{"id":"b03e11ed-2017-401e-983a-769c475fb72d","arxiv_id":"2507.00360","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A femtosecond optical pulse creates a Dresselhaus-symmetry charge current in NiMnSb, attributed to the bulk spin-galvanic effect in an inversion-asymmetric ferromagnet.","lead":"Femtosecond laser pulses sent into thin films of the magnetic compound NiMnSb generate ultrafast electric currents whose direction follows a symmetry pattern known as Dresselhaus, not just the usual Rashba pattern. The finding adds a bulk spin-to-charge conversion mechanism to ultrafast spintronics and points toward terahertz emitters and detectors that scale with film volume.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Bulk-Dresselhaus-SGE assignment is not uniquely pinned: the MgO-cap control does not exclude a vertical-spin-current ISHE or bottom-interface contribution, so a thickness series is needed.","rationale":"The reader correctly identifies the core ambiguity: the e^{-iθ} component is not uniquely tied to the spin-accumulation SGE. My concern refines that ambiguity to the most plausible alternative that the paper's controls do not eliminate. The MgO-cap experiment excludes spin transport from NiMnSb into the Ru cap, but it does not exclude a spin current generated and converted inside the NiMnSb layer, nor a contribution from the NiMnSb/InGaAs bottom interface. The paper's own text says the ISHE scenario 'makes a minor contribution,' but this conclusion rests on the MgO comparison without a mechanistic control. The proposed thickness series is the standard, decisive test for a bulk versus interface/current effect: bulk SGE should scale with the active volume, while interfacial ISHE and spin-current conversion saturate once the film exceeds the spin-diffusion length. The dynamics match with ΔR(t) is consistent with both mechanisms and therefore does not settle the question. I do not see an internal inconsistency or a fatal flaw; the observation of a Dresselhaus-symmetric THz photocurrent in NiMnSb is valuable and probably correct. The mechanism attribution, however, remains conditional pending the thickness experiment, so the existing CONDITIONAL verdict is appropriate and unchanged.","tokens_in":11290,"tokens_out":22622,"duration_ms":273772,"concrete_test":"Grow NiMnSb films of thickness 5, 10, and 20 nm on the same In0.53Ga0.47As buffer with MgO caps, and measure the e^{-iθ} Dresselhaus THz component under identical pump and detection conditions. If the amplitude scales linearly with film thickness, a bulk SGE (volume effect) is supported; if the signal saturates or remains constant with thickness, an interfacial or vertical-spin-current ISHE mechanism is indicated.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim requires the e^{-iθ} current to arise from a pump-induced spin accumulation μs converted by the bulk Dresselhaus SGE in NiMnSb. However, the same angular pattern and quasi-instantaneous dynamics are expected for a vertical spin current jsσ⊗u_z (generated by ultrafast demagnetization or spin Seebeck) converted by the Dresselhaus-type inverse spin Hall effect, either in the NiMnSb bulk or at the NiMnSb/InGaAs bottom interface. The paper's MgO-cap control (Supplemental Note 3) only suppresses transport to the Ru cap; it does not eliminate an internal vertical spin current or the bottom-interface path, because a spin current can be reflected or converted within the NiMnSb layer even when the cap is insulating. Thus the discrimination between the SGE (Eq. 1, second term) and ISHE (Eq. 1, first term) is not established. The observed similarity between ID(t) and ΔR(t) (Fig. 4) also does not discriminate, since both spin-current-driven and thermoelectric currents would follow the electron-temperature relaxation. The paper's own statement that the ISHE scenario 'makes a minor contribution' is therefore a tentative assumption, not a proven result. A thickness series would provide the missing evidence for the bulk nature of the effect.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports THz-emission spectroscopy on 10 nm NiMnSb films excited by 10 fs, 1.55 eV optical pulses. By rotating the in-plane magnetization direction θ, the emitted THz field is decomposed into components with angular dependence e^{+iθ} and e^{-iθ}, corresponding to Rashba- and Dresselhaus-type current patterns. The authors attribute the Rashba-like component to an out-of-plane photocurrent converted by the ordinary Hall effect and the Dresselhaus-like component to a pump-induced spin accumulation μ_s ∥ M converted to a charge current by the bulk spin-galvanic effect in NiMnSb. They interpret the ~0.2 ps relaxation of the Dresselhaus current as following the electron temperature, with the response time set by the ~10 fs momentum