTe only). No direct measurement of the nematic transition or nematic susceptibility in the intermediate-composition films is presented. Since the films are 40 nm thick, grown on STO or CdTe, Te-annealed, and capped with FeTe2, substrate strain, finite thickness, and annealing could plausibly shift or broaden the QCP by several percent in x. If the film QCP lies at x≈0.40 or x≈0.50, the coincidence between the |AS| peak and the assumed QCP would be weakened, and the attribution of the NFL transport to nematic fluctuations would lose its primary support. The authors should either provide direct film evidence for the nematic QCP (e.g., elastoresistance, anisotropic magnetoresistance, or x-ray measurements across x) or explicitly justify why the bulk boundary applies to the films.","section":"Fig. 4(b), Fig. 1(a), Supplement I"},{"comment":"The exponent n is computed from Eq. (1) using a residual resistivity ρ0 that is itself determined by fitting ρ=ρ0+AT^n over a low-temperature window that is not specified. The low-temperature classification into FL (n≈2) and NFL (n≈1) therefore depends on a three-parameter fit with no stated temperature range and no reported uncertainties. The color plot in Fig. 4(a) is a central piece of evidence for the claimed wide NFL range, yet the manuscript does not state how many compositions enter the plot, how the color map is interpolated, or how the result changes when the fitting window is varied. Please report the fitting windows for each composition, the resulting uncertainties in n and ρ0, and a robustness check with respect to the chosen T_max.","section":"Eq. (1), Fig. 4(a)"},{"comment":"The logarithmic slopes AS are obtained from linear fits of S/T versus ln T over a typical range of 10-50 K, but the exact fitting ranges and the uncertainties of AS are not reported. The statement that |AS| peaks sharply at x=0.45 is a comparison of fitted slopes; without error bars, it is not possible to assess whether the peak is significant relative to the scatter or to the choice of the fitting interval. The authors should provide the fit ranges, the fit results overlaid on the data, and at least standard errors for AS.","section":"Figs. 2(b)-(g) and Fig. 4(b)"},{"comment":"The Maki parameter α is determined from WHH fits with λSO fixed at 1.0. While the supplement explains the two-step fitting procedure and justifies λSO=1.0 with one representative fit (Fig. S4), the sensitivity of α to this choice and the statistical uncertainties of the fitted α values are not reported. Because the claim that α>1 in the same x range as the NFL transport is used to support the mass-enhancement interpretation, the authors should report the range of α obtained when λSO is varied over a reasonable interval, and provide confidence intervals for α from the fits.","section":"Supplement VI (WHH fittings), Fig. 4(b)"},{"comment":"The relation α≈(2m*/m0)(Δ/EF) in Eq. (2) is used to connect the large fitted α to a strong mass enhancement. This formula requires independent values of the superconducting gap Δ and the Fermi energy EF for each composition; the manuscript does not provide these quantities or discuss their x-dependence. As written, the argument is a qualitative plausibility statement rather than a quantitative derivation. The authors should either supply the relevant parameters (e.g., from ARPES, specific heat, or tunneling) or soften the conclusion to state that the large α is 'consistent with' a mass enhancement rather than 'indicative of' one.","section":"Eq. (2)"}],"minor_comments":[{"comment":"The phrase 'This behavior is characteristics of Fermi liquid transport' should read 'characteristic of'; also, 'formation of FL statea' contains a stray letter 'a' at the end of the sentence.","section":"Fig. 1(d) paragraph"},{"comment":"In the caption of Fig. S5, panel (b) says 'Same plot as (b) but plotted against the reduced parameters h and t'; this should refer to panel (a).","section":"Fig. S5 caption"},{"comment":"The notation n = ∂ ln(ρ(T)−ρ0)/∂ ln T is unusual for a logarithmic derivative; consider writing d ln(ρ−ρ0)/d ln T or defining it explicitly in the text to avoid confusion with a partial derivative.","section":"Eq. (1)"},{"comment":"The reference list appears to contain two overlapping numbering sequences: the main-text citations run from [52] upward, while the printed reference list starts at [1]. Please ensure a single consistent numbering scheme in the published version.