{"id":"a33ff5e0-6f68-4db6-8fef-a90b7cae38fe","arxiv_id":"1908.03362","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Multiple superconducting transitions up to about 6 K are observed in 20-400 nm MBE-grown α-Sn films, with sources not yet identified.","lead":"Thick films of gray tin (α-Sn) grown by molecular beam epitaxy show several superconducting transitions, including ones above the 3.7 K transition of the familiar β-Sn phase. The higher-temperature transitions are sensitive to aging, survive swapping the electrical contacts, and may reflect a strain-related superconducting phase.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Higher-Tc transitions may originate from In-Sn alloy at the α-Sn/InSb interface, a possibility the authors acknowledge they have not ruled out.","rationale":"The reader identified the same weakest assumption: the higher-Tc transitions could come from In-Sn alloy formation, which has a Tc range overlapping the observed transitions. My stress-test pass confirms this is the single most load-bearing point. The paper's own Discussion and Summary explicitly flag the In-Sn alloy possibility and admit that TEM has not ruled it out, so this is a stated limitation rather than an external speculation. The Ag-paste control is strong evidence against pressed In contacts, and the template measurements rule out the bare substrate, but neither excludes In that has segregated from the InSb layer into the α-Sn film during growth. The small volume fraction of the higher-Tc phases and their aging sensitivity are consistent with a thin interfacial alloy phase. The proposed test—growth on an In-free substrate such as CdTe, or high-resolution STEM-EDS at the interface—would directly settle the matter. Because the current evidence is insufficient to distinguish intrinsic α-Sn superconductivity from interfacial In-Sn alloy, the paper's central claim is not yet secured. However, the observation itself is likely real and the authors have already made the correct conditional caveat, so the appropriate verdict remains CONDITIONAL; my assessment does not change the reader's verdict.","tokens_in":6921,"tokens_out":3107,"duration_ms":37676,"concrete_test":"Grow α-Sn films of the same thickness range on an In-free lattice-matched substrate such as CdTe/GaAs, with the same Si capping and measurement protocol, and check whether the >4.5 K transitions still appear. If they persist in the absence of any In source, the In-Sn alloy explanation is excluded. If they disappear, the central claim of intrinsic α-Sn superconductivity above 4.5 K would be unsupported. A complementary check is cross-sectional STEM-EDS line scanning across the α-Sn/InSb interface on the exact samples showing the 4.96 K and 6 K transitions: detection of an In-Sn intermetallic layer or In concentration gradients at the interface would directly confirm the alternative source.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim is that MBE-grown α-Sn films several tens to hundreds of nanometers thick show superconducting transitions above 4.5 K that are not attributable to β-Sn, contacts, or oxides. The most load-bearing assumption is that these transitions arise from the α-Sn film itself rather than from an In-Sn alloy formed by In segregation from the InSb template. This is not a hypothetical concern: In-Sn alloys have Tc values from 3.4 K to 7.3 K, which fully bracket the observed 4.5–6.5 K transitions. The authors explicitly state in the Discussion that In segregation from InSb and In-Sn alloy formation at the interface is a possible source, and that they have not observed evidence for it in TEM. The Ag-paste control in Figure 3(a) rules out the pressed In contacts, but it does not rule out In that has already diffused into the film during growth or over time from the InSb layer. The template measurements in Figure 5 also do not address this, because the templates lack the α-Sn/InSb interface where such an alloy would form. The aging sensitivity of the higher-Tc transitions is qualitatively consistent with slow In diffusion or interfacial phase evolution. Therefore, the existence of an intrinsic superconducting phase in α-Sn at these thicknesses is not established unless In-Sn alloy formation is excluded by direct compositional and structural evidence.