{"id":"d554f584-830a-42ad-82e9-53d96a91e490","arxiv_id":"2602.19093","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Multi-layer square-planar nickelates with n=4–8 nickel-oxygen layers show superconducting signatures at ~10–13 K, extending the nickelate superconducting family beyond infinite-layer compounds.","lead":"Thin-film nickelate superconductors were made in several new layer thicknesses (n=4–8) and all show superconducting-like resistive signatures at 10–13 K. The results suggest that different nickelate crystal structures share a common electron-count window for superconductivity.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Oxygen-stoichiometry uncertainty in the nominal d-filling x-axis could make the multilayer/infinite-layer superconducting overlap an artifact.","rationale":"The reader's weakest_assumption matches my independent read. I considered whether the lack of zero resistance/Meissner should be the top concern, but the field-dependent suppression and angle-dependent anisotropy in Figs. 3A-B provide strong evidence of superconducting transport, and n=3 serves as a control. The oxygen-stoichiometry assumption is more load-bearing because it underlies the phase diagram's x-axis and the unifying overlap with infinite-layer nickelates; it is also explicitly acknowledged in the manuscript. Since the authors flag it and the reader's CONDITIONAL verdict already accounts for it, no verdict change is needed. A concrete stoichiometry measurement would settle it.","tokens_in":14782,"tokens_out":6251,"duration_ms":57998,"concrete_test":"Measure the actual oxygen content (and if possible hydrogen) of the n=4,5,6,7,8 films—e.g., by Rutherford backscattering spectrometry or calibrated STEM-EELS—and/or estimate Ni valence by X-ray absorption at the Ni L3 edge. Recompute each n's x-position from measured stoichiometry. If the n-dependent shift exceeds ~0.02 in d-filling relative to the infinite-layer dome's nominal positions, the overlap claim is not robust. A simpler internal check: compare the measured Ni L3 white-line shift across n to the nominal d-filling sequence.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central overlap claim (Fig. 2A; abstract) plots all multilayer data at nominal Ni 3d filling d^{9−1/n}, computed from ideal Nd_{n+1}Ni_nO_{2n+2} stoichiometry and formal Nd^{3+}/O^{2−} valences. The paper itself flags 'empirical uncertainties in oxygen stoichiometry' in the Discussion. If the CaH2 topochemical reduction leaves n-dependent oxygen off-stoichiometry (residual apical oxygen, vacancies, or hydrogen incorporation), the actual d-filling at each n differs from the nominal value by an n-dependent offset. Because the headline result includes overlap with the infinite-layer superconducting dome, and because the infinite-layer points are also nominal x values, the apparent overlap could be a mismatch artifact. This is not an internal inconsistency, but it is a measurement/assumption risk that directly supports the paper's own caution. The resistive-drop identification of superconductivity is comparatively well-supported by field suppression and the n=3 non-superconducting control, so I do not carry that as the primary concern.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports synthesis of the multi-layer square-planar nickelates Nd_{n+1}Ni_nO_{2n+2} for n=3–8 and presents transport, STEM, RIXS, and DFT results. The principal claim is the discovery of superconducting signatures for n=4–8, with Tc,onset between 9.9 and 12.7 K (maximum 12.9 K at n=6), forming a superconducting regime in the nominal nickel d-filling phase diagram that overlaps with the chemically doped infinite-layer nickelates. It further reports damped magnetic excitations persisting into the overdoped n=3 compound, and interprets an anomalous anisotropy of the magnetoresistance as a consequence of Nd 4f moments.","tokens_in":14993,"tokens_out":5759,"duration_ms":51261,"significance":"If confirmed, the result would be significant: it would establish a structurally tunable family of nickelate superconductors that does not require chemical doping, and would suggest a common doping window for square-planar nickelates. The paper has notable strengths: the synthesis of the full n=3–8 series with atomically precise control, direct STEM imaging of the layered structure and local lattice expansion, a detailed RIXS study identifying magnetic excitations, and DFT support for electronic structure trends. The experimental data are presented in a mostly reproducible way, and the authors are candid about limitations (oxygen stoichiometry, n=8 weakness). However, the central claim of a 'superconducting regime' rests on resistive downturns and field