{"id":"c43d9a92-57c1-45e3-9aec-ca9b6831dd16","arxiv_id":"2607.27104","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Tip-induced tensile strain in monolayer FeSe/SrTiO3 drives a two-stage superconducting-gap rise (17.8→23.6 meV) via correlation enhancement then dz2–dxy hybridization, without changing carrier density.","lead":"STM tip pressure on monolayer FeSe/SrTiO3 expands the lattice and pushes the deep dz2 band up, enlarging the superconducting gap in two stages from 17.8 to 23.6 meV. The result offers a doping-free route to tune pairing via strain, correlations, and orbital hybridization in a flagship interfacial superconductor.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"The secondary gap stage is tied to a Γ-point spectral merger whose assignment as dz2–dxy hybridization that reconstructs M-point pairing bands remains correlative and unmeasured in k-space.","rationale":"The reader correctly isolates the interpretive chain (STS peak assignment \to Γ merger = hybridization \to M-point pairing reconstruction + unmeasured strain) as the weakest load-bearing assumption. That chain is required for the paper’s distinctive claim of a previously unrecognized dz2-driven, orbital-selective second stage; the first-stage correlation enhancement and the doping-exclusion via invariant kF,app are on firmer ground. Because the manuscript already flags the k-space limitation and supplies no direct strain metrology, the concern is internal to the argument rather than an external consensus objection. A single k-resolved check under strain would settle whether the secondary stage is causal or coincidental, so the CONDITIONAL verdict is unchanged and appropriate.","tokens_in":11333,"tokens_out":519,"duration_ms":12737,"concrete_test":"Under identical tip-approach conditions that produce the secondary gap jump, acquire quasiparticle interference (QPI) or Fourier-transform STS maps of the M-point electron pockets (or, if feasible, micro-ARPES) before and after the putative hybridization current; if the orbital character, hybridization gaps, or Fermi-surface topology at M show no reconstruction while the gap still jumps, the orbital-reconstruction half of the dual-enhancement claim is unsupported.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central two-stage claim requires that the stronger secondary gap rise (above ~1.25 nA) is caused by hybridization of the upward-shifted dz2 band with the flat dxy band, which then reconstructs the pairing-active bands at M. The data show only that an STS peak (assigned dz2 by ARPES analogy) merges with kinks (assigned dxz/yz, dxy) near −80 to −90 meV at the same current where the gap slope steepens (Fig. 2C–D). STS is momentum-integrated and Γ-weighted; no k-resolved measurement under tip strain exists, the absolute lattice expansion is never quantified (SI Note S2), and the paper itself notes that whether dz2 participates at M “remains elusive.” Without that link the secondary stage collapses to a correlative coincidence of a high-energy feature shift with gap size, leaving only the weaker first-stage correlation argument.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript reports that reducing the STM tip–sample distance on monolayer FeSe/SrTiO3 enlarges the superconducting gaps in two stages (outer gap from 17.8 to 23.6 meV). The authors attribute the first stage to tip-induced in-plane tensile strain that strengthens electronic correlations (tracked by an upward shift of a spectral feature assigned to the dz2 band), and the second, stronger stage to hybridization of that upward-shifted dz2 band with dxy (and dxz/yz) signatures near −80 to −90 meV, which is argued to reconstruct pairing-active d bands. Invariant apparent Fermi wave vectors extracted from YSR spatial oscillations are used to exclude carrier-doping changes. Supporting observations include reproducibility on multiple samples and two STM–MBE systems, an Ag control without analogous peak shifts, a non-monotonic dz2 energy shift at larger distances that qualitatively tracks a force–distance curve, and tip manipulation of a subsurface Fe vacancy.","tokens_in":11574,"tokens_out":1476,"duration_ms":38596,"significance":"If the two-stage mechanism is established, the work would be significant for iron-based and interfacial superconductivity: it would show that the deep dz2 orbital can be strain-tuned into a role that amplifies pairing, offer a doping-independent route to enlarge the gap in FeSe/STO beyond the usual charge-transfer