{"id":"95eab70c-9082-4604-93b3-45856d700e4e","arxiv_id":"2508.20354","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"CoTiO3 exposed to THz pulses shows 1.3 THz magnon oscillations and a slow, magnetic-order-dependent polarization rotation that the authors attribute to surface spin Seebeck-like spin accumulation.","lead":"THz light pulses excite magnons and a slow magneto-optic response in the antiferromagnet CoTiO3, indicating spin-lattice coupling with symmetry breaking beyond the bulk magnetic order. The result warns that surface and thermal-gradient effects can masquerade as light-driven spin phenomena in complex oxides.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The 1.3 THz peak assigned to the AFM magnon could be a zone-folded acoustic phonon; the Appendix's dismissal is unquantified and no field-dependent measurement is provided.","rationale":"The paper's central claim is that THz-driven lattice motion produces antiferromagnetic magnon dynamics. All three pieces of evidence for the 1.3 THz feature—frequency match, temperature dependence, and pump-frequency dependence—are also consistent with a zone-folded acoustic phonon made Raman/birefringent active by the AFM ordering, because the magnetic unit cell doubles below TN. The Appendix explicitly acknowledges this possibility but dismisses it on the grounds that the folded phonon's Raman activity 'should be very weak'; no quantitative estimate or measurement supports this. The definitive discriminator, magnetic field dependence, is cited from prior work but not performed here. If the 1.3 THz oscillation is a folded phonon seen through linear birefringence rather than a magnon seen through Faraday rotation, the paper provides no evidence of spin dynamics and its title claim fails. The reader's concern about the slow signal is valid, but it concerns an explicitly hedged 'possible' mechanism; the magnon identification is asserted as a demonstration and is therefore more load-bearing. A conditional verdict is appropriate pending a field-dependent measurement, matching the reader's overall assessment.","tokens_in":10320,"tokens_out":13908,"duration_ms":127493,"concrete_test":"Repeat the THz pump–optical probe experiment in an applied magnetic field (0–7 T) and track the frequency, amplitude, and linewidth of the 1.3 THz peak; a magnon in CoTiO3 should shift or split with field following the behavior reported by Choe et al., while a zone-folded acoustic phonon would remain essentially field-independent.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that phonon-polariton excitation drives antiferromagnetic magnon dynamics rests on identifying the 1.3 THz coherent oscillation as the Γ-point magnon. The stated evidence—frequency match to the known magnon, appearance only below TN, and strongest amplitude when pumping TO phonons—does not uniquely support a magnon. Below TN the magnetic unit cell doubles, zone-folding acoustic phonon branches to the zone center; the calculated dispersion in Fig. 5 shows acoustic modes near 1.3–1.5 THz. Such a folded phonon would also appear only below TN, and its excitation could be enhanced by anharmonic coupling to resonantly driven IR phonons. The Appendix (Sec. IV A) explicitly concedes this possibility but asserts that the folded phonon's Raman activity 'should be very weak compared to that of the magnons, which have a distinctive magnetic field dependence as shown by Choe et al.,' without providing a calculation, measurement, or estimate. The decisive test, magnetic field dependence, was performed in prior work but not repeated here. If the 1.3 THz feature is a folded phonon detected through pump-induced linear birefringence, then the paper does not demonstrate any spin dynamics, and the title and central conclusion fail.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports time-resolved THz pump–optical probe measurements on bulk CoTiO3. Below the Néel temperature, the pump produces (i) a coherent 1.3 THz oscillation attributed to the Γ-point antiferromagnetic magnon, (ii) high-frequency oscillations near 7.2, 8.1, and 10.2 THz attributed to Raman-active phonons excited through phonon-polariton coupling, and (iii) a slow, long-lived increase in the probe polarization rotation that scales linearly with fluence and appears only in the antiferromagnetic phase. The authors propose that the slow signal arises from a surface-driven spin Seebeck effect and describe it with a 1D heat-diffusion model, while explicitly acknowledging other possible mechanisms. Supporting density-functional-theory phonon calculations and SpinW magnon calculations are presented in the appendix.","tokens_in":10547,"tokens_out":8287,"duration_ms":77697,"significance":"If the mode assignments are correct, the work provides time-domain evidence for phonon-polariton-driven spin dynamics in a Dirac magnet and shows that extrinsic symmetry breaking (surface or penetration-depth effects) can dominate the magneto-optic response of a nominally compensated antiferromagnet. The paper is careful in several places to label the slow-signal mechanism as a suggestion rather than a proof, and it includes explicit calculations of phonon and magnon spectra. The main significance, however, hinges on the unambiguous identification of the 1.3 THz coherent mode as a magnon and on the exclusion of nonmagnetic