{"id":"157c19c6-0b47-490b-a572-e489bb271e03","arxiv_id":"1908.11126","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"FeSe-based superconductors, and in particular the FeSe/SrTiO3 interface, sit close to the BCS-BEC crossover unitary point, and hole doping appears to move the interface toward a pre-formed-pair (pseudogap) regime.","lead":"The paper places FeSe-based superconductors, especially a single layer of FeSe on strontium titanate, on the BCS-BEC crossover diagram, where superconducting electrons transition from loosely bound Cooper pairs to tightly bound molecules. It argues the interface sits near the strongest-interaction unitary point and reports STM evidence that hole doping pushes it toward a pre-paired pseudogap phase.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The unitary-point placement of 1uc-FeSe/STO rests on a 3D cold-atom Δ/EF↔kFξpair mapping applied to quasi-2D ARPES data; that mapping is neither derived nor validated for the strongly renormalized lattice system.","rationale":"The paper is a phenomenological placement of FeSe-based superconductors, especially 1uc-FeSe/STO, on a BCS-BEC crossover diagram. The reader's conditional verdict already captures the main risk: the absolute position on that diagram depends on importing cold-atom calibrations and using ARPES-derived Fermi energies. My reading agrees that this is the most load-bearing assumption. If the Δ/EF↔kFξpair mapping is not valid for a quasi-2D lattice with strong renormalization, then the headline statement that 1uc-FeSe/STO is closest to the unitary point loses its quantitative meaning. The paper's own caveat about missing in-situ TC for the TCNQ-doped samples supports keeping the verdict conditional rather than raising it to acceptance. The pseudogap observation is suggestive but not decisive: a constant gap with disappearing coherence peaks can also be explained by local disorder or altered tunneling paths near molecules. However, that experimental part is explicitly presented as a demonstration of tunability, not as the sole basis of the central claim. Therefore, the conditional verdict is appropriate, and I do not see a reason to change it. The load-bearing concern is the untested transfer of the 3D unitary-gas relation to this quasi-2D system, exactly as the reader identified.","tokens_in":11547,"tokens_out":7436,"duration_ms":73906,"concrete_test":"Use determinant quantum Monte Carlo or cluster DMFT to compute the pairing gap Δ, Fermi momentum kF, and pair coherence length ξpair for the 2D attractive Hubbard model at the band filling and interaction strength appropriate to 1uc-FeSe/STO. Construct the Δ/EF versus kFξpair curve and check whether Δ/EF=0.36 maps to kFξpair≈1, as the paper's unitary-point claim requires; if the mapping differs from the 3D contact formulas in SM I, the placement is not established.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central quantitative claim is that 1uc-FeSe/STO has Δ/EF≈0.36 and TC/TF≈0.1 and therefore sits 'closer to the crossover unitary' (Δ/EF≈0.44, TC/TF≈0.167) than other FeSe-based superconductors. This requires the assertion in Section 1 and SM I that Δ/EF is in monotonic one-to-one correspondence with kFξpair, equivalently with 1/(kFa), and that the 3D unitary-gas calibrations transfer to a quasi-2D lattice system. The SM derives this mapping only from 3D continuum BCS and BEC formulas; the text's statement that for quasi-2D systems 'the phase diagram discussion based on the Fermi momentum and scattering length is still valid' is an assertion, not a derivation. In a 2D lattice with BKT physics the crossover is quantitatively different, and 'unitary point' is not a unique 3D-universality calibration. In addition, the EF=56 meV used for 1uc-FeSe/STO is the ARPES quasiparticle band-bottom energy; strong band renormalization means it is not the non-interacting Fermi energy of the cold-atom mapping. If the Δ/EF↔kFξpair mapping changes, a measured Δ/EF=0.36 may correspond to kFξpair far from unity, and the headline placement is not established. The TCNQ hole-doping experiment has a separate weakness: the gap remains unchanged while coherence peaks disappear as the tip approaches a molecule, which is also compatible with local disorder or molecular tunneling; the manuscript explicitly states that in-situ TC is unavailable. The pseudogap interpretation is therefore conditional, but the mapping issue is more fundamental because it underpins the entire BCS-BEC diagram.