{"id":"7da3f259-3f7b-4665-97df-1dc26468bd57","arxiv_id":"2608.11707","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Thickness-driven loss of superconductivity in (Cu,C)-1234 films is partially reversed by proximity coupling to YBCO, with recovery strongest for 18 nm and suppressed near 1.2 nm.","lead":"This paper reports that ultrathin films of the cuprate superconductor (Cu,C)-1234 become insulating below about 18 nm thickness, but regain superconductivity when layered on a thicker YBCO film. A generalist might read it because it tests whether a neighboring superconductor can restore phase coherence in a disordered, insulating cuprate.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The interface-circuit measurements lack a control against parallel conduction through the underlying YBCO film; without a top-layer-removed or YBCO-normal reference, S3/S4 zero resistance cannot be attributed to proximity-induced superconductivity in the (Cu,C)-1234 layer.","rationale":"The reader correctly identifies shunting as the load-bearing concern; my reading agrees. The internal evidence often cited against shunting—the interface Tc0 (62.3 K for S3, 18.7 K for S4) lying below the sole-YBCO Tc0 (78 K)—is not conclusive. The sole-YBCO value is measured on a separate uncoated region; the YBCO under the top layer could have a lower Tc due to oxygen loss or interface damage, and no local measurement is reported. In the shunting scenario, zero voltage on top contacts is expected even if the top layer is an insulator. Therefore the central claim that the proximity effect restores phase coherence in (Cu,C)-1234 is not yet secured. A control with an insulating top layer is the decisive experiment. Because the paper's other observations (thickness-driven SIT in isolated films, structural characterization) are internally consistent and the claim is testable, the appropriate verdict remains conditional, matching the reader's assessment.","tokens_in":9958,"tokens_out":5637,"duration_ms":58557,"concrete_test":"Fabricate a control heterostructure identical to S3/S4 except replace the (Cu,C)-1234 top layer with an insulating, nonsuperconducting layer of similar thickness (e.g., amorphous LAO or STO), using the same contact layout and shadow-mask geometry. Measure the same 'interface' four-probe R-T curve. If the control shows zero resistance below the YBCO transition, the S3/S4 zero-resistance states are explained by YBCO shunting and the central claim fails; if the control stays resistive to base temperature while S3/S4 show zero resistance, the shunting objection is answered.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim (Section 3.3, Fig. 4) is that the 'interface' transport circuit shows restored superconductivity in (Cu,C)-1234 for S3 (18 nm) and S4 (~1.2 nm), which are insulating when isolated. The load-bearing premise is that this circuit actually forces current through the top layer and the interface. The paper says 'To avoid possible parallel conduction [20] and ensure the current flows through the interface, the process in Fig. 1 was taken,' but the shadow-mask procedure only creates separate regions; it does not eliminate the conducting YBCO layer underneath. In the four-probe geometry with contacts on the top layer, if the top layer is highly resistive, the current path is: top contact -> interface -> YBCO -> interface -> other top contact. The voltage probes on the top layer then measure essentially the YBCO potential (zero whenever YBCO is superconducting, since the voltmeter draws negligible current), regardless of whether the top layer is superconducting. The observed Tc0 values (62.3 K and 18.7 K) being below the sole-YBCO value of 78 K do not rule this out, because the YBCO underneath the top layer may be locally degraded during top-layer growth and was not measured separately. No control with the top layer removed, with an insulating non-superconducting top layer, or with YBCO driven normal is provided. Thus the S3/S4 zero-resistance states are also fully consistent with YBCO shunting.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports a thickness-driven superconductor-insulator transition (SIT) in isolated (Cu,C)-1234 films and claims that superconductivity is re-established in (Cu,C)-1234 when it is coupled to a YBCO bottom layer in a shadow-masked heterostructure. The authors grow YBCO (150 nm) and (Cu,C)-1234 top layers with nominal thicknesses 188, 87, 18, and ~1.2 nm, and measure four-probe resistance of the isolated films and of a circuit intended to pass current through the (Cu,C)-1234/YBCO interface. Isolated films show a SIT between 87 nm and 18 nm, with the 18 nm film insulating. In the interface circuit, samples S1-S3 show zero resistance with Tc0 = 67.5, 60.0, and 62.3 K, while S4 shows a much lower Tc0 of 18.7 K and a negative temperature coefficient. The