{"id":"e32e3695-7072-4e45-86dc-bb99ed4947d9","arxiv_id":"2501.07412","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Correlation effects via DFT+U stabilize the P-62m structure of the kagome ferromagnet MnRuAs, which then exhibits quasi-1D Fermi surface sheets and a nodal sphere in the absence of spin-orbit coupling.","lead":"This paper uses DFT+U calculations to show that adding a strong on-site Coulomb repulsion (U_eff = 4 eV) removes imaginary phonon modes in MnRuAs, stabilizing the experimentally observed P-62m crystal structure and a ferromagnetic ground state. It then predicts quasi-one-dimensional Fermi surface pockets and a spherical nodal surface from spin-up/spin-down band crossings, and computes terrace-dependent surface states for future experiments.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The stabilization hinges on Ueff=4 eV, yet the paper does not prove that physical U exceeds the ~2.5 eV threshold; the U that matches the measured Mn moment is closer to 2 eV.","rationale":"I focused on the stabilization claim because it is the title and abstract claim; the FM ground state, magnon dispersion, Fermi surface, and nodal sphere are all calculated within the same DFT+U model. The paper is transparent and uses standard methods, and the reader's conditional verdict is appropriate. My concern sharpens the reader's weakest assumption with two internal quantitative checks the paper itself provides but does not exploit: the experimental magnetic moment and lattice constants. The suggested cRPA test can settle the issue. I recommend keeping the conditional verdict rather than accepting unconditionally, since the load-bearing parameter is not independently anchored.","tokens_in":20651,"tokens_out":7324,"duration_ms":76951,"concrete_test":"Compute U for Mn-3d in MnRuAs with a parameter-free cRPA calculation (or a fully self-consistent constrained random-phase approximation), then repeat the phonon calculation at that U, scanning U between the computed value and the threshold. If the computed U lies below ~2.5 eV, or if phonons remain imaginary at the computed U, the stabilization claim fails. As a complementary check, determine which U reproduces the experimental Mn moment and lattice constants; if that U is below the stability threshold, the DFT+U model is internally inconsistent.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that real MnRuAs is dynamically stable because correlations (Ueff=4 eV) remove the P-62m soft modes. For this to be true, the physical U for Mn-3d in this compound must lie above the stabilization threshold seen in Fig. S1, roughly 2-2.5 eV. The paper never establishes that. Its linear-response estimate of 5.44 eV is dismissed as overestimated; the literature value of 4.14 eV is for Mn oxides, not MnRuAs; and the chosen U=4 eV is selected because it 'stabilizes the structure.' There is an internal tension: Table S1 gives the Mn moment as 4.29 uB at U=4 eV versus the experimental 3.96 uB, so matching the measured moment would put U near 2 eV, below the threshold; lattice constants at U=0 (a=6.528, c=3.601 A) are closer to experiment (a=6.518, c=3.619 A) than at U=4 (a=6.623, c=3.671 A). Thus the only robust indication that U exceeds U_c is the very stability the paper aims to prove, making the central argument circular unless an independent U determination is supplied.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper studies MnRuAs in the noncentrosymmetric P-62m (Fe2P-type) structure using DFT+U (PBE+Dudarev), phonon calculations via Phonopy, Wannier-based tight-binding modeling, exchange-coupling extraction with TB2J, and surface Green-function calculations. The central claim is that strong electronic correlations, represented by Ueff = 4 eV on Mn-3d, stabilize the experimentally observed P-62m structure that is dynamically unstable at lower U, while also yielding a ferromagnetic ground state, dominant exchange coupling along the c axis, quasi-one-dimensional Fermi-surface pockets, and a spin-up/spin-down nodal sphere that becomes a nodal ring under SOC. The paper presents phonon spectra for Ueff = 2 and 4 eV, a U-scan of phonon soft modes in the Supplemental Material, exchange couplings and magnon dispersion at Ueff = 4 eV, electronic band structure and Fermi surface at the same U, and surface spectral functions for both terminations of the (001) surface.","tokens_in":20881,"tokens_out":4870,"duration_ms":49538,"significance":"If the stabilization claim is correct, the paper is a useful example of