{"id":"ab6506dc-3dba-4bcd-88a5-296bc99ed14d","arxiv_id":"2607.11189","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":5.5,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"In epitaxial Bi(110)/Ni, spin Hall conductivity and spin diffusion length both increase on cooling, indicating Elliott-Yafet relaxation and skew-scattering-dominated SHE.","lead":"Temperature-dependent second-harmonic Hall measurements on epitaxial Bi(110)/Ni bilayers show that both spin Hall conductivity and spin diffusion length rise as temperature falls. This points to Elliott-Yafet spin relaxation and skew-scattering as the main charge-to-spin conversion channel in oriented bismuth.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.5","headline":"Thickness-independent ρ_Bi assumption is load-bearing for the σ_SH–λ_Bi linearity that underpins the skew-scattering claim.","rationale":"The Reader correctly isolates the constant-ρ_Bi assumption as the weakest link. The temperature trends of μ0 H_DL and σ_SH^eff themselves are robust (Figs. 3–4), the SHH separation of thermal artifacts is careful, and the OHE and interfacial-Rashba alternatives are reasonably excluded. What is not robust is the quantitative extraction of λ_Bi(T) and the subsequent linear σ_SH–λ_Bi relation that converts those trends into a microscopic mechanism. Because that relation is the sole direct evidence offered for skew scattering under Elliott-Yafet relaxation, the untested resistivity assumption is load-bearing. The concrete re-fit proposed above would settle the issue without new growth; until it (or an independent λ_Bi measurement) is performed, the CONDITIONAL verdict remains appropriate. No stronger objection (e.g., data fabrication, internal inconsistency of the SHH analysis) is warranted.","tokens_in":11728,"tokens_out":722,"duration_ms":9085,"concrete_test":"Re-fit the full σ_SH^eff(t_Bi,T) data set of Fig. 5a after allowing a simple surface-scattering form ρ_Bi(t) = ρ_bulk(T)(1 + λ_s/t_Bi) with a single free λ_s (or, equivalently, use the measured bilayer sheet resistance to constrain the Bi contribution under a parallel-resistor model). If the resulting σ_SH vs λ_Bi plot loses its linear correlation or the low-T upturn in λ_Bi disappears, the skew-scattering conclusion is no longer supported by the data.","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim rests on extracting σ_SH(T) and λ_Bi(T) from the thickness series of σ_SH^eff via Eq. (4), then showing that σ_SH scales linearly with λ_Bi (Fig. 5d). That extraction explicitly assumes ρ_Bi is independent of t_Bi (and that interface transparency is thickness-independent). The authors state that single-layer Bi(110)_R cannot be grown on insulating substrates, so the assumption is unchecked. In thin Bi films, surface scattering and possible residual intermixing commonly make ρ_Bi rise as t_Bi falls; if that occurs, the sech(t_Bi/λ_Bi) fit systematically misattributes the curvature, shifting both extracted λ_Bi and the apparent σ_SH–λ_Bi slope. Because the Elliott-Yafet + skew-scattering assignment is inferred precisely from that slope (and from λ_Bi rising with falling T), a thickness-dependent ρ_Bi would remove the principal evidence for the mechanism.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.5","summary":"The manuscript reports temperature-dependent second-harmonic Hall measurements of damping-like spin-orbit torque in epitaxial Bi(110)R/Ni bilayers. After separating thermal contributions (ANE/SSE, ordinary Nernst, planar Nernst), the authors extract an effective spin Hall conductivity σ_SH^eff that rises with decreasing temperature for Bi thicknesses ≳ 5.5 nm. Fitting the thickness series of σ_SH^eff to the standard sech form (Eq. 4) yields both the intrinsic spin Hall conductivity σ_SH and the spin diffusion length λ_Bi; both quantities increase upon cooling, and σ_SH scales linearly with λ_Bi. From this the authors conclude that spin relaxation is Elliott–Yafet and that the SHE is dominated by skew scattering, with only a minor intrinsic (Dirac-electron) contribution. Orbital-Hall and interfacial Rashba contributions are argued to be negligible on the basis of prior Bi/Fe ST-FMR data and theory.","tokens_in":11991,"tokens_out":1248,"duration_ms":11148,"significance":"If the extraction of σ_SH(T) and λ_Bi(T) is robust, the work supplies a concrete microscopic assignment for the large, orientation-dependent charge-to-spin conversion previously reported in Bi(110)R. Establishing Elliott–Yafet + skew scattering as the dominant channel would place Bi(110)R in the same extrinsic regime as high-conductivity Pt and would guide materials design for low-temperature spin-orbit-torque devices. The careful angular- and field-dependent separation of thermal artifacts is a methodological strength relative to ST-FMR, and the data set is openly archived.","major_comments":[{"comment":"Section III and Eq. (4): the extraction of σ_SH and λ_Bi rests on the explicit assumption that the Bi resistivity