{"id":"def1c1ff-cd51-4b2c-8dde-ae212b846654","arxiv_id":"2412.04391","paper_version":4,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Biaxial tensile strain reproduces the pressure-like flat d_z2 band at the Fermi level in Ruddlesden-Popper nickelates (n=2-5), while compressive strain produces a cuprate-like electronic structure.","lead":"This paper uses computer simulations to show that stretching or squeezing thin-film nickelate crystals changes their electronic structure in a way that is not simply tied to the crystal tilts. The results suggest tensile strain could recreate the electronic features seen under high pressure, offering a possible path to nickelate superconductivity at normal pressure.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The n=4 and n=5 strain trends, including the tensile-strain gamma-pocket recovery, rest on unverified tetragonal I4/mmm ambient structures; if real structures have octahedral tilts, the 'up to n=5' claim may not survive.","rationale":"The reader's weakest assumption is also the place I would attack: the n=4 and n=5 calculations start from unverified I4/mmm structures. This is the only step in the argument that is not backed by either experiment or a symmetry cross-check comparable to the bilayer Amam/Fmmm comparison. The paper is otherwise careful: it checks both symmetries for n=2, provides Wannier fits, and shows consistent trends across n. My concern does not touch the n=2,3 core, which is independently supported by the Geisler et al. preprint for strained La3Ni2O7. It does, however, make the 'up to n=5' extension conditional on the ambient structure being representative. A single computational experiment, relaxing n=4 and n=5 under +3% tensile strain from tilted starting structures, would settle whether the gamma pocket is robust. If it survives, the paper can stand as written; if not, the abstract should be narrowed. Hence I recommend CONDITIONAL rather than a flat ACCEPT: the main claim for bilayer and trilayer is solid, but the general claim across n=2-5 should be accepted only with that verification or with explicit qualification.","tokens_in":15059,"tokens_out":8573,"duration_ms":94845,"concrete_test":"Repeat the n=4 and n=5 +3% tensile-strain relaxations of Sec. III C starting from symmetry-broken initial structures that carry the Bmab-type tilts of La4Ni3O10 (or P21/a-like rotations) instead of I4/mmm, and plot the dz2-projected band along M-Gamma. If the pure-dz2 gamma pocket still crosses E_F at +3% strain in every tilted starting structure, the concern is resolved; if the band drops below E_F in any case, the 'up to n=5' conclusion should be narrowed to n=2,3 or re-derived after experimental structure determination.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim is made for n=2-5, but Sec. III C extends it to n=4 and n=5 using ambient-pressure I4/mmm structures from ref. 29. These structures have no octahedral tilts and have not been experimentally resolved. The paper justifies them by the observation that, for n=2 and n=3, the tetragonalized ambient structures give an electronic structure close to the orthorhombic/monoclinic cells; that analogy is not established for n=4 and n=5. The load-bearing feature is a Fermi-surface topology change under +3% tensile strain (appearance of the pure-dz2 gamma pocket). Fermi-surface topology is sensitive to small hoppings and crystal-field splittings, both of which are modulated by octahedral rotations. A tilted ground state for n=4 or n=5 could place the dz2 bonding band at a different energy relative to E_F and change its strain response, so the 'up to n=5' conclusion is conditional on the assumed starting structure. The n=2 and n=3 core is not affected; the Note added (ref. 61) independently supports the tensile-strain trend for the bilayer.