{"id":"9a7933b3-c667-4ab6-8a0b-6d9e96423a9d","arxiv_id":"2608.09207","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"A nano-skyrmion lattice of monolayer Fe on Ir(111)/Nb(110) completely quenches the proximitized superconducting gap, attributed to strong local exchange coupling.","lead":"Monolayer iron islands hosting a nano-skyrmion lattice were grown on a superconducting iridium film on niobium, and tunneling spectroscopy shows the superconductor's energy gap disappears on the iron islands. The result warns that the strong magnetic exchange coupling needed for topological superconductivity can also destroy the superconducting gap.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The fixed-Δ BdG model shows Shiba-state gap filling, not order-parameter quenching; the causal claim that exchange coupling destroys superconductivity is therefore not established by the calculation.","rationale":"The paper reports a compelling experimental observation: a monolayer Fe nano-skyrmion lattice on proximitized Ir(111)/Nb(110) shows a filled superconducting gap, while uncovered Ir nearby shows a partially suppressed gap. The tight-binding calculation demonstrates a clear trend: increasing J fills the spectral gap. The text is careful to say the calculation is not a direct fit. My read of the argument is that the weakest point is not the assumed spin texture (the SP-STM images in Fig. 2 directly reveal the skyrmion lattice on the thin film, so the texture is not merely assumed), but rather the inference from a fixed-Δ BdG calculation to the strong claim that exchange 'quenches superconductivity.' The calculation sets Δ as an input and never checks whether the superconducting order parameter would survive at the large J values invoked. In the strong-coupling Shiba regime, the local DOS can become gapless while the condensate remains finite, so the experimental spectra on the Fe island are equally compatible with a dense Shiba band. A self-consistent calculation is a natural and decisive check. If Δ(r) survives, the title and conclusions overstate the result; if Δ(r) collapses, the exchange-quenching mechanism is supported. The reader's emphasis on stray fields is reasonable but less decisive given the direct imaging of the texture and the 'nearly compensated' argument. I also note the internal inconsistency between the text (μ = 0) and Fig. 4 caption (μ = -1.0t), which should be fixed but does not itself change the verdict.","tokens_in":10161,"tokens_out":11255,"duration_ms":126748,"concrete_test":"Perform a self-consistent BdG calculation for the same periodic skyrmion lattice and parameters (μ = -1.0t, α = 0.02t, initial Δ = 0.02t, J = 0→3t), updating the local order parameter Δ_r = V⟨c_{r↓}c_{r↑}⟩ at each site until convergence. If the self-consistent |Δ| at the skyrmion sites remains near its J = 0 value for the J values used to match the experimental spectra, then the observed gap filling is a Shiba-band spectral effect rather than quenching of the superconducting condensate, and the conclusions should be reframed. If |Δ| collapses to zero on the skyrmion sites, the exchange-quenching interpretation is supported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim rests on the BdG calculation in the Theoretical calculations section, which keeps the pairing potential Δ fixed at 0.02t while J is varied. At J=2–3t the calculated DOS fills in, but a fixed-Δ model cannot distinguish a destroyed condensate from a dense Shiba/Andreev band superimposed on a finite order parameter. Both produce a flat, gapless local DOS and suppressed coherence peaks, which is exactly the experimental signature on the Fe island. The paper's conclusion that Fe/Ir(111)/Nb(110) is in a strong-exchange regime 'strong enough to destroy the superconducting gap' therefore goes beyond what the calculation demonstrates. The observation itself is valuable, but the mechanistic claim that exchange coupling quenches superconductivity, rather than merely obscures the spectral gap, requires a self-consistent treatment. This distinction is not cosmetic: it changes the design constraint for skyrmion-based topological superconductivity, since the relevant question is whether the pairing amplitude survives or is destroyed.