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Resonant inelastic x-ray scattering at the oxygen K edge detects dispersive collective charge oscillations in the metallic low-valence nickelate Pr4Ni3O8, and these plasmons are slower, more heavily damped, and soften with temperature compa

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2026-08-03 09:50 UTC pith:4F7GCDU6

load-bearing objection First credible RIXS observation of plasmons in a nickelate; the qualitative story is convincing, but the quantitative t/Vc extraction is conditional on an incomplete RPA treatment and missing data. the 3 major comments →

arxiv 2601.12160 v3 pith:4F7GCDU6 submitted 2026-01-17 cond-mat.str-el cond-mat.supr-con

Observation of correlated plasmons in low-valence nickelates

Y. Shen , W. He , J. Sears , Xuefei Guo , Xiangpeng Luo , A. Roll , J. Li , J. Pelliciari
show 8 more authors
Xi He I. Bozovic Junjie Zhang J. F. Mitchell V. Bisogni M. Mitrano S. Johnston M. P. M. Dean
This is my paper
classification cond-mat.str-el cond-mat.supr-con
keywords plasmonsnickelatesresonant inelastic x-ray scatteringcharge dynamicsscreened Coulomb interactionPr4Ni3O8cupratescollective excitations
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 claims that plasmons—collective charge-density oscillations—exist in the metallic low-valence nickelate Pr4Ni3O8 and can be observed with oxygen K-edge resonant inelastic x-ray scattering. Compared with a similarly doped cuprate, these plasmons have a lower velocity and are more heavily damped, which the authors attribute to weaker electronic hopping and stronger screening of long-range Coulomb interactions. The plasmons also soften as temperature rises, whereas cuprate plasmons stay at nearly fixed energy and simply broaden. If correct, these measurements provide the first quantitative constraints on screened Coulomb interactions in nickelates and sharpen the comparison with cuprate superconductors, with direct implications for understanding unconventional superconductivity in nickelates.

Core claim

The central claim is that dispersive plasmons in the low-valence nickelate Pr4Ni3O8 are detectable with oxygen K-edge RIXS, and that their dispersion and damping differ characteristically from those of cuprates at comparable doping. The measured in-plane plasmon velocity is roughly 1.2 eV·Å, about half the value in La2−xSrxCuO4 (δ=0.35), and the linewidth is systematically broader. RPA simulations of the charge susceptibility, using a trilayer model with tight-binding parameters anchored to angle-resolved photoemission, reproduce the in-plane dispersion, the lack of resolvable out-of-plane dispersion, and a reduced long-range Coulomb scale. The authors attribute the differences to suppressed

What carries the argument

The central object is the plasmon, the collective charge-density oscillation of itinerant electrons, probed by resonant inelastic x-ray scattering (RIXS) tuned to the oxygen K-edge pre-peak. The interpretation is carried by the random-phase-approximation (RPA) charge susceptibility of a layered electron gas with an anisotropic long-range Coulomb interaction; for Pr4Ni3O8 a three-layer model with intra-trilayer hopping is used. The same calculation, with parameters constrained by the measured dispersions and photoemission Fermi surfaces, is what identifies the observed peak as a plasmon and converts the raw dispersion into estimates of hopping integrals and Coulomb screening.

Load-bearing premise

The load-bearing assumption is that the dispersive RIXS peak is a collective plasmon accurately described by an RPA charge susceptibility with electron-hole matrix elements neglected; if matrix-element effects or correlations beyond RPA dominate the cross-section, the mode identification and extracted hopping or screening parameters could shift.

What would settle it

A concrete check would be to compute the full RIXS cross-section at the oxygen K-edge including the resonant matrix elements; if the predicted intensity or dispersion of the mode changes dramatically, the simple RPA identification fails. Alternatively, measuring the same material with momentum-resolved electron energy loss spectroscopy should reproduce the same dispersion and damping, and a mismatch would indicate the RIXS peak reflects a different charge excitation.

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

If this is right

  • The first quantitative measure of screened Coulomb interactions in a low-valence nickelate is established, enabling direct comparison with cuprates.
  • Reduced electronic hopping and enhanced screening emerge as defining differences between low-valence nickelates and cuprates, likely extending to the broader nickelate family.
  • The temperature softening of the nickelate plasmon, potentially connected to stripe fluctuations, provides a new observable for charge-order instabilities.
  • The contrast in plasmon behavior places constraints on which parameters are essential for unconventional superconductivity and may explain the lower superconducting transition temperatures in low-valence nickelates.
  • The observation supports an appreciable hole density on oxygen sites, consistent with the mixed charge-transfer/Mott-Hubbard character of these materials.

