{"id":"e6f88411-741d-4ee2-843b-e66350f8b96b","arxiv_id":"2502.08269","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"Orbital pumping is identified in Nb/Ni bilayers via a voltage sign reversal that cannot be explained by spin pumping alone.","lead":"The paper reports that in nickel/niobium structures, a spinning magnet appears to send orbital electron motion into the niobium, producing a voltage with a distinctive sign. This gives experimenters a way to distinguish that 'orbital pumping' from ordinary spin effects and background noise.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Central claim hinges on unverified sign of Nb's orbital Hall angle imported from self-cited prior work; if that sign were opposite, the Nb/Ni sign reversal would match conventional spin pumping and the orbital pumping interpretation collapses.","rationale":"The reader's weakest assumption identifies exactly the same load-bearing concern: the sign of Nb's OHA is taken from self-cited prior work and, if wrong, the sign reversal in Nb/Ni could be conventional spin pumping from a positive-SHA Nb. I agree with this assessment after a good-faith reading of the manuscript. The paper presents a plausible experimental identification with useful methodological additions, including the angular decomposition in Eq. (1) and the spatial dependence in Figure 3, but those tools do not independently determine the OHA sign. The spin Hall angle sign is inferred from the same experiment, so the external OHA sign is the only unverified premise that can uniquely break the chain from observation to orbital pumping. The proposed concrete test, a direct independent measurement or computation of the OHA sign for the same Nb films, would settle whether the concern actually lands. Since this concern is the same one the reader flagged, the verdict remains CONDITIONAL; no change to the reader's recommendation is necessary.","tokens_in":9719,"tokens_out":12241,"duration_ms":129746,"concrete_test":"Determining the OHA sign of the identical Ta(1)/Nb(4) films by an independent technique, e.g., harmonic Hall voltage measurements or magneto-optical Kerr effect detection of orbital accumulation in Nb/Cu/FM reference stacks, with Pt as a control; if the measured OHA sign is opposite to Pt, the orbital-pumping interpretation of the Nb/Ni sign reversal is disproven, whereas if it matches Pt the central claim is supported.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central argument is that the sign reversal of the symmetric Lorentzian voltage in Nb/Ni relative to Nb/FeCoB cannot be explained by conventional spin pumping and therefore demonstrates orbital pumping. This logic requires two material-specific sign assignments: (i) Nb has a negative spin Hall angle (SHA) relative to Pt, and (ii) Nb has a positive orbital Hall angle (OHA), i.e., the same sign as Pt. The SHA sign is inferred internally from the observed negative VS in Nb/FeCoB, but the OHA sign is imported from Ref. [24], a self-cited prior experiment, and theory in Ref. [19]. The exact sputtered Ta(1)/Nb(4) films used here are not independently characterized for their OHA sign in this work. If the OHA sign of these Nb films were actually opposite to Pt, then the same sign of VS in Nb/Ni and Pt would be exactly the conventional spin-pumping signal from a Nb with positive SHA, and the orbital-pumping attribution would be unsupported. Because the angular-dependent and spatial-dependent methods are applied to interpret the sign reversal, they cannot rescue the conclusion if the OHA sign premise is false. This concern is load-bearing because it is the single external assumption on which the identification of orbital pumping rests, and it is not verified in the specific samples or geometry of this study.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports spin-pumping and orbital-pumping measurements in Nb/Ni and Nb/FeCoB bilayers compared with Pt/Ni and Pt/FeCoB controls. The central observation is a sign reversal of the symmetric Lorentzian voltage VS in Nb/Ni relative to Nb/FeCoB, which the authors attribute to orbital pumping dominating over spin pumping because Nb is assumed to have a spin Hall angle opposite to Pt but the same orbital Hall angle. The authors also present angular-dependent measurements fitted by Eq. (1) and a gap-width dependence of the extracted coefficients, which they propose as methods to separate pumping signals from spin rectification effects (SREs).","tokens_in":9963,"tokens_out":5158,"duration_ms":52064,"significance":"If the interpretation is correct, the work provides a practical scheme for identifying orbital