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper places FeSe-based superconductors on a BCS-BEC crossover phase diagram parameterized by Δ/EF and TC/TF, using literature values for Fermi energy, superconducting gap, and critical temperature. The central claims are that the single-layer FeSe/SrTiO3 interface (1uc-FeSe/STO) sits closer to the unitary point (Δ/EF≈0.36, TC/TF≈0.1) than other FeSe-derived superconductors, and that hole doping by TCNQ molecules drives the interface toward a pre-paired-electron (pseudogap) regime, based on STS showing loss of coherence peaks with an unchanged gap. The paper also proposes that matching the substrate optical-phonon energy to EF can tune the crossover position, and lists candidate oxide substrates for this purpose.","tokens_in":11908,"tokens_out":8625,"duration_ms":84070,"significance":"If the placement is correct, the paper offers a tunable solid-state platform for studying strong many-body pairing and a concrete, falsifiable design rule relating substrate phonon energy to the BCS-BEC crossover position. A strength of the analysis is that the phase-diagram positions are computed from externally measured EF, Δ, and TC values with no fitted parameters, and the authors are appropriately cautious in labeling the TCNQ result as 'possible' pre-pairing and in noting that in situ TC is unavailable. The significance of the paper is, however, conditional on the validity of the quasi-2D mapping from Δ/EF to kFξpair and on the uniqueness of the STS interpretation.","major_comments":[{"comment":"The central placement of 1uc-FeSe/STO at Δ/EF≈0.36 and TC/TF≈0.1 relies on two unproven transfers: the monotonic one-to-one mapping between Δ/EF and 1/(kFa), which SM I derives only from 3D continuum BCS/BEC formulas, and the cold-atom unitary calibrations Δ/EF≈0.44 and TC/TF≈0.167. The statement that the phase diagram 'is still valid' for quasi-2D lattice systems is an assertion, not a derivation; BKT physics and the discrete lattice change the crossover quantitatively. Moreover, EF=56 meV for 1uc-FeSe/STO is the ARPES quasiparticle band-bottom energy and is not the non-interacting Fermi energy of the cold-atom mapping. Since this mapping is load-bearing for the headline claim, please supply a derivation or numerical validation for the 2D lattice case with renormalized bands, or explicitly frame the placement as a qualitative estimate with a quantified uncertainty.","section":"Section 1 and SM I"},{"comment":"The values in Table SI are internally inconsistent with the text. For 1uc-FeSe/STO, the table lists kFξpair=4.20 (with ξpair replaced by the phase coherence length), which is not consistent with the claim of proximity to the unitary point kFξpair=1. For FeSe-e, the table gives kFξpair=3.25, while the main text states that FeSe-e is 'over the unitary point towards the BEC limit.' These contradictions make it unclear whether kFξpair or Δ/EF is intended to define proximity to the unitary point. Please reconcile the table with the text, or remove the kFξpair column if it is not used quantitatively.","section":"Table SI and Fig. 2"},{"comment":"The interpretation of the TCNQ hole-doping experiment as evidence for pre-formed electron pairs is not uniquely supported by the data. The disappearance of coherence peaks with an unchanged gap as the tip approaches a molecule is also compatible with local suppression of superconductivity, disorder, or molecular tunneling effects, and the paper explicitly states that in situ TC is unavailable. Without temperature-dependent STS or a measurement of the pairing temperature Tpair, the pre-pairing conclusion remains a conjecture. In addition, the claim that EF decreases from 56 to 37 meV at point #5 is not demonstrated by the STS data shown; please state how this EF shift is obtained and its uncertainty, or soften the conclusion.","section":"Section 3 and Fig. 3(c)"}],"minor_comments":[{"comment":"No uncertainties are reported for Δ/EF and TC/TF; because the comparative claim rests on small differences (0.36 vs 0.44 for Δ/EF and 0.1 vs 0.167 for TC/TF), error propagation should be provided.","section":"Table SI"},{"comment":"The displayed equations for the BCS and BEC limits of Δ/EF are garbled in the text and should be typeset as readable formulas; the current rendering makes the derivation difficult to follow.","section":"SM I"},{"comment":"The paper should define TF consistently: for multi-band or strongly renormalized systems, the relation TF=EF/kB may not yield the same value as a band-structure-based TF, and the convention used for each compound should be stated explicitly.","section":"Table SI"},{"comment":"The dashed curve is described as schematic, but since data points are plotted on the same axes, a reader may interpret it as a theoretical TC/TF curve; please state explicitly that it is only a guide to the eye.","section":"Fig. 2(b)"}],"recommendation":"major_revision","confidential_remarks":"The quantitative inputs for 1uc-FeSe/STO come mainly from the authors' own previous work (Refs. 30-33 and 42). This is not circular, since those are independent measurements, but independent confirmation of EF, Δ, and the TCNQ-induced EF shift would materially strengthen the paper. The novelty is primarily in the placement and tunability claims rather than in new experimental data; the TCNQ data are from an earlier publication. The paper is within the journal's scope as a phenomenology/analysis contribution, provided the mapping and STS-interpretation issues above are addressed."