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper uses first-principles DFT (VASP relaxations, WIEN2K band structures, and Wannier downfolding) to study the effect of biaxial strain (from -3% to +3%) on the structural and electronic properties of Ruddlesden-Popper nickelates La_{n+1}Ni_nO_{3n+1} with n = 2-5. The central finding is that strain decouples the structural and electronic-structure effects of hydrostatic pressure: compressive strain suppresses octahedral tilts (bringing the apical Ni-O-Ni angle closer to 180 degrees) but shifts the d_{z^2} flat band below the Fermi level, resulting in a cuprate-like electronic structure; tensile strain increases tilts and lowers the out-of-plane Ni-Ni distance, yet recovers the pure d_{z^2} gamma pocket at the Fermi level that is the hallmark of pressurized bilayer and trilayer RP nickelates. The authors propose tensile biaxial strain as a route to replicate at ambient pressure the fermiology associated with pressure-induced superconductivity, and compressive strain as a route to investigate cuprate-like physics and possible d-wave pairing.","tokens_in":15385,"tokens_out":3410,"duration_ms":36477,"significance":"If the conclusions hold, the paper provides a concrete and experimentally actionable prediction: tensile-strained thin films of RP nickelates (including higher-order members up to n = 5) should display the same gamma-pocket fermiology that accompanies pressure-induced superconductivity, while compressive strain should yield a cuprate-like electronic structure. The work is strengthened by the use of standard, well-documented computational methods, the comparison of Amam/Fmmm and Bmab/I4/mmm starting symmetries for n = 2 and n = 3, the explicit Wannier-fit benchmarks in Appendix A, and the analysis of hopping ratios that connect to pairing-symmetry discussions in the literature. The predictions are falsifiable by ARPES on strained thin films, and the Note added (ref. 61) provides independent support for the tensile-strain trend in the bilayer. The main weakness is the reliance for n = 4 and n = 5 on tetragonal I4/mmm starting structures that have not been experimentally resolved; the paper itself acknowledges this, but the 'up to n = 5' claim is conditional on that assumption.","major_comments":[{"comment":"The claim that tensile strain recovers the pure-d_{z^2} gamma pocket 'up to n = 5' rests on the tetragonal I4/mmm ambient-pressure structures from ref. 29, which have not been experimentally resolved. The paper justifies this by noting that for n = 2 and n = 3 the tetragonalized phases give electronic structures close to those of the orthorhombic/monoclinic cells, but this analogy is not demonstrated for n = 4 and n = 5. Since the appearance of the gamma pocket is a Fermi-surface topology change that is sensitive to octahedral rotations and small interlayer hoppings, a tilted ground state for n = 4 or n = 5 could place the d_{z^2} bonding band at a different energy relative to E_F and modify its strain response. I recommend either (a) performing additional calculations for n = 4 and n = 5 starting from tilted (e.g., Bmab-like) structures to test the robustness of the gamma-pocket recovery, or (b) explicitly softening the 'up to n = 5' wording to 'for the assumed ideal tetragonal structures' and stating this conditional in the abstract and conclusions.","section":"Sec. III C and Fig. 5"}],"minor_comments":[{"comment":"The title contains a grammatical error: 'Strain to mimic the effects pressure' should read 'Strain to mimic the effects of pressure'. The same phrase appears in the first line of the abstract.","section":"Title and Abstract"},{"comment":"In the second paragraph, the sentence 'the straightening of the Ni-O-Ni bond angle along the c-axis under pressure described above is likely related to the emergence of superconductivity in due to the associated enhancement' contains a typo ('in due' should likely be 'due').","section":"Sec. III A"},{"comment":"There are minor spacing/LaTeX artifacts in the text, e.g., 'V ASP' should be 'VASP' and 'the Hellman-Feynman force' should be 'the Hellmann-Feynman force'.","section":"Sec. II"},{"comment":"The space group notation is inconsistent: 'F mmm' appears with a space in several places (e.g., Fig. 1 caption, Sec. III A text); it should be 'Fmmm' for consistency with the rest of the paper.","section":"Fig. 1 caption"},{"comment":"The notation for the Fermi-surface sheets (α, β1, β2, γ, δ) is introduced in the text and used in figures, but the δ pocket is not explicitly described in the text for the trilayer case; a brief description would improve readability.","section":"Sec. III B and Fig. 4"}],"recommendation":"major_revision","confidential_remarks":"The paper is well within the scope of the journal and presents a valuable computational benchmark. The n = 2 and n = 3 results are solid and the tensile-strain gamma-pocket trend is independently supported. The only substantive issue is the unverified starting structure for n = 4 and n = 5; this is a limitation that can be addressed by additional sensitivity calculations or by a more cautious wording of the claim. I do not see a need to reject the paper; a major revision is appropriate to address this load-bearing point."