{"id":"f0c8e849-afac-4a63-8dfb-1b9353ff0d67","arxiv_id":"1908.05648","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Gate-dependent Raman and spatially resolved VEELS show FeSe phonons soften with gate voltage and FeSe plasmons red-shift near the FeSe/SrTiO3 interface, suggesting nonlocal interfacial coupling.","lead":"This paper uses Raman light scattering and electron energy loss spectroscopy to watch one layer of iron selenide on a strontium titanate crystal vibrate and conduct electrons under an applied electric field. The results suggest the substrate changes the film's vibrations and electron behavior at the interface, which may help explain its high superconducting temperature.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"VEELS red-shift is compatible with carrier depletion, so the abstract's effective-mass claim overreaches; a core-level EELS cross-check would settle it.","rationale":"The reader's verdict (CONDITIONAL) correctly targets the most fragile premise: the conversion of a VEELS red-shift into an effective-mass enhancement without ruling out carrier depletion. The paper's own hedging in the body confirms this, and the abstract overstates the result. I considered an alternative concern that the Raman ambipolar softening could be merely elastic strain transfer from the piezoelectric STO substrate rather than a nonlocal electronic coupling, but the paper's interpretation is at least plausible and the strain mechanism is still an interfacial coupling of the lattice; the VEELS effective-mass claim is more clearly load-bearing because it is offered as the independent electronic confirmation. My proposed core-level EELS check would directly discriminate between the two viable explanations for the red-shift, and it is feasible on the same specimen. Because the observations themselves seem credible and the authors explicitly flag the ambiguity in the body, the appropriate verdict remains CONDITIONAL: the paper should be published only if the abstract is aligned with the hedged interpretation and the alternative carrier-depletion explanation is resolved or explicitly retained as unresolved.","tokens_in":8433,"tokens_out":5067,"duration_ms":56035,"concrete_test":"Perform spatially resolved core-level EELS on the same FeTe/1UC FeSe/STO cross-section, along the same line scan used for VEELS, to map the Fe L3 and Ti L2,3 edge positions. A blue-shift of the Fe L3 edge would indicate increased Fe electron density; a red-shift or no shift would indicate depletion or no density change. If the Fe L3 edge shows electron accumulation in FeSe while the plasmon red-shift persists, the effective-mass interpretation is supported; if the Fe L3 edge shows depletion or no change, the carrier-depletion alternative is favored and the central effective-mass claim is not established.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim's electron-side support rests on interpreting the VEELS red-shift near the FeSe/STO interface as a local enhancement of the FeSe electron effective mass. The paper itself acknowledges the ambiguity: 'The red-shifts on either side of the interface could be attributed either to local depletion in electron concentration or to local enhancement of the electron effective mass' and that 'additional experiments would be necessary to further clarify this point.' Yet the abstract states that effective mass enhancement is 'determined from the red-shift in the FeSe VEELS spectrum.' Since the effective-mass enhancement is offered as the key evidence for interfacial electron-phonon coupling, this ambiguity is load-bearing. Furthermore, the comparison with 'calculated' VEELS spectra relies on a dielectric model with an ideal step-function interface (Methods, ref. [43]); the actual interface is graded due to charge transfer, and the observed red-shift could be an interface-loss or gradient artifact rather than a change in bulk plasma frequency. Without an independent measurement of carrier density, the effective-mass interpretation is not the unique and supported conclusion.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports low-temperature, voltage-gated Raman spectroscopy and low-temperature valence electron energy loss spectroscopy (VEELS) measurements on FeTe-capped one-unit-cell (1UC) and eight-unit-cell (8UC) FeSe films grown on SrTiO3 (STO) substrates. The Raman measurements show that several FeSe vibrational modes soften symmetrically for both positive and negative backgate voltages, and the authors note that the STO substrate modes exhibit similar ambipolar softening, which they attribute to an inverse piezoelectric effect in STO that is transmitted to the FeSe layer through interfacial coupling. The VEELS line scans across the FeSe/STO interface reveal a red-shift of both the STO and FeSe plasmon peaks relative to their bulk positions, and the authors propose that this red-shift may reflect an enhancement of the FeSe electron