REVIEW 3 major objections 5 minor 1 cited by
Unveiling the anisotropy of linear and nonlinear charge-spin conversion in Weyl semimetal TaIrTe4
T0 review · 3 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The paper reports simultaneous observation of the nonlinear planar Hall effect and spin-orbit torque in one TaIrTe4/Py heterostructure, with the two effects peaking along different crystal axes.
desk verdict Simultaneous NPHE and SOT in a single TaIrTe4/Py device with an interesting opposite anisotropy, but the quantitative separation lacks thermal controls and error bars. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The method rests on the harmonic Hall decomposition in Eqs. (1)-(3): the second harmonic signal is split into a field-like term $R_{FL+Oe}$ proportional to $\cos\varphi\cos 2\varphi$ and a damping-like plus nonlinear term $R_{DL+NPHE}$ proportional to $\cos\varphi$, with the NPHE component identified by its linear dependence on the in-plane magnetic field $H$. The nonlinear Hall coefficient $\chi = \rho_{xy}^{2\omega}/(E_x H_y)$ quantifies the strength of the nonlinear charge-spin conversion. The separation is made field-dependent: at low fields the signal is dominated by $R_{FL+Oe}$, while at high fields the linear-in-$H$ NPHE term grows and must be removed before the damping-like field $H_{DL}$ is obtained from the remaining $1/(H_K+H)$ dependence.
What would settle it
Repeat the high-field harmonic Hall measurement on a bare TaIrTe4 flake (without the Py layer) in the same Hall geometry; if the linear-in-$H$ $\cos\varphi$ second-harmonic resistance remains, the NPHE attribution is supported, whereas if it disappears, the term assigned to NPHE is actually a magnetization-related artifact of the ferromagnetic layer.
Extended reading notes
Core claim
The central claim is that in a TaIrTe4/Py bilayer the second harmonic Hall resistance $R_{xy}^{2\omega}$ contains a detectable nonlinear planar Hall contribution that is linear in magnetic field and follows $\cos\varphi$, which must be subtracted before the spin-orbit torque efficiencies are extracted. After this subtraction, the field-like SOT efficiency $\theta_{\mathrm{FL}}$ is found to be $0.57$ along [100] and the damping-like efficiency $\theta_{\mathrm{DL}}$ to be $0.93$ along [010], while the nonlinear Hall coefficient $\chi = \rho_{xy}^{2\omega}/(E_x H_y)$ takes values of $0.74$ and $0.15$ mΩ·V$^{-1}$·μm$^2$·T$^{-1}$ along [100] and [010], respectively. The authors interpret the stronger NPHE and field-like torque along [100] as a signature of the spin-momentum-locked Fermi arc surface states, and the stronger damping-like torque along [010] as a signature of the bulk spin Hall effect.
Load-bearing premise
The load-bearing premise is that the second harmonic Hall voltage is a clean sum of three known angular- and field-dependent terms, namely field-like torque, damping-like torque, and a nonlinear planar Hall term that is linear in magnetic field, so that any additional contribution sharing the same symmetries would be misassigned and would distort the reported anisotropies.
Editorial extensions
If this is right
- TaIrTe4 can serve as a single material in which both nonlinear charge-spin conversion and spin-orbit torque are measurable, with crystal orientation selecting which effect dominates.
- The nonlinear Hall coefficient $\chi$ is five times larger along [100] than along [010], so current-direction engineering can tune the efficiency of nonlinear charge-spin conversion.
- The opposite anisotropies of field-like and damping-like torques point to different mechanisms, surface spin-momentum locking versus bulk spin Hall effect, that can be investigated separately in the same device.
- The simultaneous measurement provides a direct way to correlate linear and nonlinear conversion efficiencies without sample-to-sample variations.
- The extracted spin Hall angles of 0.57 (field-like) and 0.93 (damping-like) are large enough to matter for SOT applications in topological semimetal devices.
Reading between the lines
- If the decomposition is robust, the same field-dependent harmonic Hall procedure could be applied to other type-II Weyl semimetals and topological semimetals to map their charge-spin conversion anisotropy without fabricating separate samples for each effect.
- The positive correlation between NPHE and field-like SOT along [100] suggests a shared surface origin; a thickness series of TaIrTe4 or a controlled surface-state modification (for example via capping layers) could test whether suppressing the Fermi arc contribution weakens both effects together.
