{"id":"2bb86620-867b-4907-9730-e075b1c91926","arxiv_id":"2501.15800","paper_version":1,"verdict":"REJECT","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"high","formal_verification":"none","parameter_count":5,"one_line_summary":"X-ray photoemission of LSMO/YBCO films shows core-level shifts and enhanced Fermi-level intensity, which the authors attribute to interfacial hole transfer overdoping YBCO.","lead":"This paper grows superconducting YBCO films on a magnetic manganite layer and uses X-ray photoemission to argue that electrons move across the interface, leaving YBCO more hole-doped than usual. The work suggests oxide interfaces as a route to overdoped cuprates, but the measurement geometry weakens the central claim.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"XPS probe depth is ~5 nm while the claimed hole transfer is at an interface 500 nm below the surface; no depth-resolved or interface-sensitive data connects the measured surface spectral shifts to overdoped bulk YBCO.","rationale":"The reader's weakest assumption identifies the same load-bearing concern: the surface-sensitive XPS measurements are performed on a 500 nm YBCO cap, while the claimed doping mechanism operates at the buried LSMO/YBCO interface. For the central claim to hold, the measured electronic-structure changes must be directly attributable to interfacial hole transfer. The paper's own discussion of surface oxygen effects on Y 3d, Cu 2p, and O 1s spectra makes a surface-stoichiometry artifact a serious alternative explanation, and the lack of any depth profile or interface-specific probe leaves the conclusion unsupported. The REJECT verdict is therefore appropriate: the reported spectral shifts are real observations, but they do not by themselves demonstrate overdoped bulk or interface YBCO. A cross-sectional STEM-EELS measurement would settle the question by directly probing the interface region with nm-scale spatial resolution. If that experiment confirms interfacial hole accumulation, the work could be revised with additional evidence; as written, the central claim is not established.","tokens_in":7871,"tokens_out":6490,"duration_ms":70540,"concrete_test":"Perform a cross-sectional STEM-EELS line scan across the LSMO/YBCO interface on the same films, recording O K-edge and Cu L2,3-edge fine structure from the YBCO region within 5–20 nm of the interface and at increasing distances up to the surface. Compare with identical scans on a single-layer YBCO film and with the XPS shifts. If the interfacial YBCO shows the same 0.6 eV-like core-level/chemical shift and increased Cu 3d hole signal, the overdoping claim is supported; if the interface region is indistinguishable from bulk YBCO, the surface XPS differences are not interfacial charge transfer.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that the LSMO/YBCO bilayer is overdoped YBCO via hole transfer across the buried interface. All the supporting spectral evidence, however, comes from XPS of the free surface of a 500 nm YBCO cap layer. With monochromatic Al Kα radiation and the stated 25–45° emission angles, the information depth is only a few nanometers; even 'normal emission' probes the top ≈2–10 nm, not the interface 500 nm away. The paper never bridges this gap. The comparison is between two separately grown film surfaces (YBCO/STO and YBCO/LSMO/STO), so a different surface termination, excess surface oxygen (which the authors explicitly identify as affecting Y 3d and Cu 2p spectra), or a different surface strain state could equally produce the observed 0.6 eV core-level shifts and enhanced εF intensity. No quantitative doping measurement (e.g., Hall coefficient, c-axis lattice parameter, or oxygen stoichiometry) is provided to support 'overdoped.' Consequently, the assertion that the interface transfers holes into YBCO while keeping Cu-O chains and Cu-O planes intact is not established by the data as presented. This is an internal gap between the observable (surface XPS) and the conclusion (buried interface charge transfer), not a disagreement with consensus.