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports transport measurements on MBE-grown α-Sn films on InSb-based templates (type A and type B) with thicknesses from 20 to 400 nm. The authors observe multiple superconducting transitions: a β-Sn-like transition near 3.7 K and additional transitions at 4.5–6.5 K. They rule out pressed In contacts as the source by comparing with Ag paste contacts, rule out surface oxides by capping with Si, and show that bare InSb/GaSb/GaAs templates are not superconducting. They also characterize aging effects and magnetic field dependence, extracting critical fields and fitting the angular dependence to the Tinkham model, noting 2D character and a high in-plane critical field of about 0.82 T for the higher-Tc transition in a 50 nm sample. The authors propose that the higher-Tc transitions may come from α-Sn-related phases, but they explicitly acknowledge in the Discussion that In-Sn alloy formation at the α-Sn/InSb interface remains a possible source.","tokens_in":7184,"tokens_out":5302,"duration_ms":50975,"significance":"If the higher-Tc transitions are intrinsic to strained α-Sn films, the result would be significant for topological superconductivity and would extend previous stanene work to much larger thicknesses. The paper includes useful controls for contacts and oxidation, and the aging measurements provide a careful characterization of sample evolution. However, the central attribution is not yet established because In-Sn alloy formation at the α-Sn/InSb interface has not been excluded. The observations are interesting and meritorious, but the strength of the title and the claim that 'superconductivity in α-Sn thin films up to this thickness range has never been reported' exceed what the current evidence supports.","major_comments":[{"comment":"The In-Sn alloy scenario is not excluded and is load-bearing for the central claim. The Ag-paste control in Figure 3(a) rules out In contacts but not In that segregates from the InSb layer during growth or over time. The template measurements in Figure 5 lack the α-Sn/InSb interface and therefore do not address this possibility. Since In-Sn alloys have Tc values from 3.4 to 7.3 K (Refs. 29, 30) that bracket the observed higher-Tc transitions (4.5–6.5 K), and since the aging sensitivity of the higher-Tc transitions is qualitatively consistent with In diffusion, the statement in the Discussion that 'The superconductivity in α-Sn thin films up to this thickness range has never been reported' is not supported. The authors should provide direct evidence excluding interface In-Sn alloy formation—for example, EDX or electron energy-loss spectroscopy mapping across the interface, SIMS depth profiling, or a control film grown on an In-free substrate such as CdTe—or explicitly reframe the claim as 'superconducting transitions observed in α-Sn films whose origin remains to be identified.'","section":"Discussion, 'Another possible source...' and Figure 5"},{"comment":"The extracted critical fields lack uncertainty estimates and the fitting procedure is incompletely described. The paper states in the caption of Figure 4 that the critical field is decided at the field where the resistance reaches 90% of its value at 4 T, but the fits to Eq. (1) and the Tinkham model Eq. (2) are not reported with the fitted parameters (Bc⊥, Bc∥, Tc) and their errors. The in-plane critical field of 0.82 T for the higher-Tc transition is a key quantitative claim, and it should be accompanied by the full fit parameters and an uncertainty estimate so that it can be meaningfully compared with the critical fields of candidate secondary phases such as In-Sn alloys.","section":"Figure 4 and equations (1)–(2)"}],"minor_comments":[{"comment":"The text contains placeholder question marks in 'out-of-plane residual strain(?)' and 'plastic flow model (?)' in the Method section; these should be replaced with proper terms and references.","section":"Method"},{"comment":"The caption says 'two type B samples grown on GaAs and GaSb substrates,' but the type B structure in Figure 1(a) uses GaAs substrates with GaSb and InSb buffer layers; the caption should be clarified to specify the actual difference between the two samples.","section":"Figure 2(b) caption"},{"comment":"The sentence 'The critical field increases with the angle of the magnetic field relative to the normal of the sample surface as shown in the inset of Figure 4(b)' is slightly ambiguous; explicitly stating that 0° is normal and 90° is parallel to the surface would improve readability.","section":"Results, Figure 4"},{"comment":"The Summary states that 'Multiple transitions above 4 K with larger critical fields should come from other possible sources such as In contacts, In-Sn alloys or oxides,' which is inconsistent with the earlier Discussion statement that 'more than one superconducting phases exist in our α-Sn samples besides β-Sn'; the