Where Pith is reading between the lines

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

  • If the RPA-based identification of the RIXS peak as a plasmon is correct, momentum-resolved electron energy loss spectroscopy (EELS) should reproduce the same dispersion and damping; a mismatch would signal that RIXS matrix elements or correlations beyond RPA are shaping the response.
  • The temperature softening could be a general signature of proximity to stripe order; testing a related compound such as La4Ni3O8, which exhibits charge order, would sharpen this connection.
  • The framework suggests a route to extract Coulomb parameters in other layered nickelates, including infinite-layer superconductors, if suitable single crystals or epitaxial films are available.
  • The reduced long-range Coulomb interaction measured here may feed into theories of superconductivity that rely on the balance between repulsion and attractive extended interactions, helping identify whether attractive interactions can dominate in nickelates.

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 / 4 minor

Summary. The paper reports O K-edge RIXS measurements on the low-valence nickelate Pr4Ni3O8 and on overdoped La2−xSrxCuO4 (δ=0.35), identifying a dispersive low-energy charge excitation in the nickelate that the authors interpret as a plasmon. The central observational claims are that the nickelate plasmon has a lower velocity and is more heavily damped than the cuprate plasmon at comparable doping, that it shows no resolvable out-of-plane dispersion, and that it softens with increasing temperature while the cuprate mode does not. The authors compare the data with RPA calculations of the charge susceptibility for a single-layer model (LSCO) and a trilayer model (Pr4Ni3O8), obtaining parameters in Table I that imply reduced in-plane hopping t and strongly reduced long-range Coulomb interaction Vc in the nickelate. They further argue that the reduced plasmon velocity reflects suppressed hopping and enhanced screening, and that the temperature softening may be related to stripe correlations.

Significance. If the quantitative interpretation is accepted, this is the first observation of dispersive plasmons in a low-valence nickelate and provides the first experimental constraints on the screened Coulomb interaction in this family. The comparison with cuprates at similar doping is directly relevant to ongoing debates about the degree of cuprate-like behavior in nickelates. The qualitative experimental content is substantial: the dispersive mode is clearly visible in the raw RIXS maps; the contrast with LSCO is immediately apparent; and the temperature dependence is a new observation. The paper also benefits from a transparent fitting procedure and from explicitly discussing the main caveat—the absence of predicted upper plasmon branches—rather than hiding it. However, the quantitative conclusions about t and Vc rest on an unverified identification between the RIXS cross-section and the RPA charge susceptibility, and the fitted parameters are reported without uncertainties. Those limitations are acknowledged in part by the authors, but they are load-bearing for the abstract's stronger 'reduced hopping and enhanced screening' claim.

major comments (3)
  1. [Sec. III and Eq. (D6)] The quantitative parameter extraction assumes that the RIXS intensity is proportional to Im χc(q,ω), computed from RPA with no q- or ω-dependent matrix elements. The paper itself shows this approximation is incomplete: in Fig. 3(c), at L≈1.5, RPA predicts an additional low-energy branch with appreciable intensity, but it is not observed, and matrix-element suppression or self-energy broadening are invoked ad hoc. If matrix elements can suppress predicted branches, they can also reweight the observed branch and bias the fitted t, Vc, and linewidth. The qualitative identification of a dispersive charge mode is independent of this issue, but the quantitative claims 'reduced electronic hopping and enhanced screening' are conditional on an unverified equality. I ask the authors to either provide a fuller Kramers-Heisenberg treatment for the relevant O K-edge RIXS channel, or to soften the qua
  2. [Table I and Appendices C–D] The fitted parameters are quoted without uncertainties. The authors note in Appendix C that reducing Vc can be partly compensated by increasing t, and that ARPES is used to select among degenerate solutions; Table I nevertheless gives t and Vc as sharp numbers (t=0.21 vs 0.39 eV; Vc=1.26 vs 5.85 eV). No covariance or sensitivity analysis is provided, and the self-energy parameters κ0 and κ introduced in Appendix D are not listed in Table I. Without error bars or a robustness statement, the central quantitative comparison between nickelate and cuprate cannot be evaluated. Please report uncertainties, the range of (t,Vc) pairs consistent with the RIXS data and ARPES constraints, and the sensitivity to κ0,κ.
  3. [Data Availability and Supplemental Material] The Data Availability statement gives a Zenodo accession code '[to be assigned]', and the main-text reference to the Supplemental Material (Ref. [27]) is a placeholder with '[URL will be inserted by publisher]'. The fitting code, raw spectra, and RPA implementation are therefore not currently accessible, so the fits in Figs. 1–4 and the parameter decompositions in Fig. S4/S5 cannot be independently checked. Since one of the paper's contributions is quantitative constraints on screened interactions, I request that the data and code be made available before publication, and that the placeholders be resolved in the revised version.
minor comments (4)
  1. [Appendix A] The sentence on LSCO spacing reads 'Layers are spaced by d = c/26.8Å' in the manuscript text; this is likely a typographical corruption of 'd = c/2 ≈ 6.8 Å'. Please correct.
  2. [Fig. 1(f) and Sec. S2] It would help to state explicitly whether the FWHM values in Fig. 1(f) are deconvoluted from the 30 meV resolution or are raw fitted widths. The shaded quasi-elastic regime suggests a resolution cutoff, but the text is unclear.
  3. [Appendix D, Eq. (D6)] The sign convention for Im χc is not defined. The text writes Imχc = −Σ Im χαβ; if this means the trace over the analytically continued susceptibility, the sign should be consistent with the spectral representation. A brief definition of the sign convention would remove ambiguity.
  4. [Sec. IV] The temperature-dependent comparison is made at a single Q point for each material. Given that the peak position and FWHM are extracted from multi-component fits, a statement about systematic fitting uncertainty versus statistical uncertainty would strengthen the temperature-softening claim.

