{"id":"81762678-9eb1-42bd-83c4-c6b86adc1489","arxiv_id":"2502.00830","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"63/65Cu NMR shows the 50 K relaxation anomaly in YbCuS2 weakens with magnetic field and low-temperature spin fluctuations are enhanced above about 3 T.","lead":"Researchers used copper NMR to probe how magnetic fluctuations in the zigzag-chain semiconductor YbCuS2 change under magnetic fields. They found the known 50 K fluctuation maximum is suppressed by field, while low-temperature fluctuations grow above 3 T, indicating field-driven changes even in the paramagnetic state.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Field suppression of the 50 K 1/T1 anomaly is measured at different Larmor frequencies (ν=γH); without a 63Cu/65Cu isotope test, frequency dependence is an uncontrolled confound.","rationale":"The Reader's weakest assumption is the T-independence of D in Eq. (3). That concern is real but secondary: it affects the Knight-shift proportionality claim, not the headline field dependence of 1/T1. The most load-bearing assumption is that H and Larmor frequency can be varied independently in the NMR measurement; they cannot in a single-isotope H-swept experiment. The reported field dependence of 1/T1 at 50 K across 0.1-10 T spans 1.1-113 MHz, two decades, making a frequency effect a concrete alternative. The paper's rejection of BPP applies to the T-dependence at zero field and does not rule out a frequency-dependent spectral density at finite fields. The proposed 63Cu/65Cu test is standard and would settle the issue. Because this possible confound is untested, the paper should remain CONDITIONAL, not be accepted as definitive; the current CONDITIONAL verdict is therefore retained, although the justification differs from the Reader's. The partial-orientation analysis and the H-independent K are useful supporting results and were given due weight in this assessment.","tokens_in":12326,"tokens_out":11432,"duration_ms":126370,"concrete_test":"Measure 1/T1 on both 63Cu and 65Cu at 50 K in two configurations: (i) same Larmor frequency, e.g., 40.4 MHz, requiring μ0H = 3.58 T for 63Cu and 3.34 T for 65Cu; and (ii) same magnetic field, e.g., 3.5 T, giving 39.5 MHz (63Cu) and 42.3 MHz (65Cu). If 1/T1 is equal at the same field for the two isotopes, the field effect is confirmed; if it instead tracks the Larmor frequency (or if 63Cu at 3.58 T equals 65Cu at 3.34 T), the Fig. 5 suppression is partly or wholly a frequency artifact. A reasonable fallback is to compare 63Cu 1/T1 at two fields onto which 65Cu resonances at the same frequency fall, using the 7.1% gyromagnetic ratio difference.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim that magnetic fields suppress the 50 K 1/T1 anomaly and induce a low-T upturn is based on Fig. 5, where each field point is measured at a different 63Cu Larmor frequency: 0.1 T -> 1.1 MHz, 1.7 T -> 19.5 MHz, 2.7 T -> 31.4 MHz, 3.5 T -> 40.4 MHz, 5.6 T -> 64.2 MHz, 10 T -> 113 MHz. Because the H-swept method fixes ν = γH, the observed decrease of 1/T1 at 50 K with H could in principle be a frequency dependence of the relaxation rate rather than a Zeeman-driven modification of the paramagnetic state. The paper considers the Bloembergen-Purcell-Pound frequency-dependent form only to reject it for the zero-field temperature dependence (Sec. 3); it never tests whether the field dependence in Fig. 5(b) is frequency dependence. A 63Cu-only measurement cannot separate these. This is the load-bearing gap, since the 50 K anomaly suppression is the main result. The Reader's D(T) concern, by contrast, affects only the K analysis and is partly mitigated by the reported H-independence of K across frequencies; it does not touch 1/T1. The orientation-selection issue is also secondary because the linear D plot in Fig. 3(b) suggests a stable spectral feature. Thus the central unresolved assumption is the field/frequency entanglement.","agreement_with_reader":"disagree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript reports 63/65Cu-NMR measurements on the Zigzag-chain semiconductor YbCuS2 in magnetic fields up to 10 T. The authors analyze H-swept spectra to extract a Knight shift K(T), find K proportional to bulk susceptibility with a hyperfine coupling of 0.14 T/μB, and deduce that the ac plane is an easy plane from partial sample orientation. The central relaxation result is that the broad maximum of 1/T1 at ~50 K (the 50 K anomaly) is suppressed by magnetic fields, and that for fields above 3 T a low-temperature upturn of 1/T1 appears, indicating field-induced modification of the paramagnetic state. The paper also compares with the H-T phase diagram and discusses possible connections to a field-induced spin-fluctuating