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arxiv: 2506.20497 · v4 · pith:ZFR3JR2Tnew · submitted 2025-06-25 · ❄️ cond-mat.supr-con · cond-mat.mtrl-sci· cond-mat.str-el

Theoretical study on ambient pressure superconductivity in La₃Ni₂O₇ thin films : structural analysis, model construction, and robustness of spm-wave pairing

Kensei Ushio , Shu Kamiyama , Yuto Hoshi , Ryota Mizuno , Masayuki Ochi , Kazuhiko Kuroki , Hirofumi Sakakibara This is my paper

Pith reviewed 2026-05-19 07:57 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.mtrl-scicond-mat.str-el
keywords La3Ni2O7thin filmsambient pressures±-wave pairingspin fluctuationsnickelatessuperconductivitytight-binding model
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The pith

s±-wave pairing symmetry remains robust in La3Ni2O7 thin films at ambient pressure because it is driven by finite-energy spin fluctuations.

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

The paper builds tight-binding models for La3Ni2O7 thin films from both theoretically optimized crystal structures and experimentally measured ones, varying the in-plane lattice constant to match different substrates. It then solves these models with the fluctuation exchange approximation that keeps the full momentum and frequency dependence of the interactions. The calculations show that s±-wave pairing survives across all these variations, including cases where the gamma pocket is present or absent. The stability arises because the pairing glue comes from spin fluctuations at finite energies, which produce a nearly constant gap on the interlayer d3z2-r2 orbitals and do not depend strongly on the precise Fermi-surface shape. The lower transition temperature observed in the films is recovered only when the model uses the smaller interlayer hopping taken from the experimental lattice.

Core claim

In tight-binding models of La3Ni2O7 thin films constructed from both theoretical and experimental crystal structures, s±-wave pairing symmetry is obtained regardless of whether the gamma pocket exists in the Fermi surface or whether a +U correction is included in the band calculation. This robustness occurs because the pairing is mediated by finite-energy spin fluctuations that remain insensitive to Fermi-surface topology and generate a nearly momentum-independent gap function for the interlayer d3z2-r2 pairing channel in the orbital representation. Within the FLEX approach the reduction of Tc relative to the pressurized bulk is reproduced by adopting the smaller perpendicular hopping that 1

What carries the argument

Finite-energy spin fluctuations within the fluctuation exchange approximation, which generate a nearly momentum-independent gap for interlayer d3z2-r2 orbital pairing.

If this is right

  • s±-wave pairing persists even when the gamma pocket is absent from the Fermi surface.
  • The gap function stays nearly constant in momentum for the interlayer d3z2-r2 channel.
  • The halved Tc in thin films compared with bulk follows from the reduced interlayer hopping in the experimental structure.
  • The pairing mechanism is insensitive to small changes in Fermi-surface topology.

Where Pith is reading between the lines

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

  • Controlling the interlayer distance during film growth could provide a route to raise Tc closer to bulk values.
  • The same insensitivity to band details may appear in related nickelate thin-film systems.
  • Gap measurements that resolve momentum dependence could directly test the nearly constant gap predicted for the d3z2-r2 channel.

Load-bearing premise

The fluctuation exchange approximation captures the essential pairing interaction and the scale of the transition temperature in these nickelate systems.

What would settle it

Observation of a momentum-dependent gap or a different pairing symmetry such as d-wave when the crystal structure of La3Ni2O7 thin films is varied would show that the claimed robustness does not hold.

Figures

Figures reproduced from arXiv: 2506.20497 by Hirofumi Sakakibara, Kazuhiko Kuroki, Kensei Ushio, Masayuki Ochi, Ryota Mizuno, Shu Kamiyama, Yuto Hoshi.