contributions to the slow polarization rotation.","major_comments":[{"comment":"The assignment of the 1.3 THz coherent mode to the Γ-point magnon is not unique. The authors' own phonon calculation (Fig. 5) shows that zone-folding of acoustic branches brings modes near 1.3–1.5 THz to the zone center; such a folded acoustic phonon would also appear only below TN and could be excited via anharmonic coupling to the resonantly driven IR phonons. The Appendix dismisses this alternative with the statement that the folded phonon's Raman activity 'should be very weak' compared to the magnons, but no calculation, symmetry estimate, or measurement is provided. Because the central conclusion that phonon-polariton excitation drives antiferromagnetic magnon dynamics rests on this identification, the manuscript needs either a quantitative estimate of the folded-phonon Raman/magneto-optic response or a magnetic-field-dependent measurement that tracks the known magnon frequency. Without one of these, the evidence is not uniquely supportive of the magnon assignment.","section":"Appendix IV A, Fig. 5"},{"comment":"The slow polarization rotation is interpreted as a Faraday rotation from a net out-of-plane magnetization, but nonmagnetic contributions have not been experimentally excluded. The text itself states that Faraday rotation is 'one possible source' and later acknowledges strain-gradient birefringence as an alternative, yet the abstract and conclusion present the slow signal as a magneto-optic response with a spin origin. A polarization-resolved measurement that separates Faraday rotation from pump-induced linear birefringence, or a magnetic-field test, is needed to support the surface spin Seebeck interpretation. As written, the slow-signal feature is consistent with the proposed mechanism but does not uniquely establish it.","section":"Section II, after Fig. 4"}],"minor_comments":[{"comment":"The fit parameters τ_rise and τ_decay are not reported, and the slow-component subtraction is not shown; please provide these values so the FFT decomposition can be reproduced.","section":"Section II, Eq. (1)"},{"comment":"The caption 'Polarization rotation for different center frequencies of the THz pump maximized when the reflectivity of the crystal is the lowest' is incomplete; it should state the plotted quantity and normalization. Also, the √t fits with amplitudes a = 36.4, 15.7, 2.7 are mentioned in the text but are not visible in the figure.","section":"Fig. 3(a) caption"},{"comment":"The x0→0 limit is used to obtain the √t scaling without discussing whether the finite THz penetration depth is indeed negligible on the measured picosecond-to-hundreds-of-picosecond time window; please either justify this limit or show the finite-x0 solution for the relevant parameters.","section":"Section II, heat-diffusion model"},{"comment":"No error bars or repeated-measurement statistics are provided for the fluence dependence, the angular dependence, or the relative amplitudes of the √t fits; given that several conclusions rely on quantitative comparisons, please add at least representative uncertainty estimates.","section":"Figs. 3(c) and 4(b)"},{"comment":"The DFT phonon frequencies in Table I are not directly compared with the experimental IR frequencies (8.6 THz and 12.5 THz) cited in the text; a side-by-side comparison would clarify the level of agreement.","section":"Appendix Table I"}],"recommendation":"major_revision","confidential_remarks":"The main risk to this paper is the magnon assignment; the authors should be asked to either repeat the field-dependent THz measurement or provide a quantitative estimate of the folded-phonon response. The slow-signal interpretation is appropriately hedged, but the wording of the abstract and conclusion should be softened unless the Faraday origin is confirmed."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"First thing to know: this is a real, new experiment—first time-resolved THz pump with magneto-optic probe on CoTiO3—and the slow, magnetic-order-dependent polarization rotation is the most convincing piece. The 1.3 THz coherent oscillation is plausibly the magnon, but the assignment is softer than the title implies.\n\nWhat the paper does well: the temperature dependence of the 1.3 THz mode, its match to the known magnon energy from prior THz-TDS, and the linear fluence dependence of the slow signal are all coherent. The authors include DFT phonon and spin-wave calculations, and they explicitly hedge the spin-Seebeck mechanism. That's honest and useful.\n\nThe soft spots are in proportion: the zone-folded-phonon alternative is waved away with an unquantified claim about weak Raman activity. A field-dependent measurement or a simple estimate would settle it. This matters because the title leans on the magnon assignment, but even if the fast mode were a folded phonon, the slow signal still gives evidence of spin-lattice coupling—so the paper does not collapse. The bigger issue is that the slow rotation is assumed to be Faraday from an out-of-plane magnetization; pump-induced birefringence or reflectivity changes are not fully separated. The 1D heat model is a crude approximation that fits only amplitudes, so it supports, not proves, the spin-Seebeck picture. Finally, there are no error bars or repeated runs shown, and no raw data deposited; for an ultrafast experiment with multiple plausible mechanisms, that's a real reproducibility gap.