authors should make their attribution consistent throughout.","section":"Summary"},{"comment":"The statement that 'The details about the growth and structural characterization are presented elsewhere' should include a citation or be expanded, since the growth quality and interface structure are important to the claims in the paper.","section":"Method, growth details"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within scope for a condensed-matter physics journal, and the experimental controls are a strength. The main risk is that the higher-Tc transitions may be due to In-Sn alloy formation at the α-Sn/InSb interface, a possibility the authors explicitly list but do not rule out. The title and the 'never been reported' claim are stronger than the current evidence supports. A major revision that either provides direct evidence against the In-Sn alloy scenario or clearly reframes the central claim as an unresolved origin of multiple superconducting transitions would be appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The one genuinely new thing here is that α-Sn films tens to hundreds of nanometers thick show resistive transitions above 4.5 K, up to about 6 K. Earlier stanene work stops at 10 nm, and β-Sn proximity studies don't explain transitions above the bulk β-Sn Tc. That observation, if real, would be a useful entry point for topological superconductor candidates. The paper deserves credit for clean transport data, sensible controls (Ag paste vs In contacts, template samples, Si capping), and honest reporting of aging effects. The authors also explicitly list the most obvious alternative source—In segregation from the InSb substrate forming an In-Sn alloy—which is more than many experimental papers do.\n\nThe soft spot is exactly that alternative source, and it is not minor. In-Sn alloys can superconduct up to 7.3 K, which brackets the observed 4.5–6 K transitions. The Ag paste control rules out the pressed indium contacts, but it does not rule out indium that diffused into the film during growth or over time from the InSb layer. The template samples in Figure 5 do not address this, because they lack the α-Sn/InSb interface where such an alloy would form. The authors say TEM has not shown signs of In-Sn, but no TEM data are presented. The aging sensitivity of the higher-Tc transitions is qualitatively consistent with slow indium diffusion or interfacial phase evolution. The transitions are also incomplete, involve small volume fractions, and the critical field data have no error bars. The strain-effect claim rests on two samples with different InSb thicknesses, which is suggestive but hardly conclusive.\n\nI would not call this a fatal flaw—the paper is an honest experimental report, and the authors do not overclaim. But the central claim of superconducting phases in thick α-Sn is conditional at best. Right now the most economical reading is that the higher-Tc transitions come from an interfacial In-Sn alloy, not from intrinsic α-Sn.\n\nThis paper is for specialists in tin films and topological superconductivity. It deserves a serious referee, because the observation is concrete and the artifact discussion is instructive. A referee should ask for direct compositional and structural evidence—EDX, TEM, or a control with a different buffer—to rule out In-Sn alloy. Without that, the paper can still be published as a cautionary study, but the title should not imply intrinsic α-Sn superconductivity. I'd engage with it, but I would not cite it as evidence for intrinsic superconductivity until the alloy question is settled.","headline":"A clean transport report of superconducting transitions in thick α-Sn films, but the higher-Tc phases look more like In-Sn alloy than intrinsic α-Sn; the authors leave that door open.","tokens_in":7740,"tokens_out":2036,"would_cite":false,"duration_ms":22811,"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":"MBE-grown alpha-tin films show multiple superconducting transitions, including phases above 4.5 K that are not caused by contacts or oxide.","keywords":["gray tin","alpha-Sn thin films","molecular beam epitaxy","multiple superconducting transitions","strain effects","aging effects","topological superconductivity","critical field anisotropy"],"falsifier":"Cut a fresh, silicon-capped 100 nm type B film contacted with silver paste and map the composition of the region where the higher-Tc transitions appear using transmission electron microscopy with element-sensitive spectroscopy; finding indium-tin alloy or indium-rich precipitates there would settle that the transitions are extrinsic, while