{"id":"2aa4f6fe-5b37-4fda-ac24-faed6cc6f908","arxiv_id":"2608.00526","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Spin-resolved ARPES reveals Rashba-type spin-momentum locking in both bismuthene and its precursor, plus an unexpected out-of-plane spin polarization in bismuthene.","lead":"This paper uses spin-resolved photoemission to show that electrons in two bismuth structures at a graphene-silicon carbide interface have their spin direction locked to their motion, confirming a key signature of a quantum spin Hall insulator. It also finds an unexpected out-of-plane spin signal in the topological phase that theory does not explain, pointing to measurement effects.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Unexplained constant P_y offset in the precursor spin-resolved EDC undermines the claimed opposite spin polarization of E1/E2","rationale":"The reader's weakest assumption targeted the DFT comparison (omitted graphene, 0.3 eV offset, coverage model). That is a valid concern, but it concerns the interpretation layer rather than the raw experimental evidence. My concern is more load-bearing because it is an acknowledged internal inconsistency in the measured spin polarization itself: the precursor data show a P_y component that does not reverse between E1 and E2 as expected for two opposite spin states. Since the paper's central claim is the direct observation of spin-momentum locking, an unexplained offset in the very data used to draw the spin-texture arrows means the claim is not fully supported without additional analysis. This does not overturn the paper; the spin splitting and the P_x reversal may still be correct, and the bismuthene data are more complete. But it strengthens the case for a conditional verdict pending a quantitative re-analysis and a control measurement. Therefore I recommend keeping the reader's CONDITIONAL verdict, i.e., UNCHANGED.","tokens_in":43866,"tokens_out":5313,"duration_ms":66951,"concrete_test":"Re-analyze the raw spin-resolved EDC for the precursor MΓM cut (SI Fig. S7): reconstruct P_y for the E1 and E2 energy windows from the four coil/rotator configurations, propagate the scan-to-scan statistical uncertainty, and compare the two P_y values. If they are not significantly different (e.g., within 2σ), then the claimed opposite P_y is not supported. Additionally, acquire a spin-resolved EDC on a clean Bi-free epitaxial graphene/SiC sample under identical experimental geometry and photon energy; if a similar constant P_y offset appears, it is a common-mode instrumental artifact and the precursor spin-texture arrows must be corrected before comparison with DFT.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that spin-resolved ARPES resolves two oppositely spin-polarized valence bands in the precursor rests on two EDCs: one showing P_y reversal (Fig. 2f-g) and one showing P_x reversal (Fig. 2i-j). However, in the same second EDC, the measured P_y is described as a 'nearly constant negative offset ... independent of whether the signal originates from E1 or E2,' and the authors state that 'the origin of this behavior remains unclear' (Section II). This directly contradicts the expectation of two oppositely polarized states at a given k for a Rashba-like system, and it is a limitation explicitly acknowledged in the manuscript. If the offset is an instrumental/geometric artifact, it may also bias the P_x and P_y values used to draw the spin-texture arrows in Figs. 2a-b; if it is real, then the two bands are not oppositely polarized at that k-point, weakening the abstract's claim of 'two oppositely spin-polarized contributions' for the precursor. The DFT comparison cannot resolve this, because the discrepancy is internal to the experimental spin-polarization analysis.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports spin- and angle-resolved photoemission experiments on the Bi precursor and bismuthene phases at the graphene/SiC interface. The central claim is that spin-resolved EDCs and MDCs resolve two oppositely spin-polarized valence bands in both phases, demonstrating Rashba-type spin-momentum locking and Kramers doublets. The paper also revises the precursor Bi coverage from 2/3 ML to 1/3 ML, reinterpreting the hydrogenation-induced transition as a lateral contraction, and reports a finite out-of-plane spin polarization in bismuthene that is attributed to final-state photoemission effects rather than to the initial state. Experimental spin textures are compared with DFT predictions throughout.","tokens_in":44182,"tokens_out":7062,"duration_ms":93086,"significance":"If the observations hold, this is a valuable direct confirmation of the spin texture of bismuthene and its precursor, lending support to the quantum-spin-Hall picture of bismuthene. The paper has notable strengths: the raw spin-resolved data are provided in the SI, the measurement setup and polarization extraction are described in detail, and the main spin-texture comparison is not circular—the DFT spin textures were not fitted to the spin-resolved data, and the 0.3 eV offset and 1/3 ML model were chosen to match spin-integrated dispersions, not the spin polarizations. However, the quantitative support for the key claims is weakened by an unexplained constant P_y offset in one precursor EDC, by the absence of error bars or statistical tests on all P traces, and by DFT comparisons that omit the graphene overlayer. These issues need to be addressed before the claims can be considered fully