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the preparation of monolayer Fe islands on Ir(111) thin films grown epitaxially on Nb(110), the identification of the atomic-scale nano-skyrmion lattice by spin-polarized STM/STS, and the observation that the proximitized superconducting gap is completely filled on the Fe islands while the surrounding Ir surface retains a partially suppressed gap. To interpret this, the authors perform tight-binding Bogoliubov-de Gennes calculations with a fixed induced pairing potential Delta and show that increasing local s-d exchange coupling J progressively fills the density-of-states gap and suppresses coherence peaks. They conclude that the Fe nano-skyrmion lattice lies in a strong exchange-coupling regime in which exchange coupling destroys the local superconducting gap.","tokens_in":10378,"tokens_out":4310,"duration_ms":50802,"significance":"The experimental part is valuable: it extends the Fe/Ir(111) nano-skyrmion lattice to a proximitized superconducting thin film and provides spatially resolved spectra showing complete gap disappearance on the skyrmion lattice. The non-fit parameter study is honest, and the robust trend across mu and alpha is useful. However, the mechanistic conclusion that exchange coupling quenches superconductivity goes beyond what the fixed-Delta calculation demonstrates. If the claim is established, it sets an important constraint for skyrmion-based topological superconductivity; as presented, the manuscript needs a self-consistent treatment or a softened conclusion.","major_comments":[{"comment":"The calculation keeps Delta fixed at 0.02t while varying J; it therefore demonstrates spectral-gap filling by in-gap Shiba/Andreev states, not the destruction of the pairing amplitude. The experimental signature on the Fe island - flat local DOS and absent coherence peaks - can be produced by a dense Shiba band over a finite order parameter. The conclusion that 'superconductivity gets quenched' by exchange coupling is therefore not established. Please either perform a self-consistent BdG calculation that allows Delta to respond to J and show that Delta goes to zero at the relevant coupling, or revise the concluding causal claim to state that exchange coupling fills the spectral gap rather than destroying the condensate.","section":"Theoretical calculations, Eq. (1) and Fig. 4"},{"comment":"There is a direct inconsistency in the reported simulation parameters: the text states that a representative set of parameters with mu = 0 and alpha = 0.02t was used, while the Fig. 4 caption lists mu = -1.0t for panels (a-d) and (e), with no mu = 0 case shown. This prevents the reader from reproducing the calculation and must be corrected; the actual parameter values used in each panel should be stated explicitly.","section":"Theoretical calculations, first paragraph; Fig. 4 caption"},{"comment":"The attribution of the gap quenching to exchange coupling, rather than to disorder or stray fields, rests on the assumption that the thin-film Fe/Ir(111) nano-skyrmion lattice is identical to the single-crystal system and that stray-field pair breaking is negligible. The SP-STM images in Fig. 2 support the same periodicity, but the manuscript provides no quantitative estimate of stray fields, disorder, or finite-size effects for the islands. Given that the causal claim is load-bearing, please add a concrete test - for example, a self-consistent calculation with disorder, a control experiment with a magnetic island lacking the skyrmion texture, or an estimate of the stray-field pair-breaking rate - or weaken the causal language in the conclusions.","section":"Experimental results and discussion; Conclusions"}],"minor_comments":[{"comment":"Throughout the experimental sections, gap values, ratios, and spectral features are reported without error bars or statistical measures; the statement that numerous other spectra show the same behavior should be quantified, for example by giving the number of spectra and islands measured and the standard deviation of the LDOS ratio on and off the Fe islands.","section":"Experimental results and discussion"},{"comment":"The axis labels and legend of Fig. 4(e,f) should define the DOS ratio and the parameter values more explicitly; the caption is currently ambiguous about which curve corresponds to which mu or alpha value.","section":"Fig. 4(e,f)"},{"comment":"There are typographical errors in the figure captions: 'flock-in' should be 'lock-in' in Figs. 1 and 3, and 'n Fe island' should be 'an Fe island' in Fig. 3.","section":"Figure captions"},{"comment":"Reference [41] lacks an article number; please complete the citation for the Phys. Rev. B paper.","section":"References"},{"comment":"The text refers to Figs. S1-S4, but the supplementary material is not included with the manuscript; please ensure it is available for review and referenced consistently.","section":"Supplementary information"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is from an established group and the experimental data appear carefully acquired; the self-citations are appropriate rather than excessive. The main risk