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports x-ray photoemission spectroscopy (XPS) measurements on high-quality, single-crystalline YBCO films grown on STO and on LSMO-buffered STO, with the YBCO layer approximately 500 nm thick in both cases. The authors observe a roughly 0.6 eV rigid shift of O 1s, Ba 4d, and Y 3d core levels toward the Fermi level in the LSMO/YBCO sample, enhanced valence-band intensity at the Fermi level, and modified Cu 2p lineshapes. They interpret these observations as a Fermi-level shift caused by hole transfer across the buried YBCO/LSMO interface, and conclude that the heterostructure realizes an overdoped YBCO state while keeping the Cu-O chains and planes intact. The paper also cites a previously reported reduction of Tc from 89 K to 86 K in the bilayer as supporting evidence. Structural quality is documented by pole figures, reciprocal-space maps, and XRD, and the authors acknowledge that the temperature dependence of the O 1s spectra is puzzling and requires further study.","tokens_in":8149,"tokens_out":6750,"duration_ms":70161,"significance":"If established, the claim would be significant: it would offer a route to overdoped YBCO without changing oxygen stoichiometry and would open a new doping axis for studying cuprate superconductivity. The paper has clear strengths: the films are structurally characterized, the XPS data are presented at several temperatures and emission angles, the core-level shifts are raw observations rather than fit outputs, and the authors are candid about unexplained spectral evolution. However, the central claim is not supported by the present data. The XPS measurements probe the top few nanometers of a free YBCO surface, while the alleged hole transfer occurs at an interface buried roughly 500 nm below that surface; the comparison is between two separately grown film surfaces for which surface termination and oxygen content may differ. No bulk-sensitive or interface-sensitive measurement connects the surface spectra to interfacial charge transfer. The quantitative Cu 2p analysis also depends on assumed model parameters and multi-peak fits. The central result therefore remains unestablished by the evidence presented.","major_comments":[{"comment":"The YBCO layer is about 500 nm thick in both samples, and the Al Kα XPS measurements with 25-45 degree emission angles probe only the top few nanometers of the film surface. The measured O 1s, Ba 4d, Y 3d, valence-band, and Cu 2p spectra therefore sample the free YBCO surface, not the YBCO/LSMO interface located roughly 500 nm below. The angular dependence changes the surface-to-bulk weight within the near-surface region but does not reach the interface. The paper provides no depth-resolved or interface-sensitive measurement (for example, a YBCO thickness series, hard X-ray photoemission, cross-sectional EELS, or resonant standing-wave experiment) and no bulk-sensitive doping probe (Hall coefficient, c-axis lattice parameter, or oxygen stoichiometry) that ties the surface observations to hole transfer at the buried interface. Without such a bridge, the central claim that the bilayer is overdoped via interfacial charge transfer is not established.","section":"Growth and experimental methods; Figs. 2-5"},{"comment":"The comparison is between two separately grown film surfaces, and the authors themselves show that surface chemistry differs between them: the 152.5 eV Y 3d feature is attributed to excess surface oxygen and disappears at 120 K, and the narrow 932.5 eV Cu 2p feature at 300 K is attributed to Cu3+ contributions from excess surface oxygen. Differences in surface termination, oxygen overlayer, surface strain, or surface band bending between YBCO/STO and STO/LSMO/YBCO could produce rigid core-level shifts and enhanced near-Fermi-level intensity without any bulk or interface charge transfer. The attribution of the uniform 0.6 eV shift to a Fermi-level shift therefore requires control measurements that exclude these chemical and electrostatic alternatives; the paper does not provide them.","section":"Y 3d and Cu 2p analysis; Figs. 3 and 5"},{"comment":"The quantitative inference that LSMO/YBCO has a larger density of nonlocal ligand holes and a larger 3d9 weight rests on the integrated areas of the fitted A-E peaks and on assumed model parameters tpp = 0.5 eV, t = 1.5 eV, and Udc = 6-8 eV. The paper reports no uncertainties in the fit areas, no variation over the allowed parameter range, and no sensitivity analysis for background subtraction and peak assignment. The reported changes are modest (A/B ratio 0.475 vs 0.55; Ir 0.38 vs 0.47), so the doping conclusion from the Cu 2p analysis is model-dependent even though the raw lineshape differences may be real.","section":"Cu 2p fits and model; Eqs. (1)-(3) and Fig. 5(c)-(d)"},{"comment":"The reduction of Tc from 89 K to 86 K, quoted from a prior publication, is used as supporting evidence for overdoping. However, proximity to a ferromagnetic LSMO layer, interface disorder, interfacial strain, and changes at the YBCO surface can all suppress Tc in YBCO/LSMO heterostructures without producing bulk overdoping. This observation therefore does not discriminate between hole transfer and other proximity effects, and it cannot compensate for the absence of a direct hole-concentration measurement in the YBCO layer.","section":"Summary and use of Tc from Ref. [11]"}],"minor_comments":[{"comment":"The