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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.

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2026-08-02 21:43 UTC pith:EQ3UX5WZ

load-bearing objection Careful experimental mapping of a new superconducting family, but the 'universal doping window' overlap claim rests on a stoichiometry assumption the authors themselves flag. the 3 major comments →

arxiv 2602.19093 v1 pith:EQ3UX5WZ submitted 2026-02-22 cond-mat.supr-con cond-mat.mtrl-scicond-mat.str-el

Superconducting phase diagram of multi-layer square-planar nickelates

Grace A. Pan , Dan Ferenc Segedin , Sophia F. R. TenHuisen , Lopa Bhatt , Harrison LaBollita , Abigail Y. Jiang , Qi Song , Ari B. Turkiewicz
show 16 more authors
Denitsa R. Baykusheva Abhishek Nag Stefano Agrestini Ke-Jin Zhou Jonathan Pelliciari Valentina Bisogni Hua Zhou Mark P. M. Dean Hanjong Paik David A. Muller Lena F. Kourkoutis Charles M. Brooks Matteo Mitrano Antia S. Botana Berit H. Goodge Julia A. Mundy
This is my paper
classification cond-mat.supr-con cond-mat.mtrl-scicond-mat.str-el
keywords nickelate superconductivitysquare-planar nickelatesmulti-layer nickelatesphase diagramstructural doping4f magnetismmagnetic fluctuationsRIXS
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

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.

Core claim

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

What carries the argument

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.

Load-bearing premise

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.

What would settle it

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.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

If this is right

  • 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.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 5 minor

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.

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 (3)
  1. [Fig. 2A; Discussion] 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.
  2. [Abstract; Fig. 2A] 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.
  3. [Section 'Characteristics of the superconducting state'; Fig. 2B] 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.
minor comments (5)
  1. [Fig. 3F] 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.
  2. [Magnetic excitations; Fig. 5D,E] 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.
  3. [Characteristics of the superconducting state] 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.
  4. [Fig. 2A] 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.
  5. [Fig. 4C] The subscript in the caption for the d_{x^2-y^2} orbital is garbled in the manuscript text; please fix the rendering.

Circularity Check

0 steps flagged

No significant circularity; the paper is an experimental report whose claims rest on new measurements, with only normal methodological self-citations.

full rationale

The paper's central claims—superconducting signatures for n = 4–8, the n-dependence of the phase diagram, and the overlap with infinite-layer nickelates—are supported by new transport, STEM, RIXS, and DFT results, not by a derivation that reduces to its inputs. The nominal d-filling axis d^(9−1/n) is obtained by formal valence counting and ideal stoichiometry, which is a modeling assumption explicitly flagged by the authors ('empirical uncertainties in oxygen stoichiometry'); it is not a fitted parameter disguised as a prediction, and the overlap claim is a comparison on a shared nominal scale rather than a consequence of that scale by construction. The RIXS single-magnetic-branch model is standard data analysis, and the 4f-moment anisotropy interpretation borrows an external model (Ref. 29) without claiming to derive it. Self-citations to the authors' prior synthesis and quintuple-layer reports are methodological and provide reproducible experimental context; they are not invoked as uniqueness theorems or as the sole justification for the new observations. No equation or fitted quantity is shown to be equivalent to the paper's own inputs, and the resistive superconducting identification is corroborated by field suppression and the non-superconducting n = 3 control. The acknowledged oxygen-stoichiometry uncertainty is a measurement/assumption risk appropriate for a correctness discussion, not evidence of circular reasoning.

Axiom & Free-Parameter Ledger

1 free parameters · 5 axioms · 0 invented entities

The paper's load-bearing assumptions are experimental-interpretation assumptions: formal charge counting for the doping axis, resistive signatures as superconductivity evidence, RIXS peak assignment as magnetic, and a prior model for Nd 4f effects. Only the RIXS branch parameters are fitted numbers in the main text; no new physical entities are introduced.

free parameters (1)
  • RIXS single-magnetic-branch mode energy and damping = E(q) ≈ 80 meV at q|| ≈ (-0.45, 0); Γ(q) extracted per q, roughly 25–100 meV
    Broad RIXS loss features are modeled as one damped magnon-like branch to claim magnetic excitations persist in n=3 and n=5; the fitted energy and damping are the empirical content of Fig. 5.
axioms (5)
  • domain assumption Nominal doping is 1/n holes per Ni site, derived from formal Nd3+/O2- valences and ideal oxygen stoichiometry.
    The entire phase diagram and the overlap with infinite-layer nickelates are plotted against this nominal doping. The paper itself acknowledges 'empirical uncertainties in oxygen stoichiometry' in the Discussion.
  • domain assumption Resistive downturns without zero resistance or Meissner effect are accepted as superconducting signatures.
    The central claim of a superconducting regime for n=4–8 rests on transport signatures and a Tc-onset criterion from the supplement; n=8 lacks a clear downturn.
  • domain assumption The ~80 meV RIXS feature is magnetic, as established by incident-energy and polarization dependence.
    The persistence of magnetic fluctuations into the overdoped regime depends on this identification, but the supporting analysis is only referenced to the supplementary material.
  • domain assumption Neodymium 4f moments enhance magnetic permeability and enhance paramagnetic depairing along the in-plane direction.
    This prior model (Ref. 29) is used to explain the reversed superconducting anisotropy; the present paper does not independently test the microscopic mechanism.
  • domain assumption DFT predictions of hybridization trends are reliable for Ndn+1NinO2n+2.
    DFT is used to support the claim that lower-n compounds become more cuprate-like; no experimental band structure is provided to verify the computed gaps.