relaxation time and a spin-lattice relaxation time τ_sl ≈ 1 ps, and conclude that this is the first ultrafast observation of the Dresselhaus spin-galvanic effect.","tokens_in":11575,"tokens_out":9396,"duration_ms":105522,"significance":"If the identification is correct, this is the first femtosecond observation of a bulk Dresselhaus spin-galvanic effect and provides a promising route to volume-scaled spintronic THz emitters and detectors. The paper has clear strengths: the symmetry analysis is anchored to the independently known mm2 point group and leads to the compact decomposition in Eq. (3); the Rashba-only W|CoFeB|Pt reference sample provides a clean baseline; the MgO-cap control addresses the top interface; and the comparison with reflectance dynamics is a sensible way to connect the current to the electronic temperature. The Fourier decomposition in θ is unusually direct and makes the Rashba/Dresselhaus separation transparent. However, the central identification of the e^{-iθ} component with the bulk SGE is an inference rather than a direct measurement: the discrimination from vertical-spin-current ISHE and from bottom-interface contributions is not fully closed by the presented controls.","major_comments":[{"comment":"The assignment of the e^{-iθ} current component to the bulk Dresselhaus SGE is not uniquely pinned by the data. The paper argues that a pump-induced spin current converted by a Dresselhaus-type ISHE makes a minor contribution because the MgO cap suppresses transport to the Ru cap. However, the cap experiment only blocks transfer of spin angular momentum from NiMnSb to the top cap; it does not eliminate an internal vertical spin current generated by ultrafast demagnetization and absorbed within the NiMnSb film, nor does it address a bottom-interface contribution at the NiMnSb/InGaAs interface, where the zincblende InGaAs could provide Dresselhaus-type spin-orbit coupling. Since both the ISHE and SGE terms in Eq. (1) are constrained by the same mm2 point group, the angular pattern in Eq. (3) alone cannot distinguish them. A thickness series of NiMnSb (where a bulk SGE should scale with the film volume while interface and vertical-spin-current contributions scale differently) is required to substantiate the bulk origin.","section":"Rashba- and Dresselhaus-like signal components (Eq. (1), Eq. (3), Supplementary Note 3)"},{"comment":"The dynamics analysis is partly circular. Equation (17) is written assuming the two-subsystem spin-transfer model and uses τ_sl as a free parameter; the fitted value τ_sl ≈ 1 ps and the conclusion that μ_s relaxation is governed by electron-phonon cooling rather than spin-lattice relaxation therefore follow from the assumed model rather than from an independent test. The similarity between I_D(t) and ΔR(t) is also not discriminating, because any current that follows the electron temperature, including thermoelectric or spin-current-driven currents, would show the same qualitative dynamics. The proportionality chain in Eq. (4), I_D ∝ μ_s ∝ μ_s^M, is an assumption that should be presented as such and not as a derived result.","section":"Dresselhaus current dynamics (Eq. (4), Eq. (17))"}],"minor_comments":[{"comment":"The abstract and conclusion state the Dresselhaus SGE identification categorically, while the main text introduces it as a tentative assumption (\"we tentatively assume that the suggested scenario... prevails\"). The wording should be harmonized so the level of certainty is consistent throughout.","section":"Abstract and Conclusion"},{"comment":"Reference [30] is a duplicate of reference [8] (both cite Ganichev et al., \"Experimental Separation of Rashba and Dresselhaus Spin Splittings in Semiconductor Quantum Wells\"); please remove the duplicate.","section":"References"},{"comment":"Equation (17) is not displayed in the main text; readers are referred to the Supplement. Please present the equation and a short derivation in the main text, since it is central to the claim about the relaxation time scales.","section":"Equation (17)"},{"comment":"Figure 4(b,c) would benefit from labeled axes with units; the current \"(arb. units)\" notation makes quantitative comparison of I_D(t) and ΔR(t) difficult.","section":"Figure 4"},{"comment":"The sentence stating that measurements on NiMnSb|MgO stacks indicate the spin-current scenario \"makes a minor contribution\" should specify the measurement conditions of the MgO-capped sample and report the residual signal amplitude, so the reader can judge the sensitivity of the control.","section":"Supplemental Note 3"}],"recommendation":"major_revision","confidential_remarks":"The data quality and symmetry analysis are strong, and the paper addresses an important question. However, the novel claim—the first ultrafast observation of the bulk Dresselhaus SGE—rests on an indirect identification that the current controls do not uniquely establish. A thickness series and a more explicit symmetry argument distinguishing bulk SGE from vertical-spin-current ISHE and bottom-interface contributions would substantially strengthen the paper. I therefore recommend major revision rather than rejection."