abstract contains 'plains' where 'planes' is intended, and the Introduction uses 'it's heterostructure' where 'its heterostructure' is intended. These should be corrected.","section":"Abstract and Introduction"},{"comment":"The energy-scale calibration and the way in which binding energies are referenced to the Fermi level are not described; this matters because a common rigid shift of all core levels can also arise from sample charging or surface band bending.","section":"Experimental section"},{"comment":"The caption states 'The line in (a) represents LSMO/YBCO NE data' but does not identify the line style in the figure; the figure should be self-contained for the reader.","section":"Fig. 4 caption"},{"comment":"The paper acknowledges that the temperature evolution of the O 1s spectra is puzzling, yet the central Cu 2p fits are performed on 35 K data. The unexplained temperature dependence should be resolved or at least explicitly stated as a caveat before the 35 K fits are used as the basis for quantitative doping claims.","section":"Conclusions"}],"recommendation":"reject","confidential_remarks":"This is a surface XPS study whose central claim concerns a deeply buried interface. The mismatch between the probe depth and the interface location is not a presentation issue; closing the gap would require new experiments such as a YBCO thickness series, interface-sensitive spectroscopy, or a direct bulk doping measurement. I would be willing to reconsider a substantially revised version containing such evidence, but on the present data the central conclusion is not supported."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Punchline: the paper reports careful XPS data on 500 nm YBCO films grown on STO and on LSMO/STO, and the observed shifts and Fermi-level intensity changes are real. But the dressed-up conclusion—that the bilayer is an overdoped YBCO produced by hole transfer across the buried LSMO/YBCO interface—does not follow from these measurements. With Al Kα and 25–45° emission, you are looking at the top few nanometers of the free YBCO surface, about 500 nm away from the interface. No depth-resolved data, no thinning, no cross-section, and no transport or bulk doping probe connects the surface spectra to the interface.\n\nWhat is good: the films are well characterized (XRD, pole figures, RSM, and the prior transport/magnetization work in ref. 11), and the core-level fits are detailed. The observation that the surface/bulk distinction changes in the bilayer is interesting. The Cu 2p satellite/main analysis is a real attempt to quantify ligand holes, even if it leans on assumed tpp, t, Udc values. If the authors repositioned the paper as a surface-sensitive comparison of two YBCO film surfaces, it would be a solid contribution.\n\nSoft spots: (1) The 0.6 eV core-level shift in all levels is attributed to a Fermi-level shift, but a different surface termination, surface oxygen content (the authors themselves discuss excess surface oxygen affecting Y 3d and Cu 2p), or strain state between the two separately grown films could produce the same. (2) No quantitative doping measurement—no Hall, c-axis parameter, or oxygen stoichiometry—supports 'overdoped'. (3) The argument that holes transferred at the interface would affect the surface of a 500 nm film is physically implausible without a mechanism; interface charge transfer is typically confined to a few nm. (4) The Cu 2p fit parameters (tpp=0.5, t=1.5, Udc=6-8 eV) are assumed, not measured, so the inferred ligand-hole enhancement is model-dependent.\n\nThe paper is honest about puzzling temperature behavior and does not hide the surface-bulk complexity, but the central claim overreaches. Who is this for: specialists in cuprate heterostructures who want to see what XPS reveals on these bilayers. It deserves a serious referee because the fabrication and measurement are non-trivial and the interpretation, while flawed, is worth a rigorous exchange. My recommendation: send to peer review, but the referee should require either interface-sensitive evidence (HAXPES, thin YBCO layers, or a depth profile) or a softened claim limited to surface electronic structure differences.","headline":"A careful XPS study whose central claim outruns the probe depth: the measured surface spectra of a 500 nm YBCO cap cannot see the buried LSMO/YBCO interface, so the 'overdoped YBCO via interfacial hole transfer' conclusion is unsupported.","tokens_in":8755,"tokens_out":3026,"would_cite":false,"duration_ms":29806,"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":"This paper claims that growing YBCO on a metallic manganite underlayer transfers holes into the YBCO film, producing an overdoped cuprate without adding oxygen or changing the crystal