pumping in metallic bilayers using sign reversal and angular/spatial fingerprints, building on the recently predicted orbital pumping mechanism. The device geometry with the sample placed in the waveguide slot is a useful methodological contribution, and the explicit treatment of ST-FMR as a source of symmetric voltage is a valuable caveat for the field. However, the central conclusion rests on an external, self-cited sign assignment for the orbital Hall angle of Nb that is not verified on the specific films used here, and the key fits lack error analysis. The result is therefore plausible but not yet load-bearing without further experimental support.","major_comments":[{"comment":"The identification of orbital pumping in Nb/Ni hinges on the assumption that Nb has the same sign of the orbital Hall angle (OHA) as Pt and an opposite spin Hall angle, as stated in the text: 'We chose Nb and Pt as they exhibit opposite signs for the spin Hall angle (SHA), yet have the same sign for the orbital Hall angle (OHA) [24].' This OHA sign is imported from Ref. [24], a prior study by the same group, and is not verified for the specific Ta(1)/Nb(4) films used in this work. If the OHA sign in these films were opposite to Pt, the positive VS in Nb/Ni would be indistinguishable from conventional spin pumping with a positive SHA in Nb, and the orbital-pumping claim would collapse. The authors should either directly characterize the OHA sign on identical films (e.g., via harmonic Hall or orbital torque measurements) or explicitly restrict the conclusion to the assumed sign convention and specify a measurement that could falsify the orbital-pumping interpretation.","section":"RESULTS AND DISCUSSION, paragraph after Figure 2"},{"comment":"The angular dependence in Eq. (1) is fitted to the data in Figures 2(e-h) and 3, but no error bars, confidence intervals, or goodness-of-fit metrics are reported for the fitted coefficients. The central observable is the sign of V_S^pump, the coefficient of the sin φ term; without uncertainty estimates the reader cannot judge whether the sign reversal between Nb/Ni and Nb/FeCoB is statistically significant, nor whether the magnitudes of the SRE terms are reliably separated from the pumping term. Please report standard errors or confidence intervals for all fitted coefficients, and show representative residuals.","section":"Eq. (1) and Figures 2(e-h), 3"},{"comment":"The claim that the pumping signal V_S^pump scales as 1/d^m with m ≈ 1.0, whereas the ST-FMR term scales as m ≈ 2.0, is used to argue that the sin φ term is a true pumping signal rather than a rectification artifact. However, no uncertainty on m is reported, and the number of devices and error bars per gap width are not stated. Without this information, the spatial-dependence separation is not quantitatively established. Please provide the individual data points with uncertainties, the fit with confidence bands, and specify how many independent measurements were taken.","section":"Figure 3 and 'RESULTS AND DISCUSSION' text on gap-width dependence"},{"comment":"The paper states that the sign reversal 'cannot be explained by the conventional spin pumping effect', but no quantitative estimate of the expected spin pumping voltage in Nb/Ni is provided. To support this statement, the authors should estimate the spin pumping contribution from the known (negative) SHA of Nb and show that it cannot account for the observed positive VS. Without such a comparison, the argument rests entirely on the assumed sign of the OHA, and other sources of positive VS (e.g., thermal or Nernst contributions) are not excluded.","section":"RESULTS AND DISCUSSION, paragraph after Figure 2"}],"minor_comments":[{"comment":"The phrase 'chiral magenetic skyrmions' contains a typo; it should be 'chiral magnetic skyrmions'.","section":"Introduction, first paragraph"},{"comment":"The text reads 'variouscurrent-inducedmagneticfields' without spacing; please correct the formatting to 'various current-induced magnetic fields'.","section":"RESULTS AND DISCUSSION, paragraph on SREs"},{"comment":"Figure 3 uses the symbol d for gap width, but the experimental section uses 'gap width' without defining d; please define d explicitly in the caption or text.","section":"Figure 3 and text"},{"comment":"The notation V_S^pump is used as a coefficient in Eq. (1) but is not explicitly defined as the amplitude of the sin φ term; please add a sentence defining this notation.","section":"Eq. (1)"}],"recommendation":"major_revision","confidential_remarks":"The paper relies heavily on the authors' prior work (Ref. [24]) for the sign of