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Hi [Name],\n\nYou should know about this one: it's the first femtosecond signature of a Dresselhaus-type spin-galvanic current in a bulk ferromagnet, and the core experimental work is solid. The authors do THz emission from NiMnSb and decompose the magnetization-angle dependence into a Rashba-like and a Dresselhaus-like component. The Fourier analysis in theta is clean, anchored to the mm2 point group, and backed by a Rashba-only reference sample. The strongest piece of evidence is the field dependence: the Rashba component grows with |B_ext|, as expected for an out-of-plane current converted by the ordinary Hall effect, while the Dresselhaus component saturates with M. That is a real, useful separation.\n\nThe weakness is the mechanistic assignment. The paper attributes the Dresselhaus current to a pump-induced spin accumulation mu_s converted by the bulk SGE. The MgO-cap control rules out spin transport to the Ru capping layer, but it does not rule out an internal vertical spin current in the NiMnSb itself--generated by ultrafast demagnetization or spin-Seebeck-like processes--converted by a Dresselhaus-type ISHE, either in the bulk or at the bottom InGaAs interface. That alternative would produce the same angular pattern and, to a first approximation, similar dynamics. The comparison of I_D(t) with Delta-R(t) is suggestive but not discriminating, since both spin accumulation and spin current would scale with the electron temperature. The two-subsystem dynamics model in Eq. (17) assumes the SGE picture and introduces tau_sl as a free parameter, so that part is circular.\n\nI don't think this kills the paper. The observation itself--a magnetization-dependent photocurrent with exact Dresselhaus symmetry that is field-independent above saturation--is new and will matter for THz spintronics. The authors are transparent about the alternative scenario, and they've done the obvious controls. What's missing is an experiment that distinguishes bulk SGE from internal ISHE, most directly a thickness series: a bulk SGE should scale linearly with thickness, while an interface ISHE would not. The authors should be asked for that or at least an acknowledgment of the ambiguity.\n\nWho is this for? Anyone working on ultrafast spin-charge conversion, half-metals, or spintronic THz emitters. It's a good paper for a strong journal, not a desk reject. I'd send it to a competent referee with the request to pin down the ISHE alternative.\n\nBest.","headline":"First ultrafast observation of a Dresselhaus-symmetry photocurrent in a bulk ferromagnet, with a solid symmetry analysis but a mechanism assignment that is indirect and needs a thickness series.","tokens_in":12226,"tokens_out":6229,"would_cite":true,"duration_ms":66675,"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":"Femtosecond light pulses on NiMnSb produce a charge current assigned to the bulk Dresselhaus spin-galvanic effect.","keywords":["femtosecond photocurrents","spin-galvanic effect","Dresselhaus spin-orbit coupling","half-metallic Heusler NiMnSb","THz emission spectroscopy","spin-to-charge conversion","ultrafast spin dynamics"],"falsifier":"Grow a series of NiMnSb films with thicknesses from about 5 to 50 nm under identical capping and measure the $e^{-i\\theta}$ Fourier amplitude of the THz signal at fixed pump fluence. A bulk spin-galvanic current must scale linearly with NiMnSb thickness, while an interfacial or Hall-type current would saturate; the absence of linear scaling would invalidate the bulk-Dresselhaus assignment.","tokens_in":11099,"feed_emoji":"⚡","tokens_out":9220,"duration_ms":90933,"temperature":0.7,"pith_summary":"This paper reports that femtosecond optical pulses on the ferromagnetic half-metal NiMnSb generate a charge current that emits terahertz radiation. By rotating the magnetization direction $\\theta$ relative to the crystal axes and Fourier-analyzing the emitted field, the authors separate the current into a Rashba-type part perpendicular to the magnetization and a Dresselhaus-type part flowing at the mirror angle $-\\theta$. They attribute the Dresselhaus part to the bulk spin-galvanic effect: the pump heats the electrons, creating a transient excess of spin $\\mu_s \\parallel M$, and the spin-momentum-locked states of NiMnSb convert that spin accumulation into a charge current within roughly 10 fs. The current then decays with the electron-cooling time