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The paper is empirically rich and could be a strong contribution if the authors can close the gap between the bulk-derived nematic QCP and the actual films. The missing direct measurement of the nematic phase boundary in the films is the most serious issue; a shift of the QCP by even 0.05 in x would undermine the central coincidence argument. The lack of uncertainties on the fitted quantities (AS, n, α) is a secondary but important issue for a quantitative claim of a sharp peak at x=0.45. I recommend major revision and would be willing to look at a revised version that provides direct film evidence or a very clear justification for the bulk boundary, along with fit-range and uncertainty reporting."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a careful MBE thin-film transport study across the full FeSe_xTe_1-x range, and the genuinely new content is the composition sweep showing |A_S| peaking near x=0.45 and the orbital-to-Pauli crossover in Hc2 coinciding with the NFL regime. The raw data look internally consistent, and the qualitative FL vs NFL classification is plausible. Credit where due: the authors cite prior work on Fe(Se,S) and FST thermoelectricity honestly, so they do not oversell the individual signatures as new; the supplemental has real substance (MIR estimate, WHH fitting details, two-step fitting for lambda_SO); and shipping a systematic dataset over dozens of compositions is itself a contribution.\n\nThe soft spots are real but not fatal. First, the key fitted quantities—n, A_S, alpha—are reported without uncertainties, and the fitting windows are not specified in the main text. That matters because the sharp peak in |A_S| at x=0.45 is the load-bearing coincidence. Without error bars it is hard to know how sharp the peak really is. Second, the QCP location at x=0.45 is imported from bulk Tnem data (ref 64); the films have no direct nematic susceptibility or order-parameter measurement in the intermediate composition range. The lattice-constant match and the FeSe endpoint anomaly help, but strain or thickness effects could shift the film QCP by a few percent in x. If the true film QCP were at x=0.40 or 0.50, the |A_S| peak would no longer pin the QCP, and the 'dominant role' claim would soften to 'consistent with'. I call that a moderate concern, not a fatal one. The paper's own discussion of extended quantum criticality over 0.06<=x<=0.45 shows the authors are aware the NFL region is wider than a sharp QCP window, and the peak in |A_S| is still a real feature even if the QCP label is approximate. Third, the mass-enhancement inference goes through the Maki parameter and a BCS formula with independently known parameters; that is reasonable but indirect, and the WHH fits fix lambda_SO at 1, justified in the supplemental. The low-T deviations for x=0.06 and 0.13 are discussed openly rather than hidden.\n\nOverall: the central argument holds up as a strong circumstantial case, and the heavy-fermion analogy is drawn carefully. The paper deserves a serious referee. My main demands would be error bars on n, A_S, alpha; explicit fitting windows; and ideally one direct film-level probe of nematic order, or a clear statement that none is available. If those are added, I would be comfortable with this as a solid PRB or similar contribution.","headline":"A systematic MBE thin-film transport study with a credible but not airtight case that nematic fluctuations drive the NFL transport near x=0.45; worth refereeing with requested uncertainty and QCP-probe additions.","tokens_in":19712,"tokens_out":2527,"would_cite":true,"duration_ms":20977,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The paper claims that in FeSexTe1−x thin films, nematic fluctuations — not magnetic ones — drive non-Fermi-liquid transport, and that the same fluctuations swell the effective mass enough to switch the superconducting upper-critical-field…","keywords":["non-Fermi liquid transport","nematic quantum critical point","FeSexTe1-x thin films","Seebeck coefficient","thermoelectricity","upper critical field","Maki parameter","molecular beam epitaxy"],"falsifier":"Measure the elastoresistance (or another nematic-susceptibility proxy) on identical MBE-grown FeSexTe1−x films from x = 0.06 to 0.72; if the divergent nematic response peaks at a composition other than x = 0.45, or if the peaks in |AS| and the α > 1 region persist away from where Tnem extrapolates to zero, the attribution of the NFL transport to the nematic QCP is