the orbital Hall angle in Nb, which is the key external input. This is not inappropriate in itself, but given that the central claim collapses if that sign is wrong, the authors should provide a direct verification in this manuscript or clearly reduce the claim to a conditional statement. The lack of error bars on all fitted coefficients is also a concern that should be addressed before publication. The paper fits the scope of the journal and the methods proposed are potentially useful, so a major revision is appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The one thing you should know: this paper reports a clean sign reversal in the pumped DC voltage for Nb/Ni compared with Nb/FeCoB, and reads it as orbital pumping dominating over spin pumping. That is a new and potentially useful observation. The same community already saw orbital pumping in W, but the Nb/Ni result and the sign-reversal discrimination scheme are genuinely new extensions.\n\nThe device work is solid. Putting the NM/FM wire inside a waveguide slot suppresses the usual ST-FMR background, and the angular fit separates the pumping term (sin phi) from AMR-type rectification terms. The gap-width scaling is a nice additional check: the pumping signal falls like 1/d while the ST-FMR term falls faster. The comparison across Pt, Nb, and Ru is the right control set.\n\nThe soft spot is the one the stress test flagged. The whole interpretation leans on the sign of Nb's orbital Hall angle relative to Pt, imported from Ref. [24] by the same group. They do measure the spin Hall angle sign internally through Nb/FeCoB, but they do not independently calibrate the OHA sign on these exact Ta(1)/Nb(4) films. If that sign were opposite, the Nb/Ni signal would look like conventional spin pumping from a positive-SHA Nb, and the orbital pumping claim would collapse. This is a load-bearing assumption, not a footnote. It is not fatal, because the prior work and theory support the assumed sign, but it would be much stronger with an in-situ measurement or a non-self-cited cross-check.\n\nSmaller issues: the angular fit has no error bars on the extracted coefficients, the gap-width exponent is based on a limited set of points, and data are only available on request. These are minor-to-moderate. I do not think the paper is overclaiming; it says the sign reversal 'strongly suggests' orbital pumping, which is the right level of caution given the external sign dependence.\n\nWho is this for? People working on orbitronics, especially electrical detection of orbital currents, and anyone designing experiments to separate pumping from rectification. The paper deserves a serious referee: the claim is plausible, the method is useful, and the load-bearing assumption is exactly what a referee should push on. I would not desk-reject it.","headline":"A plausible but sign-dependent orbital pumping identification in Nb/Ni, worth refereeing despite relying on a self-cited OHA sign.","tokens_in":10541,"tokens_out":1812,"would_cite":true,"duration_ms":20436,"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":"A sign flip fingerprints orbital pumping in Nb/Ni bilayers.","keywords":["orbital pumping","inverse orbital Hall effect","spin pumping","spin rectification effects","niobium","ferromagnetic resonance","orbital Hall effect","orbitronics"],"falsifier":"Measure the sign of the orbital Hall angle of these exact Nb(4) films by an independent technique, for example harmonic Hall or orbital-torque magnetometry on Nb/CoFeB with a heavy-metal capping layer. If the orbital Hall angle of this Nb is found to have the opposite sign to Pt, the observed sign reversal in Nb/Ni would have to be re-assigned to spin pumping or another mechanism rather than to orbital pumping.","tokens_in":9518,"feed_emoji":"🧲","tokens_out":5639,"duration_ms":50444,"temperature":0.7,"pith_summary":"The paper sets out to show that the recently predicted effect of orbital pumping—a precessing magnet emitting a pure orbital current into an adjacent metal—can be identified in experiment despite large backgrounds from spin pumping and spin rectification. Its evidence is a sign reversal of the symmetric Lorentzian voltage in Nb/Ni compared with Nb/FeCoB, which conventional spin pumping cannot explain. Because Nb's spin Hall angle and orbital Hall angle have opposite signs, the reversal indicates that the orbital current converted by the inverse orbital Hall effect dominates in Nb/Ni. The paper also establishes angular- and spatial-dependence procedures that separate the pumping signal from rectification artifacts.","feed_headline":"A sign flip fingerprints orbital