{"id":"30f0af0d-7a03-4d7c-9d53-083ad47ef3d2","arxiv_id":"2501.08687","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In a triclinic iron pnictide superconductor, a spin resonance precursor, Nernst signal, and NMR density-of-states reduction all point to preformed Cooper pairs persisting to T* = 45 K, above Tc = 30 K.","lead":"This paper reports multiple measurements on a quasi-2D iron pnictide superconductor, CaLa-10-3-8, suggesting that electrons pair into preformed Cooper pairs above the superconducting transition temperature, up to about 45 K. The significance is that it extends the pseudogap and pre-pairing picture, previously seen in cuprates and iron selenides, to an iron pnictide.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The precursor-resonance evidence rests on an unquantified subtraction of a linear high-temperature background; if the residual intensity at 13 meV is normal-state spin fluctuation weight, the INS-based case for preformed pairs above Tc in CaLa-10-3-8 is not established.","rationale":"The reader identified the same weakest assumption that I consider most load-bearing: the residual INS intensity at ER = 13 meV, defined by subtracting a linearly fitted high-temperature background, is the key evidence for a precursor of the spin resonance above Tc. This matters because the abstract and summary present the precursor as a central piece of the preformed-pair picture: 'the precursor of spin resonance is highly related to the preformed Cooper pairs.' If that residual is ordinary normal-state spin fluctuation intensity or a background-subtraction artifact, the INS experiment does not demonstrate a pairing-related mode above Tc. The field-suppression data are more direct, but they too are interpreted through the lens of a precursor, and their statistical significance is not quantified.\n\nThe other evidence in the paper, notably the BKT transport and the Nernst signal, supports strong phase fluctuations in this quasi-2D system, which is consistent with—but not conclusive for—preformed pairs. The NMR 1/T1T drop below T* is explicitly acknowledged by the authors to rely on ignoring q-dependent spin fluctuations, and no Knight shift data are available, so that probe cannot independently establish a pairing gap above Tc. Thus, the INS precursor is the most important new observation that would elevate the interpretation from phase fluctuations to preformed Cooper pairs.\n\nSince the paper is a multi-probe study with real but incomplete evidence, I do not think the concern warrants rejection. However, the analysis must be made quantitative before the central claim can be accepted: alternative background models, error propagation, and a clear definition of the residual are needed. The reader's verdict of CONDITIONAL is appropriate, so I recommend no change to the verdict, but I emphasize that the condition should explicitly require the INS reanalysis described above.","tokens_in":22280,"tokens_out":7837,"duration_ms":89167,"concrete_test":"Reanalyze the 13 meV and 10 meV χ″(T) data of Fig. 6(a) with an alternative normal-state background (e.g., a broad Lorentzian response normalized to T > 150 K), propagating Poisson counting errors from the raw INS intensities, and test whether the residual is significantly positive between Tc and T* and extrapolates to zero at T*. If the residual is within noise, the precursor claim is unsupported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that a precursor of the 13 meV spin resonance exists above Tc depends on identifying an 'extra magnetic excitation' as the difference between the measured χ″(13 meV, T) and a straight-line extrapolation of the high-temperature damping background (Sec. III B, Fig. 6(a)). This subtraction is not unique: in iron pnictides, χ″(Q, ω) has a broad, non-linear temperature dependence in the normal state, so a linear fit to T > 100 K can produce an apparent excess wherever the data are convex, without implying a pairing-related mode. No error bars are reported for the residual, and the T* anomaly in the FWHM (Figs. 6(c,d)) and in dχ″/dT (inset of Fig. 6(f)) may be artifacts of Gaussian fitting or numerical differentiation of noisy data. If the residual is simply the tail of normal-state spin fluctuations, the precursor of the spin resonance is not established; the INS evidence then reduces to a conventional resonance below Tc. The field-suppression data (Fig. 6(e,f)) are less model-dependent, but their interpretation as evidence for pairing fluctuations rests on the existence of the zero-field precursor. Because the abstract and summary explicitly invoke the precursor as a key observation, this is the most load-bearing uncertainty in the paper.