state.","tokens_in":12699,"tokens_out":2705,"duration_ms":29012,"significance":"If the central claim holds, the paper provides microscopic evidence that magnetic fields modify the paramagnetic state of a frustrated Yb zigzag-chain material at temperatures far above TN, which is relevant for understanding the field-induced phases below TN (1/3 plateau, up-up-down order, and the high-field phase). Strengths of the paper include direct relaxation measurements without a fitted model for the main 1/T1 trends, the reproduction of the spectra with powder-pattern simulations, the demonstration of H-independence of the Knight shift across several frequencies, and a clear comparison with the published H-T phase diagram. However, the main claim rests on an uncontrolled frequency/field correlation in the H-swept 1/T1 measurements, as detailed below, so the significance can only be assessed after that issue is addressed.","major_comments":[{"comment":"The central claim that the 50 K anomaly is suppressed by magnetic fields and that a low-T upturn appears above 3 T is based on measurements at different Larmor frequencies. Because the H-swept method fixes the frequency at ν = γH, the data at 0.1, 1.7, 2.7, 3.5, 5.6, and 10 T correspond to 63Cu frequencies of 1.1, 19.5, 31.4, 40.4, 64.2, and 113 MHz, respectively. The observed decrease of 1/T1 at 50 K with increasing nominal field could thus be a frequency dependence rather than a Zeeman-driven modification of the paramagnetic state. The paper discusses the Bloembergen-Purcell-Pound frequency-dependent form only to reject it for the zero-field temperature dependence, and it never tests the field dependence against a frequency dependence. A 63Cu-only measurement cannot separate these two variables. This is load-bearing because the suppression of the 50 K anomaly is the main result. The authors should either present data at a fixed frequency for different fields (e.g., by using both 63Cu and 65Cu at the same Larmor frequency, or by using a different NMR nucleus), or at minimum explicitly quantify the possible frequency dependence of 1/T1 over this range using the same sample and temperature.","section":"Sec. 3, Fig. 5"},{"comment":"The temperature dependence of the Knight shift is extracted using the assumption that the second-order quadrupole coefficient D is temperature-independent and equal to D(50 K) = 12.81 MHz^2. If D varies with temperature, the K(T) values would be systematically shifted, and the conclusion that K is proportional to bulk susceptibility and H-independent would need revision. The assumption is stated without an experimental test. Although the reported H-independence of K at fixed T (Fig. 3(b)) partially mitigates this concern for the H-dependence, it does not test the T-dependence. The authors should verify the constancy of D by performing the multi-frequency analysis at at least one low temperature (e.g., 10 K or 4.2 K) and comparing with the 50 K value, or provide an estimate of the systematic error in K from a plausible T-dependence of D.","section":"Sec. 3, Eq. (3)"},{"comment":"The 1/T1 data in Figs. 5(a) and 5(b) are presented without statistical error bars, and the text states that 'T1 was evaluated from the reliable fitting of the relaxation curve with the single component of T1' without showing residuals or goodness-of-fit parameters. The relaxation functions in Eqs. (7) and (8) are sums of multiple exponentials, so a single-component fit is nontrivial. The authors should provide at least representative fit residuals and error bars on the plotted 1/T1 values, especially for the low-temperature upturn points that support the central claim. Without these, it is difficult to assess whether the observed field-induced changes are statistically significant.","section":"Sec. 3, Fig. 4 and Fig. 5"}],"minor_comments":[{"comment":"There are several typographical errors in the title and abstract: 'Inv estigated' and 'Depertment' in the header, and 'e ffect' with a stray ligature. These should be corrected.","section":"Title and affiliations"},{"comment":"The text says '63/65Cu-NMR 1/T1 was also measured at the peak with the highest intensity', but only 63Cu T1 data are shown. Please specify whether 65Cu T1 was measured and, if not, which isotope was used throughout.","section":"Sec. 2"},{"comment":"The label 'Se' in Fig. 1(a) appears to be a typo for 'S'. Also, in the caption of Fig. 2(a), the simulation parameters (K = 1%, 63νzz = 9.14 MHz, 65νzz = 8.48 MHz, η = 0.32) are given, but the range of the frequency axis in the simulations is not stated; please add this or refer to the text for details.","section":"Sec. 3, Fig. 2"},{"comment":"The paper uses the term 'H-swept' but in the experimental section describes 'H-swept 63/65Cu-NMR spectra at several different frequencies'. It