effective mass mediated by interfacial STO phonons. The paper claims that these observations support a nonlocal interfacial coupling between STO and FeSe lattice and electronic degrees of freedom, potentially relevant to the enhanced superconductivity of monolayer FeSe on STO.","tokens_in":8576,"tokens_out":3887,"duration_ms":41671,"significance":"If the observations and their interpretation hold, the paper would provide new mode-resolved and spatially resolved spectroscopic evidence about interfacial coupling in the FeSe/STO system, a central model system for interface-enhanced superconductivity. The raw measurements appear to be new and technically challenging: gated Raman spectroscopy at 4 K on FeTe-capped monolayer FeSe, and low-temperature VEELS with spatial resolution across the interface. The analysis introduces no fitted free parameters and compares against standard dielectric-function formulas, which is a strength. The Raman softening of FeSe modes mirroring STO modes is an interesting empirical finding. However, the central electron-side conclusion, effective mass enhancement inferred from the VEELS red-shift, is explicitly acknowledged in the text to be ambiguous and not uniquely determined by the data, and the abstract overstates this result. The significance therefore rests on the Raman pillar and on the possibility that the VEELS red-shift is a genuine interfacial electronic effect; further experiments are required to disambiguate the interpretation.","major_comments":[{"comment":"The abstract and the concluding paragraph state that the FeSe electron effective mass enhancement is 'determined from the red-shift in the FeSe VEELS spectrum.' However, the main text explicitly acknowledges that the observed red-shifts 'could be attributed either to local depletion in electron concentration or to local enhancement of the electron effective mass' and that 'additional experiments would be necessary to further clarify this point.' Since the effective-mass interpretation is the electron-side evidence for interfacial electron-phonon coupling, this ambiguity is load-bearing. The paper should either soften the abstract to match the acknowledged ambiguity, or provide an independent measurement of the local carrier density (for example, core-level EELS as in Ref. [33]) to justify the effective-mass assignment.","section":"Abstract and VEELS results (Figure 3 and following paragraph)"},{"comment":"Eq. (3) is presented as determining the variation of plasma frequency 'caused by the change in electron density,' and it assumes a fixed effective mass because ΔN is proportional to ω_pe^2 - ω_pc^2 with the bare electron mass m. Yet the paper's central VEELS interpretation is a local change in effective mass, not density. The analysis therefore conflates density and effective-mass effects: the red-shift is used qualitatively to infer mass enhancement, while the formula that would quantify it assumes the mass is unchanged. This internal inconsistency needs to be addressed, for example by writing the full expression with both N(x) and m*(x) and stating what assumptions are needed to extract either quantity.","section":"Methods, Eqs. (1)-(3) and the VEELS analysis"},{"comment":"The reported FeSe peak positions are extracted from raw spectra by locating zeros and quasi-discontinuities of the first derivative (Figure 2c), and the peak-position versus gate-voltage curves in Figure 2d are presented without error bars or uncertainties from the Lorentzian fits shown in Figure S3. The ambipolar softening spans only a few cm^-1, so without uncertainty quantification it is not possible to assess whether the effect is statistically significant or to compare quantitatively the positive- and negative-bias asymmetries. The same applies to the STO mode positions in Figure S2c. Adding error bars, or at least a discussion of the measurement precision, is essential for the central Raman claim.","section":"Gated Raman spectroscopy (Figure 2 and Figure S3)"},{"comment":"The comparison between experimental and 'calculated' VEELS spectra relies on a dielectric model (Moreau et al., Ref. [43]) that assumes the FeSe/STO interface is an ideal geometrical plane with a step-function dielectric response. The authors themselves note that the actual intermediate layer cannot be sharp due to charge transfer. A graded interface can produce interface-loss features or apparent plasmon shifts that are not captured by the step-function reference calculation. Therefore, the observed red-shift could be an artifact of the graded interface rather than a change in the bulk plasma frequency of FeSe. To support the claim that the red-shift is an intrinsic FeSe property, the analysis should compare against a graded-interface model or provide an independent local probe of the carrier density and dielectric profile.","section":"VEELS comparison with calculated spectra (Methods, Ref. [43])"}],"minor_comments":[{"comment":"The