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's my take. The paper is a clean, honest piece of phenomenology: it collects the published Δ, EF, TC values for FeSe-based superconductors, puts them on a Δ/EF vs TC/TF diagram, and argues that monolayer FeSe/SrTiO3 sits closer to the BCS-BEC unitary point than the others. It also adds a new STS experiment—TCNQ molecules hole-dope the surface, the gap remains but coherence peaks wash out near the molecules—and the authors interpret that as a possible pre-pairing signature. That is genuinely new and worth discussing.\n\nWhat the paper does well: it is transparent about its inputs, it does not fit parameters, and it is careful to say the pseudogap interpretation is not proven and in-situ TC is missing. The compilation of parameters in the table is useful. The correlation between boson energy and position on the diagram is admittedly speculative, and they say so.\n\nThe soft spot is the mapping. The entire placement on the BCS-BEC diagram relies on the assumption that Δ/EF is in one-to-one monotonic correspondence with kFξpair, and that the 3D cold-atom calibrations (Δ/EF ~ 0.44, TC/TF ~ 0.167 at unitarity) carry over to a quasi-2D lattice with strong band renormalization. The supplementary derives the mapping only from 3D continuum formulas; the text asserts the quasi-2D extension is 'still valid' rather than showing it. So the quantitative distance to the unitary point is not established—if the mapping shifts, Δ/EF = 0.36 might not mean kFξpair close to 1. That said, the comparative statement among FeSe-based materials is more robust, because the same mapping is applied to all of them. The TCNQ experiment is a separate, softer claim: unchanged gap with vanishing coherence peaks could also come from local disorder or the molecule's tunneling channel, and the authors acknowledge this.\n\nNet: this is a solid contribution for the FeSe and BCS-BEC crossover community, not a breakthrough. The placement is plausible and the experimental observation is interesting but not conclusive. I'd send it to a serious referee, with the request that the authors either provide some justification for the quasi-2D unitary calibration or soften the 'closest to the unitary point' wording to 'closest in this phenomenological mapping.' The TCNQ data can stay as reported, but the pre-pairing interpretation should be clearly labeled as speculative, which it already is.\n\nI would not cite it as a definitive placement, but I would reference the parameter compilation. A good paper to bring to a reading group.","headline":"A clean phenomenological synthesis placing monolayer FeSe/STO near the BCS-BEC crossover, with a new but not conclusive TCNQ hole-doping STS experiment; the main soft spot is the validity of the 3D cold-atom mapping to a quasi-2D lattice.","tokens_in":12482,"tokens_out":2334,"would_cite":false,"duration_ms":22801,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A single layer of FeSe on SrTiO3 sits closer to the BCS-BEC unitary point than any other FeSe-based superconductor, putting a solid-state interface in the regime of strongest many-body pairing.","keywords":["BCS-BEC crossover","FeSe/SrTiO3 interface","single-layer FeSe","unitary point","preformed electron pairs","pseudogap","molecular hole doping","interfacial superconductivity"],"falsifier":"Measure the superconducting coherence length $\\xi_{\\mathrm{pair}}$ of 1uc-FeSe/STO directly, for example from the upper critical field or from fluctuation diamagnetism, and compute $k_F\\xi_{\\mathrm{pair}}$ with the ARPES Fermi momentum. If $k_F\\xi_{\\mathrm{pair}}$ is far from 1 while $\\Delta/E_F$ stays near 0.36, then the claimed one-to-one mapping between the two ratios is not valid for this system and the unitary-point placement collapses.","tokens_in":11348,"feed_emoji":"⚛️","tokens_out":15422,"duration_ms":126780,"temperature":0.7,"pith_summary":"This paper argues that the BCS-BEC crossover, developed for ultracold atoms, can be used to organize FeSe-based superconductors by two experimentally accessible ratios: the superconducting gap divided by the Fermi energy, $\\Delta/E_F$, and the critical temperature divided by the Fermi temperature, $T_C/T_F$. On that diagram, a single layer of FeSe on SrTiO3 (1uc-FeSe/STO) sits closest to the unitary point, where many-body pairing effects are strongest, with $\\Delta/E_F \\approx 0.36$ and $T_C/T_F \\approx 0.1$. The authors also report that depositing TCNQ molecules to remove electrons lowers the Fermi energy from 56 to 37 meV while the gap size stays roughly unchanged, which they read as a move toward the preformed-pair (pseudogap) side of the crossover. A sympathetic reader would care because it makes a single oxide interface a tunable solid-state platform for studying strong pairing, normally accessible mainly in cold atomic gases.","feed_headline":"A one-layer FeSe film sits closest to the strongest-pairing point","feed_subtitle":"Measured ratios put it near the unitary point, and hole doping pushes it toward preformed electron pairs","key_machinery":"The working device is the BCS-BEC phase diagram drawn in $\\Delta/E_F$--$T_C/T_F$ coordinates. The authors rely on a monotonic one-to-one mapping between $\\Delta/E_F$ and $k_F\\xi_{\\mathrm{pair}}$ (the Fermi momentum times the pair size), so each material gets a coordinate without needing a direct measurement of the scattering length. The unitary point is calibrated by cold-atom experiments: $\\Delta/E_F \\approx 0.44$ and $T_C/T_F \\approx 0.167$. They add the pairing-temperature curve $T_{\\mathrm{pair}}/T_F$, whose regime above $T_C/T_F$ defines preformed pairs or pseudogap, and use local scanning-tunneling spectra near TCNQ molecules as a probe of that regime. The machinery's work is to convert published ARPES, STM/STS, and transport numbers for FeSe-derived superconductors into a single map that exposes how substrates, doping, and boson energies move a material through the crossover.","core_discovery":"The central claim is that the superconductivity at the interface between single-layer FeSe and SrTiO3 is located closer to the BCS-BEC crossover unitary than other doped FeSe-based materials. Using angle-resolved photoemission spectroscopy (ARPES) values $E_F = 56$ meV and $\\Delta = 20$ meV, with $T_C \\approx 65$ K, the paper places 1uc-FeSe/STO at $\\Delta/E_F \\approx 0.36$ and $T_C/T_F \\approx 0.1$, next to the unitary reference of $\\Delta/E_F \\approx 0.44$ and $T_C/T_F \\approx 0.167$. It explains this position as the result of interface charge transfer and of an extra bosonic pairing glue, the 97 meV SrTiO3 optical phonon, cooperating with FeSe magnetic excitations. The same logic places bulk FeSe's electron pocket past the unitary point toward BEC, while electron-doped (LiFe)OHFeSe and K0.8FeSe move toward BCS. In the TCNQ hole-doping experiment, the reduced Fermi energy moves the system toward the BEC side, and the persistent gap with suppressed coherence peaks is proposed as the spectral signature of pre-paired electrons.","pith_inferences":["Testable extension: measure $T_C$ of TCNQ-doped 1uc-FeSe/STO directly, for instance with superconducting scanning tunneling microscopy at lower temperatures or with transport on gated devices; if the gap survives while $T_C$ drops sharply, the preformed-pair assignment would be strongly supported.","If the same two-ratio mapping is assumed, the screening could be applied to other monolayer superconductors on polar substrates to identify new candidates for strong-pairing physics.","Because the unitary calibrations come from a three-dimensional atomic gas, a lattice-specific BCS-BEC calculation could shift the absolute coordinates; the paper's ordering of materials is the part most likely to survive such a shift.","The phonon-energy correlation suggests a design rule the paper leaves implicit: substrates with phonon energy below the Fermi energy should give more BCS-like interfaces, and substrates with phonon energy above it should push toward BEC; growing the same FeSe layer on the listed oxides would test that rule."],"forward_implications":["If the placement is correct, 1uc-FeSe/STO is one of the closest solid-state realizations of the unitary Fermi regime, where pairing fluctuations dominate and normal Fermi-liquid descriptions break down.","Hole doping by molecular adsorbates should act as a switch toward the BEC side: the Fermi energy drops, the gap persists, and coherence peaks vanish, indicating preformed pairs rather than a weakened superconductor.","Choosing oxide substrates with optical phonon energies below, near, or above the FeSe Fermi energy should move the interface through the crossover, giving a materials-design route to tune pairing strength.","The FeSe family becomes a rare condensed-matter platform in which one can traverse a large section of the BCS-BEC diagram by doping and substrate engineering.","The observed positive correlation between the boson energy ratio and $\\Delta/E_F$ suggests that the pairing glue's energy scale, not just electron density, helps set the crossover coordinate."],"supporting_citations":[{"why":"It establishes the single-electron-pocket Fermi surface and interfacial charge transfer that set the 1uc-FeSe/STO Fermi energy.","marker":"[27]"},{"why":"It