correlation-driven dynamical stability in a kagome magnet and provides concrete, testable predictions: the FM ground state with strong c-axis coupling, flat Fermi-surface sheets, a nodal sphere near the Fermi level, and termination-dependent surface states. The computational setup is largely standard and well documented (400 eV cutoff, dense k-point grids, systematic U scan for phonons, comparison with experimental lattice constants and moments, and a linear-response U estimate). However, the significance is conditional because the central structural conclusion rests on a single effective U value that is partly selected for producing stability, and the independent experimental constraints (lattice constants and Mn moment) point toward lower U values that would be dynamically unstable in the calculation.","major_comments":[{"comment":"The central stabilization claim is not yet established because the physical U for Mn in MnRuAs is not independently constrained. The text states \"We adopt Ueff = 4 eV for our subsequent calculations, as this value stabilizes the structure,\" and the linear-response value of 5.44 eV (Fig. S3) is dismissed as overestimated without a quantitative justification. The literature value of 4.14 eV (Ref. [88]) is for Mn oxides, not for this intermetallic. Table S1 shows that matching the experimental Mn moment of 3.96 uB would place Ueff between roughly 1.5 and 2.0 eV, below the stabilization threshold inferred from Fig. S1 (between 2 and 2.5 eV), and the lattice constants at U = 0 (a = 6.528 A, c = 3.601 A) are closer to experiment (a = 6.518 A, c = 3.619 A) than those at U = 4 (a = 6.623 A, c = 3.671 A). The authors should either provide an independent U determination for MnRuAs (e.g., constrained RPA or a properly justified linear-response calculation) or demonstrate that the stabilization and all qualitative conclusions are robust across a U range compatible with independent observables.","section":"Section III.B, Fig. S1, Table S1"},{"comment":"The exchange couplings, magnon dispersion, Fermi surface, and nodal-sphere features are computed only at Ueff = 4 eV, with no sensitivity analysis for other physically plausible U values. Since the phonon instability disappears in the range 2-2.5 eV, it is essential to show whether the qualitative electronic and magnetic predictions (dominant c-axis exchange, flat Fermi-surface sheets, nodal sphere) survive at U values that are not selected by the stabilization criterion, such as U = 2.5 or 3 eV, or at the linear-response value of 5.44 eV. Without such tests, the paper has not demonstrated that these properties are intrinsic to MnRuAs rather than artifacts of the specific DFT+U parameter choice.","section":"Sections III.C and III.D"},{"comment":"The nodal sphere is claimed as a key electronic property, but its existence is not shown to be robust or symmetry-protected. The crossings arise between spin-up and spin-down bands in a metallic ferromagnet at a specific Ueff; no symmetry/irrep analysis or U-dependence study is presented, and the SOC gap at the crossings is not quantified numerically. The authors should clarify whether the nodal surface is protected by a symmetry of the P-62m structure or is accidental, and should demonstrate that it remains a closed sphere for a reasonable range of U and lattice parameters.","section":"Section III.D, Fig. 6"}],"minor_comments":[{"comment":"The text says that for \"Ueff < 2.5 eV\" imaginary modes appear and \"Ueff > 2 eV\" stabilizes the acoustic branches, leaving the critical value ambiguous; the caption or text should state the actual threshold obtained from the calculations.","section":"Section III.B and Fig. S1 caption"},{"comment":"The sentence \"The resulting tight-binding model, consisting of 39 orbitals and 78 bands.\" is a grammatical fragment and should be completed, e.g., by adding \"is used for the subsequent analysis.\"","section":"Section II"},{"comment":"The statement \"For Mn atoms, the SOC is ~4 meV on p orbitals\" is confusing because Mn in the PAW setup has no valence p states and the following sentence discusses As-p orbitals; please clarify which atomic orbitals are meant.","section":"Section III.D"},{"comment":"The high-symmetry labels g, h, i, m, n, o are used in the band-structure panels but are not defined in the text or in the Brillouin-zone diagrams; please define these points explicitly.","section":"Figures 6(b)-(c)"},{"comment":"The notation P-bar-62m is rendered inconsistently as \"P¯62m\" and \"P-62m\"; please unify to the standard crystallographic notation, e.g., P-62m.