and interfacial phonon channels, and motivate multi-orbital theories that include strain-activated dz2–dxy hybridization. The experimental platform (local tip strain with simultaneous gap and high-energy spectral tracking, plus YSR-based kF check) is a concrete strength and is in principle transferable to other 2D multiband systems. The claim is currently stronger on phenomenology (reproducible gap rise correlated with spectral evolution) than on microscopic proof of M-point band reconstruction.","major_comments":[{"comment":"Results, Fig. 2C–D and accompanying text: The secondary-stage claim rests on identifying the STS peak/kink merger near −80 to −90 meV as dz2–dxy (and dxz/yz) hybridization that reconstructs pairing-active bands at M. STS is momentum-integrated and Γ-weighted; orbital labels are assigned by analogy to ARPES on unstrained films (SI Note S1, Fig. S4). No k-resolved measurement under tip strain is provided, and the text itself states that whether dz2 participates at M “remains elusive.” The coincidence in setpoint current between the spectral merger and the steepening of Δ(I) is therefore correlative. Either strengthen the link (e.g., additional controls, modeling of how Γ hybridization implies M reconstruction, or clearer bounds) or revise the central wording so that the secondary stage is presented as a correlation with high-energy spectral merger rather than demonstrated M-point orbital-s","section":"Results (Fig. 2C–D; d-orbital band reconstruction)"},{"comment":"Results and SI Note S2: The mechanical premise—that repulsive tip–sample force produces a sufficiently uniform local in-plane tensile strain (and out-of-plane compression) whose magnitude explains the large dz2 shift—is supported only indirectly (non-monotonic dz2 shift vs distance, Fe-vacancy manipulation, comparison to prior ARPES lattice trends). Absolute lattice expansion under the tip is not measured. Because both the correlation-enhancement stage and the hybridization stage are tied to this strain, the manuscript should quantify or bound the strain (or the strained area) more tightly, or state explicitly that strain magnitude remains an inference from the dz2 shift and prior ARPES calibrations.","section":"Results (Repulsive tip-sample interaction); SI Note S2"},{"comment":"Discussion / invariant kF: Unchanged kF,app from YSR oscillations (Fig. 4) is a valuable control against doping change, but it is reported for two setpoints and relies on the relation n_s a^2 = (k_F a)^2/2π with the assumption that the oscillation period tracks the M-pocket kF under local strain. Clarify whether strain-induced changes in lattice constant a or in the YSR scattering geometry could mask a small density change, and whether kF,app was checked across the full current range that defines both gap stages (including below and above the ~1.25 nA transition).","section":"Results (Fig. 4); SI Note S3"}],"minor_comments":[{"comment":"Fig. 1H and Fig. 2: The transition current (~1.25 nA at V = 30 mV) is treated as a physically meaningful onset, yet the text notes that the critical current varies with tip and sample. State more clearly that the invariant is the coincidence of gap-slope change with spectral merger, not a universal current value.","section":"Fig. 1H; Fig. 2"},{"comment":"Gap extraction: Gaps are defined as the mean of positive and negative coherence-peak positions. Briefly justify this choice versus peak-to-peak/2 or Dynes fits, especially given mild asymmetry.","section":"Fig. 1 caption; Materials and Methods"},{"comment":"Fig. 2E–F schematics omit hybridization details “unresolved in dI/dV.” Consider a more cautious schematic or a panel that shows only the measured peak/kink trajectories to avoid over-interpreting band inversion/hybridization topology.","section":"Fig. 2E–F"},{"comment":"Interfacial electron–phonon coupling is invoked as a possible synergistic factor when the tip reduces the FeSe/STO distance, but no direct phonon or replica-band evidence under tip approach is shown. Keep this as a speculative aside or add a control.","section":"Discussion"},{"comment":"Typos/clarity: “DOSs” → DOS; ensure consistent notation for Δ1/Δ2 and for setpoint (V, It) across figure captions; SI Appendix figure callouts are dense—consider a short roadmap in the main text for S1–S18.","section":"Throughout"}],"recommendation":"major_revision","confidential_remarks":"The experimental reproducibility (two STM–MBE systems, Ag control, YSR kF check) is above average for an STM strain study and should be weighed heavily. The main risk is over-claiming a microscopic orbital-selective pairing mechanism from Γ-weighted STS correlations. Major revision that tightens causal language around the secondary stage and the unmeasured strain would make the paper appropriate; rejection would be disproportionate given the solid phenomenology. Fit to a high-impact condensed-matter journal is reasonable if the mechanism claims are calibrated to the evidence."