pith-pipeline@v1.3.0-alltime-deepseek · 14551 in / 12912 out tokens · 116514 ms · 2026-08-02T21:43:56.512332+00:00 · methodology

0 comments
read the original abstract

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of cuprate-like superconductivity. Experimental investigations however have largely been limited to the infinite-layer $R$NiO$_2$ ($R$=rare-earth) nickelates. Here, we construct a phase diagram of multi-layer square-planar Nd$_{n+1}$Ni$_n$O$_{2n+2}$ compounds and discover signatures of superconductivity for $n$ = 4 - 8. Upon decreasing the dimensionality $n$, the superconducting anisotropy evolves due to 4$f$ electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the over-doped, non-superconducting regime. Remarkably, the superconducting regime overlaps with that of chemically-doped infinite-layer nickelates, demonstrating underlying commonalities and distinct differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

Figures

Figures reproduced from arXiv: 2602.19093 by Abhishek Nag, Abigail Y. Jiang, Antia S. Botana, Ari B. Turkiewicz, Berit H. Goodge, Charles M. Brooks, Dan Ferenc Segedin, David A. Muller, Denitsa R. Baykusheva, Grace A. Pan, Hanjong Paik, Harrison LaBollita, Hua Zhou, Jonathan Pelliciari, Julia A. Mundy, Ke-Jin Zhou, Lena F. Kourkoutis, Lopa Bhatt, Mark P. M. Dean, Matteo Mitrano, Qi Song, Sophia F. R. TenHuisen, Stefano Agrestini, Valentina Bisogni.

Figure 1
Figure 1. Figure 1: Structure of the Ndn+1NinO2n+2 compounds. (A) Crystal structure schematic of the Ndn+1NinO2n+2 compounds. n repeats of NdNiO2 are sandwiched between charged (NdO2) - fluorite layers. The fluorite layers soak up negative charge, nominally redistributing charge across the NdNiO2 layers to 1/n holes per nickel. The compounds are mapped onto the doping-dependent phase diagram of the chemically-doped NdNiO2 nic… view at source ↗
Figure 2
Figure 2. Figure 2: Correlated phase diagram of multi-layer Ndn+1NinO2n+2 nickelates. (A) Summarized phase diagram of the multi-layer square-planar nickelates Ndn+1NinO2n+2 as a function of nominal layer-averaged nickel d-electron count. Superconducting phase is in purple; Tc, onset is determined using the criteria described in Ref. (36). Open circle for the n = 8 compound represents superconducting correlations without a cle… view at source ↗
Figure 5
Figure 5. Figure 5: Magnetic excitations in Ndn+1NinO2n+2 using RIXS. (A) Schematic of the RIXS scattering geometry used to access trajectories in momentum space. Purple represents the sample, blue the scattering plane. (B) Real- and momentum-space cartoons of the direction of the momentum scan direction in (D) and (E). (C) Example low-energy loss spectrum highlighting the magnon-derived contribution. Energy (D) and damping (… view at source ↗

discussion (0)

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Forward citations

Cited by 6 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Persistent structural distortions and absent superconductivity in trilayer nickelate thin films

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    Compressive strain suppresses density waves in n=3 nickelate films but persistent layer-inequivalent octahedral rotations prevent superconductivity.

  2. Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

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    Superconductivity in bilayer nickelates occurs in SDW-free oxygen-stoichiometric regions, with an interlayer five-spin polaron proposed as the ground state.

  3. Electronic structure of higher-order layered palladates: La$_{n+1}$Pd$_{n}$O$_{2n+2}$ $(n = 4-7)$

    cond-mat.str-el 2026-03 accept novelty 6.0

    Higher-order square-planar palladates show larger bandwidths, stronger p–d hybridization and reduced R-d Fermi-level interference than nickelates, making them closer cuprate analogs.

  4. Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

    cond-mat.str-el 2026-05 unverdicted novelty 5.0

    Resonant x-ray scattering on La2PrNi2O7 films reveals superconductivity in SDW-free oxygen-stoichiometric regions with distinct c-axis electronic structure, proposing an interlayer five-spin polaron ground state.

  5. Squeezing dynamical singlets in bilayer nickelates

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    Dynamical singlets from 3z²-r² orbitals hybridized with x²-y² orbitals control the distinct pressure versus strain responses in bilayer nickelates.

  6. Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films

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    The review covers experimental and theoretical progress on superconductivity in Ruddlesden-Popper nickelates, emphasizing ambient-pressure thin-film results in La3Ni2O7.

Reference graph

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