{"id":"bbfdc2fd-3474-4cc0-940b-137ce694e079","arxiv_id":"2412.18937","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"By decomposing the spin Hall conductivity of RuO2 into time-reversal-odd and time-reversal-even parts, the authors find the nonrelativistic spin current is polarized nearly along the Néel vector.","lead":"The authors use spin Hall magnetoresistance to separate two kinds of spin current in the altermagnet RuO2 and find that the magnetic, nonrelativistic component is polarized along the Néel vector. The result supports a magnetic origin for unconventional spin currents and offers a temperature-based method for isolating them.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The T-odd/T-even decomposition rests on an unverified constant-even and 1/T-odd assumption; the low-temperature odd polarization angle may be an artifact of the extrapolated two-parameter fit.","rationale":"The Reader's weakest-assumption analysis identified the same load-bearing point: the decomposition into T-odd and T-even spin Hall components depends on assumed temperature scalings and a two-parameter fit over a limited range, which is then extrapolated to 5 K. My stress-test reading confirms that this is the most critical step for the central claim. The paper's own text states the assumptions: the even SHC is taken to be weakly temperature dependent and the odd SHC is taken to follow tau proportional to 1/T between 150 K and 300 K, with no independent low-temperature constraint on the even component. If either assumption fails, the residual odd component and its polarization angle change, potentially moving away from the Neel-vector direction. The proposed concrete test, allowing a linear temperature variation in the even component and propagating uncertainties, would directly reveal whether the claimed 35-degree polarization is an artifact of the fitting model. Since this concern matches the Reader's verdict and I found no additional objection that would change the overall assessment, the conditional verdict should stand unchanged.","tokens_in":11568,"tokens_out":18402,"duration_ms":191724,"concrete_test":"Refit the Fig. 4 data with a model that does not assume a strictly T-independent even component, e.g., sigma_total(T) = A/T + B(1 + alpha T) for each component over the full 5-360 K range, and recompute theta_s^odd with propagation of the fit covariance. Alternatively, pin B_y and B_z using the 360 K SMR point or a separate nonmagnetic control sample (e.g., IrO2 of the same orientation). If theta_s^odd deviates by more than about 5 degrees from 35 degrees at any temperature, or if the propagated error bars exceed about 10 degrees, the decomposition is not robust and the unconditional claim should be softened.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires that the SMR-derived total spin Hall conductivity components are exactly the sum of a T-odd term proportional to 1/T and a T-even term independent of temperature. The authors fit sigma_zx^z(T) = A_z/T + B_z and sigma_zx^y(T) = A_y/T + B_y only over 150-300 K, then use B_y and B_z to define the even part at all temperatures. This is the load-bearing step, and it is not independently verified. First, a T-even spin Hall conductivity in a low-symmetry metal need not be strictly T-independent: Fermi-surface broadening, phonon-assisted side-jump, and skew scattering can introduce temperature dependence. Second, the T-odd conductivity may include an intrinsic, relaxation-time-independent contribution, so the 1/T form is itself an assumption. Third, a two-parameter fit provides no handle on these alternatives, and no error bars or goodness-of-fit are reported. If, for example, B_z is even slightly overestimated because the even component grows at low temperature, the residual odd z-component is reduced and theta_s^odd moves away from the claimed 35 degrees. Because the paper explicitly claims unambiguous evidence of magnetic origin, this unvalidated extrapolation from 150-300 K to 5-360 K is the soft spot.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports SMR measurements on (101)-oriented RuO2/CoFeB bilayers and uses the temperature dependence of the measured spin Hall conductivity to separate a time-reversal-odd (T-odd) component, assumed to scale as 1/T, from a time-reversal-even (T-even) component, assumed to be temperature independent. The authors find that the T-odd spin current polarization angle is approximately arctan(c/a) = 35 degrees over 5 to 360 K, which they identify as the Néel vector direction, and conclude that the nonrelativistic spin current in RuO2 has a magnetic origin. They also find that the T-even component becomes dominant at high temperature, which they use to explain the tilted spin polarization observed in earlier RuO2 experiments.","tokens_in":11856,"tokens_out":3182,"duration_ms":33923,"significance":"If