pumping in Nb/Ni bilayers","feed_subtitle":"Symmetric-voltage reversal in Nb/Ni, unlike Nb/FeCoB, points to orbital current, offering a clean test of orbitronics.","key_machinery":"The load-bearing objects are (i) the bilayer device placed in the slot of a coplanar waveguide, which produces a uniform radio-frequency field and lets the induced current path be distinguished from the pumping path; (ii) the angular decomposition $V_S(\\phi) \\approx V_S^{\\mathrm{pump}}\\sin\\phi + V_{S,\\mathrm{AMR}}^{\\mathrm{ST-FMR}}\\cos\\phi\\sin 2\\phi + V_{S,\\mathrm{AMR}}^{\\mathrm{NL}}\\sin 2\\phi$, which assigns the $\\sin\\phi$ term to the pumping signal and the other terms to spin-torque and nonlocal rectification; and (iii) the sign competition between the spin Hall angle (negative) and orbital Hall angle (positive) of Nb relative to Pt, which makes orbital pumping reverse the sign of the symmetric voltage. The inverse orbital Hall effect in the Nb layer is the conversion mechanism that turns the pumped orbital current into a measurable voltage.","core_discovery":"The central claim is that the sign reversal of the symmetric Lorentzian voltage observed in Nb(4)/Ni(6) compared with Nb(4)/FeCoB(8) cannot be explained by the conventional spin pumping effect, and therefore identifies orbital pumping via the inverse orbital Hall effect in Nb. The same sign of $V_S$ in Nb/Ni and Pt/(Ni or FeCoB), combined with the opposite sign in Nb/FeCoB, is read as evidence that the injected orbital current dominates the measured voltage in Nb/Ni, while spin pumping sets the sign in Nb/FeCoB and Pt. The angular dependence of $V_S$ is fitted to $V_S(\\phi) \\approx V_S^{\\mathrm{pump}}\\sin\\phi + V_{S,\\mathrm{AMR}}^{\\mathrm{ST-FMR}}\\cos\\phi\\sin 2\\phi + V_{S,\\mathrm{AMR}}^{\\mathrm{NL}}\\sin 2\\phi$, and the $\\sin\\phi$ coefficient is shown to be positive in Nb/Ni, indicating orbital-pumping dominance; the gap-width dependence of this coefficient scales as $1/d^{m}$ with $m\\approx 1$, as expected for a pumping signal, separating it from the $m\\approx 2$ scaling of the rectification term.","pith_inferences":["A direct test would be to measure the orbital Hall angle of the specific sputtered Nb films used here by an independent orbital-torque experiment; if the sign differs from the assumption taken from earlier work, the sign-reversal interpretation would need revision.","The $\\sin\\phi$ sign criterion could be used as a fast screening tool to rank candidate nonmagnetic metals by their orbital-pumping efficiency without needing separate spin- and orbital-Hall calibrations.","If orbital currents indeed survive in light metals with weak spin-orbit coupling, the same geometry could be used to probe orbital pumping in stacks where conductivity mismatch would otherwise hide spin pumping.","The gap-width scaling suggests that measuring the same device at several waveguide distances is a general separator; one could extend it to extract the orbital diffusion length from the $d$-dependence at short separations."],"forward_implications":["Orbital pumping generates a pure orbital current from a precessing ferromagnet without any electrical current injection, offering a route to orbital-current sources free of conductivity-mismatch losses.","Ni emits a detectable orbital current into Nb, and the sign of the symmetric voltage can be used as a fingerprint of the dominant pumped angular-momentum channel.","The angular-dependence decomposition isolates the pumping contribution from spin-torque and nonlocal rectification terms in the same measurement.","The gap-width scaling ($m\\approx 1$ for pumping vs $m\\approx 2$ for ST-FMR rectification) provides a spatial test that separates pumping from rectification artifacts.","The approach should transfer to other NM/FM pairs whose spin and orbital Hall angles have opposite signs, enabling material screening for orbital pumping."],"supporting_citations":[{"why":"Supplies the sign assignment that Nb and Pt have opposite spin Hall angles but the same orbital Hall angle sign, the load-bearing premise of the sign-reversal interpretation.","marker":"[24]"},{"why":"First-principles calculation of intrinsic spin and orbital Hall effects in metals that supports the sign of the inverse spin Hall voltage in Nb relative to Pt.","marker":"[19]"},{"why":"Theoretical prediction of orbital pumping incorporating orbital angular momentum and position, providing the mechanism the paper claims to detect.","marker":"[33]"},{"why":"Theoretical prediction of orbital pumping by magnetization dynamics in ferromagnets, defining the orbital-pumping effect.","marker":"[34]"},{"why":"Establishes