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a multi-probe investigation of the triclinic iron pnictide superconductor (Ca0.85La0.15)10(Pt3As8)(Fe2As2)5 with Tc ≈ 30 K, and claims that Cooper pairs preform above Tc below a crossover temperature T* ≈ 45 K. The evidence includes BKT-type nonlinear I-V characteristics, a 13 meV neutron spin resonance whose intensity decreases continuously through Tc with an anomaly at T*, a field suppression of the 13 meV intensity that persists up to T*, a Nernst signal extending to T*, and a reduction of 1/T1T below T* attributed to a loss of electronic density of states. The paper interprets these observations as phase-fluctuation-driven preformed pairs, analogous to the cuprate pseudogap scenario, and argues that the precursor of the spin resonance is the magnetic signature of incoherent pairing.","tokens_in":22562,"tokens_out":3557,"duration_ms":37370,"significance":"If the central claim holds, the paper would provide the first evidence in an iron pnictide that pairing and phase coherence are separated, extending the preformed-pair picture beyond cuprates and FeSe-based systems. The manuscript has notable strengths: high-quality co-aligned crystals, a clean observation of a spin resonance and spin gap in the superconducting state, a BKT analysis indicating quasi-2D superconductivity, and a multi-probe consistency of the T* ≈ 45 K crossover. The field-dependent neutron data above Tc are unusual and, if robust, would be of considerable interest. However, the central claim is an interpretation that rests on an unquantified subtraction of a high-temperature background in the neutron data, and several supporting probes have alternative normal-state explanations that are not quantitatively excluded. The paper is therefore suggestive rather than definitive at the current level of analysis.","major_comments":[{"comment":"The identification of a precursor of the spin resonance above Tc rests on the residual intensity at E = 13 meV after subtracting a straight-line extrapolation of the high-temperature damping background. No error bars, fit ranges, or residuals are reported, and in iron pnictides the normal-state χ″(Q, ω) is generally not linear in temperature over this range. The text itself hedges with 'the extra magnetic excitation at ER may be the precursor of SRM.' Because the abstract and summary invoke this precursor as a key observation, the subtraction must be shown to be robust against alternative normal-state forms (e.g., a convex or saturating background), with uncertainties propagated into the claimed excess.","section":"Sec. III B, Fig. 6(a)"},{"comment":"The FWHM kink at T* ≈ 45 K is presented as a 'clear' anomaly, but no quantitative criterion is given for locating the kink, and the FWHM values come from Gaussian fits to constant-energy scans that also include a spurious peak near Q = (0.8, 0, 2). The reported kink should be supported by fitting details, goodness-of-fit measures, and an estimate of the uncertainty in the kink position, especially because the same T* is then used to calibrate the interpretation of the other probes.","section":"Sec. III B, Figs. 6(c) and 6(d)"},{"comment":"The reduction of 1/T1T below T* is interpreted as a loss of density of states, but 1/T1T in iron pnictides is strongly affected by antiferromagnetic spin fluctuations, and the non-monotonic temperature dependence could reflect the development of spin correlations rather than a pairing gap. The authors themselves note in Sec. IV that 'it is kind of tricky to define the initial dropdown point of 1/T1T above Tc due to the crossover-like feature.' This is a load-bearing issue for the NMR-based DOS claim, and the manuscript should provide a quantitative decomposition (e.g., separating a Korringa contribution from a q-dependent spin-fluctuation contribution) or explicitly retreat from the DOS-loss interpretation.","section":"Sec. III D, Fig. 8(d)"},{"comment":"The inference that the field suppression of the 13 meV intensity above Tc is evidence for pairing fluctuations depends on the conditional statement 'if we believe that the spin fluctuation indeed acts as the pairing glue in FeSCs.' This conditionality undercuts the central claim as stated. In addition, the Nernst signal above Tc is argued to arise from phase fluctuations because Gaussian fluctuations 'cannot account' for it, but no comparison with a normal-state multiband baseline or a quantitative fluctuation model is given; the nonzero linear Sxy/B persisting to 300 K is attributed to multiband effects without a criterion separating that contribution from the proposed vortex signal. These alternatives need to be addressed for the T* onset to be credible.","section":"Sec. IV and Sec. III C"}],"minor_comments":[{"comment":"The name 'Halprin-Nelson' in the text should be 'Halperin-Nelson'; the reference [110] is correct but the in-text spelling is not.","section":"Sec. III A"},{"comment":"The text refers to 'Fig. 6(d)' when describing the temperature dependence of the Nernst coefficient; the relevant panel is Fig. 7(d).","section":"Sec. III C"},{"comment":"The sentence 'The statistic on superconducting transition is about Tc = 30 ± 3 K' is awkwardly phrased; 'statistic' should likely be 'statistical spread' or 'distribution.'","section":"Sec. II"},{"comment":"The