would be clearer to explicitly state that the field was swept at constant frequency, so that each field value corresponds to a distinct Larmor frequency (as noted in the major comments).","section":"Sec. 3"},{"comment":"The phase diagram in Fig. 1(b) includes the newly determined Tmax and T* points, but the figure is reproduced from a previous work. Please ensure the new data points are clearly distinguished (e.g., by different symbols or color) and that the figure caption explains the added points.","section":"Sec. 3, Fig. 1(b)"}],"recommendation":"major_revision","confidential_remarks":"The main concern is the frequency/field entanglement in the 1/T1 measurements. If the authors can provide a fixed-frequency comparison or an isotope test that rules out frequency dependence, the paper would be acceptable. The D(T) assumption is secondary but should be addressed. The manuscript is otherwise clearly written and fits the scope of JPSJ."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nQuick take: this is a competent, honest NMR paper from a group that clearly knows this material. The new results are real observations: the field suppression of the 50 K 1/T1 anomaly, the low-temperature upturn above 3 T, and the partial-orientation evidence for the ac easy plane. The Knight shift analysis is standard and the K-χ proportionality with bulk susceptibility is clean. The paper also does a good job situating the work in the existing YbCuS2 literature and in the broader frustrated-chain context. Credit where due: the 1/T1 data are directly measured, not model-extracted, and the phase diagram update with Tmax and T* is useful.\n\nNow the soft spots, in order of real weight.\n\nThe biggest one is the field-frequency entanglement. Because the H-swept method fixes ν = γH, each field point in Fig. 5 is also a different frequency: 0.1 T corresponds to NQR (low frequency), 1.7 T to 19.5 MHz, 10 T to 113 MHz. The paper rejects the BPP frequency-dependent form only for the zero-field temperature dependence of the anomaly; it never asks whether the field dependence of 1/T1 at 50 K could be a frequency dependence. This matters because the central claim—that the 50 K anomaly is suppressed by H—rests on comparing points at very different frequencies. The fact that the anomaly is still visible at 19.5 and 31.4 MHz but gone at 40.4 MHz suggests a sharp frequency cutoff, which is not impossible but needs an explicit check. A 63Cu/65Cu isotope comparison at the same field would disentangle field and frequency cleanly, and its absence is the main gap.\n\nThe other weaknesses are minor by comparison. There are no statistical error bars on 1/T1, and the single-component fit quality is asserted rather than shown with residuals. The D(T) independence assumption in Eq. (3) is untested, but it only affects the absolute K values, and the H-independence of K is already supported by the multi-frequency consistency in Fig. 3(b). The orientation conclusion depends on a simulation with fixed η and νzz, but the linear D plot in Fig. 3(b) suggests the spectral assignment is stable.\n\nThe interpretation linking the paramagnetic changes to the field-induced ordered phases is speculative, and the authors say so. That is fine. The paper is not overclaimed.\n\nWho is this for? Experimentalists working on Yb-based frustrated magnets and anyone building the H-T phase diagram of YbCuS2. It deserves a serious referee, but the referee should ask for the isotope test or an explicit frequency-dependence analysis before the suppression claim is taken as settled. I would not cite the suppression claim as established in its current form, but I would read the follow-up.\n\nRecommendation: send to peer review, with the field/frequency issue raised as the main required revision.","headline":"A careful NMR study of YbCuS2 with genuinely new observations, but the central field-suppression claim is entangled with a frequency dependence the paper never tests.","tokens_in":13223,"tokens_out":2347,"would_cite":false,"duration_ms":21461,"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":"Magnetic fields suppress the broad 50 K maximum of the nuclear spin-lattice relaxation rate in the paramagnetic state of YbCuS2 and, above 3 T, induce a new low-temperature enhancement of spin fluctuations.","keywords":["YbCuS2","zigzag chain","63Cu NMR","nuclear spin-lattice relaxation","Knight shift","field-induced paramagnetic state","frustrated magnet","50 K anomaly"],"falsifier":"Measure the field-swept NMR spectra at multiple frequencies at several fixed temperatures (for example, 10 K, 50 K, and 100 K) and extract D(T) directly as the slope of δν/(γHres) versus (γHres)^{-2}; if D varies by more than a few percent over the measured temperature range, the reported K(T) and