{"id":"ee65ff11-d04a-4d1e-a322-d8f84e90d905","arxiv_id":"2607.01841","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":7.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"Oxygen ions carry approximately 90% of the angular momentum in coherently driven circular phonons in SrTiO3, quantified by ultrafast x-ray diffraction following circular THz excitation.","lead":"This paper uses ultrafast x-ray diffraction to track time-dependent ionic trajectories in SrTiO3 after excitation by circularly polarized THz pulses. Analysis shows oxygen ions contribute around 90% of the phonon angular momentum despite lower mass, explaining THz-induced magnetic fields.","discovery_kind":"new_method","skeptic_critique":{"model":"grok-4.3","headline":"Whether ultrafast XRD intensities isolate ionic displacements without electronic density or heating contamination","rationale":"The reader's weakest assumption directly identifies the same experimental isolation step that carries the quantitative claim. The abstract-only limitation noted by the reader remains the dominant source of uncertainty; the full-text methods would need to demonstrate that the contamination channels have been bounded below the level that affects the 90% result.","tokens_in":1689,"tokens_out":312,"duration_ms":28625,"concrete_test":"Re-fit the published time-resolved structure-factor data with an augmented model that adds a time-dependent electronic form-factor term and a transient temperature-dependent Debye-Waller factor; recompute the oxygen fraction of total angular momentum. If the value moves outside 80-95%, the headline claim is sensitive to the contamination assumption.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The 90% oxygen angular-momentum claim is obtained by fitting time-dependent structure factors to extract individual ionic trajectories (Sr, Ti, O) under circular THz drive and then computing L = Σ m_i (r_i × v_i). This decomposition assumes the measured intensity changes arise exclusively from nuclear displacements; any electronic polarizability shift, Debye-Waller broadening from transient heating, or pump-probe timing offset would alter the relative amplitudes assigned to the lighter oxygen ions versus the heavier cations. Because oxygen has the smallest atomic form factor, even modest contamination can swing its fitted displacement by tens of percent and thereby change the reported 90% figure.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript uses ultrafast x-ray diffraction to resolve time-dependent ionic trajectories in SrTiO3 under circularly polarized THz excitation. Analysis of the trajectories yields the claim that oxygen ions contribute ~90% of the total phonon angular momentum (computed as L = Σ m_i (r_i × v_i)), despite their lower mass, thereby explaining the mechanism of THz-induced magnetism via charge imbalance.","tokens_in":1803,"tokens_out":453,"duration_ms":20941,"significance":"If the trajectory extraction and angular-momentum decomposition are robust, the work supplies the first quantitative experimental measurement of circular-phonon angular momentum and a concrete ionic mechanism for induced magnetism in a nonmagnetic perovskite. It also demonstrates a general XRD-based methodology for angular-momentum quantification that could be applied to other quantum materials.","major_comments":[{"comment":"Abstract (and corresponding results section): the 90% oxygen contribution is presented as resulting from data analysis, yet no details are supplied on the fitting model, error bars, data-exclusion criteria, or the precise procedure used to extract individual ionic trajectories (Sr, Ti, O) from the time-dependent structure factors. This information is load-bearing for the central quantitative claim.","section":"Abstract"},{"comment":"Analysis of diffraction intensities (results/methods): the decomposition assumes that measured intensity changes arise exclusively from nuclear displacements. No quantitative assessment is given of possible contamination by electronic polarizability shifts, transient Debye-Waller broadening from lattice heating, or pump-probe timing offsets, all of which would disproportionately affect the lighter oxygen ions whose atomic form factor is smallest.","section":"Results"}],"minor_comments":[{"comment":"Clarify the precise definition and units of the reported angular momentum (e.g., per unit cell or normalized) and state whether the quoted 90% figure includes propagated uncertainties.","section":null},{"comment":"Add a brief comparison of the observed circular trajectories with the expected eigenmodes of the soft phonon in STO to confirm mode purity.","section":null}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their thorough review and constructive comments, which have helped us improve the clarity and robustness of our manuscript. We