the decomposition were independently validated, the result would resolve an important puzzle: the apparent mismatch between theoretically predicted and experimentally observed spin current polarization in RuO2 would be explained by a large conventional T-even spin Hall component, while the T-odd component would be shown to align with the Néel vector. The SMR-based technique, with the Pt/CoFeB calibration and the use of two crystallographic current directions, is a useful experimental contribution, and the paper explicitly ships a falsifiable prediction: the T-odd polarization angle should track the Néel vector orientation. However, the central decomposition rests on an assumed 1/T and constant functional form that is fitted only over 150-300 K and then extrapolated, and no independent verification or error analysis is provided. For this reason the significance is conditional on the validity of that temperature-dependence assumption.","major_comments":[{"comment":"The load-bearing step is the decomposition σ_zx^z(T) = A_z/T + B_z and σ_zx^y(T) = A_y/T + B_y, with A_z, B_z, A_y, B_y fixed by two-parameter fits restricted to 150-300 K and then used to define the T-even background at all temperatures down to 5 K. This assumes, without independent support, that the T-even spin Hall conductivity is strictly temperature independent below 150 K and that the T-odd part follows exactly 1/T. Both assumptions are questionable: extrinsic T-even mechanisms such as phonon-assisted side jump and skew scattering can be temperature dependent, and the T-odd conductivity can contain an intrinsic relaxation-time-independent contribution. No error bars, goodness-of-fit values, or residuals are reported, so it is not possible to assess whether the data actually discriminate between this model and alternatives. If, for example, the true T-even z-component grows at low temperature, B_z is overestimated and θ_s^odd would move away from 35 degrees. This extrapolation is the basis of the central claim, so it needs to be verified, e.g., by fits with additional functional forms, by showing the decomposition is stable below 150 K, or by an independent measurement of one of the components.","section":"Fig. 4(b,c) and text after Eq. (1)"},{"comment":"The conclusion that the T-odd spin current is 'polarized along the Néel vector' requires knowledge of the Néel vector orientation in the measured films, but no magnetic characterization is presented. The angle θ_N = arctan(c/a) = 35 degrees is computed from the lattice parameters and assumes a particular antiferromagnetic domain state. This assumption is especially delicate because the authors themselves cite recent reports (Refs. [56-58]) suggesting a nonmagnetic ground state in RuO2, and they attribute the possible magnetism in their films to strain, oxygen vacancies, interface charge transfer, or anti-site defects, which is speculative. The claim of 'unambiguous evidence of the magnetic origin' therefore needs direct evidence that the films are antiferromagnetic and that the Néel vector has the assumed orientation, for example from magnetometry, neutron or muon measurements, or a control experiment that reverses the Néel vector.","section":"Discussion after Fig. 4(d)"},{"comment":"The conversion from the SMR magnitude to the spin Hall conductivity uses Eq. (1), which assumes zero longitudinal spin absorption and a transparent RuO2/CoFeB interface, and then uses a spin diffusion length λ_sf = 12.2 nm taken from Ref. [62] for a different RuO2 film. These assumptions can affect both the magnitude and the phase of the extracted spin current polarization, and no uncertainty propagation is given. The paper should state how sensitive the final θ_s^odd values are to these modeling choices, or at least provide error bars that include them.","section":"Eq. (1) and derivation of σ_zx"}],"minor_comments":[{"comment":"There are several grammatical errors: 'spin degree of freedoms' should be 'spin degrees of freedom'; 'the T-odd spin current is indeed have a magnetic origin' should be 'the T-odd spin current indeed has a magnetic origin'; and 'the 1/T form is itself an assumption' is repeated in different wording. The writing should be polished throughout.","section":"Abstract and Introduction"},{"comment":"The figure captions refer to current along '[1_01]' and '[010]' but the text uses the overline notation inconsistently and the subscripted digits are not rendered correctly in several places, which may confuse readers about the crystallographic directions.","section":"Fig. 3(c,d) caption and text"},{"comment":"The plot in Fig. 4(d) would benefit from error bars and a horizontal line at θ_N = 35 degrees, so that the reader can directly judge how close the extracted θ_s^odd is to the Néel vector direction over the full