symbol δε0 is used in Eq. (1) but appears as δ0 in the OCR text; please ensure the notation is consistent and define all symbols in the text surrounding the equation.","section":"Methods, Eq. (1)"},{"comment":"The phrase 'the absence of hole pockets in intercalated or 1UC FeSe' is stated without qualification. Some ARPES studies have reported small hole-like bands in related systems; a more nuanced formulation with citations would avoid overgeneralization.","section":"Introduction, first paragraph"},{"comment":"The temperature-dependent Raman peak positions for FeSe and FeTe modes are plotted only in the lower panels of Figure S1, but the main text refers to them in a way that suggests a quantitative discussion; please state explicitly what change with temperature is observed and how it compares with bulk values.","section":"Figure 1 and Figure S1"},{"comment":"The transport data are reproduced from Ref. [S1] without a description of the measurement uncertainty or the criteria used to define Tc. A brief statement about how Tc was determined (onset, midpoint, zero resistance) would improve the reproducibility of the superconductivity claim.","section":"Transport section (Figure S5)"},{"comment":"The paper uses '1UC' and '8UC' without defining 'UC' in the main text; please spell out 'unit cell' at first use for readers outside the immediate field.","section":"General notation"}],"recommendation":"major_revision","confidential_remarks":"The manuscript reports interesting and novel measurements, but the central VEELS interpretation is explicitly ambiguous and the Raman analysis lacks uncertainty quantification. The abstract overstates the effective-mass conclusion relative to the text's own caveats. These issues are fixable in revision if the authors either provide additional data (e.g., core-level EELS, error bars, graded-interface modeling) or carefully restrict the claims. I do not see a need for rejection at this stage; the raw observations are valuable and the Raman finding, if confirmed with error bars, could stand on its own. However, the authors should ensure that the abstract and conclusions accurately reflect the level of support that the data provide."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here is my read.\n\nThe gated Raman half of the paper is the real contribution. Low-temperature voltage-gated Raman on FeTe-capped 1UC FeSe/STO shows ambipolar softening of FeSe modes that tracks the STO substrate modes, while FeTe modes barely move. The appearance of the STO \"silent\" 261 cm-1 mode under bias, the mirroring of the STO strain/softening behavior, and the qualitative reproducibility of the spectra make a credible case that the substrate influences FeSe lattice dynamics. The VEELS experiment is also new and well executed at the level of observation: low-temperature line scans show a red-shift of both FeSe and STO plasmons within about a nanometer of the interface, opposite to the simple dielectric calculation. No fitted parameters are used in the central analysis, and the transport confirmation is clearly labeled as reproduced from their own earlier work [S1]. Citation pattern looks fine.\n\nThe soft spot is exactly where the stress-test note and the body text put it. The red-shift is compatible with local electron depletion as well as with effective mass enhancement. The paper says this explicitly in the body, but the abstract says the effective mass enhancement is \"determined\" from the red-shift. That overstates the case, and since the effective-mass enhancement is offered as the electron-side support for interfacial phonon coupling, the ambiguity is load-bearing. A core-level EELS cross-check or an independent carrier-density probe would settle it. Also, no error bars are given for peak positions or plasmon frequencies; the trends look systematic, but modest asymmetries, like the B1g difference between positive and negative bias, need error bars before being leaned on. Finally, the calculated spectrum uses an ideal step-function interface, while the real interface is graded by charge transfer; the red-shift is exactly where a depletion layer would sit, so an interface-loss or gradient artifact is not fully excluded. That is not fatal, but it should be addressed quantitatively.