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Read the paper. The core result is real and worth knowing: for the bilayer and trilayer RP nickelates, biaxial tensile strain recovers the pure-dz2 gamma pocket at the Fermi level that pressure produces, while compressive strain kills it even though it straightens the octahedra. That decoupling of tilt angle from the dz2 band position is the genuinely new thing here, and it is supported by clean DFT work: VASP relaxations, WIEN2K bands, Wannier downfolding with good fits, and a symmetry check (Amam vs Fmmm) for the bilayer. The authors also compute hopping ratios that track the same trend, which strengthens the story. They are appropriately careful not to claim superconductivity; they frame strain as a way to test the role of the gamma pocket.\n\nThe soft spots are real but not fatal. The n=4 and n=5 results start from tetragonal I4/mmm structures that have never been resolved experimentally. The paper justifies that choice by saying the tetragonalized cells work for n=2 and 3, but that analogy is not tested for the higher members. Because the gamma pocket is a Fermi-surface topology feature, it is sensitive to small hoppings and crystal-field splittings that octahedral tilts would change. So the 'up to n=5' claim is conditional on the assumed ambient structure. The paper does state this assumption, but it is load-bearing for the generalization, and a referee should ask for a sensitivity check or a softened claim. The other weakness is the usual one: no input structures or Wannier files are provided, so the numbers are not directly checkable. More minor: the link between the gamma pocket and s± pairing is inherited from the literature, and the authors know it.\n\nThe stress-test note lands on the n=4,5 point. I do not think it breaks the central contribution. For n=2 and 3, the trend is well established, and the concurrent Geisler preprint independently supports the tensile-strain direction for the bilayer on STO.\n\nThis paper is for the nickelate community and anyone thinking about strain engineering of superconductors. It deserves a serious referee. My recommendation: send it out, and ask the authors to either test n=4,5 with a tilted starting structure or explicitly mark that part as speculative. With that revision, I would be comfortable with it.","headline":"Solid DFT study showing tensile strain recovers the pressure-like dz2 pocket in RP nickelates; the n=4,5 legs rest on assumed structures, but the n=2,3 core holds up.","tokens_in":15830,"tokens_out":2324,"would_cite":true,"duration_ms":27263,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["74.70.-b","71.18.+y","71.15.Mb"],"model":"deepseek-v4-flash","headline":"Biaxial tensile strain reproduces the pressure-induced electronic structure of Ruddlesden-Popper nickelates at ambient pressure.","keywords":["Ruddlesden-Popper nickelates","biaxial strain","high-pressure superconductivity","d_z2 flat band","gamma pocket","density functional theory","La3Ni2O7","La4Ni3O10"],"falsifier":"Angle-resolved photoemission on a La3Ni2O7 film grown under roughly +3% tensile strain should show the pure $d_{z^2}$ gamma pocket at the M point if the prediction is right; its absence would falsify the electronic-structure claim. Separately, diffraction on n=4 and n=5 films could check whether the assumed I4/mmm ambient structures are real, since wrong starting structures would undermine the higher-order predictions.","tokens_in":14820,"feed_emoji":"🧲","tokens_out":10198,"duration_ms":90288,"temperature":0.7,"pith_summary":"The paper uses density-functional theory to show that biaxial strain can decouple the structural and electronic-structure effects that hydrostatic pressure produces in Ruddlesden-Popper nickelates. Compressive strain straightens the Ni-O-Ni octahedral angle toward 180 degrees, but it pushes the flat Ni-$d_{z^2}$ band below the Fermi level, producing a more cuprate-like Fermi surface. Tensile strain increases the tilts, yet it restores the pure $d_{z^2}$ band crossing the Fermi level, the