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"The usable core here is experimental: they continuously tune tip–sample distance on monolayer FeSe/STO and watch both the superconducting coherence peaks and the high-energy d-orbital edges move together. Outer gap goes 17.8 → 23.6 meV in two stages, the steeper second stage lining up with the assigned dz2 feature merging into the −80/−90 meV kinks. Reproduced on multiple samples/tips and two STM–MBE systems. Ag control shows no analogous shift. YSR oscillations give unchanged kF,app, so they have a decent argument against extra doping. Non-monotonic dz2 motion at larger distance tracks a force–distance curve and they can shove subsurface Fe vacancies, so repulsive tip force and local lattice expansion are plausible.\n\nWhat is actually new is the simultaneous current-dependent map of gap + high-energy edges and the claim that the secondary rise is locked to dz2–dxy hybridization that reconstructs pairing bands. Prior FeSe/STO work (including from overlapping groups) already had interfacial doping, e-ph, correlation/smectic physics, and lattice-constant sensitivity of dz2. The two-stage lock and the explicit “dz2 now participates” framing are the increment.\n\nSoft spots are real but proportionate. Peak/kink assignment is by ARPES analogy, not in-situ k-resolved data under the tip. STS is momentum-integrated and Γ-weighted; they never measure absolute strain (SI Note S2 admits it); and they themselves say whether dz2 actually enters the M-point pairing bands “remains elusive.” So the secondary-stage mechanism is correlative coincidence of a high-energy spectral merger with a gap-slope change, not a demonstrated reconstruction of the pairing Fermi surface. First-stage correlation-under-expansion argument is on firmer ground. Gap extraction (mean of ±peaks) and the exact transition current are free parameters but not load-bearing. Citation pattern is heavy on the group’s prior FeSe work; that is normal context, not circularity.\n\nThis is for people who already live in interfacial iron chalcogenides or multi-orbital pairing. They will want the raw spectra and the YSR control. It deserves a serious referee, not a desk reject—data quality and the doping-null result are enough. I would bring it to reading group as a methods-plus-interpretation discussion, cite the gap-vs-current and kF results if I am writing on strain or orbital-selective pairing, and expect revision that separates observation from the hybridization claim more cleanly.","headline":"Real tip-tunable two-stage gap data on FeSe/STO with clean doping control; the dz2–pairing causal story is still correlative.","tokens_in":12236,"tokens_out":621,"would_cite":true,"duration_ms":17638,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["74.70.Xa","74.78.-w","73.20.-r","68.37.Ef"],"model":"grok-4.5","headline":"Tip strain lifts the buried dz2 band in FeSe/SrTiO3 and enlarges the superconducting gap in two stages from 17.8 to 23.6 meV.","keywords":["FeSe/SrTiO3","superconducting gap enhancement","dz2 orbital","tip-induced strain","electronic correlation","orbital hybridization","multiband pairing","STM/STS"],"falsifier":"An independent measurement (ARPES under controlled strain, or a second local probe) that either shows the Fermi wave vector changing with tip approach or shows the dz2 band remaining far below −80 meV while the gap still enlarges would falsify the claimed mechanism.","tokens_in":12156,"feed_emoji":"🧲","tokens_out":968,"duration_ms":22780,"temperature":0.7,"pith_summary":"Monolayer FeSe on SrTiO3 already hosts a large superconducting gap, yet the dz2 orbital band sits far below the Fermi level and is normally written out of the pairing story. This paper shows that an STM tip can apply local tensile strain that first expands the in-plane lattice, strengthens electronic correlations, and modestly enlarges the gap, then drives the dz2 band upward until it hybridizes with the flat dxy band near −80 meV. That hybridization reconstructs the