\n\nBottom line: the paper deserves peer review. The Raman result carries it, and the VEELS result is a suggestive, explicitly hedged observation that would strengthen with one more measurement and an honest abstract. I would not desk-reject it.","headline":"The gated Raman data are the real new result; the VEELS effective-mass claim is an acknowledged ambiguity that the abstract overstates.","tokens_in":9164,"tokens_out":3294,"would_cite":true,"duration_ms":30619,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A SrTiO3 substrate nonlocally alters the phonons and the effective electron mass of a superconducting FeSe film within about one nanometer of the interface.","keywords":["Gated Raman spectroscopy","Valence electron energy loss spectroscopy","One unit-cell FeSe on SrTiO3","Ambipolar phonon softening","Interfacial electron-phonon coupling","Plasmon red-shift","Effective mass enhancement","Superconducting heterostructures"],"falsifier":"Measure the local carrier density across the FeSe/STO interface independently of the plasmon frequency, for example with spatially resolved Hall or quantum-oscillation measurements on patterned one-unit-cell devices, and compare the density profile with the VEELS red-shift. If the red-shift persists where the density is flat, the effective-mass enhancement is supported; if the red-shift tracks a density dip, the red-shift is electron depletion and the central electron-side claim fails.","tokens_in":8230,"feed_emoji":"🔬","tokens_out":8077,"duration_ms":73110,"temperature":0.7,"pith_summary":"This paper tries to show that the SrTiO3 (STO) substrate does not passively host the one-unit-cell FeSe superconductor: it actively couples to the FeSe lattice and electrons across the interface. The authors use back-gated Raman spectroscopy at 4 K to show that FeSe vibrational modes soften symmetrically for both positive and negative gate voltages, mimicking the STO substrate's own modes, which they read as a nonlocal interfacial coupling that affects FeSe lattice dynamics. They add valence electron energy loss spectroscopy (VEELS) showing an unexpected red-shift of the FeSe plasmon peak within roughly one nanometer of the interface, which they interpret as a possible enhancement of the FeSe electron effective mass through interaction with STO phonons. If true, the effect lives in exactly the one to two atomic layers believed to host the enhanced superconductivity, giving a concrete spatial and phononic mechanism for the interface's role.","feed_headline":"SrTiO3 bends FeSe phonons one nanometer deep","feed_subtitle":"Gated Raman and electron-loss maps tie the interface's one-nanometer reach to enhanced superconductivity.","key_machinery":"The load-bearing mechanism is ambipolar phonon softening: the frequency of a Raman-active phonon decreases for both electron and hole doping by a gate voltage, which the paper observes in FeSe modes and in STO modes and attributes to interlayer coupling rather than to doping alone. The second leg is the spatial map of the valence plasmon, whose frequency depends on the ratio of electron density to effective mass; the observed red-shift near the interface is the observable that would reveal an effective-mass enhancement if the density is not depleted. Together the two spectroscopies tie a phonon-mediated interfacial effect to the superconducting layers.","core_discovery":"The central claim is that the STO substrate exerts a nonlocal interfacial coupling on FeSe: voltage-gated Raman shows the FeSe Eg, A1g, and B1g phonons soften ambipolarly with gate voltage just as the STO modes do, while FeTe modes in the same stack remain almost unaffected; and VEELS shows both STO and FeSe plasmon peaks red-shift as the interface is approached, opposite to the slight blue-shift expected from dielectric theory. The authors propose that the FeSe red-shift is at least partly a local enhancement of the FeSe electron effective mass mediated by STO phonons, confined to a roughly one-nanometer region comparable to the first two FeSe layers. They present this as evidence that interfacial electron-phonon coupling, not just charge transfer, contributes to the enhanced superconductivity of 1UC FeSe/STO.","pith_inferences":["Inference: applying the same two-probe protocol to FeSe on other polar oxide substrates would test whether the nonlocal coupling is a general property of such interfaces or specific to STO.","Inference: a density-resolved measurement, for example local Hall or quantum oscillations, could separate the two explanations of the red-shift; the paper's own interpretation predicts the red-shift survives with density held constant.","Inference: because the STO modes themselves soften under gate voltage via piezoelectric strain, a control experiment on a non-piezoelectric substrate would reveal how much of the FeSe softening is strain-mediated rather than electronically mediated.","Inference: if the coupling is indeed confined to the first layer, thickness-dependent Tc studies should show a crossover dominated by the interface layer rather than a uniform enhancement across the film."],"forward_implications":["The STO substrate actively participates in FeSe lattice dynamics, so models that treat it as an inert template omit a relevant interaction.","The interfacial response is confined to roughly one nanometer, matching the FeSe thickness range that thickness-dependent studies associate with superconductivity; the coupling is therefore localized in the layers that matter for Tc.","A positive backgate slightly strengthens the FeSe B1g softening, indicating that the interfacial