temperature range.","section":"Fig. 4(d)"},{"comment":"Reference [41] is cited as evidence of tilted spin current in RuO2, but the paper does not explicitly quantify the discrepancy between its own θ_s^odd and the earlier reported tilt angle; adding a direct quantitative comparison would strengthen the claim that the decomposition resolves the earlier puzzle.","section":"References"}],"recommendation":"major_revision","confidential_remarks":"The paper addresses an important question and the experimental dataset appears rich, but the central temperature-decomposition assumption is currently unvalidated and the magnetic state of the films is not independently established. I would encourage the authors to add a sensitivity analysis of the decomposition, report uncertainties, and ideally provide a magnetic characterization or a domain-reversal control. If the current data cannot support those additions, the conclusions should be softened from 'unambiguous evidence' to 'consistent with a magnetic origin under the assumed decomposition.'"},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Dear colleague,\n\nThe one thing to know about 2412.18937 is that it has a genuinely new experimental idea: use the temperature dependence of spin Hall magnetoresistance to separate the time-reversal-odd (magnetic) and time-reversal-even (SOC) spin Hall conductivities in an altermagnet. The paper then claims that the odd component in (101)-RuO2 is polarized along the Néel vector, which would resolve the previous discrepancy between theory and the tilted polarization seen in earlier RuO2 experiments.\n\nWhat it does well: the SMR measurement is clean and the symmetry checks are convincing. Current along [010] gives a phase shift with both y and z polarization; current along [1-01] gives no shift, as expected from mirror symmetry. The data show a clear temperature trend: the phase shift and SMR magnitude both fall as T rises, consistent with the odd component fading and the even SOC part dominating at high T. The A/T + B decomposition is transparent, and the extracted odd polarization angle stays near θ_N ≈ 35° from 5 K to 360 K. That is a striking, testable result.\n\nThe soft spots are real and load-bearing. The separation into odd and even components assumes the odd part scales exactly as 1/T, the even part is strictly T-independent, and both hold down to 5 K even though the fit is only over 150–300 K. For a metallic film, τ should not keep growing as 1/T to 5 K; it should saturate once impurity scattering dominates. If the even part also drifts at low T (via phonon-assisted side-jump, Fermi-surface smearing, etc.), the subtraction changes and the low-T odd polarization angle is no longer reliably pinned to 35°. No error bars are given on σ(T) or on A and B, and no goodness-of-fit is reported, so the abstract's phrase \"unambiguous evidence\" overreaches. The single spin diffusion length used at all T is another assumption, though it probably affects magnitudes more than angles.\n\nNone of this means the claim is wrong. But the central inference is only as strong as the temperature scalings, and those are not independently verified. A control experiment, such as varying RuO2 thickness or using a nonmagnetic low-symmetry metal like IrO2, would help. This is a promising, important-if-true result that needs better error analysis and a more measured statement of its assumptions.\n\nWho is it for? Anyone working on altermagnets, spin Hall physics, or RuO2 films. It deserves a serious referee: the idea is novel, the data are original, and the discrepancy it addresses matters. The referee should push hard on the decomposition.\n\nRecommendation: send to peer review with a request for substantial revisions, not a desk reject.","headline":"Clever SMR-based split of magnetic and conventional spin Hall effects in RuO2, but the central claim leans on temperature scalings that are fitted, not proven.","tokens_in":12412,"tokens_out":7399,"would_cite":false,"duration_ms":70414,"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":"RuO2's magnetic spin current is polarized along the Néel vector, once the ordinary spin Hall contribution is subtracted.","keywords":["altermagnetism","spin Hall magnetoresistance","nonrelativistic spin current","Néel vector","ruthenium dioxide","time-reversal-odd spin Hall effect","spin current polarization"],"falsifier":"Measure the spin Hall magnetoresistance phase shift in RuO$_2$/CoFeB down to 5 K while independently determining the electron scattering time, and also measure the ordinary spin Hall contribution in a nonmagnetic structural analogue; if the extracted time-reversal-even conductivity at low temperature departs from the constant $B$ fitted at 150–300 K, or if $\\theta_s^{\\mathrm{odd}}$ at 5 K changes with film growth conditions beyond the fit