suppression rather than zero resistance or Meissner effect, and the phase-diagram x-axis relies on nominal formal valences. These issues must be addressed before the broad conclusions can be accepted.","major_comments":[{"comment":"The x-axis of the phase diagram is the nominal nickel 3d filling d^{9−1/n}, computed from ideal Nd_{n+1}Ni_nO_{2n+2} stoichiometry and formal Nd^{3+}/O^{2−} valences. As the manuscript itself notes in the Discussion, there are 'empirical uncertainties in oxygen stoichiometry.' This is load-bearing: the headline overlap of the multilayer superconducting regime with the infinite-layer dome would be compromised if oxygen content varies with n by even a few percent. Please provide an estimate of the oxygen stoichiometry uncertainty (e.g., from Rutherford backscattering, neutron reflectivity, or titration) and show how the phase diagram and the overlap claim change under reasonable oxygen off-stoichiometry. At minimum, the nominal-doping caveat should be stated in the caption of Fig. 2A and in the abstract.","section":"Fig. 2A; Discussion"},{"comment":"The paper claims a 'superconducting regime' for n=4–8, but the evidence is resistive downturns and their suppression by magnetic field; no zero-resistance or Meissner measurements are reported. The Fig. 2A caption itself states that the n=8 compound shows 'superconducting correlations without a clear superconducting downturn.' This point is therefore qualitatively different from n=4–7, yet it is included in the same shaded region. Please separate the n=8 point (e.g., with an open or hatched marker), and soften the phase-diagram label to 'superconducting correlations' or 'superconducting signatures' unless bulk thermodynamic evidence is added. This is not a wording nuance: the continuity and extent of the claimed regime depend on it.","section":"Abstract; Fig. 2A"},{"comment":"Tc,onset is determined according to 'criteria described in Ref. (36)' (the supplementary). Since the reported Tc values and the phase diagram depend on this definition, the criterion should be stated in the main text or the relevant supplementary section should be clearly reproduced. Without this, the reader cannot assess whether the Tc values are consistent across n, and the phase diagram is not reproducible from the present text.","section":"Section 'Characteristics of the superconducting state'; Fig. 2B"}],"minor_comments":[{"comment":"The angle-dependent magnetoresistance is min-max normalized; please specify the normalization range in the caption and state whether the minimum corresponds to in-plane or out-of-plane orientation for each sample.","section":"Fig. 3F"},{"comment":"The single-branch fit has free parameters; the manuscript notes that this may be an average of 2n modes. Please state the fit parameters (mode energy, damping) and the confidence intervals for the n=3 and n=5 data, and make the raw RIXS spectra available for all q values.","section":"Magnetic excitations; Fig. 5D,E"},{"comment":"The sentence 'the n = 4, 8 compounds likely represent the edges of the superconducting region accessible by dimensional doping' is speculative; suggest rephrasing as 'may represent' to avoid overstatement.","section":"Characteristics of the superconducting state"},{"comment":"The infinite-layer data from Refs. 26,27,16 are plotted against nominal Sr content; those x-values also carry stoichiometry uncertainty. Please note this in the caption.","section":"Fig. 2A"},{"comment":"The subscript in the caption for the d_{x^2-y^2} orbital is garbled in the manuscript text; please fix the rendering.","section":"Fig. 4C"}],"recommendation":"major_revision","confidential_remarks":"This is a strong experimental advance and the authors are honest about limitations. However, the two central claims—superconductivity for n=4–8 and the overlap with the infinite-layer dome—rest on evidence that needs to be consolidated. Given the paper's profile, I recommend major revision, with particular attention to quantifying oxygen stoichiometry and to clearly separating the n=8 point. The supplementary material must be part of the review process."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Main news: this is the first systematic experimental survey of the multi-layer square-planar nickelates across n=3–8, and it reports resistive superconducting signatures for n=4–7, with weaker correlations at n=8. That extends the previously known n=5 compound into a family, and the phase diagram based on nominal d-filling is a useful organizing framework. The STEM and RIXS work are solid: the films look clean, the local lattice expansion near the fluorite layers is a nice observation, and the persistence of ~80 meV magnetic excitations into overdoped n=3 is a genuine data point for the field. The paper is careful to label the n=8 signal as correlations rather than a transition, and it flags the oxygen stoichiometry uncertainty itself.