structure.","keywords":["YBCO","LSMO","overdoped cuprate","heterostructure","charge transfer","x-ray photoemission","Cu 2p core level","high-temperature superconductivity"],"falsifier":"Perform a depth-resolved photoemission or a thickness series of YBCO on fixed LSMO (e.g., 10 nm, 50 nm, 500 nm): if the 0.6 eV core-level shift and Fermi-level enhancement do not grow or scale with proximity to the LSMO interface, the overdoping is not interfacial. Alternatively, controlled removal of surface layers before XPS would show whether the shifted spectra originate at the interface rather than at the free surface.","tokens_in":7646,"feed_emoji":"🔬","tokens_out":7025,"duration_ms":58922,"temperature":0.7,"pith_summary":"The paper tries to establish that a bilayer of La0.67Sr0.33MnO3 (LSMO) and YBa2Cu3O7 (YBCO) pushes the YBCO layer into an overdoped state by transferring holes across the interface. The evidence comes from x-ray photoemission: the O 1s, Ba 4d, and Y 3d core levels all shift about 0.6 eV toward the Fermi level, the valence-band intensity at the Fermi level grows, and a Cu 2p lineshape analysis indicates more holes on the copper sites. This matters because fully oxygenated YBCO is already slightly overdoped and cannot be doped further by adding oxygen, since the Cu-O chains are full; an interface route would open a new way to reach the overdoped regime and study the exotic physics there. The paper also notes that superconductivity survives in the bilayer with a slightly reduced $T_c$, which it calls puzzling.","feed_headline":"Magnetic underlayer overdopes YBCO without changing its structure","feed_subtitle":"Core-level shifts and added Fermi-level weight point to hole transfer across the interface.","key_machinery":"The central object is the YBCO/LSMO interface as a site of hole transfer, read through core-level and valence-band photoemission. The quantitative workhorse is a cluster model of the CuO$_4$ plaquette in which the energy gap between the main Cu 2p line and its satellite is $\\Delta E = \\sqrt{(\\Delta - U_{dc} + 2t_{pp})^2 + 4t^2}$ and the satellite-to-main intensity ratio is $I_r = \\tan^2(\\theta-\\phi)$, with $\\Delta$ the charge-transfer energy, $U_{dc}$ the Cu 2p-3d Coulomb interaction, and $t$ the Cu-O hopping. Fitting the Cu 2p spectra to this model shows $I_r$ rising from 0.38 in YBCO to 0.47 in the bilayer, which the paper takes as quantitative evidence that more holes sit on the Cu sites.","core_discovery":"On its own terms, the paper claims that the LSMO/YBCO heterostructure is \"a unique case of overdoped YBCO keeping Cu-O chains and Cu-O planes intact.\" The core-level spectra of the bilayer shift uniformly by about 0.6 eV toward the Fermi level relative to bare YBCO, the spectral weight at the Fermi level is significantly enhanced, and a fit of the Cu 2p spectra with a cluster model gives a larger satellite-to-main intensity ratio and a larger density of delocalized ligand holes, which the authors interpret as an increase in the Cu 3d hole content of the Cu-O planes. That increase in hole concentration, without any change in oxygen content or lattice structure, is the overdoping claim. The paper also reports that the bilayer remains superconducting with $T_c \\approx 86$ K, about 3 K lower than the bare YBCO film.","pith_inferences":["An unstated testable consequence is that the amount of transferred charge should depend on the LSMO thickness or its surface termination; a thickness series would separate interfacial charge transfer from substrate strain effects.","Because the 500 nm YBCO top layer is far thicker than the XPS probe depth, the paper's surface spectra may reflect the free-surface electronic structure rather than the interface; depth-resolved or cross-section measurements would directly test the attribution.","A similar bilayer with a metallic but non-magnetic underlayer could show whether the magnetic ordering of LSMO is essential or incidental to the hole transfer."],"forward_implications":["If the claim is right, overdoped YBCO can be made without changing oxygen stoichiometry or breaking the Cu-O chains, bypassing the usual doping bottleneck.","The drop in $T_c$ from 89 K in YBCO to 86 K in the bilayer is consistent with the YBCO layer moving further past optimal doping, as expected for added holes.","The same interface-charge-transfer mechanism could be used to tune doping in other cuprates that cannot be overdoped by oxygenation alone.","The simultaneous presence of superconductivity and ferromagnetism in the bilayer suggests the interface hosts competing orders, a regime the paper presents as newly accessible."],"supporting_citations":[{"why":"Supplies the film growth details, interface sharpness, and the measured $T_c$ values (89 