a clean map with no such phase would support intrinsic strained alpha-Sn superconductivity.","tokens_in":6728,"feed_emoji":"❄️","tokens_out":7104,"duration_ms":70136,"temperature":0.7,"pith_summary":"This paper reports that molecular-beam-epitaxy-grown alpha-tin (gray tin) films, 20 to 400 nanometers thick, can host several distinct superconducting transitions, including ones above 4.5 K and up to about 6 K, well above the familiar 3.7 K transition of beta-tin. The authors argue that these higher-temperature transitions are not produced by indium contacts, surface oxide, or the InSb substrate, and they show the transitions survive when indium is replaced by silver paste and the surface is capped with silicon. If the claim holds, strained alpha-tin is superconducting well beyond the few-layer stanene regime, with critical fields larger than bulk beta-tin and with a two-dimensional character. The result matters because strained alpha-tin is predicted to be topologically nontrivial, so these phases may be candidates for topological superconductivity.","feed_headline":"Gray tin films show superconductivity above 4.5 K","feed_subtitle":"MBE-grown alpha-tin hosts extra superconducting phases beyond beta-tin, with higher critical fields and 2D behavior.","key_machinery":"The central object is the fully strained alpha-Sn film grown by MBE on InSb(001)-based templates, with the strain set by the relative thickness of the alpha-Sn layer and the InSb stabilizer layer. The argument is carried by transport measurements that resolve multiple resistance drops: distinct superconducting transitions are identified as separate features in resistance-versus-temperature curves and by matching the number of critical-field branches in resistance-versus-magnetic-field sweeps at several temperatures. Control comparisons do the load-bearing work: the same sample measured with pressed-indium versus Ag-paste contacts, with and without a silicon cap, and templates without alpha-Sn.","core_discovery":"The central claim is that MBE-grown alpha-Sn films in the 20-400 nm thickness range, not just monolayer stanene, show multiple superconducting transitions. Besides a transition near 3.7 K attributed to beta-Sn inclusions, the films show sharp transitions at 4.96 K and near 6 K, with smaller volume fractions (below about 20 percent), higher critical fields, and stronger aging sensitivity. Control experiments rule out indium contacts, surface oxide, and the InSb template as the source, since Ag-paste contacts on the same sample reproduce all three transitions, silicon capping preserves them, bare templates show no superconductivity, and the transitions persist over days at low temperature. The authors interpret the higher-temperature transitions as superconductivity from phases of the strained alpha-Sn film itself, possibly connected to its topological band structure, while leaving indium-tin alloy formed during growth as an acknowledged alternative they have not yet ruled out.","pith_inferences":["If the In-Sn alloy alternative is excluded by nanoscale composition mapping, the remaining explanation is an intrinsic superconducting phase of strained alpha-Sn, which would make the film a promising platform for searching for topological superconductivity.","The incomplete transitions and the small 6.48 K feature interpreted as granular fluctuations suggest these phases may be filamentary; whether zero resistance is achieved may depend on percolation, so patterned or thinner samples might reveal different transition completeness.","Because the high-Tc transitions age faster than the beta-Sn transition, a direct time-resolved correlation between lattice strain and superconducting critical temperature on the same sample would test whether strain relaxation is what kills the new phase."],"forward_implications":["Superconductivity in alpha-Sn is not limited to few-layer stanene; films tens to hundreds of nanometers thick show transitions above the beta-Sn critical temperature.","The higher-Tc transitions carry larger critical fields than the beta-Sn transition and show two-dimensional anisotropic field behavior, giving a clear transport signature to identify them.","Strain from the InSb template stabilizes the alpha-Sn structure and suppresses the beta-Sn fraction, while the transition near 5 K persists, pointing to strain as a controlling parameter.","Freshness matters: room-temperature aging affects the higher-Tc transitions far more than the beta-Sn transition, so time-controlled measurements are needed to characterize these phases.","The possibility that strained alpha-Sn