{"id":"0db1d0c4-b85c-4cd9-9d9c-46788442f638","arxiv_id":"2608.06602","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":0,"one_line_summary":"Non-resonant EPR signals from a FeSe single crystal are interpreted as derivatives of the superconducting transition, yielding Tc to be about 8 K, a transition width of about 4 K, and Hc1 of about 27 Oe, with resonant signals attributed to Fe2+.","lead":"Using an EPR spectrometer, this paper measures non-resonant signals from a FeSe single crystal and claims these signals reveal the superconducting transition temperature, transition width, and first critical field. It also assigns two resonant EPR features to Fe2+ ions with g-factors of 4.8 and 2.0.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Hc1 claim hinges on unproven mapping of the 12 Oe EPR phase flip to vortex entry; a same-sample magnetization measurement would settle it.","rationale":"The reader's verdict identifies the unproven interpretive step connecting the 180-degree phase change to vortex entry, and our independent stress-test converges on the same point. We considered whether other aspects of the paper—such as the form of Eq. (1), the g-factor assignment to Fe2+, or the extraction of Tc and transition width from Fig. 4—might be more load-bearing. The g-factor assignment is peripheral and not essential to the claimed method for determining Hc1. The Tc and transition-width extraction from temperature derivatives is model-dependent but is supported by the MPMS-measured onset of 8.8 K and is discussed as a plausible contactless proxy. The Hc1 determination, however, is the most novel and externally compared result. The phase-flip-to-Hc1 step is the weakest link because no quantitative model links the sign of the non-resonant EPR signal to the sample's vortex state, and the only validation is agreement with a literature value obtained on a different crystal. A same-sample magnetization measurement would directly calibrate the first-penetration field and would settle whether the 12 Oe feature is Hc1 or an experimental artifact. Since the paper otherwise reports plausible data and the claim is testable, the CONDITIONAL verdict is appropriate; the proposed test would provide the missing validation without requiring rejection of the manuscript.","tokens_in":6921,"tokens_out":6956,"duration_ms":65886,"concrete_test":"Use the same MPMS-XL5 to record isothermal magnetization M(H) on the same FeSe crystal at T = 6 K with the field along the c-axis, from 0 to 30 Oe in small steps. Determine the first-penetration field Hp as the field at which M(H) first deviates from linear Meissner behavior (or at which hysteresis first appears), and compare Hp directly with the 12 Oe phase-jump field. If Hp agrees with 12 Oe within experimental uncertainty (say ±3 Oe), the phase-jump-to-Hc1 assignment is supported. If Hp is significantly different, the Hc1 claim is not supported. As a secondary control, repeat the EPR measurement with a reduced modulation amplitude (e.g., 2 Oe instead of 20 Oe) to verify that the 12 Oe phase-flip field is independent of detection parameters.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central Hc1 claim rests on the statement in Section 3 that 'the magnetic field corresponding to a 180-degree phase jump of the spectrometer signal in a field of 12 Oe at a temperature of about 6 K is equal to the first critical field of the single crystal.' The data show only that the low-field EPR signal changes sign between 11.72 Oe and 14 Oe, and the authors interpret this sign change as a 180-degree phase shift caused by Abrikosov-vortex entry. This assignment is not supported by a quantitative cavity-perturbation model, an independent phase calibration, or a control experiment on the same crystal. The sign flip could equally arise from other field-dependent changes in the sample-resonator system, such as surface impedance changes, cavity detuning from Meissner screening currents, or modulation artifacts, none of which are directly tied to Hc1. The only external support is the agreement of Hc1 ≈ 27 Oe, obtained via Eq. (1) with d/w = 0.2, with the value 25 Oe measured on a different sample in ref. [11]. That agreement is a consistency check, not a validation of the physical assignment. Since the quoted Hc1 value and the associated claim of determining the first critical field depend entirely on this unproven mapping, the central claim is not established by the data as presented.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports EPR measurements on a FeSe single crystal (2.5 x 2 x 0.4 mm3) in magnetic fields up to 6000 Oe at temperatures from 3.5 K to 25 K, supplemented by MPMS magnetization used to determine the onset of superconductivity. The authors claim that non-resonant EPR signals in zero and weak fields can determine Tc, the superconducting transition width, and Hc1. The central Hc1 value is obtained by interpreting a 180-degree phase jump of the EPR signal near H = 12 Oe as the onset of Abrikosov-vortex penetration, then applying the demagnetization relation Hp approximately Hc1(d/w)^0.5 to obtain Hc1 ~ 27 Oe, which is compared with ~25 Oe from the literature. Resonant features at 1400 and 3400 Oe are assigned to Fe2+ ions with g-factors of about 4.8 and 2.0.","tokens_in":7144,"tokens_out":6991,"duration_ms":66306,"significance":"If the phase-jump identification were quantitatively established, the paper would offer