ρ_Bi is independent of thickness. The authors note that single-layer Bi(110)R cannot be grown on insulating substrates, so the assumption is unchecked. In thin Bi films surface scattering commonly makes ρ_Bi rise as t_Bi falls; any such dependence systematically distorts the sech curvature, shifts the extracted λ_Bi, and can manufacture or destroy the linear σ_SH–λ_Bi relation of Fig. 5(d) that underpins the skew-scattering claim. A quantitative sensitivity analysis (or an independent estimate of ρ_Bi(t) from bilayer conductance) is required before the mechanism assignment can be regarded as secure.","section":null},{"comment":"Section III, discussion of μ0Ms: the conversion of HDL into σ_SH^eff (Eq. 3) uses a fixed room-temperature value μ0Ms = 0.61 T for all temperatures, while the authors themselves note that Ms(0 K)/Ms(300 K) ≈ 1.06. Because both σ_SH and λ_Bi are obtained from the same thickness series of σ_SH^eff, a temperature-dependent Ms rescales the entire vertical axis of Fig. 5(d) and can alter the apparent slope. The manuscript should either measure Ms(T) on the actual stacks or propagate the known Ms(T) correction through the fits and re-evaluate the linearity.","section":null},{"comment":"Section III, final paragraphs: the authors acknowledge that band-gap shrinking at the L-point can enhance the intrinsic SHE upon cooling, yet still assert that skew scattering dominates solely on the basis of the linear σ_SH–λ_Bi plot. Without an independent estimate of the intrinsic contribution (e.g., from the known L-point gap temperature dependence or a comparison with the Bi-Sb data of Ref. [39] under identical analysis), the relative weight of intrinsic versus extrinsic channels remains qualitative. A short calculation or bounding argument would strengthen the claim.","section":null}],"minor_comments":[{"comment":"Fig. 4 caption: several data points (Ni-only at 50 K/10 K, Bi(9.5 nm) at 10 K) are omitted because fits failed; a brief statement of the failure criterion would help the reader assess completeness.","section":null},{"comment":"Eq. (2): the symbols a∇T and b∇T are introduced without units or typical magnitudes; a short parenthetical estimate would clarify that they are sub-dominant.","section":null},{"comment":"Page 4: the phrase “the high-purity sample negates the side-jump mechanism” is slightly overstated; high purity suppresses side-jump relative to skew, but does not rigorously exclude it.","section":null},{"comment":"References: a few recent works on temperature-dependent SHE in other Dirac/semimetal systems could be added for context, but this is optional.","section":null}],"recommendation":"major_revision","confidential_remarks":"The central physics claim is interesting and the experimental care with thermal artifacts is above average, but the load-bearing resistivity assumption is currently untested. I would not recommend acceptance until the authors either constrain ρ_Bi(t) or demonstrate that the σ_SH–λ_Bi linearity survives plausible thickness-dependent resistivity models. The paper is otherwise well within the scope of a solid-state/spintronics journal."},"author_rebuttal":null,"desk_editor":{"model":"grok-4.5","letter":"The new result is the temperature series: both σ_SH and λ_Bi rise on cooling in epitaxial Bi(110), and the authors read the linear σ_SH–λ_Bi plot as skew scattering under Elliott-Yafet relaxation. That assignment is new for this orientation and useful, because the large room-temperature SHE they already reported (PNAS 2023) needed a microscopic mechanism.\n\nWhat they do well is the measurement. SHH with full angle and field dependence lets them separate damping-like SOT from ANE/SSE, ordinary Nernst, and planar Nernst; the concave ADL curves are clear at every temperature. They also give a solid argument against orbital Hall contamination by comparing Bi/Ni and Bi/Fe efficiencies and citing the Kubo calculation. Data are deposited, self-citation is background only, and circularity is low.\n\nThe soft spot is real but not fatal. Extraction of σ_SH(T) and λ_Bi(T) from the thickness series of σ_SH^eff uses Eq. (4) under the explicit assumption that ρ_Bi is thickness-independent. They cannot grow single-layer Bi(110) on insulators, so the assumption is unchecked. Surface scattering or residual intermixing would make ρ_Bi rise at small t_Bi and systematically shift both extracted parameters and the slope that underpins the skew-scattering claim. Missing error bars on the fit parameters and a few omitted low-T points are secondary. The qualitative rise of λ_Bi with cooling still favors EY over Dyakonov-Perel, and the overall temperature trend of the conversion is robust; only the quantitative linearity is load-bearing.