caption says 'The open shape in (c) shows the background' but the open symbols appear to be in panel (a) or (c); please clarify which panel contains the background data.","section":"Fig. 4(a) caption"},{"comment":"The phrase 'unambiguously reveal a spin resonance peak' is stronger than the conditional discussion in Sec. IV; consider aligning the abstract with the stated caveats.","section":"Abstract"}],"recommendation":"major_revision","confidential_remarks":"The experimental data set is valuable and the multi-probe convergence is intriguing, but the manuscript currently overstates the certainty of the preformed-pair interpretation. The most serious issue is the unquantified neutron background subtraction, which is the basis for the claimed precursor resonance; without a robustness analysis or error bars, the central claim is not established. The authors also appear aware of several weaknesses (e.g., the difficulty of defining T* from 1/T1T and the conditional reliance on spin fluctuations as the pairing glue), and these admissions should be moved from passing remarks to systematic treatments in a revision. The paper fits the journal's scope, but the conclusions need to be either hardened with quantitative analysis or softened substantially."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Bottom line: this is a data-rich study of a quasi-2D iron pnictide, and the new measurement is the claim of a precursor of the 13 meV spin resonance surviving above Tc up to T* ~ 45 K, with an in-plane field suppression. If true, it's the first such case in a pnictide and would extend the preformed-pair picture beyond FeSe. The paper does a lot well: the crystals are well characterized, the BKT analysis is clean, and the resonance below Tc is convincingly identified with spectral-weight transfer and a gap.\n\nThe soft spot is exactly the one the stress-test flags. The 'extra' intensity at 13 meV above Tc is obtained by subtracting a straight-line extrapolation of the high-T background. No error bars are given for the intensity points, let alone for the residual. In iron pnictides the normal-state chi''(Q,omega) is not guaranteed to be linear in T, so a convex shape could mimic a precursor. The FWHM kink at 45 K and the d-chi''/dT anomaly are small effects and could come from the fitting or numerics. I would want to see the raw residuals, a bootstrap or Monte-Carlo error, and at least one alternative background (e.g., a power law or a fit that includes a broad normal-state mode) before I'd call the precursor established.\n\nThe other probes are individually weaker than the abstract implies. The 1/T1T drop below 45 K is consistent with a DOS reduction, but q-dependent spin fluctuations are ignored and there is no Knight shift. The Nernst signal above Tc overlaps with normal-state multiband contributions, which the authors concede. The field-suppression data are the most robust piece after the resonance itself, since they don't rely on the high-T background, but they do assume field-insensitive backgrounds.\n\nNone of this makes the paper unserious. The measurements are extensive, the authors acknowledge the crossover ambiguity in 1/T1T, and the citation pattern looks appropriate. I just think the central claim is an interpretation held together by several suggestive but individually non-quantitative probes. It deserves peer review—editors should send it out—but referees should demand a quantitative treatment of the INS background and error bars. If the precursor survives that, this becomes an important paper; if not, the rest of the data still has value for the 10-3-8 family.\n\nMy vote: send to review, with a strong request for revised statistics and a more falsifiable analysis.","headline":"New claim of a precursor spin resonance above Tc in an iron pnictide, backed by a multi-probe dataset but undercut by unquantified INS background subtraction; deserves peer review but not acceptance as-is.","tokens_in":23207,"tokens_out":3786,"would_cite":false,"duration_ms":38986,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["74.70.Xa","74.25.nj","78.70.Nx","74.72.Kf"],"model":"deepseek-v4-flash","headline":"In a layered iron pnictide, Cooper pairs begin forming at 45 K while superconductivity only locks in near 30 K, a separation of pairing from phase coherence that resembles the cuprate pseudogap regime.","keywords":["preformed Cooper pairs","spin resonance mode","phase fluctuations","quasi-2D superconductivity","iron pnictide","Nernst effect","nuclear magnetic resonance","Berezinskii-Kosterlitz-Thouless transition"],"falsifier":"Measure the 13 meV response in an isostructural non-superconducting 10-3-8 compound, or one with $T_c$ suppressed by controlled disorder, using identical background subtraction: if the same residual hump, the same peak-width kink at 45 K, and the same field suppression persist, the precursor is not specific to Cooper pairing. Alternatively, a high-statistics time-of-flight measurement showing that the residual intensity does not