a contactless, all-EPR route to Tc, transition width, and Hc1 in small superconducting single crystals, which would be of practical interest to the FeSe and iron-chalcogenide community. The reported Fe2+ EPR resonances and g-factor values are also a useful addition. The manuscript is, however, exploratory and largely qualitative: the central Hc1 conclusion rests on an unproven interpretation of the phase jump, the Tc and transition-width values are extracted by visual comparison with model curves, and the data are given in arbitrary units without error bars. The use of MPMS magnetization to anchor Tconset is a strength, as is the clear presentation of the raw field and temperature dependences.","major_comments":[{"comment":"The central claim that the phase jump at H ~ 12 Oe equals the first critical field is not established. The text states that vortex formation 'can lead' to a 180-degree phase change, but it provides no cavity-perturbation model relating the resonator Q-factor and phase to vortex density, no phase calibration, and no control experiment on the same crystal. The observed sign change between 11.72 and 14 Oe could also arise from Meissner screening currents changing the cavity detuning, from surface-impedance changes at the onset of flux penetration, or from field-modulation artifacts; notably, the modulation amplitude of 20 Oe is larger than the claimed Hp = 12 Oe. A same-sample magnetization measurement of Hc1 using the already available MPMS-XL5, or a quantitative model, is required to support the assignment.","section":"§3, paragraph on the 180-degree phase jump (around Fig. 4)"},{"comment":"The paper correctly states that an EPR signal is proportional to the first field derivative of the absorbed microwave power, but then asserts that the temperature dependences in Fig. 4 'represent a family of temperature derivatives of this transition'. This step is unjustified: at fixed field, the lock-in output is the field derivative evaluated as a function of temperature, not the temperature derivative. The relation between dP/dH at fixed H and dR*/dT must be derived or demonstrated before the extracted values Tc ~ 8 K, transition width ~ 4 K, and zero-resistance temperature 4.0 K can be accepted. The comparison with the model curves in Fig. 5 is visual and qualitative, with no fit metric or uncertainty estimate.","section":"§3, interpretation of Fig. 4 as temperature derivatives"},{"comment":"The closeness of Hc1 = 27 Oe to the literature value of 25 Oe is presented as confirmation of the phase-jump interpretation, but it is only a weak consistency check. Hp is quoted as '12 Oe' even though the data only bracket it between 11.72 and 14 Oe; no uncertainty is given for Hc1, and the literature value from ref. [11] is also quoted without an error bar. In addition, Eq. (1) is applied without specifying the orientation of the EPR magnetic field relative to the c axis of the crystal and without justifying the demagnetization form-factor approximation for this particular sample shape. The apparent agreement therefore does not independently validate the assignment of the phase jump to Hc1.","section":"§3, Eq. (1) and comparison with ref. [11]"}],"minor_comments":[{"comment":"The abstract states that non-resonant EPR signals above 8 K are field independent, while Section 3 says they are field independent 'with the exception of two local features' at 1400 and 3400 Oe; please reconcile these statements.","section":"Abstract"},{"comment":"Please specify whether the curves in Fig. 4 are raw data or vertically shifted, and define the sign convention used to identify a 180-degree phase change; without this, the field at which the jump occurs cannot be read off precisely.","section":"Fig. 4 caption and Section 3"},{"comment":"Typos such as 'Referenses' and 'trasition' should be corrected, and the inconsistent notation 'Tconset'/'Tсonset' should be unified; all symbols in Eq. (2), including nu, mu0, muB, and h, should be defined at first use.","section":"Throughout"},{"comment":"Since the experimental EPR signals have no numerical vertical scale, please describe how the model curves in Fig. 5 were normalized before the visual comparison with the experimental curves in Fig. 4.","section":"§3, Fig. 5"}],"recommendation":"major_revision","confidential_remarks":"The central novelty of the paper is the identification of the 180-degree phase jump with Hc1, and this step is currently unsupported by a quantitative model or an independent same-sample measurement. The authors already have MPMS-XL5 capability, so a direct magnetization-based Hc1 determination on the same crystal would be a natural and decisive addition. The Tc and transition-width determination also needs a more explicit justification of the relation between the EPR signal and the temperature derivative of the resistance. These are load-bearing issues, but they appear fixable within the scope of the manuscript, so major revision rather than rejection seems appropriate."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe paper is worth a look if you work on contactless characterization of superconductors. The authors have real EPR data on a FeSe single crystal, and the clever bit is treating the non-resonant signal as the temperature derivative of the superconducting transition. From that they read Tc around 8 K, a transition width around 4 K, and an inflection near 6 K. Those numbers are consistent with their own magnetization measurement and with the shape of the R(T) curves they model. That part is credible.