of about 0.25 ps rather than the slower spin-lattice relaxation of about 1 ps, consistent with the half-metallic nature of NiMnSb. If correct, this is the first observation of an ultrafast Dresselhaus spin-galvanic effect, and it suggests a bulk, volume-scaling route to terahertz spintronic emitters.","feed_headline":"Femtosecond light reveals bulk Dresselhaus spin effect in NiMnSb","feed_subtitle":"A 10-femtosecond spin-galvanic response in a half-metal points toward volume-scaled THz emitters.","key_machinery":"The argument is carried by the spin-galvanic tensor $\\chi$ of the mm2 point group, which forces the in-plane relations $-\\chi_{xy}=\\chi_{yx}$ and $\\chi_{xx}=-\\chi_{yy}$, reducing the current to the compact complex form $I_c = I_R e^{i(\\theta+90^\\circ)} + I_D e^{-i\\theta}$. Fourier transformation with respect to $\\theta$ cleanly separates the Rashba amplitude (angular frequency $+1$, imaginary coefficient) from the Dresselhaus amplitude (frequency $-1$, real coefficient), so the two currents can be extracted from the data without assuming a microscopic model. A two-subsystem relaxation model then links the Dresselhaus current to the spin dynamics: the pump-induced spin excess in the magnetization-carrying subsystem transfers to a second subsystem whose spin-momentum-locked states provide the SGE, giving $I_D(t) \\propto \\mu_s(t) \\propto \\mu_s^M(t)$.","core_discovery":"The paper claims that the measured THz photocurrent in a 10-nm NiMnSb film is a superposition $I_c = I_R e^{i(\\theta+90^\\circ)} + I_D e^{-i\\theta}$, with $\\theta$ the angle of the in-plane magnetization $\\boldsymbol{M}$ with respect to the [100] axis. The Rashba term $I_R$ is found to grow with magnetic-field magnitude and is assigned to an out-of-plane photocurrent converted by the ordinary Hall effect. The Dresselhaus term $I_D$ is independent of field magnitude, reverses with $\\boldsymbol{M}$, and its waveform matches the pump-induced reflectance change. The authors conclude that $I_D$ arises from a pump-induced spin accumulation $\\boldsymbol{\\mu}_s \\parallel \\boldsymbol{M}$ converted to charge by a Dresselhaus-symmetric spin-galvanic effect in the NiMnSb bulk, with the current responding on the electron momentum-relaxation time of about 10 fs and relaxing as the electrons cool.","pith_inferences":["Editorial inference: A direct thickness series (for example 5-50 nm NiMnSb with identical caps) should show the $e^{-i\\theta}$ Fourier amplitude growing linearly with thickness if the SGE is truly bulk; saturation at small thickness would point to an interface contribution.","Editorial inference: The two-subsystem picture predicts that the Dresselhaus current tracks the pump-induced change in electron spin population rather than the net magnetization, so separating spin and non-spin contributions in time-resolved MOKE at multiple probe wavelengths could test the model.","Editorial inference: The same angular Fourier analysis could map the bulk Dresselhaus SGE tensor in other noncentrosymmetric conductors, including half-Heusler compounds, where the sign of the $e^{-i\\theta}$ coefficient would encode the spin texture.","Editorial inference: If volume scaling holds, NiMnSb-based emitters may outperform ferromagnet/heavy-metal bilayers for ultrabroadband THz emission, but the net efficiency also depends on optical absorption depth and conductivity, so this remains to be measured."],"forward_implications":["The Dresselhaus spin-galvanic effect responds on the electron momentum-relaxation time of about 10 fs, making it a genuinely femtosecond-scale spin-to-charge conversion channel.","Because the spin accumulation relaxes through electron-phonon cooling (~0.25 ps) rather than the slower spin-lattice relaxation (~1 ps), the emitted THz waveform can serve as a time-resolved probe of hot-electron cooling in half-metals.","Since the effect resides in the NiMnSb bulk rather than at interfaces, thicker films should emit proportionally stronger THz fields, a scaling behavior unavailable to ferromagnet/heavy-metal bilayers.","Fourier decomposition of photocurrents in the magnetization angle is a general tool for isolating Rashba- and Dresselhaus-type spin-charge conversion in other inversion-asymmetric ferromagnets."],"supporting_citations":[{"why":"Defines the spin-galvanic effect, the conversion of a spin accumulation into a charge current that the paper invokes.","marker":"[3]"},{"why":"Establishes that NiMnSb has a bulk Dresselhaus-type spin-orbit contribution to spin-charge conversion, making it the chosen material.","marker":"[4]"},{"why":"Supplies the THz-emission spectroscopy method and the spintronic-emitter framework used to detect the photocurrents.","marker":"[10]"},{"why":"Reports ultrafast spin-charge conversion at Rashba interfaces, the