authors interpret the S3/S4 behavior as superconducting proximity recovery in an insulating cuprate, with thickness-dependent disorder and confined pairing explaining the suppression at ~1.2 nm.","tokens_in":10177,"tokens_out":6101,"duration_ms":63059,"significance":"If the central claim is correct, this is a valuable experimental contribution: it would demonstrate that a strongly disordered, phase-incoherent cuprate can regain long-range phase coherence through proximity coupling to a robust superconductor, and it introduces a cuprate S-S' heterostructure platform with a thickness-tunable crossover. The isolated-film SIT in (Cu,C)-1234 is itself an interesting result, and the simultaneous growth of component films and heterostructure regions on one substrate is a good experimental design. The structural characterization (XRD, SEM, HAADF-STEM/FFT) is a genuine strength. However, the proximity-recovery claim currently rests on an unverified assumption about current path in the interface circuit, and the thinnest-sample evidence is incomplete.","major_comments":[{"comment":"The central claim that S3 and S4 show superconductivity re-established in the (Cu,C)-1234 layer is not supported without a control against shunting by the underlying YBCO film. The statement \"To avoid possible parallel conduction [20] and ensure the current flows through the interface, the process in Fig. 1 was taken\" describes only a spatial separation of growth regions; it does not make the YBCO layer non-conducting in the overlap region. In the interface four-probe geometry, current injected into a resistive top layer can enter the superconducting YBCO at one contact, travel through YBCO, and leave through another contact, so the inner voltage contacts can read near zero whenever YBCO is superconducting, regardless of the state of the (Cu,C)-1234 layer. The fact that Tc0(S3)=62.3 K and Tc0(S4)=18.7 K lie below the isolated-YBCO value of 78 K is not decisive, because the YBCO underneath the top layer is not measured independently and could be locally degraded by the second deposition. I request at least one of the following controls: (i) an identical interface circuit measured after removal of the top layer; (ii) an interface circuit with an insulating, non-superconducting top layer of similar thickness; or (iii) measurement of the S3/S4 interface circuit with the YBCO layer driven normal by magnetic field or by operating above its Tc. Without such a control, the S3/S4 zero-resistance states are fully consistent with YBCO shunting.","section":"Section 3.3, Fig. 4"},{"comment":"The evidence for the thinnest sample is incomplete. The thickness of S4 is only \"estimated by others\" (Section 3.3), and the transport data for the isolated ~1.2 nm (Cu,C)-1234 film are not shown (\"unavoidable test error\", Section 3.2). Consequently, the claim that the S4 top layer is itself insulating, and that its low Tc0 of 18.7 K reflects pairing degradation in (Cu,C)-1234 rather than a property of the YBCO or interface, is not directly established. Direct thickness determination (e.g., cross-sectional STEM or X-ray reflectivity) and an R(T) trace for the isolated ultrathin film are needed to substantiate Figs. 5(d) and the discussion of S4.","section":"Section 3.2, Fig. 3 and Fig. 4(b)"}],"minor_comments":[{"comment":"The sentence \"The recovery of superconductivity in S3 is stronger than S4 which is inconsistentwith\" is grammatically incomplete and should be finished; as written it obscures the argument against a simple Cooper-pair-injection picture.","section":"Section 4"},{"comment":"The caption reads \"10Kⅹ SEM image\" and should be \"10k× SEM image\"; in addition, the many panels (a)-(h) are not all explained in the caption, making the figure difficult to interpret.","section":"Fig. 3 caption"},{"comment":"There are numerous typographical errors, including \"Tnset\" for \"Tconset\", \"coherent length\" for \"coherence length\", and \"4.5md\" for \"4.5 mm\" in Section 2.2; a careful proofreading pass is needed.","section":"Throughout"},{"comment":"Reference [20], concerning Bi2223 tapes in parallel connection, does not support the claim that the procedure in Fig. 1 avoids parallel conduction; the citation should be replaced or removed.","section":"Section 3.3, reference [20]"},{"comment":"The normalization convention for the normalized R-T curves is not stated; the authors should specify the normalization point (e.g., value at 300 K) so that the curves can be compared quantitatively.","section":"Fig. 4(a)"},{"comment":"For the 188 nm isolated film, the comment that the R-T curve \"got fluctuation because of connection problem\" is concerning; if contact instabilities were present, the reported Tc0 for that sample should be qualified.","section":"Section 3.2"}],"recommendation":"major_revision","confidential_remarks":"The