uncertainty, the Néel-vector-locking conclusion would fail.","tokens_in":11376,"feed_emoji":"🧲","tokens_out":6906,"duration_ms":55483,"temperature":0.7,"pith_summary":"The paper claims that the nonrelativistic (time-reversal-odd) spin current in altermagnetic RuO$_2$ is polarized essentially along the Néel vector, and that a large conventional time-reversal-even spin Hall effect had masked this in earlier experiments. Using spin Hall magnetoresistance in RuO$_2$/CoFeB from 5 to 360 K, the authors separate the two contributions by their temperature dependence and find the odd component's polarization angle stays near $\\theta_N = \\arctan(c/a) \\approx 35^\\circ$. This would establish the magnetic origin of the nonrelativistic spin current and explain why earlier measurements saw a tilted polarization.","feed_headline":"RuO2's magnetic spin current is polarized along the Néel vector","feed_subtitle":"Separates magnetic from ordinary spin Hall effects, explaining the earlier tilted polarization.","key_machinery":"The central object is the spin Hall magnetoresistance (SMR) measurement with an added phase-shift analysis. In a RuO$_2$/CoFeB bilayer, a charge current along RuO$_2$[010] generates a spin current whose polarization makes an angle $\\Delta\\beta$ with the $y$ axis, shifting the SMR angular dependence to $\\Delta R\\cos(2(\\beta-\\Delta\\beta))$; calibrating with a Pt/CoFeB reference fixes $\\Delta\\beta$, giving the $y$ and $z$ components of the spin Hall conductivity $\\sigma_{zx}$. The decomposition then uses the disparate temperature dependence: the odd, magnetic component scales with the electron lifetime $\\tau \\propto 1/T$ (fitted between 150 and 300 K as $A/T + B$), while the even, spin-orbit component is assumed temperature independent, so the constant $B$ is subtracted to isolate the odd part over the full range.","core_discovery":"On the paper's terms, the central discovery is that after subtracting the temperature-independent, spin-orbit-coupled even component, the remaining odd spin Hall conductivity in (101)-oriented RuO$_2$ has a polarization angle $\\theta_s^{\\mathrm{odd}} = \\arctan(\\sigma_{zx}^{y,\\mathrm{odd}}/\\sigma_{zx}^{z,\\mathrm{odd}})$ close to $\\theta_N \\approx 35^\\circ$ across 5–360 K, pointing along the lattice-derived Néel vector. The measured total polarization is tilted because the even component is not negligible; it even dominates at high temperature. The authors take this as evidence that the nonrelativistic spin current has a magnetic origin and that magnetic order survives in their films despite reports questioning bulk RuO$_2$'s ground state.","pith_inferences":["If the odd polarization is exactly locked to the Néel vector, one could use the SMR phase shift itself as a magnetometer for the antiferromagnetic order parameter, reading the Néel orientation from transport alone.","A sharper test would be to repeat the decomposition on (110)-oriented RuO$_2$, where theory permits a different set of polarization components, and check whether $\\theta_s^{\\mathrm{odd}}$ follows the corresponding Néel projection.","The two-parameter $A/T + B$ fit could be validated by measuring the ordinary spin Hall contribution separately in a nonmagnetic structural analogue and comparing its low-temperature magnitude with the assumed constant $B$.","Below 150 K, where the fit is extrapolated, the paper's conclusion rests on the unverified stability of the even component; a direct low-temperature measurement of that component would settle whether the Néel-vector locking persists to 5 K."],"forward_implications":["The previously reported tilted spin polarization in RuO$_2$ is explained as a superposition of a large conventional $y$-polarized spin Hall current and the Néel-vector-polarized odd current, without invoking exotic physics.","If the odd spin current is locked to the Néel vector, spin-orbit-torque switching experiments on (101)-oriented RuO$_2$ can use the polarization direction to infer the Néel orientation, making current-driven switching deterministic with respect to it.","The temperature dependence offers a practical fingerprint: magnetic spin Hall signals weaken on heating while ordinary spin Hall signals persist, so the SMR phase-shift method can identify magnetic-origin spin currents in other altermagnets and noncollinear antiferromagnets.","Film-growth effects such as strain and oxygen vacancies appear to stabilize magnetism in RuO$_2$ films even if bulk RuO$_2$ is nonmagnetic, making the magnetic state a film-dependent property."],"supporting_citations":[{"why":"Supplies the theoretical prediction of a time-reversal-odd spin current in RuO2 polarized along the magnetic order parameter.","marker":"[32]"},{"why":"Reports observation of spin-splitter torque in RuO2, evidence of an unconventional polarized spin