established.","major_comments":[{"comment":"The nearly constant negative P_y offset is explicitly described as 'independent of whether the signal originates from E1 or E2' and its origin is stated to be unclear. This directly contradicts the expectation of two oppositely polarized states at a given k, and it is load-bearing for the precursor spin-momentum-locking claim. Since the arrows in Figs. 2(a)-(b) are determined from the P_x and P_y values measured at the E1 and E2 maxima in Figs. 2(g) and 2(j), the unexplained offset is folded into the displayed spin texture. Please either identify and remove the instrumental/geometric origin of the offset, or restrict the claim to the cleanly reversing P_x component and show explicitly how the offset affects the arrow directions.","section":"Section II, Fig. 2(k)-(l)"},{"comment":"No error bars or statistical significance tests are reported for any spin-polarization trace. This is particularly important for the new out-of-plane P_z claim in Fig. 5(g), where a 'significant' polarization and sign reversal are asserted. The raw data in SI Sec. VI contain multiple scans; please propagate counting statistics through the asymmetry and Sherman-function analysis, and state significance thresholds for the sign reversals and for the E1/E2 component separations.","section":"All spin-resolved figures, e.g. Figs. 2(d-g), 4(d-g), 5(c-g)"},{"comment":"The DFT calculations omit the graphene overlayer, and the precursor bands are rigidly shifted by 0.3 eV to match experiment. The claimed agreement of the measured spin texture with DFT (Figs. 2, 4, 5) depends on this model. If the graphene overlayer or a different adsorption configuration changes the spin texture or band ordering, the agreement would be less compelling. Please justify the omission or test its robustness, for example by including graphene in at least one calculation or by comparing spin textures for the T4 and H3 adsorption models.","section":"Methods and SI Sec. I"},{"comment":"The revised 1/3 ML coverage for the precursor is presented as an important structural conclusion with implications for the phase transition, but the evidence in this manuscript appears to be the DFT overlay in Fig. 1(a) rather than an independent experimental determination. Please provide direct evidence (e.g., quantitative core-level analysis, XSW, or STM) or explicitly state that the coverage is inferred from the DFT match. As written, the lateral-contraction interpretation of the hydrogenation transition rests on an assumption that is not established here.","section":"Section II, coverage revision"},{"comment":"The out-of-plane P_z is attributed to final-state interference, but no model, calculation, or photon-energy/polarization dependence is provided to support this attribution. The text already calls this a hypothesis, which is appropriate, but the summary states it more strongly as the established explanation. Please either add a quantitative test of the final-state mechanism or soften the summary to clearly identify the final-state origin as a tentative interpretation, not a demonstrated result.","section":"Section III, Fig. 5(g) and SI Sec. IV"}],"minor_comments":[{"comment":"The raw-data figures appear to contain garbled or unreadable labels in the version provided, making it difficult to verify which coil/rotator configuration corresponds to each panel. Please ensure the figure text is legible and that the arrangement described in the text matches the displayed labels.","section":"SI Sec. VI, Figs. S6-S13"},{"comment":"The Sherman function is given as S = 0.29, but no uncertainty is stated. Since all P_i values and the derived C±i intensities depend linearly on S, please quote the uncertainty of S or the resulting systematic error on P_i.","section":"Methods, spin-polarization formalism"},{"comment":"The arrows indicating measured spin polarization would benefit from a statement of their uncertainty and from a clear indication that some arrows include the unexplained P_y offset (precursor) or are based on a single EDC per momentum point. This would help readers judge the significance of the tangential versus non-tangential components.","section":"Figs. 2(a)-(b) and 4(a)-(b)"},{"comment":"The argument that equal averaging of S2 and S2* terraces prevents cancellation is plausible, but the assumption of exactly equal signal is stated without support. A brief justification or an estimate of the domain imbalance would strengthen the interpretation.","section":"Section III, K/K' domain averaging"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"This is the first spin-resolved ARPES study of both bismuthene and its precursor, and the central observation—resolving two oppositely spin-polarized valence bands—is likely right. The raw data are in the Supplementary Information, the methods are described in detail, and the authors are honest about limitations. That counts for a lot. The in-plane spin texture of bismuthene agrees qualitatively with DFT, the out-of-plane Pz observation is genuinely new (even if the final-state-effect explanation is a hypothesis), and the coverage revision to 1/3 ML is an interesting claim that fits the dispersion and a recent STM work.