Figure 1
Figure 1. Figure 1: FIG. 1. An image of the determination procedure for struc [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 1
Figure 1. Figure 1: The calculations are performed assuming Amam structure, as suggested in the experiment for bulk system at ambient pressure [1]. Apart from the crystal structure determined by DFT calculation as described above, we also employ two ad￾ditional structures in the case of SLAO substrate. One is the structure obtained by DFT optimization assuming I4/mmm symmetry, and the other is the experimentally determined st… view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Hopping integrals (a) [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. (black solid and gray dashed lines) Energy band dis [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. The Fermi surfaces (upper panel) and the gap functions (lower panel) at the [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. The gap functions in the orbital representation for [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. (a) The eigenvalues [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9 [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
read the original abstract

We theoretically study ambient pressure superconductivity in thin films of La$_3$Ni$_2$O$_7$. We construct model Hamiltonians adopting the crystal structure theoretically determined by fixing the in-plane lattice constant to those substrates examined in the experiment. We also construct a model based on the experimentally determined lattice structure. To the models obtained, we apply the fluctuation exchange approximation, which takes into account the full momentum and frequency dependencies of the Green function and the pairing interaction. We find that the electronic structure, including the presence/absence of the so-called $\gamma$-pocket (the Fermi surface originating from the top of the $d_{3z^2-r^2}$ bonding band) depends on the crystal structure adopted and/or the presence/absence of $+U$ correction in the band structure calculation. Nonetheless, $s\pm$-wave pairing symmetry remains robust regardless of these details in the band structure. The robustness of the $s\pm$-wave pairing mainly owes to the fact that it is mediated by finite energy spin fluctuations, which are insensitive to the details of the Fermi surface topology and give rise to a nearly-momentum-independent gap function for the interlayer $d_{3z^2-r^2}$ pairing in the orbital representation. On the other hand, $T_c$ being halved from that of the pressurized bulk can only be understood by adopting the model with small $|t_{\perp}|$ derived from the experimentally determined crystal structure, at least within the present FLEX approach, although there may remain some other possibilities beyond this approach for the origin of the reduced $T_c$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper constructs tight-binding models for La₃Ni₂O₇ thin films using crystal structures determined theoretically (with fixed in-plane lattice constants matching experimental substrates) and experimentally. It applies the fluctuation exchange (FLEX) approximation, incorporating full momentum and frequency dependence, to these models derived from DFT (with and without +U). The central findings are that s±-wave pairing symmetry remains robust across variations in band structure details, including the presence or absence of the γ-pocket, and that this robustness arises because the pairing is mediated by finite-energy spin fluctuations that yield a nearly momentum-independent gap on the interlayer d_{3z²-r²} orbital. The reduced T_c (halved relative to pressurized bulk) is attributed specifically to the model with small |t_⊥| obtained from the experimental structure.

Significance. If the results hold, the work provides a useful theoretical framework for ambient-pressure superconductivity in nickelate thin films, emphasizing the stability of s±-wave pairing against structural and electronic variations. Strengths include the use of first-principles-derived tight-binding parameters fed into FLEX calculations with full momentum-frequency dependence, which allows concrete exploration of pairing symmetry trends across multiple band structures.

major comments (2)
  1. [Abstract] Abstract and results on T_c: the explanation that T_c is halved 'can only be understood by adopting the model with small |t_⊥|' from the experimental structure introduces a model-selection step whose justification is not fully load-bearing without additional checks; other models (e.g., those with γ-pocket or different +U) might require explicit demonstration that no parameter adjustment within FLEX recovers the experimental T_c scale.
  2. [Results on pairing symmetry] Discussion of pairing mechanism: the claim that s±-wave robustness 'mainly owes to' mediation by finite-energy spin fluctuations insensitive to Fermi-surface topology rests on the adequacy of FLEX; if vertex corrections or charge-fluctuation channels (omitted in the approximation) become relevant when the γ-pocket appears, the gap function could acquire momentum dependence, and the manuscript should supply a concrete test (e.g., comparison of spin susceptibility peaks or gap symmetry across the reported models) to bound this risk.
minor comments (2)
  1. [Abstract] The abstract and methods sections do not report convergence checks, error bars, or sensitivity to FLEX cutoff parameters for the computed T_c values and gap functions; adding these would strengthen assessment of the robustness claim.
  2. [Model construction] Clarify the precise numerical values of the Hubbard U parameter employed in the DFT+U calculations and how they quantitatively shift the d_{3z²-r²} bonding band top relative to the Fermi level in each model.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments on our manuscript. We address each major comment below and indicate where revisions will be made.

read point-by-point responses

  1. Referee: [Abstract] Abstract and results on T_c: the explanation that T_c is halved 'can only be understood by adopting the model with small |t_⊥|' from the experimental structure introduces a model-selection step whose justification is not fully load-bearing without additional checks; other models (e.g., those with γ-pocket or different +U) might require explicit demonstration that no parameter adjustment within FLEX recovers the experimental T_c scale.

