{"id":"48407d28-8840-4097-bbcf-64f3284cc6bd","arxiv_id":"2607.14677","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"ThFeAsN is predicted to be a spin-fluctuation-driven superconductor with Tc ≈ 22.4 K and a nodal dxy-wave (B2g) gap, from parameter-free superconducting density functional theory.","lead":"An ab initio calculation predicts that the iron-based superconductor ThFeAsN pairs its electrons through magnetic spin fluctuations, giving a critical temperature of about 22 K and a d-wave-like gap with nodes. The distinctive predicted signatures — a V-shaped density of states and strongly direction-dependent ultrasonic attenuation — can be checked by existing experiments.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The SF-toggle experiment is not controlled: removing spin fluctuations also changes µ (1.304 → 0.687), so the isolation of SF as the pairing glue is incomplete.","rationale":"The reader's weakest assumption (reliability of the ALDA/RPA SF kernel for FeSCs) is a legitimate external-validity concern. My stress-test identifies a more internal, fixable issue: the SF-toggle experiment in Table I is not a controlled isolation because µ changes alongside the SF kernel. This does not refute the mechanism, but it weakens the strongest piece of evidence. The paper's other strengths—parameter-free SCDFT, concrete testable predictions (V-shaped QPDOS, anisotropic ultrasonic attenuation), and internal honesty about contradictory µSR interpretations—support keeping a CONDITIONAL verdict rather than moving to ACCEPT or REJECT. The proposed test (repeat the toggle with µ fixed) is a computational experiment the authors can run with their existing code; it would settle whether the 'SF drives pairing' conclusion is an artifact of the kernel decomposition. I do not see a fatal flaw; the central claim is plausible but not yet established to the standard the abstract suggests.","tokens_in":12264,"tokens_out":7614,"duration_ms":85324,"concrete_test":"Repeat the SCDFT calculation for ThFeAsN with the spin-fluctuation pairing kernel K_SF set to zero, but keep the Coulomb kernel (µ) and the renormalization Z exactly as in the full calculation (case (c) of Table I), i.e., only omit the K_SF term from Eq. (1). Record Tc and the gap symmetry. If Tc drops to below ~1 K and the B2g solution disappears, the SF-driven mechanism is confirmed for this kernel decomposition. If Tc remains significant (e.g., >5 K) or the B2g state persists, the mechanism claim is not cleanly supported and the decomposition must be reconsidered.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that ThFeAsN is an SF-mediated superconductor rests on the computer experiments in Table I: switching off the SF kernel drops Tc from 22.4 K to 0.17 K (or 0 K when µ is included). However, the comparison is not a controlled toggle. In case (c) (EPC+µ+SF) µ = 1.304, while in case (b) (EPC+µ, no SF) µ = 0.687. Removing the SF channel therefore also changes the screened Coulomb repulsion. Since Tc in this system emerges from near-cancellation between a very large repulsive µ and the attractive SF kernel, the observed Tc and even the B2g gap symmetry may depend on how the total kernel is split between K^ee and K^SF. The manuscript does not define µ operationally, and the unexplained jump signals scheme dependence. Moreover, the paper claims RPA and ALDA give similar results but provides no numerical comparison, leaving the reader unable to assess the sensitivity of the B2g solution to the susceptibility approximation. The mechanistic conclusion 'SF drives pairing' would be much stronger if the toggle were performed with µ fixed at the full-calculation value.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports superconducting density functional theory (SCDFT) calculations for ThFeAsN, including electron-phonon coupling, screened static and dynamic Coulomb repulsion, and spin-fluctuation (SF) mediated pairing. Using a parameter-free calculation, the authors obtain a superconducting transition temperature Tc = 22.4 K, close to the experimental 29 K, and predict an even-parity B2g (dxy-wave) order parameter with sign changes between Fermi-surface sheets. They also compute a V-shaped quasiparticle density of states and direction-dependent ultrasonic attenuation, presented as experimentally testable signatures. The central mechanistic claim is that superconductivity is driven by spin fluctuations, based on Table I: the EPC-only case gives Tc = 0.17 K, EPC + Coulomb gives 0 K, the full calculation gives 22.4 K, and the no-EPC case gives 31.4 K.","tokens_in":12356,"tokens_out":3951,"duration_ms":46998,"significance":"If the calculation is correct, this is a significant contribution: it provides a first-principles, parameter-free prediction that a stoichiometric Fe-based superconductor is magnetically mediated with nodal dxy pairing, alongside concrete experimental consequences. The internal comparisons cleanly show that phonons alone cannot explain 29 K, and the absence of any fit to the target Tc is a genuine strength. However, the reliability of the central claim rests on the spin-fluctuation kernel, whose benchmark is limited to weakly correlated V and Nb, and on the treatment of the large effective Coulomb parameter µ. The manuscript is therefore of high interest but requires strengthening on control experiments and sensitivity analysis.","major_comments":[{"comment":"The core attribution of superconductivity to spin fluctuations is based on a toggle that is not controlled. Case (c) (EPC + µ + SF) uses µ = 1.304, while case (b) (EPC + µ, no SF) uses µ = 0.687. Removing the SF channel also changes the repulsive Coulomb kernel, so the comparison does not isolate the SF pairing interaction. Since Tc = 22.4 K arises from near cancellation between a large repulsive µ and the attractive SF kernel, this confound could materially affect both the value of Tc and the B2g solution. Please provide an operational definition of µ (equation and frequency dependence) and perform a controlled toggle with µ fixed at the full-calculation value, or decompose the total kernel into SF-dependent and SF-independent parts.","section":"Table I and 'computer experiments'"},{"comment":"The manuscript states that RPA and ALDA calculations give similar SC properties but provides no numerical comparison. This is load-bearing because µ is much larger than λ (1.304 vs 0.127), so a modest relative change in the SF kernel could shift Tc by tens of kelvin and possibly alter the gap symmetry. The only benchmark cited (ref. [45]) covers V and Nb, which are weakly correlated metals; no Fe-based superconductor benchmark is given. Please include RPA versus ALDA results (at least Tc and gap symmetry) or a quantitative sensitivity analysis of the B2g solution to kernel variations.","section":"Appendix A (ALDA choice)"},{"comment":"The paper mentions that a multichannel Eliashberg calculation by Schrodi et al. predicted Tc = 3.15 K for ThFeAsN, and found no superconductivity down to 2 K when EPC and charge fluctuations were included. This factor-of-seven discrepancy with the present 22.4 K is directly relevant to the central claim, yet the manuscript never discusses why the two first-principles-based approaches differ so strongly. Please add a detailed comparison of the two methods (kernel definitions, treatment of spin fluctuations, and Coulomb repulsion) and, ideally, a benchmark on a common input to show that the SCDFT machinery is not producing an artefact.","section":"Introduction, ref. [8]"}],"minor_comments":[{"comment":"The caption uses '×(1/2)' without clearly indicating which curves are scaled; this makes the temperature dependence hard to read. Please plot the raw gap values or add explicit panel labels.","section":"Fig. 2 caption"},{"comment":"The relation between the scalar µ in Table I and the k-resolved µee_nk defined in Eq. (6) is not explained. Please state how the momentum-dependent quantity is reduced to the single number µ, since this is central to the sensitivity discussion.","section":"Table I and Appendix D"},{"comment":"The orange, yellow, and red arrows indicating nesting vectors and pairing interactions are difficult to distinguish in monochrome print. Consider using different line styles or labels in the figure.","section":"Fig. 6"},{"comment":"The phrase 'fully taken into account' overstates the ALDA/RPA treatment of spin fluctuations and the approximate handling of the Coulomb kernel. Suggest softening to 'treated within the SCDFT approximation' to avoid overclaiming.","section":"Abstract and main text"}],"recommendation":"major_revision","confidential_remarks":"The paper addresses an important problem and the central claim is plausible, but the uncontrolled µ toggle and the missing RPA/ALDA numerical comparison are load-bearing issues. If the authors can supply a fixed-µ comparison and a sensitivity/noise benchmark for the SF kernel, the manuscript could be suitable for publication. The discrepancy with ref. [8] also needs a substantive explanation rather than a passing mention."