falsified.","tokens_in":18601,"feed_emoji":"⚡","tokens_out":6482,"duration_ms":59530,"temperature":0.7,"pith_summary":"The paper sets out to establish that nematic fluctuations, not magnetism, produce non-Fermi-liquid transport in FeSexTe1−x thin films, and that those same fluctuations strongly renormalize the quasiparticle mass. It reports two hallmark signatures: resistivity that is linear in temperature and a thermoelectric coefficient S/T that grows logarithmically as temperature drops, both appearing across a wide composition window. The logarithmic prefactor peaks sharply at x = 0.45, exactly where bulk crystals place the nematic quantum critical point, and the Maki parameter exceeds unity over the same composition range, signalling a switch from orbital- to Pauli-limited upper critical fields. If the paper is right, a clean, magnetism-free platform exists for studying quantum criticality, with nematic fluctuations shaping both the normal-state transport and the superconducting pair-breaking in an iron-based superconductor.","feed_headline":"Nematic fluctuations, not magnetism, drive strange-metal transport","feed_subtitle":"T-linear resistivity and S/T divergence peak where nematic order ends at x=0.45.","key_machinery":"The central object is the nematic quantum critical point at x = 0.45, probed through two experimental observables: the logarithmic prefactor AS of S/T ≈ AS ln T, which measures the strength of critical scattering, and the Maki parameter α = √2 Hc2^orb / Hc2^P, which measures the relative weight of orbital versus Pauli pair-breaking and, through α ≈ (2m*/m0)(Δ/EF), reports the effective mass. The argument is carried by the composition-space coincidence of the |AS| peak, the T-linear resistivity window, and the α > 1 region, all anchored to the bulk value of the nematic QCP.","core_discovery":"On its own terms, the central claim is that the normal state of MBE-grown FeSexTe1−x thin films is governed by the nematic quantum critical point. The authors find T-linear resistivity and a logarithmic divergence of S/T for 0.06 ≤ x ≤ 0.45, with the magnitude |AS| of the logarithmic term peaking at x = 0.45. In the same composition range, the Maki parameter α rises above unity, indicating that the upper critical field is Pauli-limited rather than orbital-limited. They interpret this coincidence as evidence that nematic fluctuations drive the NFL transport and inflate the effective mass, since α ≈ (2m*/m0)(Δ/EF) grows with mass renormalization. The authors conclude that nematic quantum criticality has a strong impact on both the normal-state transport and the superconducting properties of FeSexTe1−x.","pith_inferences":["Beyond the paper: if the mass enhancement is truly driven by nematic fluctuations, the electronic specific heat coefficient γ = C/T should peak near x = 0.45 in these same films; a low-temperature specific heat measurement on MBE films would directly test the α-based mass claim.","Beyond the paper: substrate strain and finite film thickness could shift or broaden the effective quantum critical point; comparing films on STO and CdTe substrates, which the paper shows have the same Tc dome but different lattice constants, would reveal whether the x = 0.45 coincidence is robust or strain-tuned.","Beyond the paper: the unusually wide NFL window from x = 0.06 to 0.45 might be a finite-thickness or disorder-broadened remnant of a sharper critical point rather than true extended quantum criticality; high-field experiments that access the normal state below Tc would distinguish a genuine zero-temperature extended critical regime from a crossover region."],"forward_implications":["The nematic QCP in FeSexTe1−x is isolated enough to act as a clean case study: the paper cites NMR data showing no significant antiferromagnetic fluctuations near x = 0.42, allowing nematic fluctuations alone to be tested as the driver of NFL transport.","The results place FeSexTe1−x among the few materials, such as cuprates, heavy fermions, and twisted bilayer graphene, in which NFL behavior survives over an extended composition range, suggesting extended quantum criticality rather than a sharp critical point.","The observed common onset temperature T*S ≈ T*ρ indicates that a single energy scale governs the appearance of NFL behavior in both resistivity and thermoelectricity.","The Pauli-limited Hc2 with α > 1 for out-of-plane fields is rare outside heavy-fermion systems, and the low-temperature upturns in Hc2 for Te-rich films motivate searches for exotic high-field