REVIEW 2 major objections 4 minor 15 cited by
Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$
T0 review · 2 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash
Pith's one-line read In compressively strained superconducting La2PrNi2O7 films, the flat 3dz2 band stays about 70 meV below the Fermi level, not crossing it as density functional theory predicts, so the high density of states of that band cannot be what…
desk verdict Solid ARPES benchmark showing the g band sits ~70 meV below EF in strained superconducting LPNO films, but the claimed factor-of-5-to-10 discrepancy with DFT rests on comparing to unsubstituted bulk La3Ni2O7. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the flat, strongly renormalized 3dz2 bonding band, called the gamma band, whose band top is measured by in situ synchrotron ARPES at several photon energies in both MBE- and PLD-grown films. Its position relative to the Fermi level is what distinguishes this paper's picture from earlier ones: the measured band top stays about 70 meV below EF, whereas an earlier report extrapolated it to a Fermi-level crossing. The quantitative argument is carried by comparing the measured strain-induced band shifts with DFT+U predictions computed at fixed experimental lattice constants, and the kz dispersion is captured by a 16-orbital tight-binding model that adds a small inter-bilayer 3dx2-y2 hopping term.
What would settle it
Measure the gamma-band top in the same strained film system across a continuous strain series, or in Pr-free La3Ni2O7 films at identical strain, using photon energies and matrix elements that clearly resolve the band: if the band top reaches or crosses the Fermi level in a superconducting film, or if the measured strain shift approaches the 210 meV DFT value, the paper's central claim would be refuted.
Extended reading notes
Core claim
The central discovery is that in compressively strained La2PrNi2O7 thin films, including ozone-treated films with onset Tc near 40 K, the strongly renormalized flat 3dz2 (gamma) band sits about 70 meV below the Fermi level rather than crossing it. DFT+U calculations using the measured 2% in-plane compression and 1% c-axis expansion predict this band should move down by roughly 210 meV, so the measured downward shift is a factor of 5-10 smaller. The paper therefore concludes that superconductivity in these films is not explained by the flat band's high density of states at the Fermi level, and instead notes that the 70 meV position coincides with the energy of collective modes, namely oxygen-related phonons and the peak of the spin excitation spectrum. As a separate finding, the 3dx2-y2-derived band shows a clear kz dispersion, which the paper attributes to inter-bilayer hopping enabled by buckled Ni-O planes rather than to strain relaxation or simple two-dimensional band structure.
Load-bearing premise
The load-bearing premise is that DFT+U with U = 3.5 eV, using the measured 2% in-plane compression and 1% c-axis expansion, is a reliable baseline for how much the gamma band should shift; if the effective Hubbard U, oxygen stoichiometry, or actual strain state differs, the claimed factor-of-5-to-10 discrepancy could shrink, and the comparison to bulk La3Ni2O7 could also be affected by Pr substitution rather than strain alone.
Editorial extensions
If this is right
- A Fermi-level crossing of the flat 3dz2 band is not required for 40 K superconductivity in strained bilayer nickelates, so theories built on the gamma band's density of states at EF need to be revised.
- The gamma band's position near 70 meV, matching the energy of oxygen phonons and the spin-excitation peak, suggests collective-mode coupling may be more relevant to pairing than the flat band's density of states.
- The effective pressure in these films is about 5 GPa, considerably larger than the naive expectation from the DFT-relaxed structure, which changes how strain experiments are compared with hydrostatic-pressure experiments on bulk crystals.
- The observed kz dispersion of the 3dx2-y2 band implies that three-dimensional inter-bilayer hopping and buckled Ni-O planes must be included in realistic models of the superconducting state.
- The measured band positions and renormalization factors provide a quantitative benchmark for future correlated-electron calculations of strained bilayer nickelates.
Reading between the lines
- Editorial inference beyond the paper: if the gamma band never crosses the Fermi level under any accessible strain, then the gamma band may contribute to superconductivity indirectly, for example through interlayer scattering or by mediating coupling to collective modes, rather than by supplying a large density of states; a testable consequence is that Tc would not track the gamma-band density of s
- Editorial inference beyond the paper: the kz dispersion of the 3dx2-y2 band suggests that the c-axis coherence of the electronic structure is stronger than previously assumed, so photon-energy-dependent ARPES at more kz points could map the full three-dimensional Fermi surface and test whether inter-bilayer hybridization affects the superconducting gap anisotropy.