K for YBCO, 86 K for the bilayer).","marker":"[11]"},{"why":"Establishes that YBCO doping is normally tuned through growth conditions and that oxygen content cannot readily go beyond x = 7.","marker":"[18]"},{"why":"Provides the valence-band satellite interpretation for cuprates used to read the 8.2 eV feature.","marker":"[19]"},{"why":"X-ray standing wave study showing that near-$\\epsilon_F$ intensity in YBCO is dominated by Cu1 chain contributions, used to interpret the valence band.","marker":"[20]"},{"why":"Documents charge transfer across oxide interfaces, the mechanism invoked for hole transfer into YBCO.","marker":"[22]"},{"why":"Gives hard x-ray photoemission binding energies and assignments for YBCO core levels used in the spectral fits.","marker":"[23]"},{"why":"Provides the Cu 2p lineshape components (localized versus delocalized ligand holes) used to decompose the main peak.","marker":"[30]"},{"why":"Supplies the reported main-to-satellite energy separation of about 8.5 eV that anchors the cluster-model parameters.","marker":"[33]"},{"why":"Defines the charge-transfer regime of the Zaanen-Sawatzky-Allen diagram in which the paper places YBCO.","marker":"[34]"}],"fun_headline_variants":["Charge transfer overdopes YBCO in LSMO bilayer","Magnetic layer dopes YBCO without changing lattice","Overdoped YBCO via interface charge transfer","LSMO hands holes to YBCO, keeping structure intact","Interface doping pushes YBCO further into overdoped regime"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The X-ray photoemission signal comes from the top few nanometres of a 500 nm YBCO film, while the claimed hole transfer happens at the buried interface 500 nm deeper; the paper assumes the surface electronic structure reflects that interfacial process.","fun_headline_variants_meta":{"raw":{"variants":["Charge transfer overdopes YBCO in LSMO bilayer","Magnetic layer dopes YBCO without changing lattice","Overdoped YBCO via interface charge transfer","LSMO hands holes to YBCO, keeping structure intact","Interface doping pushes YBCO further into overdoped regime"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000168,"raw_usage":{"total_tokens":1240,"prompt_tokens":905,"completion_tokens":335,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":521,"completion_tokens_details":{"reasoning_tokens":254}},"tokens_in":521,"tokens_out":335,"duration_ms":3635,"temperature":1.0,"reasoning_tokens":254,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T13:56:02.174215+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Perform a depth-resolved photoemission or a thickness series of YBCO on fixed LSMO (e.g., 10 nm, 50 nm, 500 nm): if the 0.6 eV core-level shift and Fermi-level enhancement do not grow or scale with proximity to the LSMO interface, the overdoping is not interfacial. Alternatively, controlled removal of surface layers before XPS would show whether the shifted spectra originate at the interface rather than at the free surface.","supporting_citations":[{"cited_title":"Singh et al., SciPost Phys","cited_arxiv_id":null,"evidence_quote":"Supplies the film growth details, interface sharpness, and the measured $T_c$ values (89 K for YBCO, 86 K for the bilayer)."},{"cited_title":"Arpaia et al., Phys","cited_arxiv_id":null,"evidence_quote":"Establishes that YBCO doping is normally tuned through growth conditions and that oxygen content cannot readily go beyond x = 7."},{"cited_title":"Maiti, D","cited_arxiv_id":null,"evidence_quote":"Provides the valence-band satellite interpretation for cuprates used to read the 8.2 eV feature."},{"cited_title":"Cu 2 contributions appear essentially around 3 eV binding energies","cited_arxiv_id":null,"evidence_quote":"X-ray standing wave study showing that near-$\\epsilon_F$ intensity in YBCO is dominated by Cu1 chain contributions, used to interpret the valence band."},{"cited_title":"Thiess et al., Phys","cited_arxiv_id":null,"evidence_quote":"Documents charge transfer across oxide interfaces, the mechanism invoked for hole transfer into YBCO."},{"cited_title":"Maiti et al., Phys","cited_arxiv_id":null,"evidence_quote":"Gives hard x-ray photoemission binding energies and assignments for YBCO core levels used in the spectral fits."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the Cu 2p lineshape components (localized versus delocalized ligand holes) used to decompose the main peak."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the reported main-to-satellite energy separation of about 8.5 eV that anchors the cluster-model parameters."},{"cited_title":"Steiner et al., Z","cited_arxiv_id":null,"evidence_quote":"Defines the charge-transfer regime of the Zaanen-Sawatzky-Allen diagram in which the paper places YBCO."}],"review_version":1}