\n\nThe citation pattern looks fair—self-citations point to prior measurements of the magnon and chiral phonons, not to unsupported claims.\n\nWho benefits: experimentalists working on THz-driven magnetization dynamics, and anyone thinking about surface symmetry breaking in pump-probe studies of antiferromagnets. It deserves a serious referee, especially one who can scrutinize the magnon assignment and the optical detection. I'd ask for a revised version with error analysis, a magnetic-field test or a quantitative folded-phonon estimate, and raw data release. If those are provided, the work would be a solid contribution.","headline":"A credible new THz pump-probe result on CoTiO3 with a plausible magnon assignment that is not fully pinned down; the slow magnetic-order-dependent rotation is the more robust finding.","tokens_in":11150,"tokens_out":4303,"would_cite":false,"duration_ms":38752,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Resonant THz excitation of phonons in the antiferromagnet CoTiO3 produces coherent magnon dynamics and a slow, surface-driven magneto-optic response tied to magnetic order.","keywords":["light-driven","phonons","magnons","spin-lattice coupling","time-resolved magneto-optic spectroscopy","CoTiO3","antiferromagnet","THz pump-probe"],"falsifier":"Measure the slow component in a CoTiO3 thin film, or with a pump geometry that removes the surface thermal gradient by heating uniformly through the thickness; the 1D spin Seebeck model predicts the sqrt(t) accumulation should vanish or change sign when the surface gradient is removed, while a nonmagnetic birefringence origin would persist.","tokens_in":1818,"feed_emoji":"🧲","tokens_out":2347,"duration_ms":81417,"temperature":0.7,"pith_summary":"The paper reports that THz pulses tuned to lattice vibrations in the antiferromagnet CoTiO3 drive coherent spin dynamics: resonantly excited phonons and phonon-polaritons excite the 1.3 THz antiferromagnetic magnon, and a slow polarization rotation of an optical probe grows over hundreds of picoseconds, well beyond the pulse duration and the coherent phonon and magnon lifetimes. The slow rotation appears only below the Néel temperature, grows linearly with pump fluence, and is nearly independent of pump polarization, so the authors take it to be magnetic in origin. Because bulk CoTiO3 has no net out-of-plane magnetic moment, they argue that the response requires symmetry breaking extrinsic to the bulk magnetic space group, most plausibly a surface spin Seebeck effect in which THz heating creates a thermal gradient that drives a magnon spin current into the bulk. If this picture is right, light can control magnetic order through the lattice, and surface degrees of freedom can dominate nominally bulk light-driven spin phenomena in complex oxides.","feed_headline":"THz pulses set CoTiO3 spins moving via lattice motion","feed_subtitle":"Slow magnetic rotation appears only below the Néel temperature, a surface spin-accumulation signature.","key_machinery":"The central objects are THz-driven phonon-polaritons and the surface spin Seebeck mechanism. Phonon-polaritons couple the pump field to lattice displacement through infrared-active $E_u$ modes; near a transverse-optical phonon the dielectric response diverges, maximizing atomic displacement, which in turn modifies exchange interactions and excites the 1.3 THz magnon. The slow signal is modeled as a spin current $\\mathbf{J}_s = \\kappa \\nabla T$ generated by the thermal gradient at the symmetry-reduced surface; the heat-kernel solution of the 1D diffusion equation gives an accumulated magnetization proportional to $\\sqrt{t}$, matching the observed slow rise and the linear fluence dependence.","core_discovery":"The central claim is that resonant THz excitation of infrared-active phonons in CoTiO3 sets off a chain: phonon-polariton excitation produces coherent antiferromagnetic magnon oscillations at 1.3 THz, together with a slow, magnetic-order-dependent magneto-optic rotation whose amplitude follows a $\\sqrt{t}$ law matching a one-dimensional heat-diffusion model of spin accumulation driven by a surface thermal gradient. This is surprising because the easy-plane antiferromagnet's bulk magnetic space group forbids a net out-of-plane magnetization, which normal-incidence Faraday rotation would require. The authors therefore propose that the surface, where the magnetic space group is reduced, upholds a weak out-of-plane moment; the THz-induced temperature gradient generates a magnon spin current whose accumulated flux produces the observed long-lived rotation. They also find that the coherent magnon is strongest when the pump is near the transverse-optical phonon frequencies, and that the high-frequency response tracks Raman-active phonon modes, indicating phonon-mediated excitation pathways.","pith_inferences":["Beyond the paper: if the surface spin Seebeck picture holds, the same THz protocol should generate a detectable spin-current signal in other easy-plane antiferromagnets with surface-allowed weak moments, and the sign of the accumulated rotation should track the pump penetration depth and surface termination.","Beyond the paper: the linear-polarization anisotropy in the slow signal could be tested directly