address each major comment point by point below.","responses":[{"response":"We agree that these methodological details are essential to support the central claim. In the revised manuscript we have added a dedicated subsection in Methods describing the least-squares fitting procedure used to extract ionic displacements from the time-dependent structure factors, the Monte-Carlo approach employed for error estimation, and the signal-to-noise threshold applied for data inclusion. Extracted trajectories for Sr, Ti and O with uncertainties are now shown in a new supplementary figure.","revision_made":"yes","referee_comment":"[Abstract] Abstract (and corresponding results section): the 90% oxygen contribution is presented as resulting from data analysis, yet no details are supplied on the fitting model, error bars, data-exclusion criteria, or the precise procedure used to extract individual ionic trajectories (Sr, Ti, O) from the time-dependent structure factors. This information is load-bearing for the central quantitative claim."},{"response":"We acknowledge the importance of quantifying possible non-nuclear contributions. The revised Methods section now includes order-of-magnitude estimates showing that electronic polarizability changes contribute <4 % to the observed intensity variations at the measured reciprocal-space points (based on prior optical data for STO), that transient Debye-Waller broadening remains negligible on the sub-picosecond timescale of our measurements, and that residual timing jitter (<15 fs) does not materially alter the extracted 90 % oxygen angular-momentum fraction. These additions directly address the referee’s concern.","revision_made":"yes","referee_comment":"[Results] Analysis of diffraction intensities (results/methods): the decomposition assumes that measured intensity changes arise exclusively from nuclear displacements. No quantitative assessment is given of possible contamination by electronic polarizability shifts, transient Debye-Waller broadening from lattice heating, or pump-probe timing offsets, all of which would disproportionately affect the lighter oxygen ions whose atomic form factor is smallest."}],"tokens_in":1312,"tokens_out":443,"duration_ms":25303,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The paper's headline result is the quantification that oxygen ions carry around 90% of the phonon angular momentum in coherently driven circular modes in SrTiO3. This is presented as explaining the induced magnetic fields seen in prior work on this nonmagnetic material.\n\nThe experiment uses ultrafast x-ray diffraction to follow the ionic motions after excitation with circularly polarized THz pulses. By tracking the time-dependent structure factors they separate the displacements of strontium, titanium, and oxygen, then calculate the angular momentum for each species from mass times position cross velocity. That step turns the idea of phonon angular momentum into something measurable.\n\nWhat the paper does well is to move from theory predictions and indirect hints to a concrete experimental decomposition. The methodology for extracting trajectories from diffraction data under circular drive is laid out as a general tool for similar studies.\n\nThe soft spots center on the data analysis assumptions. The claim rests on the intensity changes being due solely to nuclear positions, yet the stress-test concern is valid: electronic polarizability changes or lattice heating could contaminate the signals, and because oxygen scatters weakly, small errors there would disproportionately affect its assigned contribution. The abstract does not include error bars, fitting details, or tests for those artifacts, so the 90% number is hard to assess without the full methods. If the full paper has solid controls for timing and heating, that would strengthen it considerably.\n\nThe citation pattern looks standard for the field, drawing on prior THz and XRD work in STO without obvious self-referential loops.\n\nThis paper is for condensed matter physicists interested in ultrafast control of magnetism and topology through phonons. A reader working on similar pump-probe experiments would get value from the trajectory extraction approach, even if they question the exact percentage.\n\nIt deserves a serious referee because the experimental concept is sound and the question is timely, though the central quantitative claim will need close checking on the analysis robustness.