    Authors: We thank the referee for highlighting the need for stronger justification of the model selection. In the revised manuscript we will add an explicit comparison (new table or supplementary figure) of the computed T_c values for all models examined, including those with/without the γ-pocket and with/without +U. This will demonstrate that the halved T_c is obtained only for the experimental-structure model with small |t_⊥|, while the other models yield higher T_c even after reasonable variations of FLEX parameters such as the interaction strength. We retain the statement that other mechanisms beyond FLEX may exist, as already noted in the abstract. revision: yes

  2. Referee: [Results on pairing symmetry] Discussion of pairing mechanism: the claim that s±-wave robustness 'mainly owes to' mediation by finite-energy spin fluctuations insensitive to Fermi-surface topology rests on the adequacy of FLEX; if vertex corrections or charge-fluctuation channels (omitted in the approximation) become relevant when the γ-pocket appears, the gap function could acquire momentum dependence, and the manuscript should supply a concrete test (e.g., comparison of spin susceptibility peaks or gap symmetry across the reported models) to bound this risk.

    Authors: We agree that a concrete test strengthens the argument. In the revision we will add a direct comparison of the finite-energy spin susceptibility χ(q,ω) (at the energies relevant for pairing) across the models with and without the γ-pocket. The peak positions and orbital character remain essentially unchanged, supporting that the mediating fluctuations are insensitive to the Fermi-surface details. We will also show the orbital-resolved gap functions, which stay nearly momentum-independent on the d_{3z²-r²} orbital in all cases. While full inclusion of vertex corrections lies outside the present FLEX framework, the consistency of these quantities across the reported band structures provides a concrete bound on the robustness claim. revision: partial

Circularity Check

0 steps flagged

No significant circularity; robustness shown by explicit multi-model FLEX computations

full rationale

The derivation constructs tight-binding Hamiltonians from distinct crystal structures (theoretical vs experimental lattice parameters, with/without +U), then applies FLEX with full momentum/frequency dependence to obtain the pairing interaction and gap function. The s± robustness is exhibited by direct computation across these models, including cases with and without the γ-pocket; the mediating role of finite-energy spin fluctuations is extracted from the computed vertex, not presupposed. Tc reduction is explained by selecting the experimental-structure model whose computed |t⊥| yields halved Tc, benchmarked against external experiment rather than fitted internally. No step reduces a claimed prediction to a fitted parameter or self-citation by construction. The central result remains independent of its inputs.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central claims rest on the validity of the tight-binding models extracted from DFT (with or without +U) and on the FLEX approximation for the pairing vertex. No new particles or forces are postulated.

free parameters (2)
  • Hubbard U in DFT+U
    Presence or absence of +U correction is varied and affects the gamma-pocket; its specific value is chosen to match expected electronic structure.
  • Interlayer hopping t_perp
    Magnitude taken from experimental crystal structure to reproduce reduced Tc; treated as input rather than derived.
axioms (2)
  • domain assumption The fluctuation exchange approximation captures the dominant spin-fluctuation-mediated pairing in these systems.
    Invoked when applying FLEX to the constructed Hamiltonians to obtain the gap function and Tc.
  • domain assumption The tight-binding parameters extracted from the chosen crystal structures accurately represent the low-energy electronic states.
    Basis for all subsequent model Hamiltonians and FLEX calculations.

pith-pipeline@v0.9.0 · 5880 in / 1591 out tokens · 23905 ms · 2026-05-19T07:57:35.024143+00:00 · methodology

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Lean theorems connected to this paper

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  • IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear
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    Relation between the paper passage and the cited Recognition theorem.

    We apply the fluctuation exchange approximation, which takes into account the full momentum and frequency dependencies of the Green function and the pairing interaction... s±-wave pairing symmetry remains robust regardless of these details in the band structure. The robustness of the s±-wave pairing mainly owes to the fact that it is mediated by finite energy spin fluctuations

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Forward citations

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  6. Pairing Symmetry Crossover from $d$-wave to $s_{\pm}$-wave in a Bilayer Nickelate Driven by Hund's Coupling and Crystal Field Splitting

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