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The main issue is the parameter dependence of the central claim: the chosen Ueff = 4 eV is justified largely by the fact that it stabilizes the structure, while the experimental Mn moment and lattice constants suggest a lower U. This is a fixable but load-bearing problem; an independent U estimate or a systematic robustness study would materially strengthen the paper. If the authors can supply that, the paper would be a solid contribution to the kagome-magnet literature."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a competent DFT+U study that gives MnRuAs some genuinely new material-specific predictions — correlation-driven phonon stabilization in the P-62m phase, exchange couplings with dominant c-axis FM, quasi-1D Fermi pockets, a spin-split nodal sphere, and termination-dependent surface states. The paper is honest about its central parameter choice and gives reasons why Ueff = 4 eV is reasonable. What it doesn't do is close the loop on whether real MnRuAs sits above the stabilization threshold.\n\nThe new content is real. The FM ground state is not controversial, but the exchange constants and magnon dispersion are new, and the quasi-1D pockets tied to Ru-As chains plus the nodal sphere are specific predictions that ARPES or neutron experiments could chase. The computational setup is standard and well converged. The authors also include a linear-response estimate (5.44 eV) and cite a high-throughput Mn value (4.14 eV), which are independent grounds for U > 2.5 eV. So the circularity charge is only half right: U = 4 eV was chosen partly because it stabilizes the structure, but not arbitrarily.\n\nThe real soft spot is internal consistency, and it deserves referee attention. Table S1 shows that the U reproducing the measured Mn moment (~1.5–2 eV) sits below the phonon stabilization threshold, and the lattice constants at U = 0 are actually closer to experiment than those at U = 4 eV. The authors don't address this tension. If the physical U is near 2 eV, the central claim fails. They need robustness scans across U = 2.0–5.0 eV, and ideally a more reliable U from constrained RPA or a better linear-response calculation for this specific compound. The surface-state and nodal-sphere results would also gain a lot from one figure showing whether they survive at U = 2.5 or 3 eV.\n\nMinor: no code or data deposit, and the nodal sphere is presented without a symmetry characterization, but that is not disqualifying.\n\nVerdict: worth a serious referee. Send it to review, but the referee should push for U-robustness and a resolution of the lattice/moment tension. The audience is computational materials scientists working on kagome magnets and Mn-based ternaries.","headline":"A transparent but U-sensitive DFT+U study: the phonon stabilization claim is plausible but not yet proven, because the U values that match measured moments sit below the stabilization threshold.","tokens_in":21457,"tokens_out":2608,"would_cite":true,"duration_ms":26863,"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":"Strong electron correlation, modeled by a Hubbard $U_{\\mathrm{eff}}=4$ eV on Mn-$3d$ states, stabilizes MnRuAs in its observed $P\\bar{6}2m$ kagome structure and yields a stable ferromagnet with dominant c-axis coupling.","keywords":["MnRuAs","distorted kagome lattice","DFT+U","ferromagnetism","phonon stability","nodal sphere","quasi-one-dimensional Fermi surface","magnon dispersion"],"falsifier":"An independent parameter-free estimate of the Mn-$3d$ Hubbard $U$ in MnRuAs (for example, a constrained random-phase approximation) returning a value below about 2.5 eV would undermine the stabilization claim; alternatively, inelastic neutron or X-ray scattering that resolves soft acoustic branches along the A-L-H-A path in the $P\\bar{6}2m$ phase would directly falsify the predicted dynamical stability.","tokens_in":20460,"feed_emoji":"🧲","tokens_out":11622,"duration_ms":97044,"temperature":0.7,"pith_summary":"The paper sets out to resolve a contradiction: MnRuAs is observed to crystallize in the noncentrosymmetric $P\\bar{6}2m$ structure with a distorted kagome lattice and to order ferromagnetically near 496 K, yet ordinary density functional calculations find that structure dynamically unstable. The authors argue that strong on-site Coulomb repulsion among Mn-$3d$ electrons, captured by DFT+U with $U_{\\mathrm{eff}}=4$ eV, removes the imaginary phonon modes and stabilizes