- Editorial inference beyond the paper: the coincidence between the gamma-band energy and the 70 meV mode could mean the flat band acts as a sensitive probe of collective-mode coupling; a future experiment tracking the strength of the 70 meV renormalization kink across Tc could discriminate between phonon-mediated and spin-fluctuation-mediated pairing.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper reports an in-situ synchrotron ARPES study of compressively strained La2PrNi2O7 thin films grown by oxide MBE, with supplementary PLD-grown films, both as-grown and after ozone treatment (superconducting, onset Tc ~40 K). The central finding is that the strongly renormalized flat 3dz2-derived g band remains approximately 70 meV below the Fermi level in the strained films, rather than shifting downward by the 210–350 meV predicted by DFT+U calculations for the same nominal strain. The authors also report a photon-energy-dependent kz dispersion of the 3dx2-y2-derived b band, which they attribute to a low-symmetry Ni sublattice and a finite inter-bilayer dx2-y2 hopping, and they note that the ~70 meV g-band energy coincides with oxygen phonon and spin-excitation energies. On this basis they argue that superconductivity in these films is not driven by a Fermi-level-crossing flat band.
Significance. If the central observation holds, the paper provides a strong experimental constraint on theories of superconductivity in strained bilayer nickelates: it rules out the common scenario in which the flat 3dz2 band crosses the Fermi level and supplies the large density of states that drives pairing. The robustness of the observation is a clear strength—the g band below EF is reproduced across MBE and PLD samples, as-grown and ozone-treated states, and multiple photon energies, and the paper carefully controls matrix-element effects that had obscured the band in prior work. The kz dispersion of the dx2-y2 band, if confirmed, is also an important input for structural and electronic models. The main weakness is that the quantitative claim of a factor-of-5-to-10 discrepancy relies on comparing strained La2PrNi2O7 films to bulk La3Ni2O7 data and to DFT calculations for La3Ni2O7, without separating strain effects from Pr-substitution effects. The paper is well positioned to influence the field, but this compositional confound must be addressed.
major comments (2)
- [§2, Fig. 2h, Table I] The quantitative claim that the g band shifts by only ~20 meV instead of the predicted 210–350 meV rests on comparing strained La2PrNi2O7 thin films with bulk La3Ni2O7 ARPES data (ref. 7) and with DFT calculations performed for La3Ni2O7. This varies strain and chemical composition simultaneously. Pr substitution alone can alter the Ni-O bond lengths, octahedral tilts, and the dz2-derived g-band energy independently of epitaxial strain. The paper itself notes in §3 that its Pr content differs from a previous film report, yet it provides no unstrained or weakly strained La2PrNi2O7 reference, nor a Pr-substituted DFT calculation. Without separating the composition effect from the strain effect, the reported factor-of-5-to-10 discrepancy could be substantially overestimated. I recommend adding an ARPES measurement of an unstrained or weakly strained LPNO film (or of a La2PrNi2O7 bulk crystal) and DFT calculations that include Pr, so that the strain-driven shift is isolated from the substitution effect.
- [§2, Table I, Supplementary Information (DFT+U section)] The DFT benchmark uses U = 3.5 eV and treats rare-earth f-electrons as core states. The size of the strain-induced g-band shift is likely sensitive to these choices. Fig. S1b shows a calculation with U = 1 eV, but the text does not report the g-band position for this U, nor does it provide a systematic U sweep to demonstrate that the 210 meV shift is robust. If the predicted shift decreases at lower U or with explicit Pr 4f treatment, the claim of 'fundamental missing pieces in DFT' would be less well supported. A simple U sweep (e.g., U = 0–5 eV) for the strained and unstrained structures, and ideally a Pr-substituted supercell, would make the disagreement quantitative and less model-dependent.
minor comments (4)
- [§2, Fig. 2h] The Gaussian fits for the as-grown and ozone-treated g-band distributions are based on a small number of samples, and the as-grown distribution appears broad. The paper should state the number of samples in each histogram bin and provide error bars on the fitted centers, since the inferred ~20 meV shift is comparable to the spread between samples.
- [Supplementary Information, 'Lattice structure indicated from kz-dispersion'] The statement that 'the only reason that ARPES data shows kz-dispersion must be from the fact that the single Ni layer takes a lower symmetry case' is an inference by elimination, not a direct structural determination. The SI later acknowledges that the structure is inferred, but the main text should more carefully phrase this as a suggestion consistent with the data and DFT, rather than a definite conclusion.
- [Supplementary Information, 'Construction of a simple tight-binding model'] The tight-binding model is fitted to the measured dispersions, so the resulting kz dispersion is not an independent prediction. The SI notes this caveat, but the main text would benefit from stating explicitly that the model is a parameterization of the observed bands, not a verification of the low-symmetry sublattice scenario.