coupling can be tuned by drawing electrons to the FeSe/STO interface.","The observed red-shift, if interpreted as effective-mass enhancement, provides a microscopic route by which STO phonons could raise the superconducting transition temperature of 1UC FeSe.","Low-temperature gated Raman and VEELS can be applied to other heterostructures to reveal similar nonlocal substrate couplings."],"supporting_citations":[{"why":"establishes the 20 meV superconducting gap in 1UC FeSe/STO that motivates the search for an interfacial mechanism.","marker":"[21]"},{"why":"supplies the FeSe Eg mode assignment used to identify the FeSe Raman peak at 104 cm^-1.","marker":"[28]"},{"why":"assigns the FeSe A1g and B1g modes whose gate-driven softening is a central observation.","marker":"[29]"},{"why":"provides prior evidence for electron coupling to STO phonons, which the Raman result extends to FeSe lattice dynamics.","marker":"[30]"},{"why":"shows that substrate templating stabilizes FeSe and favors enhanced electron-phonon coupling, supporting the coupling interpretation.","marker":"[32]"},{"why":"reports the positive-backgate effect and the core-level EELS blue-shift that the VEELS red-shift is compared against.","marker":"[33]"},{"why":"supplies the dielectric-theory formalism used to compute the expected plasmon peak positions that the red-shift contradicts.","marker":"[43]"},{"why":"identifies the 1–2 unit-cell thickness range hosting superconductivity, which matches the ~1 nm interfacial region observed.","marker":"[18]"}],"fun_headline_variants":["STO tweaks FeSe phonons a nanometer deep","Why FeSe superconducts: STO's one-nanometer touch","FeSe phonons mimic STO, revealing interfacial coupling","Ambipolar phonon softening links FeSe to STO","One-nanometer coupling boosts FeSe superconductivity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the red-shift of the FeSe valence-electron-loss peak near the interface reflects a heavier electron effective mass, not a drop in local electron density; the paper itself says additional experiments are needed to tell these apart, and if the shift is depletion the electron-side evidence for interfacial phonon coupling collapses.","fun_headline_variants_meta":{"raw":{"variants":["STO tweaks FeSe phonons a nanometer deep","Why FeSe superconducts: STO's one-nanometer touch","FeSe phonons mimic STO, revealing interfacial coupling","Ambipolar phonon softening links FeSe to STO","One-nanometer coupling boosts FeSe superconductivity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000753,"raw_usage":{"total_tokens":3332,"prompt_tokens":911,"completion_tokens":2421,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":527,"completion_tokens_details":{"reasoning_tokens":2352}},"tokens_in":527,"tokens_out":2421,"duration_ms":15977,"temperature":1.0,"reasoning_tokens":2352,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:07:23.027974+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the local carrier density across the FeSe/STO interface independently of the plasmon frequency, for example with spatially resolved Hall or quantum-oscillation measurements on patterned one-unit-cell devices, and compare the density profile with the VEELS red-shift. If the red-shift persists where the density is flat, the effective-mass enhancement is supported; if the red-shift tracks a density dip, the red-shift is electron depletion and the central electron-side claim fails.","supporting_citations":[{"cited_title":"Wang et al., Chinese Physics Letters 29, 037402 (2012)","cited_arxiv_id":null,"evidence_quote":"establishes the 20 meV superconducting gap in 1UC FeSe/STO that motivates the search for an interfacial mechanism."},{"cited_title":"Kumar et al., Solid State Communications 150, 557 (2010)","cited_arxiv_id":null,"evidence_quote":"supplies the FeSe Eg mode assignment used to identify the FeSe Raman peak at 104 cm^-1."},{"cited_title":"Gnezdilov et al., Phys","cited_arxiv_id":null,"evidence_quote":"assigns the FeSe A1g and B1g modes whose gate-driven softening is a central observation."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"shows that substrate templating stabilizes FeSe and favors enhanced electron-phonon coupling, supporting the coupling interpretation."},{"cited_title":"Zhao et al., Science Advances 4 (2018), 10.1126/sciadv.aao2682","cited_arxiv_id":null,"evidence_quote":"reports the positive-backgate effect and the core-level EELS blue-shift that the VEELS red-shift is compared against."},{"cited_title":"Moreau, N","cited_arxiv_id":null,"evidence_quote":"supplies the dielectric-theory formalism used to compute the expected plasmon peak positions that the red-shift contradicts."},{"cited_title":"Tan et al., Nature Materials 12, 634 (2013)","cited_arxiv_id":null,"evidence_quote":"identifies the 1–2 unit-cell thickness range hosting superconductivity, which matches the ~1 nm interfacial region observed."}],"review_version":1}