superconducting states such as FFLO, although the paper notes the dirty limit makes that speculative."],"supporting_citations":[{"why":"Supplies the MBE growth, in-situ Te-annealing, and capping procedure that made full-composition FeSexTe1−x thin films available.","marker":"[75]"},{"why":"Locates the nematic quantum critical point at x = 0.45 in bulk single crystals, anchoring the composition axis of the paper's central coincidence.","marker":"[64]"},{"why":"Reports divergence of nematic susceptibility around x ≈ 0.5, supporting the presence of critical nematic fluctuations near the QCP.","marker":"[65, 66]"},{"why":"Provides the precedent of T-linear resistivity near a nematic QCP in Fe(Se,S), which this paper extends to the thermoelectric channel and to FeSexTe1−x films.","marker":"[70]"},{"why":"Supplies the theoretical prediction S/T ∝ ln T near a quantum critical point, the basis for using AS as a measure of quantum fluctuations.","marker":"[78]"},{"why":"Gives the Werthamer-Helfand-Hohenberg formula used to extract the orbital and Pauli limiting fields and hence the Maki parameter α.","marker":"[85]"},{"why":"Establishes the heavy-fermion benchmark where α > 1 for out-of-plane fields accompanies strong mass renormalization, the comparison underlying the mass-enhancement claim.","marker":"[86]"},{"why":"Places the antiferromagnetic QCP near x = 0.08, allowing the NFL window and the |AS| peak to be separated from magnetic criticality.","marker":"[68]"}],"fun_headline_variants":["Nematic QCP drives strange-metal transport in FeSeTe films","Thermoelectric peak reveals nematic QCP controls strange-metal state","Mass enhancement and NFL transport near nematic QCP in FeSeTe","Nematic quantum criticality dictates transport and pairing in FeSeTe"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the nematic quantum critical point sits at x = 0.45 in the MBE-grown films, a value inherited from bulk single-crystal resistivity measurements without direct measurement of nematic order or nematic susceptibility in the films themselves.","fun_headline_variants_meta":{"raw":{"variants":["Nematic QCP drives strange-metal transport in FeSeTe films","Thermoelectric peak reveals nematic QCP controls strange-metal state","Mass enhancement and NFL transport near nematic QCP in FeSeTe","Nematic quantum criticality dictates transport and pairing in FeSeTe"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001046,"raw_usage":{"total_tokens":4389,"prompt_tokens":927,"completion_tokens":3462,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":543,"completion_tokens_details":{"reasoning_tokens":3386}},"tokens_in":543,"tokens_out":3462,"duration_ms":21564,"temperature":1.0,"reasoning_tokens":3386,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T04:28:22.421754+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the elastoresistance (or another nematic-susceptibility proxy) on identical MBE-grown FeSexTe1−x films from x = 0.06 to 0.72; if the divergent nematic response peaks at a composition other than x = 0.45, or if the peaks in |AS| and the α > 1 region persist away from where Tnem extrapolates to zero, the attribution of the NFL transport to the nematic QCP is falsified.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the MBE growth, in-situ Te-annealing, and capping procedure that made full-composition FeSexTe1−x thin films available."},{"cited_title":"Mukasa, K","cited_arxiv_id":null,"evidence_quote":"Locates the nematic quantum critical point at x = 0.45 in bulk single crystals, anchoring the composition axis of the paper's central coincidence."},{"cited_title":"Licciardello, J","cited_arxiv_id":null,"evidence_quote":"Provides the precedent of T-linear resistivity near a nematic QCP in Fe(Se,S), which this paper extends to the thermoelectric channel and to FeSexTe1−x films."},{"cited_title":"Paul and G","cited_arxiv_id":null,"evidence_quote":"Supplies the theoretical prediction S/T ∝ ln T near a quantum critical point, the basis for using AS as a measure of quantum fluctuations."},{"cited_title":"Matsuda and H","cited_arxiv_id":null,"evidence_quote":"Establishes the heavy-fermion benchmark where α > 1 for out-of-plane fields accompanies strong mass renormalization, the comparison underlying the mass-enhancement claim."},{"cited_title":"Otsuka, S","cited_arxiv_id":null,"evidence_quote":"Places the antiferromagnetic QCP near x = 0.08, allowing the NFL window and the |AS| peak to be separated from magnetic criticality."}],"review_version":1}