provides the measured values EF = 56 meV, gap = 20 meV, and TC used to place 1uc-FeSe/STO on the diagram.","marker":"[33]"},{"why":"It reports the Fermi energy and superconducting parameters of 1uc-FeSe/STO that the paper combines with other measurements to fix the crossover coordinate.","marker":"[41]"},{"why":"It supplies the TCNQ hole-doping procedure and the lateral reduction of EF from 56 to 37 meV used to approach the pre-pairing regime.","marker":"[42]"},{"why":"They supply the bulk FeSe Fermi energies and gap ratios for the hole and electron pockets that anchor the BCS and BEC sides.","marker":"[17,18]"},{"why":"It gives the Fermi energy and gap of (LiFe)OHFeSe, the electron-doped point that moves toward the BCS side.","marker":"[36]"},{"why":"It gives the Fermi energy and gap of K0.8FeSe, the most BCS-like FeSe-based point in the comparison.","marker":"[37]"},{"why":"It calibrates the unitary-point critical ratio TC/TF ≈ 0.167 from a unitary Fermi gas.","marker":"[9]"},{"why":"It calibrates the unitary-point gap-to-Fermi ratio of about 0.44 from a unitary Fermi gas.","marker":"[10]"},{"why":"It identifies the 97 meV SrTiO3 optical phonon as the interfacial pairing boson that the paper treats as tunable glue.","marker":"[30]"}],"fun_headline_variants":["FeSe/STO sits closest to the unitary pairing point","Hole doping pushes FeSe/STO toward pre-paired electrons","Interface FeSe nears the strongest-pairing limit","One-layer FeSe closest to unitary in BCS-BEC crossover","FeSe/STO: a hop away from the BEC side"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the measured Fermi energy and gap of the single electron pocket give a true $\\Delta/E_F$, and that this ratio maps one-to-one onto $k_F\\xi_{\\mathrm{pair}}$ in the same way as in the three-dimensional cold-atom gas, so that the placement on the diagram is meaningful for this nearly two-dimensional crystal.","fun_headline_variants_meta":{"raw":{"variants":["FeSe/STO sits closest to the unitary pairing point","Hole doping pushes FeSe/STO toward pre-paired electrons","Interface FeSe nears the strongest-pairing limit","One-layer FeSe closest to unitary in BCS-BEC crossover","FeSe/STO: a hop away from the BEC side"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001028,"raw_usage":{"total_tokens":4360,"prompt_tokens":1001,"completion_tokens":3359,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":617,"completion_tokens_details":{"reasoning_tokens":3269}},"tokens_in":617,"tokens_out":3359,"duration_ms":22341,"temperature":1.0,"reasoning_tokens":3269,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T10:23:07.411905+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the superconducting coherence length $\\xi_{\\mathrm{pair}}$ of 1uc-FeSe/STO directly, for example from the upper critical field or from fluctuation diamagnetism, and compute $k_F\\xi_{\\mathrm{pair}}$ with the ARPES Fermi momentum. If $k_F\\xi_{\\mathrm{pair}}$ is far from 1 while $\\Delta/E_F$ stays near 0.36, then the claimed one-to-one mapping between the two ratios is not valid for this system and the unitary-point placement collapses.","supporting_citations":[{"cited_title":"Liu et al., Nat","cited_arxiv_id":null,"evidence_quote":"It establishes the single-electron-pocket Fermi surface and interfacial charge transfer that set the 1uc-FeSe/STO Fermi energy."},{"cited_title":"Zhang et al., Phys","cited_arxiv_id":null,"evidence_quote":"It provides the measured values EF = 56 meV, gap = 20 meV, and TC used to place 1uc-FeSe/STO on the diagram."},{"cited_title":"Peng et al., Nat","cited_arxiv_id":null,"evidence_quote":"It reports the Fermi energy and superconducting parameters of 1uc-FeSe/STO that the paper combines with other measurements to fix the crossover coordinate."},{"cited_title":"Tan et al., Nat","cited_arxiv_id":null,"evidence_quote":"It supplies the TCNQ hole-doping procedure and the lateral reduction of EF from 56 to 37 meV used to approach the pre-pairing regime."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It gives the Fermi energy and gap of (LiFe)OHFeSe, the electron-doped point that moves toward the BCS side."},{"cited_title":"Zhang et al., Nat Mater 10, 273 (2011)","cited_arxiv_id":null,"evidence_quote":"It gives the Fermi energy and gap of K0.8FeSe, the most BCS-like FeSe-based point in the comparison."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It calibrates the unitary-point critical ratio TC/TF ≈ 0.167 from a unitary Fermi gas."},{"cited_title":"Schirotzek, Y.‐i","cited_arxiv_id":null,"evidence_quote":"It calibrates the unitary-point gap-to-Fermi ratio of about 0.44 from a unitary Fermi gas."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"It identifies the 97 meV SrTiO3 optical phonon as the interfacial pairing boson that the paper treats as tunable glue."}],"review_version":1}