the field-independence conclusion are not robust. A single-crystal NMR measurement that resolves the orientation of the ac plane would also independently test the easy-plane assignment.","tokens_in":12143,"feed_emoji":"🧲","tokens_out":3663,"duration_ms":35958,"temperature":0.7,"pith_summary":"This paper uses 63/65Cu nuclear magnetic resonance to ask how magnetic fields alter the paramagnetic state of the zigzag-chain semiconductor YbCuS2. The authors find that the broad maximum of the nuclear spin-lattice relaxation rate 1/T1 near 50 K, which is robust against pressure and chemical substitution at zero field, is rapidly suppressed by applied fields. Above 3 T, a new low-temperature enhancement of 1/T1 appears, signaling that magnetic fields modify the spin fluctuations well above the antiferromagnetic ordering temperature of about 1 K. The results imply that the field-induced phases seen below TN have a precursor in the paramagnetic state. A sympathetic reader would care because the field-driven changes suggest the Yb zigzag chains host competing ground-state tendencies that a field can tilt.","feed_headline":"Field suppresses 50 K spin anomaly in YbCuS2","feed_subtitle":"NMR shows a new 1/T1 upturn above 3 T in the paramagnetic state of the zigzag-chain semiconductor","key_machinery":"The central observable is the 63Cu nuclear spin-lattice relaxation rate 1/T1, measured by spin-echo NMR on a powdered sample at fields from 0.1 T to 10 T. The Knight shift is extracted from field-swept spectra using the second-order quadrupolar relation δν/(γHres) ≈ K + D/(1+K)(γHres)^2, with the quadrupolar coefficient D assumed temperature-independent at its 50 K value of 12.81 MHz². The 1/T1 data are fitted with relaxation functions appropriate for spin I = 3/2, including a matrix-diagonalization form at low fields. These measurements probe low-energy spin fluctuations and their field dependence.","core_discovery":"In the paramagnetic state of YbCuS2, the broad 50 K anomaly in the nuclear spin-lattice relaxation rate is field-dependent: increasing magnetic fields suppress the 1/T1 maximum, and for fields above roughly 3 T the maximum becomes indistinct and a new low-temperature upturn appears instead. The Knight shift K is proportional to the bulk magnetic susceptibility and is independent of magnetic field, with a hyperfine coupling of about 0.14 T/μB. Spectral simulations of the partially oriented powder sample indicate that the ac plane is the easy plane. The authors interpret the 1/T1 behavior as evidence that a magnetic-field-driven modification of the paramagnetic state occurs above TN, likely related to the field-induced phases observed below TN, including the 1/3 magnetization plateau and the high-field phase.","pith_inferences":["If the quadrupolar coefficient D actually varies with temperature, the reported temperature dependence of the Knight shift, and hence the conclusion that K is field-independent, could be systematically off; a direct multi-frequency measurement of D(T) would settle this.","The identification of the ac plane as the easy plane rests on a two-dimensional powder-pattern simulation of partially oriented grains; a single-crystal NMR study would confirm the orientation and refine the hyperfine coupling tensor.","The onset of the 1/T1 upturn near 3 T coincides with the field where the antiferromagnetic phase boundary changes slope, suggesting that thermodynamic measurements in the paramagnetic regime (such as magnetostriction or specific heat) might reveal a crossover feature at this field.","The theoretical zigzag-chain model with competing J1 and J2 interactions predicts field-induced gapless states; the observed 1/T1 upturn could be a finite-temperature signature of such a state, testable by comparing with numerical calculations of the relaxation rate."],"forward_implications":["The 50 K anomaly is a field-tunable feature, unlike its robustness under pressure and chemical substitution, so magnetic field provides a distinct control parameter for the frustrated zigzag-chain physics.","Above 3 T, the paramagnetic state enters a field-induced spin-fluctuating regime that appears to be the finite-temperature counterpart of the up-up-down ordered phase seen below TN.","The Knight shift being field-independent and proportional to the bulk susceptibility indicates a single uniform spin response, with the hyperfine coupling exceeding the classical dipolar estimate and thus implying transferred hyperfine contributions.","The similarity of the 1/T1 behavior to that of one-dimensional spin-ladder systems suggests that a field-driven crossover from a gapped, dimer-like state to a gapless, Tomonaga-Luttinger-like fluctuating state may be relevant to YbCuS2."],"supporting_citations":[{"why":"Provides