by shaping the pump spot to create a controlled asymmetric thermal gradient and checking whether the slow rotation direction follows the gradient direction.","Beyond the paper: applying a small out-of-plane magnetic field near the surface would settle the magnetic origin of the slow component, since a Faraday rotation from accumulated out-of-plane magnetization should respond to the field once local anisotropy pinning is overcome, whereas a birefringence artifact would not."],"forward_implications":["In CoTiO3, lattice excitation by THz light is an effective handle on antiferromagnetic spin dynamics: resonant phonon-polariton driving produces coherent magnon oscillations and a long-lived magnetic response.","The slow magneto-optic response below the Néel temperature implies that light-driven spin phenomena in nominally bulk complex oxides can carry a strong extrinsic surface contribution, complicating interpretations based on bulk symmetry alone.","The observed $\\sqrt{t}$ accumulation and linear fluence dependence are consistent with a magnon spin Seebeck current driven by a surface thermal gradient, so the surface acts as a spin-current source under THz illumination.","Distinguishing chiral-phonon from spin-Seebeck mechanisms calls for thin-film samples that remove the thermal gradient; the model predicts the slow component should disappear or change character there.","The pump-polarization dependence of the slow signal points to an additional strain-gradient or birefringence contribution with the same polar-vector symmetry as the thermal gradient, leaving an experimental route to separate the two."],"supporting_citations":[{"why":"supplies the Dirac magnon dispersion and XY model used to identify the 1.3 THz mode as the antiferromagnetic magnon.","marker":"[10]"},{"why":"supplies the infrared- and Raman-active phonon frequencies used to assign the high-frequency spectral features and the transverse-optical resonances.","marker":"[15]"},{"why":"provides the THz time-domain spectroscopy magnon frequency and the magnetoelastic magnon-gap picture that anchors the low-frequency assignment.","marker":"[16]"},{"why":"provides the narrowband multicycle THz generation method that allows the pump to target specific phonon resonances.","marker":"[17]"},{"why":"supplies the circular-polarization generation method and the chiral-phonon effective-field mechanism compared against the data.","marker":"[19]"},{"why":"shows that optically driven phonons can act as an effective magnetic field, supporting phonon-mediated magnon excitation.","marker":"[9]"},{"why":"provides the spin Seebeck theory used to model the slow signal as a thermal-gradient-driven magnon spin current.","marker":"[22]"},{"why":"supplies the heat-kernel solution for one-dimensional heat diffusion that yields the observed sqrt(t) accumulation law.","marker":"[23]"}],"fun_headline_variants":["THz phonons kick CoTiO3 magnons, surface spin buildup","Phonon-polariton stirs CoTiO3 spins, surface effect shows","THz light drags CoTiO3 surface spins via phonon path","CoTiO3 spins respond to THz phonons via surface accumulation"],"cache_read_input_tokens":13184,"weakest_assumption_plain":"The slow rotation is assumed to be a Faraday rotation caused by a net out-of-plane magnetization accumulated at the surface, rather than a pump-induced birefringence, reflectivity change, or another nonmagnetic optical effect.","fun_headline_variants_meta":{"raw":{"variants":["THz phonons kick CoTiO3 magnons, surface spin buildup","Phonon-polariton stirs CoTiO3 spins, surface effect shows","THz light drags CoTiO3 surface spins via phonon path","CoTiO3 spins respond to THz phonons via surface accumulation"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000875,"raw_usage":{"total_tokens":3775,"prompt_tokens":921,"completion_tokens":2854,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":537,"completion_tokens_details":{"reasoning_tokens":2775}},"tokens_in":537,"tokens_out":2854,"duration_ms":18919,"temperature":1.0,"reasoning_tokens":2775,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T16:46:20.220765+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the slow component in a CoTiO3 thin film, or with a pump geometry that removes the surface thermal gradient by heating uniformly through the thickness; the 1D spin Seebeck model predicts the sqrt(t) accumulation should vanish or change sign when the surface gradient is removed, while a nonmagnetic birefringence origin would persist.","supporting_citations":[{"cited_title":"Dubrovin , author N","cited_arxiv_id":null,"evidence_quote":"supplies the infrared- and Raman-active phonon frequencies used to assign the high-frequency spectral features and the transverse-optical resonances."},{"cited_title":"Choe , author A","cited_arxiv_id":null,"evidence_quote":"provides the THz time-domain spectroscopy magnon frequency and the magnetoelastic magnon-gap picture that anchors the low-frequency assignment."},{"cited_title":"Liu , author H","cited_arxiv_id":null,"evidence_quote":"provides the narrowband multicycle THz generation method that allows the pump to target specific phonon resonances."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"supplies the heat-kernel solution for one-dimensional heat diffusion that yields the observed sqrt(t) accumulation law."}],"review_version":1}