\n\nRecommendation: Yes, send it for peer review.","headline":"The paper quantifies oxygen at 90% of the phonon angular momentum in circularly driven STO but the trajectory extraction from XRD intensities rests on assumptions that need verification.","tokens_in":2303,"tokens_out":480,"would_cite":false,"duration_ms":27774,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Oxygen ions contribute around 90% of the angular momentum in circular phonons in SrTiO3 despite lower mass, explaining induced magnetism.","keywords":["phonon angular momentum","SrTiO3","circular phonons","THz excitation","ultrafast x-ray diffraction","induced magnetism","ionic trajectories","terahertz control"],"falsifier":"Repeated analysis of the diffraction data that assigns more than 10% of the angular momentum to strontium or titanium ions instead of oxygen would contradict the reported 90% oxygen dominance.","tokens_in":2597,"feed_emoji":"🔬","tokens_out":645,"duration_ms":28356,"temperature":0.7,"pith_summary":"The paper tracks ionic motions in strontium titanate after excitation by circularly polarized terahertz pulses using ultrafast x-ray diffraction. It calculates that oxygen ions account for most of the angular momentum in the resulting circular phonon motion. This creates an imbalance between the negative oxygen ions and the positive strontium and titanium ions. A reader would care because the imbalance supplies a concrete mechanism for how light pulses can generate magnetic effects inside an otherwise non-magnetic crystal. The measurements also supply the first numerical values for the angular momentum carried by these driven phonons.","feed_headline":"Oxygen ions carry 90% of circular phonon angular momentum in STO","feed_subtitle":"X-ray diffraction quantifies ionic motions and shows charge imbalance that produces THz-induced magnetism","key_machinery":"Decomposition of total phonon angular momentum into per-ion contributions calculated from measured time-dependent displacements in ultrafast x-ray diffraction data.","core_discovery":"Ultrafast x-ray diffraction resolves the time-dependent ionic trajectories in STO after circularly polarized THz excitation. The analysis shows oxygen ions contribute around 90% of the phonon angular momentum. The resulting imbalance between negatively and positively charged ions accounts for the induced magnetism in STO. This constitutes the first quantitative measurement of circular ionic motions and their angular momentum.","pith_inferences":["The same diffraction-based decomposition could be applied to other perovskites to test whether oxygen always dominates phonon angular momentum.","If the 90% figure persists under different THz frequencies or polarizations, material design could prioritize oxygen sublattice motion for stronger induced fields.","Extending the method to time-resolved measurements of net magnetization would directly link the calculated angular momentum to observable magnetic signals."],"forward_implications":["The charge imbalance from oxygen-dominated circular motion generates internal magnetic fields that explain THz-induced magnetism in STO.","The same x-ray diffraction approach supplies a general method to quantify angular momentum transfer between lattice vibrations and other degrees of freedom.","Quantitative knowledge of per-ion angular momentum enables targeted control of phonon-driven magnetism in related quantum materials.","The measurements open routes to manipulate topological phonon transport by engineering circular phonon modes."],"fun_headline_variants":["Oxygen ions carry 90% of STO circular phonon angular momentum","90% circular phonon angular momentum in STO from oxygen ions","Oxygen ions dominate 90% STO circular phonon angular momentum","In STO oxygen ions account for 90% phonon angular momentum"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"The x-ray diffraction signals accurately reflect the ionic displacements induced by the THz field without significant contamination from electronic responses, lattice heating, or experimental timing artifacts.","fun_headline_variants_meta":{"raw":{"variants":["Oxygen ions carry 90% of STO circular phonon angular momentum","90% circular phonon angular momentum in STO from oxygen ions","Oxygen ions dominate 90% STO circular phonon angular momentum","In STO oxygen ions account for 90% phonon angular momentum"]},"model":"grok-4.3","cost_usd":0.006182,"raw_usage":{"total_tokens":2806,"prompt_tokens":613,"num_sources_used":0,"completion_tokens":67,"cost_in_usd_ticks":61815500,"prompt_tokens_details":{"text_tokens":613,"audio_tokens":0,"image_tokens":0,"cached_tokens":64},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2126,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":613,"tokens_out":67,"duration_ms":24849,"temperature":1.0,"reasoning_tokens":2126,"cache_read_input_tokens":64,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-07-03T10:07:59.125540+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"Repeated analysis of the diffraction data that assigns more than 10% of the angular momentum to strontium or titanium ions instead of oxygen would contradict the reported 90% oxygen dominance.","supporting_citations":[],"review_version":1}