exactly the experimentally observed phase. With that stabilization, MnRuAs is a stable ferromagnet whose dominant Mn–Mn exchange couplings run along the $c$ axis, giving a parabolic magnon dispersion with pronounced $k_z$ dependence. The same electronic structure produces quasi-one-dimensional Fermi-surface sheets from Ru-As chains and a spherical nodal surface from spin-up/spin-down band crossings near the Fermi level. If this is right, it provides a concrete example of correlation-controlled structural stability in a magnetic kagome-type compound and predicts observable momentum-space features.","feed_headline":"4 eV Hubbard U makes MnRuAs stable and ferromagnetic","feed_subtitle":"Adding Coulomb repulsion removes imaginary phonons and restores the kagome structure with c-axis magnetism.","key_machinery":"The load-bearing object is the effective Hubbard parameter $U_{\\mathrm{eff}}$ applied to Mn-$3d$ states within a rotationally invariant DFT+U scheme. The paper tunes this single parameter and watches the phonon spectrum: at $U_{\\mathrm{eff}}$ below roughly 2.5 eV the acoustic branches along A-L-H-A go imaginary, while at $U_{\\mathrm{eff}}=4$ eV all modes are positive, and this value is adopted for all subsequent results. Around that stable point, a maximally localized Wannier tight-binding model of 39 orbitals reproduces the DFT band structure and feeds two downstream calculations: the Mn–Mn exchange couplings $J_{ij}$ obtained from a Green's-function method that treats rigid spin rotations as perturbations, and the surface Green's functions for the (001) surface. The argument's moving parts are the $U_{\\mathrm{eff}}$-dependent phonon hardening and the c-axis-dominated exchange that together connect correlation strength to the ferromagnetic ground state.","core_discovery":"The central discovery is that strong on-site Coulomb repulsion on Mn-$3d$ orbitals, captured by DFT+U with $U_{\\mathrm{eff}}=4$ eV, stabilizes MnRuAs in the experimentally observed $P\\bar{6}2m$ structure and produces a stable ferromagnetic state. In plain DFT the phonon spectrum has imaginary soft modes along the A-L-H-A path; above a critical $U_{\\mathrm{eff}}$ of roughly 2.5 eV these modes harden, and at 4 eV all frequencies are positive. The magnetic ground state is ferromagnetic, about 0.1 eV per formula unit below the A-type antiferromagnetic state, with a Mn moment of 4.29 $\\mu_B$ (experimental value: 3.96 $\\mu_B$) and an easy plane perpendicular to $c$. Exchange couplings extracted from a Wannier-based Green's-function method are dominated by Mn pairs stacked along $c$ (8.79 meV at 3.67 Å and 4.27 meV at 5.06 Å), giving a parabolic magnon branch at $\\Gamma$ with strong $k_z$ dependence. Electronically, the Fermi surface has two flat quasi-one-dimensional pockets tied to Ru-As chains, and spin-up/spin-down band crossings form a nodal sphere around $\\Gamma$; spin-orbit coupling opens a small gap along most of the sphere, leaving an effective nodal ring in the $xy$ plane.","pith_inferences":["Inference: because the stability threshold is a parameter value rather than a structural feature, isostructural Fe2P-type compounds that show soft modes in plain DFT (for example, MnRuP or MnRhAs) may also be correlation-stabilized, and the same DFT+U prescription could be tested on them.","Inference: the quasi-one-dimensional Fermi-surface sheets with high Fermi velocity suggest a nesting-driven charge- or spin-density-wave instability along the $c$ direction; anisotropic resistivity or pressure experiments could look for a transition the paper does not itself predict.","Inference: the near-Fermi nodal sphere in a ferromagnet without inversion symmetry is a candidate source of anomalous Hall or magneto-optical response, and measuring those signals would provide an indirect check of the nodal structure.","Inference: the comparable energy scales of magnons (about 85 meV) and phonons (about 32 meV) raise the possibility of magnon-phonon hybridization, which inelastic neutron or X-ray scattering could test in the stabilized compound."],"forward_implications":["MnRuAs should be dynamically stable in the $P\\bar{6}2m$ structure at ambient conditions, resolving the earlier contradiction between DFT phonons and experiment.","The ferromagnetic ground state is stable, with a Mn moment near 4.3 $\\mu_B$ and exchange couplings dominated by Mn pairs stacked along the $c$ axis; the magnon spectrum is parabolic at $\\Gamma$ and extends to about 85 meV.","Two Fermi-surface pockets are