same $\\gamma$ pocket that appears under pressure and is linked to superconductivity in the bilayer and trilayer compounds. The paper presents this as a route to ambient-pressure superconductivity in thin films and as a way to test which structural and electronic features are actually required.","feed_headline":"Tensile strain can mimic pressure in nickelate superconductors","feed_subtitle":"DFT predictions show tensile strain recovers the pressure-like Fermi surface in Ruddlesden-Popper nickelate films.","key_machinery":"The argument is carried by constrained-biaxial-strain density-functional relaxations: the in-plane lattice constants are fixed at a chosen strain, and the out-of-plane lattice constant and atomic positions are relaxed. The electronic-structure object that matters is the flat Ni-$d_{z^2}$ bonding band and its Fermi-surface pocket, the $\\gamma$ pocket, traced across strain and pressure. The structural marker tracked alongside is the apical Ni-O-Ni angle. Wannier downfolding supplies hopping ratios that connect the strain trends to the pairing-symmetry debate in the literature, in particular the out-of-plane $d_{z^2}$ hopping relative to the in-plane $d_{x^2-y^2}$ hopping.","core_discovery":"For La$_{n+1}$Ni$_n$O$_{3n+1}$ with n = 2 to 5, the paper claims that tensile biaxial strain can mimic the electronic structure of the pressurized state even though it does not reproduce the pressure-induced structural straightening of the octahedra. At 3% tensile strain, the flat bonding band of pure $d_{z^2}$ character crosses the Fermi level, forming the $\\gamma$ pocket, in La3Ni2O7 and La4Ni3O10, and the same pocket appears in the n = 4 and n = 5 members when strained. At 3% compressive strain, the apical Ni-O-Ni angle moves closer to 180 degrees, but the $d_{z^2}$ band moves below the Fermi level and the $\\gamma$ pocket is absent, giving a cuprate-like fermiology. The paper concludes that the out-of-plane Ni-Ni distance, rather than the tilt angle, is the key parameter controlling the pocket, and that strain offers a way to decouple the structural and electronic effects of pressure.","pith_inferences":["If the $\\gamma$ pocket is the superconducting switch, tensile-strained films become the cleanest ambient-pressure laboratory for testing s$\\pm$ pairing, because photoemission and transport could be done without high-pressure cells; this experimental consequence is implicit in the paper rather than stated.","The compressive-strain cuprate-like state offers a controlled comparison with infinite-layer nickelates: if such films do not superconduct, it would strengthen the view that the $d_{z^2}$ pocket, not the NiO2 planes alone, is essential to the RP nickelate superconductivity.","The most likely quantitative weak point is the DFT level of theory used to place the flat band; adding dynamical correlations on the strained structures could shift the crossover strain at which the $\\gamma$ pocket appears, and this is a direct computational next step the paper leaves open.","A broader extension is that the same decoupling logic may apply to other pressure-tuned layered oxides, where tensile strain could recover pressure-induced fermiology without the structural collapse, though that transfers the mechanism beyond the nickelates."],"forward_implications":["A 3% tensile strain on La3Ni2O7 and La4Ni3O10 films should produce the pressure-like $\\gamma$ pocket at ambient pressure, giving an angle-resolved photoemission signature that high-pressure experiments cannot easily reach.","A 3% compressive strain should give a cuprate-like Fermi surface with the $d_{z^2}$ flat band fully occupied, so comparing tensile and compressive films would test whether the $\\gamma$ pocket is required for superconducting signatures.","The calculations identify the out-of-plane Ni-Ni distance, not the octahedral tilt angle, as the controlling parameter for the pocket, implying that films with the same c-axis compression but different tilts should behave similarly.","Higher-order members with n = 4 and n = 5, which are only stable as thin films, are predicted to show the same $\\gamma$-pocket recovery under tensile strain, making them candidate members of a thin-film superconducting family.","The Wannier-derived hopping ratios move in the direction associated with s$\\pm$ pairing under tensile strain and away from it under compressive