Circularity Check

0 steps flagged

No significant circularity: the RIXS observation and cuprate comparison are independent; RPA parameters are explicitly tuned, not predicted, and the paper concedes the missing upper branches.

full rationale

The central claim—dispersive, more damped, lower-velocity modes in Pr4Ni3O8 relative to LSCO—is extracted directly from fitted RIXS peak positions and widths (Figs. 1–2) before any model is invoked. The RPA calculations in Sec. III are not presented as predictions: Appendix C states that parameters were 'tuned tz/t and Vc/t to search for the best combination of parameters that can reproduce the La2−xSrxCuO4 (δ = 0.35) plasmon dispersions,' and Appendix D explicitly says that 'the reduced t and Vc in Pr4Ni3O8 compared to La2−xSrxCuO4 can be concluded directly from a qualitative consideration of the RIXS data, thus the simulations act to confirm and quantify a trend already evident in the raw data.' This is transparent fitting and interpretation, not a hidden circular prediction. The one model step that might look self-referential—Eq. (D6), which equates the RIXS response to Im χc with no matrix elements—is an acknowledged approximation, and Sec. III concedes that at L ∼ 1.5 RPA predicts branches with sufficient intensity 'yet they remain unobstructed in the experiment,' invoking matrix-element suppression or self-energy broadening. That is a falsifiable shortcoming, not a definitional reduction. There is no load-bearing self-citation: the RPA form and α = 3.5 come from external cuprate RIXS work, ARPES [51] provides an independent constraint on t, and self-references are for sample quality and prior nickelate spectroscopy. The quantitative t and Vc values are model-dependent fits quoted without uncertainties, but conditional accuracy is a correctness risk, not circularity.

Axiom & Free-Parameter Ledger

10 free parameters · 6 axioms · 0 invented entities

The experimental observation stands on its own, but every quantitative model conclusion rests on fitted RPA parameters and a series of modeling assumptions: RPA validity, matrix-element-free RIXS cross-section, tight-binding truncation, and phenomenological self-energy broadening. The central free parameters are the tight-binding and Coulomb parameters in Table I, tuned to match the measured dispersions; no independent physical measurement fixes Vc directly.