\n\nThe soft spots are real but not disqualifying. The superconducting identification rests on resistive downturns and field suppression, not zero resistance or Meissner; that's normal for thin-film nickelates, but it means the word 'superconducting' is slightly ahead of the evidence, especially at the dome edges. The x-axis of the phase diagram — nominal nickel d-filling — assumes ideal O stoichiometry and formal valences. The authors admit 'empirical uncertainties in oxygen stoichiometry', and if the actual oxygen content varies with n, the overlap between the multilayer dome and the infinite-layer dome could be partly an artifact of that doping scale. The main claim that these compounds superconduct doesn't hinge on this, but the 'remarkably universal' overlap claim does. Minor issue: the data availability statement says data 'will be available' rather than providing access.\n\nMy read: this is a serious experimental paper with a plausible central claim and honest acknowledgment of its own limitations. The soft spots are addressable with more measurements (zero resistance, magnetization, more careful stoichiometry) and should not block peer review. I'd send it to referees. For the reading group, it's worth discussing because it reframes the nickelate phase diagram and gives the field a new structural axis to tune. I'd cite it if I worked on nickelates, and I'd want the authors to tighten the doping axis before publication.","headline":"Careful experimental mapping of a new superconducting family, but the 'universal doping window' overlap claim rests on a stoichiometry assumption the authors themselves flag.","tokens_in":15636,"tokens_out":2655,"would_cite":true,"duration_ms":24827,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper reports that stacking n=4 to 8 layers of NdNiO2 between fluorite spacer layers produces superconductivity with onset temperatures up to 12.9 K, without any chemical doping.","keywords":["nickelate superconductivity","square-planar nickelates","multi-layer nickelates","phase diagram","structural doping","4f magnetism","magnetic fluctuations","RIXS"],"falsifier":"Measure the absolute oxygen content of the n = 4–8 films (for example by atomically resolved electron energy-loss spectroscopy or resonant X-ray scattering) and recompute the phase diagram; if the real d-filling moves the n = 4 and n = 8 points outside the infinite-layer superconducting dome, the claimed universal regime near d^9 collapses. Alternatively, synthesize the La analog La_{n+1}Ni_nO_{2n+2} across the same n range; if none superconduct, the 4f-moment and lanthanide-chemistry dependence would need to be folded into the universal picture.","tokens_in":14666,"feed_emoji":"🧲","tokens_out":5154,"duration_ms":43059,"temperature":0.7,"pith_summary":"The paper tries to establish that structural layering alone—varying the number n of square-planar nickel-oxide layers in Nd_{n+1}Ni_nO_{2n+2}—drives superconductivity for n = 4 through 8, with onset temperatures of 9.9–12.7 K and a maximum near 12.9 K at n = 6. This matters because it extends nickelate superconductivity beyond the chemically doped infinite-layer compounds and suggests a common superconducting regime near a nominal nickel d^{9−1/n} filling. The paper constructs a phase diagram in terms of this nominal filling and argues it overlaps with the superconducting dome of doped infinite-layer nickelates, implying a shared underlying physics in square-planar nickel-oxygen planes. It also reports that decreasing n makes the electronic structure more cuprate-like, that magnetic fluctuations persist into the overdoped non-superconducting regime, and that neodymium 4f moments reshape the superconducting anisotropy.","feed_headline":"Layered nickelates superconduct up to 12.9 K with no chemical doping","feed_subtitle":"Structural stacking of n=4–8 NdNiO2 layers reproduces the same 3d9 doping dome as doped infinite-layer nickelates.","key_machinery":"The structural motif is the multi-layer square-planar nickelate: n layers of NdNiO2 sandwiched between (NdO2)- fluorite layers, written as (NdNiO2)_n(NdO2). The fluorite layers nominally dope 1/n holes per Ni, shifting the nominal d-electron count from d9 (undoped) to d^{9−1/n}. Varying n therefore tunes doping and dimensionality without chemical substitution, and the paper maps each n onto a point in a phase diagram of nominal d-filling. Supporting mechanisms include the local lattice expansion near the fluorite layers and the 4f moments of neodymium, which