strain, suggesting strain can tune between pairing regimes."],"supporting_citations":[{"why":"reports the bilayer La3Ni2O7 superconducting signature under pressure and the Amam-to-Fmmm structural transition with tilt suppression that the strain study uses as its pressure baseline.","marker":"[23]"},{"why":"reports the trilayer La4Ni3O10 superconducting signature under pressure and the tetragonalization transition used as the n=3 baseline.","marker":"[26]"},{"why":"provides the theoretical I4/mmm structures of the higher-order n=4 and n=5 RPs that are the ambient-pressure starting points for the strain calculations.","marker":"[29]"},{"why":"supplies the pressurized structural and electronic-structure results for La3Ni2O7 that are compared against the strained cases.","marker":"[37]"},{"why":"supplies the trilayer La4Ni3O10 electronic structure and structural transition under pressure used for the n=3 comparison.","marker":"[48]"},{"why":"introduces the effective model with s±-wave pairing in the trilayer nickelate, tying the gamma pocket to the superconducting instability.","marker":"[33]"},{"why":"analyzes how the pairing symmetry changes when the gamma pocket disappears, supporting the interpretation of the compressive-strain regime.","marker":"[41]"},{"why":"shows interlayer-coupling-driven superconductivity and defines the J_perp/J_parallel ratio that the paper uses to connect strain to pairing symmetry.","marker":"[42]"},{"why":"demonstrates epitaxial growth of RP nickelate thin films up to n=5, which is why the strained higher-order members are experimentally realistic.","marker":"[44]"}],"fun_headline_variants":["Tensile strain recreates pressure-like Fermi surface in nickelates","Strain mimics pressure in nickelate superconductors, DFT shows","Flat d_z2 band returns under tensile strain in nickelates","Tensile strain: a route to ambient-pressure nickelate superconductivity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"For the four-layer and five-layer Ruddlesden-Popper nickelates, the calculations assume tetragonal I4/mmm ambient-pressure structures that have not been confirmed experimentally; if the real films have different octahedral tilts or layer stackings, the predicted strain trends, including the recovery of the gamma pocket, could change.","fun_headline_variants_meta":{"raw":{"variants":["Tensile strain recreates pressure-like Fermi surface in nickelates","Strain mimics pressure in nickelate superconductors, DFT shows","Flat d_z2 band returns under tensile strain in nickelates","Tensile strain: a route to ambient-pressure nickelate superconductivity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000267,"raw_usage":{"total_tokens":1707,"prompt_tokens":1128,"completion_tokens":579,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":744,"completion_tokens_details":{"reasoning_tokens":505}},"tokens_in":744,"tokens_out":579,"duration_ms":5836,"temperature":1.0,"reasoning_tokens":505,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T21:24:13.494743+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Angle-resolved photoemission on a La3Ni2O7 film grown under roughly +3% tensile strain should show the pure $d_{z^2}$ gamma pocket at the M point if the prediction is right; its absence would falsify the electronic-structure claim. Separately, diffraction on n=4 and n=5 films could check whether the assumed I4/mmm ambient structures are real, since wrong starting structures would undermine the higher-order predictions.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"reports the bilayer La3Ni2O7 superconducting signature under pressure and the Amam-to-Fmmm structural transition with tilt suppression that the strain study uses as its pressure baseline."},{"cited_title":"Yang, K.-Y","cited_arxiv_id":null,"evidence_quote":"introduces the effective model with s±-wave pairing in the trilayer nickelate, tying the gamma pocket to the superconducting instability."},{"cited_title":"Zhang, L.-F","cited_arxiv_id":null,"evidence_quote":"analyzes how the pairing symmetry changes when the gamma pocket disappears, supporting the interpretation of the compressive-strain regime."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"demonstrates epitaxial growth of RP nickelate thin films up to n=5, which is why the strained higher-order members are experimentally realistic."}],"review_version":1}