pairing-active d bands and produces a second, stronger gap jump. The two stages together raise the outer gap from 17.8 meV to 23.6 meV while the Fermi wave vector stays fixed, proving the boost is not extra doping. The result supplies a concrete, doping-independent route to strengthen superconductivity by strain-activated orbital engineering and forces pairing theories to include the once-irrelevant dz2 orbital.","feed_headline":"Tip strain lifts buried dz2 band, boosts FeSe gap 32%","feed_subtitle":"Two-stage rise to 23.6 meV comes from correlation then orbital hybridization, not doping","key_machinery":"Tip-controlled tensile strain that continuously shifts the dz2 band toward the Fermi level until it hybridizes with dxy, thereby reconstructing the M-point pairing bands while the measured Fermi wave vector remains constant.","core_discovery":"Tip-induced in-plane tensile strain in monolayer FeSe/SrTiO3 produces a two-stage superconducting-gap enhancement (outer gap 17.8 → 23.6 meV). The first stage arises from correlation strengthening under lattice expansion; the second, larger rise occurs when the upward-shifted dz2 band hybridizes with the dxy band near −80 meV, reconstructing the pairing-active d-orbital bands. Invariant Fermi wave vectors show the effect is band renormalization and reconstruction, not carrier doping.","pith_inferences":["If the secondary gap jump is truly hybridization-driven, similar tip or substrate strain should produce an analogous two-stage rise in other FeSe monolayers grown on larger-lattice perovskites.","The invariant-kF result implies that any future device exploiting this effect can keep carrier density fixed while still tuning Tc via mechanical strain alone.","The observation that dz2 participation appears only after hybridization suggests a threshold orbital mixing strength that could be mapped systematically across the iron chalcogenide family."],"forward_implications":["Pairing theories for iron-based superconductors must incorporate the dz2 orbital once it is brought near the Fermi level.","Local tensile strain becomes a practical, doping-independent knob for enlarging gaps in monolayer FeSe and related 2-D multiband films.","The same strain-plus-correlation route can be tested in other interface-doped chalcogenides where competing orders are already suppressed by substrate charge transfer.","Atomic-scale lattice control may stabilize previously hidden orbital-selective paired states in two-dimensional heterostructures."],"fun_headline_variants":["Tip strain shifts dz2 band, two-stage FeSe gap rise to 23.6 meV","Lattice expansion then dz2-dxy hybrid boosts FeSe superconductivity","Strain-activated dz2 hybridization enlarges FeSe gap without doping","Two-stage gap hike in FeSe/SrTiO3 from correlation and orbital mix","In-plane strain reconstructs d bands, lifts FeSe gap 17.8 to 23.6 meV"],"cache_read_input_tokens":128,"weakest_assumption_plain":"The spectroscopic peaks and kinks seen by the tip are correctly assigned to the Γ-point dz2, dxz/yz and dxy band edges, their apparent merger equals real interband hybridization that rebuilds the distant M-point pairing bands, and the tip force creates a uniform local tensile strain whose size is never measured directly.","fun_headline_variants_meta":{"raw":{"variants":["Tip strain shifts dz2 band, two-stage FeSe gap rise to 23.6 meV","Lattice expansion then dz2-dxy hybrid boosts FeSe superconductivity","Strain-activated dz2 hybridization enlarges FeSe gap without doping","Two-stage gap hike in FeSe/SrTiO3 from correlation and orbital mix","In-plane strain reconstructs d bands, lifts FeSe gap 17.8 to 23.6 meV"]},"model":"grok-4.5","effort":"low","cost_usd":0.004483,"raw_usage":{"total_tokens":1343,"prompt_tokens":785,"num_sources_used":0,"completion_tokens":119,"cost_in_usd_ticks":44828000,"prompt_tokens_details":{"text_tokens":785,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":439,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":785,"tokens_out":119,"duration_ms":8958,"temperature":1.0,"reasoning_tokens":439,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-30T11:00:04.236635+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"An independent measurement (ARPES under controlled strain, or a second local probe) that either shows the Fermi wave vector changing with tip approach or shows the dz2 band remaining far below −80 meV while the gap still enlarges would falsify the claimed mechanism.","supporting_citations":[],"review_version":1}