are invoked to explain the anomalous superconducting anisotropy.","core_discovery":"The central discovery is that superconductivity appears in multi-layer square-planar nickelates Nd_{n+1}Ni_nO_{2n+2} for n = 4, 5, 6, 7, and 8, with resistive signatures and a maximal onset temperature of 12.9 K at n = 6. The authors achieve this by atomic-layer-controlled synthesis followed by topochemical reduction, producing films whose nominal nickel d-filling d^{9−1/n} spans the same range as chemically doped infinite-layer nickelates (the single-layer RNiO2 compounds). They interpret the superconducting regime as overlapping with the infinite-layer dome, so that structural tuning—inserting fluorite spacer layers that nominally dope holes—can substitute for chemical doping. They further","pith_inferences":["A direct test of the paper's universal-doping claim is to measure the actual oxygen content in each n film; if oxygen stoichiometry deviates from O_{2n+2}, the x-axis shifts and the overlap with the infinite-layer dome may be an artifact.","The same structural template could be explored with other rare earths (La, Pr) to separate 4f effects from dimensionality; the paper notes La versions are not yet superconducting, suggesting strain or synthesis issues, so a testable extension is to optimize those.","If the near-3d9 regime is truly universal across structural families, one might expect superconductivity in the n = 3 compound at slightly different hole doping or under pressure, since it already shows cuprate-like hybridization and magnetic fluctuations.","The local lattice expansion near fluorite layers could be used to engineer interlayer coupling via spacer-layer chemistry, a route the paper only gestures at."],"forward_implications":["If the claims hold, structural layering is a viable alternative to chemical doping for creating square-planar nickelate superconductors, and the n = 4–8 family provides a tunable platform.","The overlap of the superconducting regime with doped infinite-layer nickelates at similar nominal d-filling suggests a common set of ingredients near 3d^9 for nickelate superconductivity.","The persistence of ~80 meV magnetic fluctuations in non-superconducting n = 3 means superconductivity can be destroyed without destroying magnetism, constraining pairing mechanisms.","The 4f-moment effect on anisotropy implies that replacing Nd with a nonmagnetic rare earth (e.g., La) could alter or clarify the superconducting anisotropy and its dimensionality."],"fun_headline_variants":["Structural doping makes nickelates superconduct at 12.9 K","Nickelate superlattices hit superconducting dome without doping","Stacking layers unlocks superconductivity in nickelates up to 12.9 K","Multi-layer nickelates superconduct via structural hole doping","No chemical doping needed: nickelate stacks superconduct at 12.9 K"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The phase diagram is plotted against a nominal nickel d-filling computed from formal ion valences and the assumption of perfectly stoichiometric oxygen content; if the actual oxygen concentration differs from O_{2n+2}, the superconducting dome shifts along the doping axis and the overlap with infinite-layer nickelates may be a coincidence of that assumed scale.","fun_headline_variants_meta":{"raw":{"variants":["Structural doping makes nickelates superconduct at 12.9 K","Nickelate superlattices hit superconducting dome without doping","Stacking layers unlocks superconductivity in nickelates up to 12.9 K","Multi-layer nickelates superconduct via structural hole doping","No chemical doping needed: nickelate stacks superconduct at 12.9 K"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000837,"raw_usage":{"total_tokens":3483,"prompt_tokens":740,"completion_tokens":2743,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":484,"completion_tokens_details":{"reasoning_tokens":2654}},"tokens_in":484,"tokens_out":2743,"duration_ms":16154,"temperature":1.0,"reasoning_tokens":2654,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-02T21:43:56.512332+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the absolute oxygen content of the n = 4–8 films (for example by atomically resolved electron energy-loss spectroscopy or resonant X-ray scattering) and recompute the phase diagram; if the real d-filling moves the n = 4 and n = 8 points outside the infinite-layer superconducting dome, the claimed universal regime near d^9 collapses. Alternatively, synthesize the La analog La_{n+1}Ni_nO_{2n+2} across the same n range; if none superconduct, the 4f-moment and lanthanide-chemistry dependence would need to be folded into the universal picture.","supporting_citations":[],"review_version":1}