free parameters (10)
  • LSCO in-plane hopping t = 0.39 eV
    Set by matching RPA plasmon dispersions to data and constrained by ARPES; controls the overall energy scale.
  • LSCO tz/t (interlayer hopping ratio) = 0.017
    Tuned to reproduce the in-plane and out-of-plane plasmon dispersions (Supp. Fig. S4).
  • LSCO Vc/t (long-range Coulomb interaction) = 15
    Tuned to reproduce the plasmon dispersion; degeneracy with t is partially broken by ARPES.
  • LSCO chemical potential μ = -0.51 eV
    Set by the doping condition 1-δ = (2/N)Σ n_F(ε_k); not independent of doping.
  • Pr438 in-plane hopping t = 0.21 eV
    RPA fit to data, constrained by ARPES; the reduced value is central to the reduced-velocity claim.
  • Pr438 tz/t (intra-trilayer hopping ratio) = 0.04
    Tuned to reproduce the trilayer model dispersion and the absence of clear out-of-plane dispersion.
  • Pr438 Vc/t (long-range Coulomb interaction) = 6
    Tuned to reproduce data; the smaller value compared to LSCO underpins the enhanced-screening claim.
  • Pr438 Δμ/t (interlayer potential difference) = -0.4
    Adjusted to match the ARPES Fermi surface and the experimental spectra.
  • Pr438 chemical potential μ = -0.28 eV
    Set by doping δ=1/3 in the trilayer model.
  • Self-energy broadening parameter(s) κ0, κ = not stated in text
    Phenomenological energy-dependent self-energy introduced to explain why the upper RPA plasmon branches are not observed (Supp. Fig. S7).
axioms (6)
  • domain assumption The RPA charge susceptibility Im χ(q,ω) captures the measured RIXS plasmon spectra.
    Eqs. C3–C6 and D3–D6 compute the RIXS response from the RPA susceptibility; correlation effects beyond RPA are only partly captured by parameter renormalization.
  • domain assumption O K-edge RIXS intensity is proportional to the RPA charge susceptibility, ignoring matrix-element effects.
    Section III states the RIXS response is obtained from the imaginary part of the analytically continued RPA susceptibility (Eq. D6); the authors later concede fuller Kramers-Heisenberg multi-orbital calculations would be needed.
  • domain assumption The single-layer (LSCO) and trilayer (Pr438) tight-binding models in Table I describe the relevant low-energy bands.
    Appendix C/D; inter-trilayer hopping is neglected because it is expected to be much smaller than the intra-trilayer term.
  • standard math Analytical continuation with a small broadening γ=5 meV gives a reliable real-frequency response.
    Appendix C uses χ(q, iω_n → ω+iγ); this is a standard but uncontrolled approximation for RPA susceptibilities.
  • ad hoc to paper The missing upper plasmon branches are obscured by self-energy broadening or matrix-element effects.
    Section III invokes phenomenological energy-dependent self-energy broadening (Supp. Fig. S7) to reconcile RPA prediction with non-observation of upper branches; this is not independently verified.
  • domain assumption Temperature softening of the plasmon may be caused by stripe fluctuations.
    Section IV: 'stripe fluctuations are a possible candidate for the temperature dependence of the plasmon'—presented as a speculative interpretation.

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read the original abstract

The discovery of nickelate superconductors has opened a new arena for studying the behavior of correlated electron liquids that give rise to unconventional superconductivity. While critical information about a material's charge dynamics is encoded in its plasmons, collective modes of the electron gas, these excitations have not yet been observed in nickelate materials. Here, we use resonant inelastic x-ray scattering (RIXS) to detect plasmons in the metallic, low-valence nickelate Pr4Ni3O8. Although qualitatively similar to those in cuprates, the nickelate plasmons are more heavily damped and have a lower velocity than those in a cuprate at comparable doping, which we attribute to reduced electronic hopping and enhanced screening of the long-range Coulomb interactions. Furthermore, the plasmons in Pr4Ni3O8 soften with increasing temperature, in contrast to the cuprate, where plasmons remain at nearly fixed energy but become more strongly damped. Taken together, these results reveal a distinct charge-screening landscape in nickelates and place quantitative constraints on analogies to cuprates.

Figures

Figures reproduced from arXiv: 2601.12160 by A. Roll, I. Bozovic, J. F. Mitchell, J. Li, J. Pelliciari, J. Sears, Junjie Zhang, M. Mitrano, M. P. M. Dean, S. Johnston, V. Bisogni, W. He, Xiangpeng Luo, Xi He, Xuefei Guo, Y. Shen.

Figure 1
Figure 1. Figure 1: FIG. 1. Dispersive plasmons in Pr [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Out-of-plane plasmon dispersions. (a),(b), RIXS spec [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Theoretical description of the dispersing plasmons. (a [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Temperature dependence of the plasmons. (a),(b), Temp [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗

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Forward citations

Cited by 3 Pith papers

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

  1. Unconventional plasmon dynamics due to strong correlations in Sr$_2$RuO$_4$

    cond-mat.str-el 2026-04 unverdicted novelty 7.0

    Strong electron correlations in Sr2RuO4 produce unconventional plasmon dispersion, intrinsic width below the electron-hole continuum, and a high-energy peak from incoherent transitions.

  2. Strongly correlated model of acousticlike plasmons persisting across the phase diagram of cuprate superconductors

    cond-mat.str-el 2026-04 unverdicted novelty 6.0

    One fixed parameter set in the layered t-J-V model accounts for acousticlike plasmon dispersions across the entire cuprate phase diagram in available RIXS data.

  3. Strongly correlated model of acousticlike plasmons persisting across the phase diagram of cuprate superconductors

    cond-mat.str-el 2026-04 unverdicted novelty 5.0

    One fixed layered t-J-V parameter set describes acousticlike plasmon dispersions from RIXS across the full doping range of La2-xSrxCuO4.

Reference graph

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