\n\nThe soft spots are real, though not fatal. The biggest one is the precursor second EDC (Fig. 2k-l): the authors report a nearly constant negative Py offset for both E1 and E2, explicitly saying the origin is unclear. That does sit awkwardly with the claim of two oppositely polarized bands and could bias the spin-texture arrows drawn for that momentum. The first EDC (Py reversal) already demonstrates the spin splitting, so the main result does not collapse, but the offset should not be left unexplained in the final version. The paper also shows no error bars or statistical tests on the polarization traces, which matters when the Pz signal in bismuthene is small and when one wants to assess whether the constant Py offset is significant. The DFT comparison uses a rigid 0.3 eV shift and omits graphene, which is acceptable for a first study but makes the spin-texture agreement qualitative rather than quantitative. And the 1/3 ML coverage revision is inferred from band alignment, not from independent structural data, so it should be presented as a model-dependent interpretation.\n\nThe paper is a solid experimental contribution for the ARPES and topological-materials community. It deserves a serious referee, and a good referee will ask for error bars, a discussion of the Py offset (possibly an instrumental asymmetry), and a more cautious framing of the coverage change. I would support sending it to peer review rather than desk rejecting it.","headline":"First spin-resolved ARPES on bismuthene and its precursor, with a defensible central result, but an unexplained constant Py offset in one precursor EDC and missing error bars are real caveats that need addressing.","tokens_in":44681,"tokens_out":3461,"would_cite":true,"duration_ms":40428,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Spin- and angle-resolved photoemission directly resolves Rashba-split valence bands in bismuthene and its precursor.","keywords":["bismuthene","spin texture","spin-momentum locking","Rashba effect","spin-orbit coupling","spin-resolved ARPES","quantum spin Hall insulator","graphene/SiC interface"],"falsifier":"Measure the out-of-plane spin polarization of bismuthene's valence bands while sweeping photon energy and light polarization: if the Pz signal changes sign or magnitude with those parameters, the final-state interference explanation is supported; if it stays fixed, an initial-state origin cannot be dismissed. Also compute DFT with the graphene overlayer included and check whether the 0.3 eV shift and the predicted in-plane spin texture survive.","tokens_in":43810,"feed_emoji":"🧲","tokens_out":6398,"duration_ms":66995,"temperature":0.7,"pith_summary":"The paper uses spin- and angle-resolved photoelectron spectroscopy to map the spin polarization of the low-energy valence bands in bismuthene, a candidate room-temperature quantum spin Hall insulator, and in its topologically trivial precursor. It resolves, for the first time in the precursor, two oppositely spin-polarized bands split in energy by roughly 0.2 eV, direct evidence that spin-orbit coupling lifts spin degeneracy. For bismuthene, the measured in-plane spin texture matches DFT calculations, including cubic Rashba deviations from purely tangential locking. A finite out-of-plane polarization is also found, but the paper argues this is a final-state photoemission artifact rather than an initial-state property. The precursor's Bi coverage is revised to 1/3 ML, implying the transition to bismuthene proceeds by lateral contraction.","feed_headline":"Bismuthene's electron spins are locked to momentum, spin-ARPES shows","feed_subtitle":"Spin-resolved ARPES resolves two oppositely polarized valence bands in both phases, confirming the Rashba spin texture.","key_machinery":"The load-bearing objects are the Rashba-split valence bands E1/E2 (precursor) and B1/B2 (bismuthene), measured with a VLEED spin detector that resolves all three spin components, and the DFT spin textures computed for the revised 1/3 ML T4 precursor and the 2/3 ML T1 bismuthene structures. The argument is carried by comparing the measured spin polarization vectors with the calculated textures: cubic Rashba terms allowed by C3v symmetry explain the non-tangential in-plane component, the mirror-plane geometry forbids out-of-plane polarization along ΓK, and final-state interference is invoked for the remaining out-of-plane signal.","core_discovery":"The central discovery is that Rashba spin-momentum locking is directly observable in both the precursor phase and the quantum spin Hall phase of bismuthene. In the precursor, spin-resolved EDCs split into two contributions with opposite in-plane polarization, separated by about 0.2 eV, matching the DFT bands E1 and E2, and the spin direction winds tangentially around the constant-energy contours. In bismuthene, the in-plane spin texture of the B1 and B2 valence bands reproduces the DFT prediction, including a non-tangential component attributed to cubic terms in the spin-orbit Hamiltonian allowed by C3v symmetry; the same winding direction at K and K' is observed, ruling out opposite spin te","pith_inferences":["If the final-state interpretation is correct, the out-of-plane polarization should vary systematically with photon energy and light geometry; this is a direct test the authors did not perform.","The revised 1/3 ML coverage suggests hydrogenation could be used as a patterning tool: partial hydrogenation would template bismuthene islands with controlled size, an idea not developed in the paper.","The same spin-resolved EDC splitting technique could reveal Rashba-split bands in other buried or capped 2D systems where the splitting is smaller than the linewidth.","Including graphene in the DFT calculation may remove the 0.3 eV offset and test whether the precursor spin texture survives a more realistic