manuscript addresses an interesting and timely question, and the isolated-film SIT is reasonably convincing. However, the proximity-recovery claim is the paper's centerpiece and currently lacks a decisive control for YBCO parallel conduction. I would like the editor to ask the authors for the control measurements described in Major Comment 1; without them, the main conclusion should not be accepted. The incomplete S4 data (Major Comment 2) should also be addressed in the revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: the paper has a genuinely interesting observation—a thickness-driven superconductor-insulator transition in (Cu,C)-1234 films and an apparent recovery of superconductivity when the insulating phase is coupled to YBCO, with a non-monotonic thickness dependence (recovery at 18 nm, suppressed near 1.2 nm). If confirmed, it gives a useful platform for phase-coherence versus pairing questions in cuprates.\n\nWhat's genuinely new: the combination of a thickness-driven SIT with an S-S' proximity recovery in this material system, and the thickness window where recovery is strongest. The isolated-film data are internally consistent: Tc0 drops from 73.5 K to 60.0 K as thickness goes 188 to 87 nm, and the 18 nm film is insulating. The structural characterization (XRD, STEM, FFT) is decent, and they did measure the YBCO after the full growth process, which shows its Tc0 degrades from 91 to 78 K.\n\nThe soft spot is the load-bearing interpretation of the heterostructure transport. The 'interface' circuit puts contacts on the top (Cu,C)-1234 layer, and the paper asserts without a control that the current flows through the top layer and interface rather than being shunted by the underlying YBCO. The shadow-mask geometry alone does not rule out parallel conduction. The measured Tc0 values for S1-S3 (67.5, 60.0, 62.3 K) are below the sole-YBCO 78 K, which does suggest the top layer contributes—if it were pure shunting you'd expect to see ~78 K. The negative temperature coefficient in S4 also looks like a top-layer signature. But the YBCO underneath the top layer could be differently degraded, and they didn't measure that region separately. A top-layer-removed control or a field-driven normal YBCO reference would settle it. Without that, the zero resistance in S3/S4 cannot be uniquely attributed to proximity-induced superconductivity in the (Cu,C)-1234.\n\nOther issues are minor: the 1.2 nm thickness is estimated, no isolated 1.2 nm data are shown, error bars are absent, and the Discussion contains an incomplete sentence ('inconsistent with' followed by nothing). These are draft-level problems.\n\nBottom line: the paper deserves a serious referee. The idea is interesting, the experiment is mostly careful, and the missing control is addressable experimentally. I'd send it to review with a clear request for the control rather than desk reject. I wouldn't cite it yet, but I'd watch for the revised version.","headline":"Interesting thickness-driven SIT in (Cu,C)-1234 with a plausible but unproven proximity-recovery claim; needs a shunting control before the headline result is credible.","tokens_in":10810,"tokens_out":4869,"would_cite":false,"duration_ms":45404,"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":"Proximity to superconducting YBCO restores zero resistance in an 18-nm (Cu,C)-1234 layer that is otherwise insulating, evidence that interfacial coupling re-establishes lost phase coherence.","keywords":["superconducting proximity effect","superconductor-insulator transition","cuprate heterostructure","(Cu,C)-1234 thin film","YBCO","phase coherence","pulsed laser deposition","thickness-driven SIT"],"falsifier":"Remove the top (Cu,C)-1234 layer from the heterostructure in the region between the voltage probes and remeasure the interface circuit: if zero resistance persists, the signal is carried by YBCO alone, not by a recovered (Cu,C)-1234 channel. Alternatively, apply a magnetic field strong enough to suppress YBCO superconductivity at the operating temperature and check whether the recovered zero-resistance state disappears with the same field scale as the top layer's expected response.","tokens_in":9681,"feed_emoji":"⚡","tokens_out":8813,"duration_ms":86429,"temperature":0.7,"pith_summary":"This paper asks whether a thickness-driven superconductor–insulator transition in a cuprate film can be reversed by placing that film on top of a superconductor. The authors find that isolated (Cu,C)-1234 films stop superconducting below roughly 18 nm thickness and become insulating, but the same insulating film in a (Cu,C)-1234/YBCO heterostructure shows restored zero resistance, with a transition near 62 K. At thickness near 1.2 nm, only a degraded superconducting state appears, with a transition near 19 K and an insulating normal state. The experiments support a picture