\n\nThis is for people already working on SHE mechanisms in Dirac semimetals or high-conductivity spin-orbit materials. It does not invent a new platform, but it tightens the interpretation of an already interesting system. I would send it to peer review; a referee can demand a resistivity check or an independent λ_Bi measurement without killing the paper. Worth reading and, with that caveat, worth citing.","headline":"Clean temperature-dependent SHH data on epitaxial Bi(110)/Ni that reasonably points to EY + skew scattering, but the key linearity rests on an unchecked constant-ρ_Bi assumption.","tokens_in":12688,"tokens_out":568,"would_cite":true,"duration_ms":5897,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.5","headline":"Both spin Hall conductivity and spin diffusion length rise as temperature falls in epitaxial Bi(110), showing skew scattering under Elliott-Yafet relaxation.","keywords":["spin Hall effect","bismuth thin films","Elliott-Yafet","skew scattering","second-harmonic Hall","spin diffusion length","spin-orbit torque","rhombohedral Bi(110)"],"falsifier":"A direct measurement of thickness-dependent resistivity on Bi(110) films (or an independent determination of spin diffusion length by another technique such as nonlocal spin valves) that breaks the observed linear relation between spin Hall conductivity and spin diffusion length.","tokens_in":12662,"feed_emoji":"🧲","tokens_out":571,"duration_ms":5201,"temperature":0.7,"pith_summary":"Rhombohedral (110) bismuth converts charge current into spin current with unusually high efficiency, yet the microscopic scattering process that produces this conversion has remained unclear. This work measures that conversion from room temperature down to 10 K in epitaxial Bi(110)/Ni bilayers by the second-harmonic Hall method, carefully subtracting thermal artifacts. Both the spin Hall conductivity and the spin diffusion length increase as temperature is lowered. The simultaneous rise of the two quantities implies that spin relaxation is of Elliott-Yafet type (spin lifetime tracks momentum lifetime) and that the dominant conversion mechanism is extrinsic skew scattering rather than intrinsic band effects or side-jump scattering. The result supplies a concrete microscopic picture for the large, orientation-dependent spin Hall effect previously reported in this crystal face and guides how purity and temperature can be used to tune the conversion for spintronic devices.","feed_headline":"Bi(110) spin Hall rises at low T via skew scattering","feed_subtitle":"Spin conductivity and diffusion length both grow on cooling, fixing the conversion mechanism","key_machinery":"Thickness- and temperature-dependent second-harmonic Hall extraction of the damping-like effective field, converted to effective spin Hall conductivity and then fitted to the sech form that isolates bulk spin Hall conductivity and spin diffusion length.","core_discovery":"In epitaxial Bi(110)/Ni bilayers the effective spin Hall conductivity and the Bi spin diffusion length both increase with decreasing temperature; their linear correlation shows that spin relaxation follows the Elliott-Yafet mechanism and that charge-to-spin conversion is dominated by skew scattering.","pith_inferences":[],"forward_implications":[],"fun_headline_variants":["Bi(110) spin Hall conductivity rises on cooling via skew scattering","Both SH conductivity and diffusion length grow as T falls in Bi(110)","Cooling boosts Bi(110) SHE; EY mechanism and skew scattering dominate","Spin relaxation in Bi(110) follows Elliott-Yafet as T drops","Charge-to-spin conversion in Bi(110) strengthens at low temperature"],"cache_read_input_tokens":128,"weakest_assumption_plain":"The resistivity of the Bi layer is assumed independent of thickness when the thickness series is fitted, even though pure Bi(110) films cannot be grown on insulating substrates to check that assumption directly.","fun_headline_variants_meta":{"raw":{"variants":["Bi(110) spin Hall conductivity rises on cooling via skew scattering","Both SH conductivity and diffusion length grow as T falls in Bi(110)","Cooling boosts Bi(110) SHE; EY mechanism and skew scattering dominate","Spin relaxation in Bi(110) follows Elliott-Yafet as T drops","Charge-to-spin conversion in Bi(110) strengthens at low temperature"]},"model":"grok-4.5","effort":"low","cost_usd":0.005096,"raw_usage":{"total_tokens":1366,"prompt_tokens":727,"num_sources_used":0,"completion_tokens":83,"cost_in_usd_ticks":50960000,"prompt_tokens_details":{"text_tokens":727,"audio_tokens":0,"image_tokens":0,"cached_tokens":128},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":556,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":727,"tokens_out":83,"duration_ms":6698,"temperature":1.0,"reasoning_tokens":556,"cache_read_input_tokens":128,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-14T06:22:57.480455+00:00","model_set":{"reader":"grok-4.5"},"falsifier":"A direct measurement of thickness-dependent resistivity on Bi(110) films (or an independent determination of spin diffusion length by another technique such as nonlocal spin valves) that breaks the observed linear relation between spin Hall conductivity and spin diffusion length.","supporting_citations":[],"review_version":1}