- The claim that damping-like SOT is bulk-dominated could be cross-checked by spin-pumping experiments on the same TaIrTe4 crystals, or by comparing the measured $\theta_{DL}$ anisotropy with ab initio calculations of the bulk spin Hall conductivity.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports harmonic Hall measurements on TaIrTe4/Py bilayers and claims the first simultaneous observation of the nonlinear planar Hall effect (NPHE) and spin-orbit torques (SOT) in a TaIrTe4-based heterostructure. The authors develop a field-dependent decomposition based on Eqs. (1)-(3) to separate the second-harmonic Hall signal into field-like SOT, damping-like SOT, and NPHE contributions, and they report distinct anisotropies: stronger NPHE and field-like SOT along the [100] direction, and larger damping-like SOT along the [010] direction. The results are interpreted in terms of Fermi-arc surface states versus bulk spin Hall contributions.
Significance. If the experimental decomposition is reliable, the paper advances the field by showing that linear and nonlinear charge-spin conversion can be characterized simultaneously in a single device, and by using the different field scalings of DL-SOT (1/(H+H_K)) and NPHE (linear in H) to separate them. The reported anisotropy pattern is a concrete, falsifiable observation that can guide theory on surface versus bulk mechanisms. The authors also provide a clear device fabrication and measurement description. However, the quantitative conclusions currently rest on unverified assumptions about the thermal background and on unspecified electrical parameters, so the significance is conditional on additional controls and error analysis.
major comments (3)
- [Sec. B, Eq. (3)] The separation of R_DL+NPHE into a 1/(H+H_K) component and a linear-in-H NPHE component requires that the thermal term R_xy^∇T be negligible or removable, but this is never demonstrated. The manuscript states the thermal contributions (anomalous Nernst and spin Seebeck effects in the 7-nm Py layer) without quantifying them, and no control experiments (TaIrTe4-only, Py-only, or Cu/Py devices) or frequency-dependent harmonic measurements are reported to bound their magnitude. Because these thermal effects produce second-harmonic signals with the same cosφ symmetry and with weak or non-linear field dependences, they can be absorbed into the fitted H_DL or χ_NPHE parameters. The linear-in-I and linear-in-H scaling shown in Figures 3(c)-(d) is consistent with NPHE but also with Joule-heating-driven thermoelectric signals, so it does not by itself rule out a thermal background. Please provide control experiments or an independent estimate of R_xy^∇T, and preferably measurements at multiple AC frequencies.
- [Sec. D, Eqs. (2)-(3) and Fig. 4(f)] The quoted efficiencies θ_FL = 0.57 and θ_DL = 0.93 depend on the current density j in the TaIrTe4 layer, the Oersted field H_Oe, the anisotropy field H_K, and the saturation magnetization M_s of Py. The manuscript does not report the values of H_K, M_s, the resistivities used for the current-sharing calculation among TaIrTe4, Py, and Cu, or the effective cross-sectional area used for j. The conversion from measured second-harmonic voltages to effective fields is therefore not reproducible, and no uncertainty is assigned to the final spin-Hall efficiencies. Please provide these parameters and error bars, and state the formula used to compute the current density in the TaIrTe4 layer.
- [Sec. E, Fig. 5] The central claim of anisotropy (five-fold difference in the nonlinear coefficient χ between [100] and [010], and opposing anisotropies for FL-SOT and DL-SOT) appears to be based on a single device per crystallographic direction, since the Supporting Information is only cited for the [010] data and no sample-to-sample variability is discussed. Without multiple devices or an explicit reproducibility statement, the quantitative ratios and the assignment of the anisotropy to intrinsic crystal properties are not fully established. Please report the number of devices measured, the spread of values, and the statistical significance of the anisotropy.
minor comments (5)
- [Throughout] The manuscript contains several typos and notation inconsistencies, including the title 'anisotrop y', the inconsistent use of 'TaIrTe₄' and 'TaIrTe4', and the spacing in 'charge –spin'. Please proofread carefully.
- [Sec. B, after Eq. (3)] The sentence 'the value of 1/H will be larger than 1/(H_K − H)' appears to have a typo: Eq. (3) contains 1/(H_K + H), so the comparison should be between 1/H and 1/(H_K + H). Please correct this.
- [Eq. (2)] The sign convention for H_FL is not defined; please specify whether H_FL is taken positive along or opposite to the Oersted field direction and how that affects the extracted value of θ_FL.