in which disorder and dimensional confinement destroy long-range phase coherence while local Cooper pairing survives, and the neighboring YBCO re-imposes phase coherence through the proximity effect. The result connects two usually separate research lines—proximity effects and thickness-tuned quantum phase transitions—and suggests interfaces can switch superconductivity on in an otherwise insulating cuprate.","feed_headline":"YBCO proximity restores superconductivity in an 18-nm insulating cuprate","feed_subtitle":"An insulating 18-nm cuprate layer regains zero resistance when coupled to YBCO, until the film nears coherence length","key_machinery":"The operative mechanism is the superconducting proximity effect acting as a phase reference between two superconductors, in a regime where the top film's Cooper pairs are localized by thickness-driven disorder. In the authors' picture, isolated (Cu,C)-1234 retains local pairing into the insulating state, but the pairs lack a common phase; contacting it with YBCO, whose condensate has a well-defined phase, re-locks the phases and restores zero resistance. The experimental machinery is a shadow-mask growth layout that produces four regions on one substrate—bare LAO, sole YBCO, sole (Cu,C)-1234, and the heterostructure—so that thickness effects and interface effects are measured under identical growth conditions.","core_discovery":"The central claim is that superconductivity is re-established in transport through the interface when an insulating ultrathin (Cu,C)-1234 layer is coupled to a thicker superconducting YBCO layer. Isolated (Cu,C)-1234 films show a clear thickness-driven SIT: the 188 nm film has a transition near 73.5 K, the 87 nm film near 60 K, and the 18 nm film is an insulator with resistance diverging at low temperature. In the heterostructure, the interface circuits S1–S3 (188, 87, and 18 nm top layers) all show zero resistance with transition temperatures of 67.5 K, 60.0 K, and 62.3 K, respectively; the nominally insulating 18 nm film has recovered superconductivity. For S4 with an estimated 1.2 nm top layer, the onset is still near 87 K but the zero-resistance transition drops to 18.7 K and the normal state has a negative temperature coefficient, which the authors take as evidence that pairing itself degrades when the thickness is comparable to the coherence length. The mechanism proposed is that YBCO provides a phase reference rather than simply injecting Cooper pairs, re-establishing long-range coherence among already-existing localized Cooper pairs in (Cu,C)-1234.","pith_inferences":["If the phase-reference mechanism is general, the same recovery should occur with other stiff superconductors in place of YBCO; testing a conventional s-wave contact would separate the phase-reference effect from d-wave pairing compatibility.","The picture predicts local pairing in the 18 nm isolated film should be visible spectroscopically as a pseudogap or precursor diamagnetism above the SIT, even though the resistance is insulating.","A thickness series with finer steps between 18 nm and 1.2 nm would map the spatial length scale over which the interface can re-establish coherence and locate where pair breaking begins.","The S4 behavior suggests that, at sub-coherence-length thickness, the bilayer behaves as a superconductor/insulator hybrid whose normal state is dominated by the top layer, so interface resistance measurements could quantify the leakage of YBCO phase stiffness into the disordered layer."],"forward_implications":["In isolated (Cu,C)-1234, superconductivity is lost between 87 nm and 18 nm; the 18 nm film shows insulating resistance that diverges at low temperature.","In the heterostructure, the 18 nm insulating layer shows zero resistance with a transition near 62.3 K, close to the values of the thicker heterostructures, which is what the phase-reference mechanism predicts for recovery of coherence rather than for simple pair injection.","At 1.2 nm thickness, comparable to the cuprate coherence length, the recovered transition is suppressed to 18.7 K and the normal state becomes insulating-like, indicating that pairing, not only coherence, is damaged.","The shadow-mask growth on a single substrate allows the isolated films and the interface to be measured under the same conditions, making the comparison between the insulating top layer and the recovered state direct."],"supporting_citations":[{"why":"Supplies the reference transition temperature near 96 K for (Cu,C)-1234 films, used to judge the quality and underdoping of the films.","marker":"[23]"},{"why":"Establishes the thickness-controlled SIT near roughly 10 nm in La2-xSrxCuO4 that the authors compare with their own critical thickness.","marker":"[15]"},{"why":"Cited for the need to avoid parallel conduction in multilayer transport measurements, supporting the