prior context that the Dresselhaus observation extends.","marker":"[13]"},{"why":"Demonstrates broadband THz generation via the inverse Rashba-Edelstein effect, another ultrafast SGE benchmark.","marker":"[14]"},{"why":"Provides the model for pump-induced spin excess and the exponential functions used to fit the Dresselhaus current dynamics.","marker":"[26]"},{"why":"Gives the estimated ~7 fs momentum relaxation time in NiMnSb that underpins the quasi-instantaneous SGE response.","marker":"[39]"},{"why":"Supplies the procedure for extracting current waveforms from THz signals and the reflectance model for the electron-cooling time.","marker":"[40]"},{"why":"Supports the slow spin-lattice relaxation expected in half-metals, used to interpret the ~1 ps time constant.","marker":"[45]"}],"fun_headline_variants":["10-fs spin-galvanic currents in NiMnSb bulk","Dresselhaus bulk effect drives femtosecond photocurrents","NiMnSb shows ultrafast bulk spin-galvanic response","Femtosecond spin-charge conversion via Dresselhaus effect","Bulk Dresselhaus SGE yields 10-fs photocurrents"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that the $e^{-i\\theta}$ current component is produced by a pump-induced spin accumulation converted through the bulk Dresselhaus spin-galvanic effect, and that no other magnetization-dependent ultrafast transport process produces the same angular pattern; the paper itself labels this a tentative assumption.","fun_headline_variants_meta":{"raw":{"variants":["10-fs spin-galvanic currents in NiMnSb bulk","Dresselhaus bulk effect drives femtosecond photocurrents","NiMnSb shows ultrafast bulk spin-galvanic response","Femtosecond spin-charge conversion via Dresselhaus effect","Bulk Dresselhaus SGE yields 10-fs photocurrents"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000196,"raw_usage":{"total_tokens":1385,"prompt_tokens":996,"completion_tokens":389,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":612,"completion_tokens_details":{"reasoning_tokens":297}},"tokens_in":612,"tokens_out":389,"duration_ms":4368,"temperature":1.0,"reasoning_tokens":297,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T21:18:20.498508+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Grow a series of NiMnSb films with thicknesses from about 5 to 50 nm under identical capping and measure the $e^{-i\\theta}$ Fourier amplitude of the THz signal at fixed pump fluence. A bulk spin-galvanic current must scale linearly with NiMnSb thickness, while an interfacial or Hall-type current would saturate; the absence of linear scaling would invalidate the bulk-Dresselhaus assignment.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Defines the spin-galvanic effect, the conversion of a spin accumulation into a charge current that the paper invokes."},{"cited_title":"Ciccarelli et al., Room -temperature spin–orbit torque in NiMnSb, Nat","cited_arxiv_id":null,"evidence_quote":"Establishes that NiMnSb has a bulk Dresselhaus-type spin-orbit contribution to spin-charge conversion, making it the chosen material."},{"cited_title":"Seifert et al., Efficient metallic spintronic emitters of ultrabroadband terahertz radiation, Nat","cited_arxiv_id":null,"evidence_quote":"Supplies the THz-emission spectroscopy method and the spintronic-emitter framework used to detect the photocurrents."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports ultrafast spin-charge conversion at Rashba interfaces, the prior context that the Dresselhaus observation extends."},{"cited_title":"Zhou et al., Broadband Terahertz Generation via the Interface Inverse Rashba-Edelstein Effect, Phys","cited_arxiv_id":null,"evidence_quote":"Demonstrates broadband THz generation via the inverse Rashba-Edelstein effect, another ultrafast SGE benchmark."},{"cited_title":"Rouzegar et al., Laser-induced terahertz spin transport in magnetic nanostructures arises from the same force as ultrafast demagnetization, Phys","cited_arxiv_id":null,"evidence_quote":"Provides the model for pump-induced spin excess and the exponential functions used to fit the Dresselhaus current dynamics."},{"cited_title":"Železný et al., Unidirectional magnetoresistance and spin-orbit torque in NiMnSb, Phys","cited_arxiv_id":null,"evidence_quote":"Gives the estimated ~7 fs momentum relaxation time in NiMnSb that underpins the quasi-instantaneous SGE response."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the procedure for extracting current waveforms from THz signals and the reflectance model for the electron-cooling time."},{"cited_title":"Mann et al., Insights into Ultrafast Demagnetization in Pseudogap Half -Metals, Phys","cited_arxiv_id":null,"evidence_quote":"Supports the slow spin-lattice relaxation expected in half-metals, used to interpret the ~1 ps time constant."}],"review_version":1}