is overclaiming from a fixed-Delta model, which is fixable by either a self-consistent calculation or a revised conclusion. I recommend major revision rather than rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read this for the experiment. The authors grow monolayer Fe on Ir(111) thin film on Nb(110), show the atomic-scale skyrmion lattice by SP-STM, and measure STS across the island: the superconducting gap disappears on Fe while remaining partially on surrounding Ir. That is a clean, reproducible observation (they show many spectra, plus a lower-coverage sample with recovery ~10 nm away). It directly challenges the earlier expectation that a symmetry-protected gap survives in a skyrmion lattice, so it is a meaningful new data point for the skyrmion-superconductor field.\n\nThe theoretical part is where I have reservations. The BdG calculation keeps Δ fixed at 0.02t and increases J. A fixed-Δ model can produce a filled local DOS from Shiba/Andreev bands while the order parameter stays finite. The paper's title and conclusions say exchange coupling 'destroys' or 'quenches' superconductivity, but the calculation only demonstrates gap filling, not order-parameter suppression. A self-consistent calculation, or a direct measurement of J, is needed to support the stronger claim. The authors do acknowledge the calculation is not a fit, but the wording goes beyond what the model shows.\n\nOther soft spots: no error bars or statistical analysis on the spectra, the chemical potential is inconsistent (text says μ=0, Fig. 4 caption says μ=−1.0t), and no data/code are made public. None of these kill the experimental result, but they make the mechanistic conclusion provisional. The edge-localized in-gap enhancement is noted as unexplained, which is honest.\n\nOverall: the experiment is the contribution. The theory is a plausible parameter scan, not a proof. For a referee, I would want the self-consistent calculation or a clear statement that the model shows Shiba-band filling, not condensate destruction, and a fix of the parameter inconsistency. The paper deserves serious review because the platform is new and the observation is likely correct.\n\nRecommendation: send to peer review with a request for revision on the theoretical interpretation and data availability.","headline":"A new experimental platform—monolayer Fe nano-skyrmion lattice on proximitized Ir(111)/Nb(110)—shows complete local gap quenching; the fixed-Δ BdG calculation supports spectral filling but does not prove order-parameter destruction.","tokens_in":10856,"tokens_out":1781,"would_cite":true,"duration_ms":60211,"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":"A monolayer Fe nano-skyrmion lattice on Ir(111)/Nb(110) completely quenches the superconducting gap, and tight-binding Bogoliubov-de Gennes calculations attribute the quenching to strong local s-d exchange coupling between the Fe spins…","keywords":["nano-skyrmion lattice","exchange coupling","superconducting gap quenching","scanning tunneling spectroscopy","Bogoliubov-de Gennes","proximity effect","skyrmion-superconductor hybrid","Fe/Ir(111)"],"falsifier":"Measure the superconducting gap on the same Fe islands while using a magnetic field to progressively unwind or polarize the nano-skyrmion lattice: if the gap stays fully quenched regardless of the spin texture, direct exchange coupling is confirmed, whereas a partial gap recovery would show that stray-field or texture-dependent pair breaking contributes. A second decisive check would be to compute the stray-field pair-breaking rate from the measured spin texture and show it is far too small to close the 1.16 meV proximity gap.","tokens_in":10000,"feed_emoji":"🧲","tokens_out":9144,"duration_ms":86253,"temperature":0.7,"pith_summary":"This paper reports that a monolayer Fe nano-skyrmion lattice, grown on an Ir(111) thin film proximitized by Nb(110), completely destroys the local superconducting gap: the coherence peaks vanish and the conductance ratio of Fermi-level to coherence-peak signal reaches one on the Fe islands. The study argues that the cause is not magnetic stray fields, because the nano-skyrmion spin texture is nearly compensated, but strong direct s-d exchange coupling between the Fe spins and the itinerant electrons of the Ir film. Tight-binding Bogoliubov-de Gennes calculations reproduce the spectral evolution, starting from a clean gap at zero exchange coupling and ending with a fully filled gap at strong coupling. This matters because skyrmion-superconductor hybrids are proposed platforms for topological superconductivity and Majorana modes, and the result shows that exchange-coupling strength is a decisive design