environment, strengthening or revising the structural assignment."],"forward_implications":["The low-energy valence bands of bismuthene are spin-split and spin-momentum locked in the direction DFT predicts, reinforcing the quantum spin Hall insulator description.","The precursor's valence bands are also Rashba-split by roughly 0.2 eV, so spin-resolved ARPES can resolve features invisible to spin-integrated ARPES.","Because the precursor is 1/3 ML rather than 2/3 ML, hydrogenation contracts the Bi layer instead of merely displacing it; bismuthene islands form inside an H-intercalated graphene matrix.","The out-of-plane spin polarization seen in bismuthene is assigned to final-state photoemission effects, meaning spin-resolved ARPES on strong spin-orbit materials can show apparent spin textures absent from the ground state.","Graphene's pi-bands remain spin-unpolarized in both phases, ruling out a strong proximity-induced spin polarization in the capping layer."],"supporting_citations":[{"why":"Establishes bismuthene on SiC as a high-temperature quantum spin Hall candidate with an 0.8 eV gap and honeycomb T1 structure, the baseline for the bismuthene phase.","marker":"[10]"},{"why":"Demonstrates reversible switching between precursor and bismuthene via hydrogenation; the precursor coverage assumption corrected here.","marker":"[11]"},{"why":"Shows graphene intercalation of bismuthene and supports the reversible conversion pathway between the two phases.","marker":"[12]"},{"why":"Earlier band-structure study of Bi-intercalated graphene/SiC that proposed an H3 adsorption site, contrasted with the T4 model adopted here.","marker":"[15]"},{"why":"STM observation of bismuthene islands embedded in an H/QFG matrix, cited as evidence for lateral contraction during hydrogenation.","marker":"[17]"},{"why":"Provide the higher-order cubic Rashba terms allowed by C3v symmetry that explain the non-tangential in-plane spin texture.","marker":"[21, 22]"},{"why":"Shows spin-dependent quantum interference in photoemission from spin-orbit coupled states, the basis for the final-state interference explanation of out-of-plane polarization.","marker":"[23]"},{"why":"States the time-reversal symmetry relation that the measured out-of-plane component violates, motivating the final-state interpretation.","marker":"[24]"}],"fun_headline_variants":["Spin-locked bands verified in bismuthene and its precursor","Rashba spin texture seen in both bismuthene phases","Spin-ARPES resolves spin-momentum locking in bismuthene","Bismuthene's electron spins are locked, spin-ARPES confirms"],"cache_read_input_tokens":2688,"weakest_assumption_plain":"The spin-texture comparison rests on DFT calculations that omit the graphene overlayer and are shifted by 0.3 eV to match experiment; if the omitted layer or the assumed 1/3 ML T4 structure changes the band ordering or spin direction, the claimed agreement would not hold.","fun_headline_variants_meta":{"raw":{"variants":["Spin-locked bands verified in bismuthene and its precursor","Rashba spin texture seen in both bismuthene phases","Spin-ARPES resolves spin-momentum locking in bismuthene","Bismuthene's electron spins are locked, spin-ARPES confirms"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000224,"raw_usage":{"total_tokens":1260,"prompt_tokens":671,"completion_tokens":589,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":415,"completion_tokens_details":{"reasoning_tokens":510}},"tokens_in":415,"tokens_out":589,"duration_ms":7264,"temperature":1.0,"reasoning_tokens":510,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T00:42:05.078720+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the out-of-plane spin polarization of bismuthene's valence bands while sweeping photon energy and light polarization: if the Pz signal changes sign or magnitude with those parameters, the final-state interference explanation is supported; if it stays fixed, an initial-state origin cannot be dismissed. Also compute DFT with the graphene overlayer included and check whether the 0.3 eV shift and the predicted in-plane spin texture survive.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes bismuthene on SiC as a high-temperature quantum spin Hall candidate with an 0.8 eV gap and honeycomb T1 structure, the baseline for the bismuthene phase."},{"cited_title":"Tilgner, S","cited_arxiv_id":null,"evidence_quote":"Demonstrates reversible switching between precursor and bismuthene via hydrogenation; the precursor coverage assumption corrected here."},{"cited_title":"Gehrig, C","cited_arxiv_id":null,"evidence_quote":"Shows graphene intercalation of bismuthene and supports the reversible conversion pathway between the two phases."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Earlier band-structure study of Bi-intercalated graphene/SiC that proposed an H3 adsorption site, contrasted with the T4 model adopted here."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"STM observation of bismuthene islands embedded in an H/QFG matrix, cited as evidence for lateral contraction during hydrogenation."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows spin-dependent quantum interference in photoemission from spin-orbit coupled states, the basis for the final-state interference explanation of out-of-plane polarization."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"States the time-reversal symmetry relation that the measured out-of-plane component violates, motivating the final-state interpretation."}],"review_version":1}