interface-circuit interpretation.","marker":"[20]"},{"why":"Shows proximity-induced gap decay with thickness in a Bi2Te3/Bi-2212 heterostructure, a comparative S-S'/S-TI system.","marker":"[10]"},{"why":"Supplies the disordered-2D-superconductor phase-coherence picture that motivates restoring coherence via a neighboring condensate.","marker":"[29]"},{"why":"Documents high-Tc superconductivity in a monolayer Bi-2212, contrasting with the thickness-driven degradation observed here.","marker":"[14]"},{"why":"Supports the view that low-dimensional disorder causes quantum breakdown of superconductivity, the mechanism behind the SIT.","marker":"[32]"}],"fun_headline_variants":["Proximity to YBCO revives superconductivity in an insulating cuprate layer","Thin cuprate insulator turns superconducting when coupled to YBCO","Interface restores superconductivity in thickness-killed cuprate film","YBCO proximity brings back zero resistance in ultrathin cuprate layer","18-nm cuprate regains superconductivity via YBCO interfacial coupling"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The measured 'interface' superconductivity would not demonstrate recovery in (Cu,C)-1234 if the four-probe circuit is actually shunted by the underlying YBCO film, and the paper provides no control measurement—such as driving YBCO normal or measuring with the top layer removed—to rule out this shunt.","fun_headline_variants_meta":{"raw":{"variants":["Proximity to YBCO revives superconductivity in an insulating cuprate layer","Thin cuprate insulator turns superconducting when coupled to YBCO","Interface restores superconductivity in thickness-killed cuprate film","YBCO proximity brings back zero resistance in ultrathin cuprate layer","18-nm cuprate regains superconductivity via YBCO interfacial coupling"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000571,"raw_usage":{"total_tokens":2793,"prompt_tokens":1128,"completion_tokens":1665,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":744,"completion_tokens_details":{"reasoning_tokens":1563}},"tokens_in":744,"tokens_out":1665,"duration_ms":12900,"temperature":1.0,"reasoning_tokens":1563,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-16T00:29:38.085670+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Remove the top (Cu,C)-1234 layer from the heterostructure in the region between the voltage probes and remeasure the interface circuit: if zero resistance persists, the signal is carried by YBCO alone, not by a recovered (Cu,C)-1234 channel. Alternatively, apply a magnetic field strong enough to suppress YBCO superconductivity at the operating temperature and check whether the recovered zero-resistance state disappears with the same field scale as the top layer's expected response.","supporting_citations":[{"cited_title":"Preparation and superconducting properties of the (Cu,C)Ba2Ca3Cu4O11+y films with zero-resistance transition temperature of 96 K","cited_arxiv_id":null,"evidence_quote":"Supplies the reference transition temperature near 96 K for (Cu,C)-1234 films, used to judge the quality and underdoping of the films."},{"cited_title":"Ultrathin Limit on the Anisotropic Superconductivity of Single-LayeredCuprateFilms.ChinesePhysLett2024;41:027401","cited_arxiv_id":null,"evidence_quote":"Establishes the thickness-controlled SIT near roughly 10 nm in La2-xSrxCuO4 that the authors compare with their own critical thickness."},{"cited_title":"Current–voltage characteristic of Bi2223 multifilamentary tapesinparallelconnection.SupercondSciTechnol2006;19:1225–8","cited_arxiv_id":null,"evidence_quote":"Cited for the need to avoid parallel conduction in multilayer transport measurements, supporting the interface-circuit interpretation."},{"cited_title":"Twofold symmetry of proximity-induced superconductivity in Bi2Te3/Bi2Sr2CaCu2O8+δ heterostructures revealed by scanning tunneling microscopy","cited_arxiv_id":null,"evidence_quote":"Shows proximity-induced gap decay with thickness in a Bi2Te3/Bi-2212 heterostructure, a comparative S-S'/S-TI system."},{"cited_title":"Quantum phase transitions in disordered two-dimensional superconductors","cited_arxiv_id":null,"evidence_quote":"Supplies the disordered-2D-superconductor phase-coherence picture that motivates restoring coherence via a neighboring condensate."},{"cited_title":"High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ.Nature2019;575:156–63","cited_arxiv_id":null,"evidence_quote":"Documents high-Tc superconductivity in a monolayer Bi-2212, contrasting with the thickness-driven degradation observed here."},{"cited_title":"Quantum breakdown of superconductivity in low-dimensionalmaterials.NatPhys2020;16:734–46","cited_arxiv_id":null,"evidence_quote":"Supports the view that low-dimensional disorder causes quantum breakdown of superconductivity, the mechanism behind the SIT."}],"review_version":1}