constraint.","feed_headline":"Exchange coupling quenches superconductivity on a skyrmion lattice","feed_subtitle":"STS shows the Fe nano-skyrmion lattice fills the gap completely; calculations trace it to strong exchange coupling.","key_machinery":"The carrying object is the nano-skyrmion lattice, a periodic array of topologically non-trivial swirling spin structures with a period of about one nanometer that forms spontaneously in monolayer Fe on Ir(111) with a nearly compensated total spin. The carrying mechanism is the onsite s-d exchange coupling $J$ in a tight-binding Bogoliubov-de Gennes Hamiltonian that couples the local Fe spin at each site to the itinerant electron spin, alongside Rashba spin-orbit coupling and an s-wave pairing potential inherited from the superconducting substrate. As $J$ grows, the calculated density of states develops in-gap weight and loses its coherence peaks; the DOS ratio defined as the Fermi-level density of states divided by the average density of states at the coherence peaks climbs to near one when $J$ reaches about $2t$. That ratio provides the direct point of comparison with the experimental conductance maps, making $J$ the single control parameter that reproduces the measured spectral evolution.","core_discovery":"The central discovery is that the field-free nano-skyrmion lattice in monolayer Fe on Ir(111)/Nb(110) leaves no trace of superconductivity in the tunneling spectra: the gap and coherence peaks disappear across the Fe island, while the uncovered Ir film still shows a partially suppressed gap. This is unexpected because the nearly compensated spin texture should minimize stray-field pair breaking, and because previous monolayer ferromagnetic and antiferromagnetic films on superconductors still displayed resolvable gap features. The accompanying tight-binding calculation shows that increasing the onsite exchange coupling $J$ progressively fills the density-of-states gap and removes the coherence peaks, with essentially complete quenching near $J \\approx 2t$. The paper concludes that this hybrid sits in a strong exchange-coupling regime and that direct magnetic hybridization, not stray fields, is what closes the superconducting gap.","pith_inferences":["If exchange coupling is really the quench mechanism, inserting a thin spacer that reduces $J$ should restore a resolvable superconducting gap on the Fe islands; this is a testable prediction the paper does not make.","The same mechanism may explain why topological superconducting phases have been hard to observe in other directly coupled monolayer magnets: the exchange coupling strong enough to create Shiba bands may also be strong enough to fill the gap before the topological phase is reached.","Stray-field pair breaking could still contribute on samples where the spin texture is not perfectly compensated or is partially polarized; a controlled comparison of the field-free skyrmion lattice with a field-polarized Fe monolayer on the same film would separate the two channels.","The near-edge in-gap enhancement, which the paper leaves unexplained, could be a boundary effect of the quenched-gap region rather than a topological signature; spin-resolved tunneling across the island edge could test that reading."],"forward_implications":["In a directly coupled magnet-superconductor hybrid, strong exchange coupling alone can quench superconductivity even when the magnetic layer is one atom thick and its spin texture is nearly compensated.","Fe/Ir(111)/Nb(110) in the as-grown strong-coupling regime cannot serve directly as a platform for skyrmion-based topological superconductivity, because the gap needed for topological states is destroyed.","Efforts to engineer topological states in skyrmion-superconductor hybrids must tune the exchange coupling downward, for example by choosing spacer layers, so that non-collinear magnetism survives while the superconducting gap remains open.","The conductance-ratio measure introduced in the paper offers a simple experimental quantity that future theoretical models of magnet-superconductor coupling can match directly.","The unexplained edge-localized in-gap enhancement near the Fe island marks a region where further theoretical work is needed to decide whether boundary effects of the quenched-gap region carry physical significance."],"supporting_citations":[{"why":"shows a monolayer magnet on a superconductor, Fe/Re(0001), where superconducting features survive, providing the contrast that makes the present quenching unexpected.","marker":"[11]"},{"why":"predicted that a full superconducting gap can be preserved inside a skyrmion lattice under the relevant symmetries, the theoretical expectation the experiment