current.","marker":"[40]"},{"why":"Reports the tilted spin current in RuO2 whose polarization deviated from the Néel vector, the discrepancy this paper resolves.","marker":"[41]"},{"why":"Reports observation of spin-splitter torque in collinear antiferromagnetic RuO2.","marker":"[42]"},{"why":"Establishes spin Hall magnetoresistance in metallic bilayers, the measurement basis of the technique.","marker":"[59]"},{"why":"Provides the SMR mechanism used to extract the spin current polarization direction.","marker":"[60]"},{"why":"Supplies the equation relating the SMR magnitude to the spin Hall angle and spin diffusion length.","marker":"[61]"},{"why":"Gives the spin diffusion length of 12.2 nm used in the quantitative decomposition.","marker":"[62]"},{"why":"Raises the possibility of a fragile or absent magnetic ground state in RuO2, which the paper's conclusion speaks against.","marker":"[56]"},{"why":"Reports absence of magnetic order in RuO2 from muon spin rotation and neutron diffraction, motivating the film-growth interpretation.","marker":"[58]"}],"fun_headline_variants":["Spin current aligns with Neel vector in RuO2 altermagnet","Magnetic spin Hall effect in RuO2 tied to Neel vector","Nonrelativistic spin current in RuO2 points along Neel vector","RuO2 reveals magnetic origin of spin current polarization"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The decomposition assumes the ordinary, spin-orbit-driven spin Hall conductivity does not change as the sample cools from 150 K to 5 K, and that the magnetic part grows exactly as $1/T$; both are fixed by a fit only between 150 and 300 K, so if the ordinary part varies at low temperature, the deduced magnetic polarization angle could shift away from the Néel vector.","fun_headline_variants_meta":{"raw":{"variants":["Spin current aligns with Neel vector in RuO2 altermagnet","Magnetic spin Hall effect in RuO2 tied to Neel vector","Nonrelativistic spin current in RuO2 points along Neel vector","RuO2 reveals magnetic origin of spin current polarization"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00074,"raw_usage":{"total_tokens":3281,"prompt_tokens":901,"completion_tokens":2380,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":517,"completion_tokens_details":{"reasoning_tokens":2316}},"tokens_in":517,"tokens_out":2380,"duration_ms":16010,"temperature":1.0,"reasoning_tokens":2316,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T04:19:05.378850+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the spin Hall magnetoresistance phase shift in RuO$_2$/CoFeB down to 5 K while independently determining the electron scattering time, and also measure the ordinary spin Hall contribution in a nonmagnetic structural analogue; if the extracted time-reversal-even conductivity at low temperature departs from the constant $B$ fitted at 150–300 K, or if $\\theta_s^{\\mathrm{odd}}$ at 5 K changes with film growth conditions beyond the fit uncertainty, the Néel-vector-locking conclusion would fail.","supporting_citations":[{"cited_title":"Smejkal, J","cited_arxiv_id":null,"evidence_quote":"Supplies the theoretical prediction of a time-reversal-odd spin current in RuO2 polarized along the magnetic order parameter."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports observation of spin-splitter torque in RuO2, evidence of an unconventional polarized spin current."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the tilted spin current in RuO2 whose polarization deviated from the Néel vector, the discrepancy this paper resolves."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports observation of spin-splitter torque in collinear antiferromagnetic RuO2."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes spin Hall magnetoresistance in metallic bilayers, the measurement basis of the technique."},{"cited_title":"Keßler, L","cited_arxiv_id":null,"evidence_quote":"Provides the SMR mechanism used to extract the spin current polarization direction."},{"cited_title":"S1 of Supplementary Note 3), λsf is the spin diffusion length of 12.2 nm [62] , tAF is the thickness of RuO2, and ε = tAF/2λsf","cited_arxiv_id":null,"evidence_quote":"Supplies the equation relating the SMR magnitude to the spin Hall angle and spin diffusion length."},{"cited_title":"Nakayama, M","cited_arxiv_id":null,"evidence_quote":"Gives the spin diffusion length of 12.2 nm used in the quantitative decomposition."},{"cited_title":"Samanta, Y.-Y","cited_arxiv_id":null,"evidence_quote":"Raises the possibility of a fragile or absent magnetic ground state in RuO2, which the paper's conclusion speaks against."},{"cited_title":"Smolyanyuk, Mazin, II, L","cited_arxiv_id":null,"evidence_quote":"Reports absence of magnetic order in RuO2 from muon spin rotation and neutron diffraction, motivating the film-growth interpretation."}],"review_version":1}