peak at the antiferromagnetic wave vector would also falsify the pairing-related precursor claim.","tokens_in":22033,"feed_emoji":"🧲","tokens_out":9686,"duration_ms":90954,"temperature":0.7,"pith_summary":"This paper argues that in the triclinic iron pnictide superconductor CaLa-10-3-8, with composition $(\\text{Ca}_{0.85}\\text{La}_{0.15})_{10}(\\text{Pt}_3\\text{As}_8)(\\text{Fe}_2\\text{As}_2)_5$, Cooper pairs begin to form near $T^*\\approx 45$ K, well above the superconducting transition at $T_c\\approx 30$ K, and only become phase coherent at $T_c$. The authors study a quasi-two-dimensional material whose zero-resistance transition follows the Berezinskii-Kosterlitz-Thouless picture, so phase fluctuations are strong enough to matter. They find that the neutron spin resonance at 13 meV, a collective magnetic excitation usually tied to Cooper pairing, does not disappear at $T_c$; its intensity, width, and magnetic-field response all track $T^*$ instead. Nernst and nuclear-magnetic-resonance measurements independently show signatures of fluctuating pairs and a reduced density of states below $T^*$. If the claim is right, pairing and phase coherence separate in this iron pnictide, making it a close cousin of the cuprate pseudogap problem.","feed_headline":"Iron pnictide forms Cooper pairs 15 K before it superconducts","feed_subtitle":"Spin resonance, Nernst signal and NMR relaxation all converge on a 45 K pairing onset.","key_machinery":"The load-bearing object is the neutron spin resonance mode (SRM), a sharp enhancement of the dynamic magnetic susceptibility $\\chi''(Q,\\omega)$ near the antiferromagnetic wave vector that in iron pnictides is regarded as the magnetic fingerprint of the paired state. Here it appears at $E_R=13$ meV and the argument follows its temperature and field dependence: a straight-line high-temperature damping background leaves residual intensity above $T_c$, the full width at half maximum of the magnetic signal develops a kink at $T^*\\approx 45$ K, and a 10 T in-plane field suppresses the intensity only below that same temperature. Supporting probes are the nonlinear current-voltage characteristics that locate a Berezinskii-Kosterlitz-Thouless transition at $T_{\\rm BKT}\\approx 28.5$ K, the Nernst signal whose nonlinear field dependence persists to $T^*$, and the $^{75}$As spin-lattice relaxation rate $1/T_1T$ whose downturn below $T^*$ signals a partial gap in the density of states. Together these measurements anchor the identification of $T^*$ as the onset of phase-incoherent pairing.","core_discovery":"The central discovery is a temperature scale $T^*\\approx 45$ K in CaLa-10-3-8 at which incoherent superconducting correlations set in, while long-range superconducting coherence only appears at $T_c\\approx 30$ K. Inelastic neutron scattering shows a spin resonance at $E_R=13$ meV with a 5 meV spin gap in the superconducting state; above $T_c$ the intensity does not drop to the normal-state baseline but continues decreasing smoothly, with a kink in the peak width exactly at $T^*$. An in-plane 10 T field suppresses this residual intensity all the way up to $T^*$, matching the field response expected for pair-breaking rather than ordinary spin fluctuations. Below $T^*$ the Nernst coefficient becomes nonlinear and field-dependent, and the NMR relaxation rate $1/T_1T$ turns downward, indicating a loss of density of states at the Fermi level. The authors conclude that the 13 meV resonance has a precursor above $T_c$ and that this precursor is intimately connected to preformed Cooper pairs generated by phase fluctuations.","pith_inferences":["Beyond the paper: one testable extension is to map $T^*$ as a function of La or Pt doping and of interlayer spacing; if $T^*$ tracks anisotropy or superfluid density rather than $T_c$, the phase-fluctuation scenario is strengthened.","Beyond the paper: a direct time-of-flight neutron measurement of the full spin-excitation spectrum could decide whether the precursor mode has the same dispersion and wave-vector structure as the superconducting-state resonance, since the present data are too limited to settle the dispersion.","Beyond the paper: the paper leaves open whether the same pre-pairing signature would appear in specific heat or in a Josephson plasma response, two probes that could independently confirm a phase-incoherent paired region.","Beyond the paper: if the residual 13 meV intensity is truly tied to pairing, it should be suppressed by controlled non-magnetic substitution in the FeAs layers that breaks pairs, a test the authors do not perform."],"forward_implications":["If preformed pairs exist from 45 K down to $T_c$, then $T_c$ in this compound is set by phase coherence, specifically vortex unbinding, rather than by the pairing energy scale itself.","The 13 meV spin resonance should be understood as a response of the paired but phase-incoherent state, not only of the superconducting condensate, so its intensity above $T_c$ is expected and carries information about $T^*$.","The coincidence of anomalies at $T^*$ across neutron, Nernst, and NMR measurements makes $T^*$ a robust, probe-independent temperature scale in this material.","The same combination of measurements applied to other quasi-two-dimensional iron-based superconductors should reveal whether a precursor resonance and a Nernst signal always appear together."],"supporting_citations":[{"why":"Supplies the theoretical basis that strong phase fluctuations create phase-incoherent preformed Cooper pairs above $T_c$.","marker":"[2]"},{"why":"Establishes the Nernst signal above $T_c$ as a probe of superconducting phase fluctuations, which the paper uses as one of its three independent signatures.","marker":"[14]"},{"why":"Demonstrates preformed-pair behavior in quasi-two-dimensional iron chalcogenide superconductors, the closest prior material example this paper extends to a pnictide.","marker":"[29]"},{"why":"Shows in underdoped cuprates that resonance-like spin fluctuations persist up to $T^*$, providing the analog for the precursor resonance reported here.","marker":"[62]"},{"why":"Shows in HgBa2CuO4+delta that resonance-like spin fluctuations persist into the pseudogap regime, another comparison anchoring the interpretation of the 13 meV precursor.","marker":"[64]"},{"why":"Shows that an in-plane magnetic field suppresses spin fluctuations in the cuprate pseudogap state, the direct analog of the field response measured here.","marker":"[66]"},{"why":"Provides the Berezinskii-Kosterlitz-Thouless resistance formula used to extract $T_{\\rm BKT}$ and confirm the quasi-two-dimensional nature of the superconductivity.","marker":"[110]"}],"fun_headline_variants":["Spin resonance reveals preformed pairs 15 K above Tc in iron pnictide","Iron pnictide shows phase-fluctuation pairing at 45 K, above Tc=30 K","Preformed Cooper pairs in iron pnictide at 45 K, Tc at 30 K","Iron pnictide mimics cuprates: pairing precedes coherence by 15 K"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The fragile step is treating the excess 13 meV neutron intensity above a straight-line high-temperature background as an intrinsic magnetic precursor tied to pairing; if that residual is ordinary normal-state spin fluctuation intensity or a background-subtraction artifact, the central evidence for a precursor resonance collapses.","fun_headline_variants_meta":{"raw":{"variants":["Spin resonance reveals preformed pairs 15 K above Tc in iron pnictide","Iron pnictide shows phase-fluctuation pairing at 45 K, above Tc=30 K","Preformed Cooper pairs in iron pnictide at 45 K, Tc at 30 K","Iron pnictide mimics cuprates: pairing precedes coherence by 15 K"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000755,"raw_usage":{"total_tokens":3408,"prompt_tokens":1049,"completion_tokens":2359,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":665,"completion_tokens_details":{"reasoning_tokens":2265}},"tokens_in":665,"tokens_out":2359,"duration_ms":18590,"temperature":1.0,"reasoning_tokens":2265,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:19:37.604244+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the 13 meV response in an isostructural non-superconducting 10-3-8 compound, or one with $T_c$ suppressed by controlled disorder, using identical background subtraction: if the same residual hump, the same peak-width kink at 45 K, and the same field suppression persist, the precursor is not specific to Cooper pairing. Alternatively, a high-statistics time-of-flight measurement showing that the residual intensity does not peak at the antiferromagnetic wave vector would also falsify the pairing-related precursor claim.","supporting_citations":[{"cited_title":"Preformed Cooper pairs in layered FeSe-based superconductors","cited_arxiv_id":"1912.03508","evidence_quote":"Demonstrates preformed-pair behavior in quasi-two-dimensional iron chalcogenide superconductors, the closest prior material example this paper extends to a pnictide."},{"cited_title":"Spin dynamics in the pseudogap state of a high-temperature superconductor","cited_arxiv_id":"0806.4134","evidence_quote":"Shows in underdoped cuprates that resonance-like spin fluctuations persist up to $T^*$, providing the analog for the precursor resonance reported here."},{"cited_title":"Hourglass dispersion and resonance of magnetic excitations in the superconducting state of the single-layer cuprate HgBa2CuO4+{\\delta} near optimal doping","cited_arxiv_id":"1610.01097","evidence_quote":"Shows in HgBa2CuO4+delta that resonance-like spin fluctuations persist into the pseudogap regime, another comparison anchoring the interpretation of the 13 meV precursor."},{"cited_title":"The connection between superconducting phase correlations and spin excitations in YBa$_2$Cu$_3$O$_{6.6}$: A magnetic field study","cited_arxiv_id":"cond-mat/0006433","evidence_quote":"Shows that an in-plane magnetic field suppresses spin fluctuations in the cuprate pseudogap state, the direct analog of the field response measured here."}],"review_version":1}