overturns.","marker":"[17]"},{"why":"established that monolayer Fe on Ir(111) hosts the spontaneous atomic-scale nano-skyrmion lattice used as the magnetic layer.","marker":"[37]"},{"why":"identified Fe/Ir(111) nano-skyrmions as a route toward skyrmion-superconductor hybrids and supplies the direct-coupling motivation.","marker":"[38]"},{"why":"provided the clean Nb(110) surface preparation and the 1.53 meV superconducting reference gap used to calibrate the proximity gap.","marker":"[41]"},{"why":"characterized the structural and superconducting properties of ultrathin Ir films on Nb(110), establishing the proximitized substrate.","marker":"[44]"},{"why":"supplied the tight-binding Bogoliubov-de Gennes framework for magnetic structures coupled to s-wave superconductors.","marker":"[48]"},{"why":"provided the effective Shiba-band and strong-coupling description used to interpret the filled gap and suppressed coherence peaks.","marker":"[49]"}],"fun_headline_variants":["Skyrmion lattice exchange coupling quenches superconductivity","Nano-skyrmion lattice fully suppresses superconducting gap","Exchange coupling fills superconducting gap on skyrmion lattice","Fe nano-skyrmion lattice kills superconductivity in hybrid","Strong exchange coupling shuts off superconductivity in skyrmion hybrid"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The causal conclusion rests on the premise that the Fe monolayer on the Ir(111) thin film has the same nearly compensated nano-skyrmion texture as on Ir(111) single crystals, so that stray-field pair breaking is negligible and the onsite exchange coupling $J$ is the only relevant quench mechanism.","fun_headline_variants_meta":{"raw":{"variants":["Skyrmion lattice exchange coupling quenches superconductivity","Nano-skyrmion lattice fully suppresses superconducting gap","Exchange coupling fills superconducting gap on skyrmion lattice","Fe nano-skyrmion lattice kills superconductivity in hybrid","Strong exchange coupling shuts off superconductivity in skyrmion hybrid"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000179,"raw_usage":{"total_tokens":1266,"prompt_tokens":875,"completion_tokens":391,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":491,"completion_tokens_details":{"reasoning_tokens":308}},"tokens_in":491,"tokens_out":391,"duration_ms":4206,"temperature":1.0,"reasoning_tokens":308,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T21:23:02.503849+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the superconducting gap on the same Fe islands while using a magnetic field to progressively unwind or polarize the nano-skyrmion lattice: if the gap stays fully quenched regardless of the spin texture, direct exchange coupling is confirmed, whereas a partial gap recovery would show that stray-field or texture-dependent pair breaking contributes. A second decisive check would be to compute the stray-field pair-breaking rate from the measured spin texture and show it is far too small to close the 1.16 meV proximity gap.","supporting_citations":[{"cited_title":"Palacio -Morales, E","cited_arxiv_id":null,"evidence_quote":"shows a monolayer magnet on a superconductor, Fe/Re(0001), where superconducting features survive, providing the contrast that makes the present quenching unexpected."},{"cited_title":"Mascot, J","cited_arxiv_id":null,"evidence_quote":"predicted that a full superconducting gap can be preserved inside a skyrmion lattice under the relevant symmetries, the theoretical expectation the experiment overturns."},{"cited_title":"Heinze, K","cited_arxiv_id":null,"evidence_quote":"established that monolayer Fe on Ir(111) hosts the spontaneous atomic-scale nano-skyrmion lattice used as the magnetic layer."},{"cited_title":"Kubetzka, J","cited_arxiv_id":null,"evidence_quote":"identified Fe/Ir(111) nano-skyrmions as a route toward skyrmion-superconductor hybrids and supplies the direct-coupling motivation."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"provided the clean Nb(110) surface preparation and the 1.53 meV superconducting reference gap used to calibrate the proximity gap."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"characterized the structural and superconducting properties of ultrathin Ir films on Nb(110), establishing the proximitized substrate."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"supplied the tight-binding Bogoliubov-de Gennes framework for magnetic structures coupled to s-wave superconductors."},{"cited_title":"Advanced Imaging of Matter","cited_arxiv_id":null,"evidence_quote":"provided the effective Shiba-band and strong-coupling description used to interpret the filled gap and suppressed coherence peaks."}],"review_version":1}