quasi-one-dimensional flat sheets tied to Ru-As chains, implying strongly anisotropic transport and possible nesting-driven density-wave instabilities along $c$.","Spin-up/spin-down band crossings form a spherical nodal surface around $\\Gamma$; spin-orbit coupling opens only a small gap, leaving a nodal ring in the $xy$ plane that should be visible to momentum-resolved probes.","Surface-sensitive measurements should see termination-dependent states: a Dirac-like crossing for the MnAs-terminated (001) surface and hole-like bands crossing the Fermi level for the RuAs termination."],"supporting_citations":[{"why":"Provides the experimental lattice constants and ferromagnetic ordering for MnRuAs that the calculations must reproduce.","marker":"[51]"},{"why":"Gives the experimental ferromagnetic ground state and Mn moment (3.96 μB) used as comparison targets.","marker":"[52]"},{"why":"Supplies the DFT+U formulation with a single effective U parameter that the stabilization argument relies on.","marker":"[62]"},{"why":"Provides the direct supercell force-constant method used to compute the phonon spectra showing the instability and its removal.","marker":"[70]"},{"why":"Gives the linear-response estimate of U (5.44 eV) that the paper judges overestimated to justify choosing 4 eV.","marker":"[87]"},{"why":"Supplies the literature linear-response value for Mn (4.14 eV) invoked as independent support for U_eff = 4 eV.","marker":"[88]"},{"why":"Provides the Green's-function method used to extract Mn–Mn exchange couplings from the tight-binding model.","marker":"[81]"}],"fun_headline_variants":["U=4 eV stabilizes MnRuAs into a ferromagnet","Correlation fixes MnRuAs, yields ferromagnetic kagome","Hubbard U transforms MnRuAs: from soft modes to magnetism","DFT+U makes MnRuAs stable with c-axis magnetism","Nodal sphere and ferromagnetism emerge in MnRuAs under U"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The argument rests on the assumption that the effective on-site Coulomb repulsion for Mn-$3d$ electrons in MnRuAs really is close to 4 eV; if the physical $U_{\\mathrm{eff}}$ falls below about 2.5 eV, the phonon calculation says the $P\\bar{6}2m$ structure would not be stable, and the main claim would collapse.","fun_headline_variants_meta":{"raw":{"variants":["U=4 eV stabilizes MnRuAs into a ferromagnet","Correlation fixes MnRuAs, yields ferromagnetic kagome","Hubbard U transforms MnRuAs: from soft modes to magnetism","DFT+U makes MnRuAs stable with c-axis magnetism","Nodal sphere and ferromagnetism emerge in MnRuAs under U"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000429,"raw_usage":{"total_tokens":2213,"prompt_tokens":985,"completion_tokens":1228,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":601,"completion_tokens_details":{"reasoning_tokens":1134}},"tokens_in":601,"tokens_out":1228,"duration_ms":9729,"temperature":1.0,"reasoning_tokens":1134,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:42:28.093400+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"An independent parameter-free estimate of the Mn-$3d$ Hubbard $U$ in MnRuAs (for example, a constrained random-phase approximation) returning a value below about 2.5 eV would undermine the stabilization claim; alternatively, inelastic neutron or X-ray scattering that resolves soft acoustic branches along the A-L-H-A path in the $P\\bar{6}2m$ phase would directly falsify the predicted dynamical stability.","supporting_citations":[{"cited_title":"Kanomata, T","cited_arxiv_id":null,"evidence_quote":"Provides the experimental lattice constants and ferromagnetic ordering for MnRuAs that the calculations must reproduce."},{"cited_title":"Kaneko, T","cited_arxiv_id":null,"evidence_quote":"Gives the experimental ferromagnetic ground state and Mn moment (3.96 μB) used as comparison targets."},{"cited_title":"Parlinski, Z","cited_arxiv_id":null,"evidence_quote":"Provides the direct supercell force-constant method used to compute the phonon spectra showing the instability and its removal."},{"cited_title":"Cococcioni and S","cited_arxiv_id":null,"evidence_quote":"Gives the linear-response estimate of U (5.44 eV) that the paper judges overestimated to justify choosing 4 eV."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the literature linear-response value for Mn (4.14 eV) invoked as independent support for U_eff = 4 eV."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the Green's-function method used to extract Mn–Mn exchange couplings from the tight-binding model."}],"review_version":1}