the angular dependence of the inverse spin Hall voltage from spin pumping, the basis for the $\\sin\\phi$ term in the fit.","marker":"[38]"},{"why":"Derives the spin pumping and anisotropic magnetoresistance voltages in magnetic bilayers, the basis for separating pumping from rectification.","marker":"[40]"},{"why":"Demonstrates the inverse spin Hall effect conversion of spin current to charge voltage, the reference for the conventional spin-pumping signal.","marker":"[46]"},{"why":"Prior observation of orbital pumping, the experiment this work extends by resolving the spin-pumping and rectification backgrounds.","marker":"[45]"},{"why":"Identifies spin-torque ferromagnetic resonance as a source of symmetric voltage, which the paper must exclude in its angular analysis.","marker":"[48]"}],"fun_headline_variants":["Sign reversal signals orbital pumping in Nb/Ni","Orbital pumping exposed by voltage sign flip in Nb/Ni","Voltage sign flip separates orbital pumping from spin effects","Nb/Ni voltage reversal tags orbital pumping","Clean test for orbital pumping via sign change in Nb/Ni"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The interpretation rests on the assumption that the specific Nb films have a spin Hall angle opposite in sign to Pt and an orbital Hall angle of the same sign as Pt, taken from prior work on orbital Hall torques.","fun_headline_variants_meta":{"raw":{"variants":["Sign reversal signals orbital pumping in Nb/Ni","Orbital pumping exposed by voltage sign flip in Nb/Ni","Voltage sign flip separates orbital pumping from spin effects","Nb/Ni voltage reversal tags orbital pumping","Clean test for orbital pumping via sign change in Nb/Ni"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000241,"raw_usage":{"total_tokens":1535,"prompt_tokens":970,"completion_tokens":565,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":586,"completion_tokens_details":{"reasoning_tokens":487}},"tokens_in":586,"tokens_out":565,"duration_ms":4889,"temperature":1.0,"reasoning_tokens":487,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-08T05:46:09.371744+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the sign of the orbital Hall angle of these exact Nb(4) films by an independent technique, for example harmonic Hall or orbital-torque magnetometry on Nb/CoFeB with a heavy-metal capping layer. If the orbital Hall angle of this Nb is found to have the opposite sign to Pt, the observed sign reversal in Nb/Ni would have to be re-assigned to spin pumping or another mechanism rather than to orbital pumping.","supporting_citations":[{"cited_title":"Detection of long-range orbital-hall torques,","cited_arxiv_id":null,"evidence_quote":"Supplies the sign assignment that Nb and Pt have opposite spin Hall angles but the same orbital Hall angle sign, the load-bearing premise of the sign-reversal interpretation."},{"cited_title":"First-principles the- ory of intrinsic spin and orbital hall and nernst effects in metallic monoatomic crystals,","cited_arxiv_id":null,"evidence_quote":"First-principles calculation of intrinsic spin and orbital Hall effects in metals that supports the sign of the inverse spin Hall voltage in Nb relative to Pt."},{"cited_title":"Angular de- pendence of inverse spin–hall effect induced by spin pumping investigated in a ni 81 fe 19/pt thin film,","cited_arxiv_id":null,"evidence_quote":"Establishes the angular dependence of the inverse spin Hall voltage from spin pumping, the basis for the $\\sin\\phi$ term in the fit."},{"cited_title":"Spin pumping and anisotropic magnetoresistance voltages in magnetic bi- layers: Theory and experiment,","cited_arxiv_id":null,"evidence_quote":"Derives the spin pumping and anisotropic magnetoresistance voltages in magnetic bilayers, the basis for separating pumping from rectification."},{"cited_title":"Con- version of spin current into charge current at room tem- perature: Inverse spin-hall effect,","cited_arxiv_id":null,"evidence_quote":"Demonstrates the inverse spin Hall effect conversion of spin current to charge voltage, the reference for the conventional spin-pumping signal."},{"cited_title":"Observation of orbital pumping,","cited_arxiv_id":null,"evidence_quote":"Prior observation of orbital pumping, the experiment this work extends by resolving the spin-pumping and rectification backgrounds."},{"cited_title":"Spin- torque ferromagnetic resonance induced by the spin hall effect,","cited_arxiv_id":null,"evidence_quote":"Identifies spin-torque ferromagnetic resonance as a source of symmetric voltage, which the paper must exclude in its angular analysis."}],"review_version":1}