- [Sec. E, definition of χ] Please define all quantities in χ = ρ_xy^2ω/(E_x H_y), including whether E_x is the applied electric field along the current direction and H_y is the in-plane magnetic field component; state the assumed angle between current and magnetic field when reporting the value of χ.
- [Sec. D, spin-Hall angle formula] In the formula θ_SH = 2e μ0 M_s t H_FL(DL)/(ℏ j), clarify the units of j (A m^-2) and state whether j is the current density in the TaIrTe4 layer only or averaged over the full heterostructure; this is needed to interpret the reported values.
Circularity Check
No significant circularity: NPHE and SOT are separated by different field regimes, not by a fit-then-predict loop; only a minor, non-load-bearing self-citation appears.
full rationale
The derivation chain is not circular. The paper's central claim (simultaneous NPHE and SOT with distinct [100]/[010] anisotropy) rests on decomposing the second-harmonic Hall signal using Eqs. (1)-(3). The NPHE amplitude is extracted in Section C from high-field data (1-6 T) where the damping-like SOT term, proportional to 1/(H+H_K), is suppressed; its linear-in-I and linear-in-H scaling is then used in Section D to subtract the 'linear NPHE term' before extracting H_DL at 250-5000 Oe. Thus the NPHE and SOT parameters are obtained from separate field regimes rather than being fitted and then re-predicted from the same subset; no quoted equation reduces a predicted quantity to a fitted input. The only self-citation found is Ref. 16 (Lao et al., overlapping authorship) in the sentence 'Charge-spin conversion can be categorized into linear and nonlinear responses based on how the spin current depends on the charge current.16,17'; this is paired with Ref. 17 and is not load-bearing for the results. The unverified assumption that the thermal term R_xy^∇T in Eq. (3) is negligible or removable is a control/identifiability concern, not a circularity, because the paper does not define the NPHE or SOT signals in terms of that assumption's conclusion.
Assumptions & free parameters
free parameters (4)
- Anisotropy field H_K of Py =
Not stated
- Oersted field H_Oe =
Not stated
- Current density J in TaIrTe4 layer =
Not stated
- Saturation magnetization M_s of Py =
Not stated
assumptions (4)
- domain assumption Harmonic Hall equations (Eq. 1-3) correctly describe the angular dependencies of FL, DL, and NPHE contributions.
- domain assumption The NPHE contribution is linear in magnetic field H over 0-6 T.
- ad hoc to paper Thermal contributions (anomalous Nernst and spin Seebeck) are negligible or can be separated.
- ad hoc to paper The Fermi arc surface states along ky are responsible for the enhanced NPHE/FL-SOT along [100].
Cite this review
Pith. "Pith review of Unveiling the anisotropy of linear and nonlinear charge-spin conversion in Weyl semimetal TaIrTe4." pith.science (2026). https://pith.science/paper/KZJQMHMN
@misc{pith2026241119062,
author = {Pith},
title = {Pith review of: Unveiling the anisotropy of linear and nonlinear charge-spin conversion in Weyl semimetal TaIrTe4},
year = {2026},
howpublished = {\url{https://pith.science/paper/KZJQMHMN}},
note = {Machine review of arXiv:2411.19062}
}
read the original abstract
In Weyl semimetals, the nonlinear planar Hall effect (NPHE) and spin-orbit torque (SOT) are prominent manifestations of nonlinear and linear charge-spin conversion, respectively. However, simultaneous investigations of these phenomena within a single material system are scarce, limiting our understanding of their intrinsic connection and underlying mechanisms. Here, we report the first simultaneous observation of NPHE and SOT in a TaIrTe4/Py heterostructure. By employing harmonic Hall measurements and developing a magnetic field-dependent method, we successfully separated the contributions from NPHE, field-like SOT, and damping-like SOT, enabling accurate characterization of both linear and nonlinear charge-spin conversion properties. Our experiments revealed significant anisotropy along the [100] and [010] crystallographic directions of TaIrTe4, with stronger nonlinear responses and field-like SOT along the [100] direction, and larger damping-like SOT along the [010] direction. The distinct directional dependence of these phenomena provides new insights into the interplay between surface and bulk contributions to charge-spin conversion in Weyl semimetals. These findings enhance our understanding of anisotropic charge-spin conversion mechanisms in Weyl semimetals, which may inform future research and development of spintronic devices based on topological materials.
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Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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