- [Table I] The column headers of Table I are difficult to parse: the row for the c-lattice constant lists three values, and it is unclear which columns correspond to 'Relaxed bulk', 'LPNO thin film', and the two DFT columns. Please reformat the table so that each column is unambiguously labeled.
Circularity Check
No load-bearing circularity: the ARPES measurement of the g band at ~70 meV below E_F is direct, and the DFT/tight-binding comparisons are benchmarks rather than fitted outputs.
full rationale
The central empirical claim is an ARPES measurement: the flat 3dz2 g band is observed ~70 meV below the Fermi level in strained La2PrNi2O7 films, in contrast to the ~350 meV shift 'anticipated by DFT'. This observation does not reduce to any fitted parameter or prior result. The DFT comparison in Table I takes 'the DFT calculated values... from ref.21', a preprint with overlapping authorship (Y. Zhong, C. Jia); this is a self-citation in the quantitative baseline. However, it is not load-bearing: the Supplementary Information independently repeats the DFT+U calculation with U=3.5 eV and experimental lattice parameters, obtaining the same ~210 meV g-band shift, and the bulk ARPES comparison is to ref.7. The tight-binding model in the SI is explicitly introduced 'to capture the observed kz dispersion', i.e., it is fitted to the data and is not presented as a prediction; the inferred ~5 GPa effective pressure comes from DFT stress at the measured lattice constants, not from the g-band energy. The paper also flags a relevant limitation ('our Pr content differs from the previous report considerably'), which bears on the strain/composition comparison but is not circularity. No equation defines the claimed band position in terms of the measured position, and no fitted parameter is renamed as a prediction.
Assumptions & free parameters
free parameters (3)
- Hubbard U on Ni (DFT+U) =
3.5 eV (1 eV also tested)
- 16-orbital tight-binding site energies and hoppings =
Values in Table SI, e.g. epsilon_dxy=-0.345 eV, t_xy=0.057 eV, epsilon_dz2=0.851 eV, t_z=0.192 eV
- t_perp_star inter-bilayer dx2-y2 hopping =
-0.051 eV
assumptions (4)
- domain assumption ARPES spectra at the used photon energies represent the intrinsic band structure, and matrix element effects do not hide the true g-band top.
- domain assumption DFT+U with PBE, PAW potentials, and f-electrons in the core is a reliable reference for the relative bulk and strained band positions.
- domain assumption Photon energies of 150 and 200 eV correspond to the intended kz cuts, and the probing depth difference between them is negligible.
- ad hoc to paper A low-symmetry, two-fold Ni sublattice is required to explain the observed kz dispersion.
invented entities (2)
-
Inter-bilayer dx2-y2 to dx2-y2 hopping t_perp_star
-
Low-symmetry (2x) Ni sublattice with a doubled unit cell
independent evidence
Cite this review
Pith. "Pith review of Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$." pith.science (2026). https://pith.science/paper/7UXTQTOX
@misc{pith2026250416372,
author = {Pith},
title = {Pith review of: Electronic structure of compressively strained thin film La$_2$PrNi$_2$O$_7$},
year = {2026},
howpublished = {\url{https://pith.science/paper/7UXTQTOX}},
note = {Machine review of arXiv:2504.16372}
}
abstract
The discovery of superconductivity in the bulk nickelates under high pressure is a major advance in physics. The recent observation of superconductivity at ambient pressure in compressively strained bilayer nickelate thin films has now enabled direct characterization of the superconducting phase through angle resolved photoemission spectroscopy (ARPES). Here we present an in-situ ARPES study of compressively strained La$_2$PrNi$_2$O$_7$ films grown by oxide molecular beam epitaxy, and the ozone treated counterparts with an onset T$_c$ of 40 K, supplemented with results from pulsed laser deposition films with similar T$_c$. We resolve a systematic strain-driven electronic band shift with respect to that of bulk crystals, in qualitative agreement with density functional theory (DFT) calculations. However, the strongly renormalized flat 3$d_{z2}$ band shifts a factor of 5-10 smaller than anticipated by DFT. Furthermore, it stays ~70 meV below the Fermi level, contradicting the expectation that superconductivity results from the high density of states of this band at the Fermi level. We also observed a non-trivial k$_z$ dispersion of the cuprate-like 3$d_{x2-y2}$ band. Combined with results from both X-ray diffraction and DFT, we suggest that the strained films are under ~5 GPa effective pressure, considerably larger than the na\"ive expectation from the DFT relaxed structure. Finally, the ~70 meV energy position is intriguingly close to the collective mode coupling more prominently seen in thin films, in the energy range of both oxygen related phonons and the maximum of the spin excitation spectrum.
Figures
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Forward citations
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Reference graph
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Reviewed August 16, 2026 · model on record in the stance chip above.
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