the magnetization data and H-T phase diagram, including the 1/3 plateau and high-field phase that the field-dependent 1/T1 is compared against.","marker":"[13]"},{"why":"Earlier NQR work reporting the zero-field 50 K anomaly in 1/T1 that this paper shows is field-suppressed.","marker":"[14]"},{"why":"NQR study of the antiferromagnetic state below TN, establishing the incommensurate helical order and gapless excitations that the field-induced paramagnetic changes are linked to.","marker":"[15]"},{"why":"Neutron diffraction determination of the ac-plane rotating helical structure, supporting the ac-plane easy-plane assignment from the NMR powder patterns.","marker":"[19]"},{"why":"WIEN2k DFT calculation that places the electric field gradient principal axis nearly parallel to the b axis, allowing the partial orientation to be identified as the ac plane.","marker":"[22]"},{"why":"Supplies the method for extracting the Knight shift K and second-order quadrupolar coefficient D from the linear relation between δν/(γHres) and (γHres)^{-2}.","marker":"[24]"},{"why":"Standard reference for the second-order quadrupolar shift formula used to derive Eq. (2).","marker":"[25]"},{"why":"Theoretical zigzag-chain model with J1 ~ J2 that predicts the up-up-down plateau and related field-induced states, providing the context for interpreting the 1/T1 changes.","marker":"[18]"}],"fun_headline_variants":["Field wipes out 50 K spin anomaly in YbCuS2","NMR: Magnetic field tunes spin relaxation in YbCuS2","YbCuS2: Fields suppress 50 K peak, reveal new upturn","Paramagnetic YbCuS2 shifts with field: 50 K anomaly dies","Field-induced change in YbCuS2 spin dynamics above 3 T"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The extraction of the Knight shift relies on assuming that the second-order quadrupolar coefficient D is independent of temperature and equal to its 50 K value of 12.81 MHz²; if D changes with temperature, the reported Knight shift values would be systematically shifted and the conclusion that K is field-independent would need to be revisited.","fun_headline_variants_meta":{"raw":{"variants":["Field wipes out 50 K spin anomaly in YbCuS2","NMR: Magnetic field tunes spin relaxation in YbCuS2","YbCuS2: Fields suppress 50 K peak, reveal new upturn","Paramagnetic YbCuS2 shifts with field: 50 K anomaly dies","Field-induced change in YbCuS2 spin dynamics above 3 T"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000312,"raw_usage":{"total_tokens":1772,"prompt_tokens":942,"completion_tokens":830,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":558,"completion_tokens_details":{"reasoning_tokens":727}},"tokens_in":558,"tokens_out":830,"duration_ms":8451,"temperature":1.0,"reasoning_tokens":727,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-09T17:31:59.883055+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the field-swept NMR spectra at multiple frequencies at several fixed temperatures (for example, 10 K, 50 K, and 100 K) and extract D(T) directly as the slope of δν/(γHres) versus (γHres)^{-2}; if D varies by more than a few percent over the measured temperature range, the reported K(T) and the field-independence conclusion are not robust. A single-crystal NMR measurement that resolves the orientation of the ac plane would also independently test the easy-plane assignment.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the magnetization data and H-T phase diagram, including the 1/3 plateau and high-field phase that the field-dependent 1/T1 is compared against."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Earlier NQR work reporting the zero-field 50 K anomaly in 1/T1 that this paper shows is field-suppressed."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"NQR study of the antiferromagnetic state below TN, establishing the incommensurate helical order and gapless excitations that the field-induced paramagnetic changes are linked to."},{"cited_title":"Ohmagari, Y","cited_arxiv_id":null,"evidence_quote":"Neutron diffraction determination of the ac-plane rotating helical structure, supporting the ac-plane easy-plane assignment from the NMR powder patterns."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"WIEN2k DFT calculation that places the electric field gradient principal axis nearly parallel to the b axis, allowing the partial orientation to be identified as the ac plane."},{"cited_title":"Saito and C","cited_arxiv_id":null,"evidence_quote":"Standard reference for the second-order quadrupolar shift formula used to derive Eq. (2)."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Theoretical zigzag-chain model with J1 ~ J2 that predicts the up-up-down plateau and related field-induced states, providing the context for interpreting the 1/T1 changes."}],"review_version":1}