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Two things to know. The paper makes the first ab initio SCDFT prediction of a nodal B2g dxy gap in an iron-based superconductor, ThFeAsN, with Tc = 22.4 K against experimental 29 K, and it lays out two concrete experimental tests. It also has a control problem in its central mechanism experiment: turning off spin fluctuations changes the effective Coulomb repulsion µ, so the SF-attribution is less clean than the abstract implies.\n\nWhat is good: this is a parameter-free calculation. They compute λ, ωln, µ, Z, and the momentum-dependent gap from DFT-derived inputs, no fitting to Tc. The internal computer experiments are informative: phonons alone give 0.17 K, phonons plus µ give 0, µ plus SF gives 31.4 K, and the full calculation gives 22.4 K. The predicted V-shaped QPDOS and direction-dependent ultrasonic attenuation are concrete and falsifiable. The dxy(±) gap structure is a genuinely new result, different from the earlier multichannel Eliashberg s± prediction (ref [8]) with Tc = 3.15 K. If the symmetry prediction survives scrutiny, it is a meaningful advance for SCDFT applied to FeSCs.\n\nSoft spots, in proportion. The un-controlled toggle is the main one. Table I lists µ = 0.687 without SF and µ = 1.304 with SF. Switching the SF channel on also reshapes µ, and the manuscript does not explain why. Since Tc depends on near-cancellation of a large repulsive term against the SF attraction, a moderate error in the kernel could shift Tc by tens of kelvin or change the gap symmetry. I would want to see the toggle with µ fixed at the full-calculation value, and the RPA/ALDA comparison given as numbers, not \"do not differ significantly.\" Second, the abstract says the results are \"consistent with experiments,\" but the text itself documents that the µSR data are fit by conflicting models (nodeless two-gap vs. s+d with nodes). The honest statement in the text is fine; the abstract overclaims. Third, the factor-of-seven Tc discrepancy with ref [8] is not addressed. It may come from different methods, but a sentence of explanation would help.\n\nOverall: this is a serious piece of work. The main claim may be right, but I would not take the gap symmetry as settled until the kernel sensitivity is quantified. It deserves a real referee, and the referee should be asked to push on the µ-fixed toggle and the RPA/ALDA numbers.","headline":"First-principles dxy prediction for ThFeAsN is new and credible, but the spin-fluctuation attribution rests on an uncontrolled toggle, and the µ jump needs explaining.","tokens_in":13083,"tokens_out":2946,"would_cite":true,"duration_ms":31391,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":["82D55"],"pacs":["74.20.-z","74.25.Kc","74.70.Xa"],"model":"deepseek-v4-flash","headline":"ThFeAsN is predicted to be a spin-fluctuation-driven superconductor with a dxy-wave gap and a Tc of 22.4 K, close to the measured 29 K.","keywords":["unconventional superconductivity","ThFeAsN","spin fluctuations","dxy-wave gap","B2g symmetry","SCDFT","iron-based superconductor","first-principles"],"falsifier":"High-resolution ARPES of ThFeAsN in the superconducting state: if the gap on the largest hole pocket shows no nodes along the [100] direction, or if the gap on the electron pockets has the same sign as the hole pockets, the dxy-wave prediction is falsified. Likewise, ultrasonic attenuation along [100] that rises exponentially at low temperature rather than nearly linearly would disagree with the predicted nodal structure.","tokens_in":11964,"feed_emoji":"🧲","tokens_out":2195,"duration_ms":24291,"temperature":0.7,"pith_summary":"The paper uses first-principles superconducting density functional theory to determine the pairing mechanism and gap symmetry of the iron-based superconductor ThFeAsN. It claims that spin fluctuations, not electron-phonon coupling, are responsible for superconductivity, and that the order parameter has B2g symmetry, forming dxy-waves with sign changes between Fermi surface sheets. The calculated critical temperature of 22.4 K agrees well with the experimental value of 29 K, and the predicted nodal gap structure yields a V-shaped quasiparticle density of states and anisotropic ultrasonic attenuation—both measurable fingerprints.","feed_headline":"Spin fluctuations drive ThFeAsN's 22 K superconductivity","feed_subtitle":"First-principles calculations predict a dxy-wave gap with line nodes, matching the measured 29 K Tc within a few kelvin.","key_machinery":"The central object is the spin-fluctuation pairing kernel K_SF, computed from the ab initio spin susceptibility within the adiabatic local density approximation. This kernel, together with the electron-phonon kernel and the screened Coulomb repulsion, enters the superconducting density functional theory gap equation. The magnetic susceptibility peaks near q = (π/a, π/a), linking hole and electron pockets and forcing a sign-changing dxy-wave solution; the nodal lines of this solution produce the predicted V-shaped density of states and non-exponential ultrasonic attenuation.","core_discovery":"The authors establish that ThFeAsN is an unconventional multiband superconductor whose pairing is mediated by spin fluctuations. Solving the superconducting gap equation with electron-phonon, screened Coulomb, and spin-fluctuation kernels, they find a Tc of 22.4 K and a gap function of B2g symmetry: quasi-two-dimensional dxy-waves with vertical line nodes and opposite signs on different Fermi surface sheets. Controlled computer experiments show that switching off spin fluctuations drops Tc to 0.17 K, while switching off electron-phonon coupling leaves Tc at 31.4 K, demonstrating that spin fluctuations are the dominant pairing glue.","pith_inferences":["A direct extension of this work would be to compute the spin-fluctuation kernel for other undoped iron-based superconductors, such as LiFeAs or NaFeAs, to see whether the B2g dxy-wave state is specific to ThFeAsN or a more general prediction.","The predicted vertical line nodes on all five Fermi sheets imply that low-temperature thermal conductivity should exhibit a quasi-linear T dependence; this could be checked against existing or future heat-transport data.","Because the screening parameter μ jumps from 0.687 to 1.304 when spin fluctuations are included, the method treats μ as an effective scheme-dependent quantity; a more transparent separation of static and dynamic screening could reduce this ambiguity.","If the dxy-wave state is confirmed, it would show that heavy hole doping is not the only route to nodal pairing in iron-based superconductors, broadening the phase diagram of gap symmetries."],"forward_implications":["If correct, superconductivity in ThFeAsN is magnetic in origin, strengthening the case that spin fluctuations are a common pairing mechanism in iron-based superconductors.","The predicted nodal dxy-wave gap with sheet-dependent signs can be tested directly by scanning tunneling spectroscopy and high-resolution ARPES.","The anisotropic ultrasonic attenuation—nearly linear at low temperature along [100] and [110]—provides a clear experimental signature that distinguishes this state from nodeless s-wave.","The success of first-principles calculations including spin fluctuations would motivate applying the same method to other stoichiometric iron-based superconductors to predict their gap symmetries.","The large screened Coulomb repulsion (μ = 1.304) means the predicted Tc emerges from near-cancellation of competing terms, so quantitative agreement with experiment is a stringent test of the approach."],"fun_headline_variants":["Spin fluctuations, not phonons, pair electrons in ThFeAsN","ThFeAsN's 22 K superconductivity traced to spin fluctuations","Spin fluctuations win in ThFeAsN: 22 K unconventional gap","ThFeAsN: spin fluctuations glue electrons at 22 K","Unconventional superconductivity in ThFeAsN: spin-fluctuation mediated"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The spin-fluctuation pairing strength computed from density functional theory is quantitatively reliable for iron-based superconductors, even though the method has only been benchmarked on simple elemental metals like vanadium and niobium.","fun_headline_variants_meta":{"raw":{"variants":["Spin fluctuations, not phonons, pair electrons in ThFeAsN","ThFeAsN's 22 K superconductivity traced to spin fluctuations","Spin fluctuations win in ThFeAsN: 22 K unconventional gap","ThFeAsN: spin fluctuations glue electrons at 22 K","Unconventional superconductivity in ThFeAsN: spin-fluctuation mediated"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000948,"raw_usage":{"total_tokens":3854,"prompt_tokens":686,"completion_tokens":3168,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":430,"completion_tokens_details":{"reasoning_tokens":3085}},"tokens_in":430,"tokens_out":3168,"duration_ms":23121,"temperature":1.0,"reasoning_tokens":3085,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T01:24:04.033063+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"High-resolution ARPES of ThFeAsN in the superconducting state: if the gap on the largest hole pocket shows no nodes along the [100] direction, or if the gap on the electron pockets has the same sign as the hole pockets, the dxy-wave prediction is falsified. Likewise, ultrasonic attenuation along [100] that rises exponentially at low temperature rather than nearly linearly would disagree with the predicted nodal structure.","supporting_citations":[],"review_version":1}