is a topological superconductor remains open, since the observed phases occur in the predicted topological regime."],"supporting_citations":[{"why":"Supplies the MBE growth route and substrate stabilization of alpha-Sn on InSb and CdTe that all samples in this paper rely on.","marker":"[7]"},{"why":"Reports superconductivity in few-layer stanene and provides the thickness trend and high in-plane field baseline that this work extends.","marker":"[24]"},{"why":"Provides the Tinkham two-dimensional critical-field formula used to fit the angular dependence of the critical field.","marker":"[25]"},{"why":"Documents proximity-induced superconductivity from beta-Sn islands, the rival interpretation the higher-Tc transitions must be distinguished from.","marker":"[26]"},{"why":"Establishes that In-Sn alloys can superconduct with critical temperatures up to about 7.3 K, the main competing source considered.","marker":"[29]"},{"why":"Supplies the composition-dependent In-Sn alloy superconductivity data used to assess whether alloy formation explains the observed Tc range.","marker":"[30]"},{"why":"Provides the granular superconducting fluctuation picture used to interpret the small transition near 6.48 K.","marker":"[31]"},{"why":"Reports similar granular-tin superconducting fluctuations on graphene and supports the fluctuation interpretation.","marker":"[32]"}],"fun_headline_variants":["Gray tin films reveal multiple superconducting transitions","Alpha-Sn shows extra superconducting phases above 4.5 K","MBE-grown gray tin superconducts in multiple phases","Strained alpha-Sn films host multiple superconductivity onsets","Alpha-Sn thin films superconduct beyond the beta-Sn transition"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The new claim stands on the premise that the 4.5-6 K transitions are not caused by indium-tin alloy or another secondary phase formed during growth, which the authors state they have not yet observed or ruled out.","fun_headline_variants_meta":{"raw":{"variants":["Gray tin films reveal multiple superconducting transitions","Alpha-Sn shows extra superconducting phases above 4.5 K","MBE-grown gray tin superconducts in multiple phases","Strained alpha-Sn films host multiple superconductivity onsets","Alpha-Sn thin films superconduct beyond the beta-Sn transition"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000263,"raw_usage":{"total_tokens":1574,"prompt_tokens":892,"completion_tokens":682,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":508,"completion_tokens_details":{"reasoning_tokens":602}},"tokens_in":508,"tokens_out":682,"duration_ms":7305,"temperature":1.0,"reasoning_tokens":602,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T14:14:50.958638+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Cut a fresh, silicon-capped 100 nm type B film contacted with silver paste and map the composition of the region where the higher-Tc transitions appear using transmission electron microscopy with element-sensitive spectroscopy; finding indium-tin alloy or indium-rich precipitates there would settle that the transitions are extrinsic, while a clean map with no such phase would support intrinsic strained alpha-Sn superconductivity.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the MBE growth route and substrate stabilization of alpha-Sn on InSb and CdTe that all samples in this paper rely on."},{"cited_title":"Liao et al., Nature Physics 14, 344 (2018)","cited_arxiv_id":null,"evidence_quote":"Reports superconductivity in few-layer stanene and provides the thickness trend and high in-plane field baseline that this work extends."},{"cited_title":"Tinkham, Physical Review 129, 2413 (1963)","cited_arxiv_id":null,"evidence_quote":"Provides the Tinkham two-dimensional critical-field formula used to fit the angular dependence of the critical field."},{"cited_title":"Didschuns, K","cited_arxiv_id":null,"evidence_quote":"Documents proximity-induced superconductivity from beta-Sn islands, the rival interpretation the higher-Tc transitions must be distinguished from."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the composition-dependent In-Sn alloy superconductivity data used to assess whether alloy formation explains the observed Tc range."},{"cited_title":"Allain, Z","cited_arxiv_id":null,"evidence_quote":"Provides the granular superconducting fluctuation picture used to interpret the small transition near 6.48 K."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports similar granular-tin superconducting fluctuations on graphene and supports the fluctuation interpretation."}],"review_version":1}