\n\nThe new claim is that a 180-degree phase jump in the low-field EPR signal at 12 Oe marks Hc1, and after a demagnetization correction gives 27 Oe, close to the 25 Oe from a literature magnetization study. That is a plausible reading, but it is not established. The text states the field is equal to Hc1 and later says the closeness confirms the correctness of the explanation. Neither follows from the data. There is no cavity-perturbation model linking the phase flip to vortex entry, no independent phase calibration, and no control experiment on the same crystal. The phase change could come from Meissner screening currents, surface impedance changes, or modulation artifacts. One data point agreeing with a literature value measured on a different sample is a consistency check, not validation.\n\nThe other soft spots are secondary: all signals are in arbitrary units with no error bars, and the Hc1 comparison uses a single published value. The g-factor assignments for Fe2+ are reasonable but not central. The paper is not circular; the phase-jump field is an observed quantity.\n\nOverall, this is a solid experimental report with one under-supported interpretive step. A good referee would send it back for either a quantitative model or a same-sample magnetization measurement of Hc1. I would not cite the Hc1 claim as established, but the Tc/transition-width method and the raw EPR data are citable. Send it to peer review; it deserves a serious referee, not a desk reject.","headline":"Real EPR data on FeSe with a plausible but unproven phase-jump assignment for Hc1.","tokens_in":7706,"tokens_out":2340,"would_cite":false,"duration_ms":21828,"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":"This paper claims that a 180-degree phase jump in the non-resonant EPR signal of a FeSe single crystal marks vortex entry and yields a contactless estimate of the first critical field.","keywords":["FeSe","superconductivity","electron paramagnetic resonance","non-resonant EPR","first critical field","mixed state","critical temperature","g-factor"],"falsifier":"A decisive test would be to record the EPR phase while monitoring the local magnetic induction on the same FeSe crystal; if the first vortex penetration, seen as the first deviation of local induction from the Meissner value, does not occur at the applied field of the 180-degree phase jump, the identification of that jump with $H_{c1}$ is wrong.","tokens_in":6709,"feed_emoji":"🧲","tokens_out":10822,"duration_ms":90057,"temperature":0.7,"pith_summary":"The paper aims to show that electron paramagnetic resonance (EPR) spectroscopy, even when no spin resonance is being excited, can serve as a contactless probe of the superconducting state of a FeSe single crystal. It argues that non-resonant signals in zero and weak magnetic fields (up to about 30 Oe) encode the critical temperature, the width of the superconducting transition, and the first critical field $H_{c1}$. The load-bearing observation is a 180-degree phase jump in the signal at about 12 Oe near 6 K, which the authors interpret as the moment Abrikosov vortices enter the crystal; after a shape correction this yields $H_{c1}\\approx 27$ Oe, close to the 25 Oe magnetization value reported in reference [11]. If the interpretation holds, EPR would offer a quick, contactless route to characterize iron-based superconductors.","feed_headline":"EPR phase jump pins down FeSe's first critical field","feed_subtitle":"A 180-degree signal swing reveals the transition temperature and the field where vortices enter an FeSe crystal.","key_machinery":"The central object is the non-resonant EPR signal: the spectrometer output recorded when no spin resonance is excited, which the paper treats as proportional to the derivative of microwave power absorbed in the resonator. The load-bearing feature is a 180-degree phase jump between negative and positive signal branches, interpreted as the signature of the crystal switching from reflecting microwaves in the Meissner state to absorbing them in the vortex state. Two quantitative relations carry the analysis: the resonance condition $\\nu = g\\mu_0\\mu_B H/h$, which converts the 1400 Oe and 3400 Oe resonances into Fe$^{2+}$ assignments, and the demagnetization identity $H_p\\simeq H_{c1}(d/w)^{0.5}$, which corrects the measured jump field for the sample's flat geometry.","core_discovery":"On the authors' account, the non-resonant EPR signal of a superconducting FeSe platelet is proportional to the field derivative of the microwave power absorbed in the resonator, so in zero and weak fields its temperature dependence traces the superconducting transition. They observe that the signal is negative below about 12 Oe and positive above about 14 Oe, with a 180-degree phase change at the crossover, and they identify this phase jump, seen near 6 K, as the transition from the Meissner state to the mixed state in which Abrikosov vortices (quantized flux tubes) enter the crystal. Taking the measured jump field $H_p=12$ Oe as the shape-dependent entry field and applying the relation $H_p\\simeq H_{c1}(d/w)^{0.5}$ with $d/w=0.4/2$ gives $H_{c1}\\approx 27$ Oe, matching the magnetization value of about 25 Oe from the literature. The paper also finds ordinary EPR resonances at 1400 Oe and 3400 Oe, assigns them to Fe$^{2+}$ ($3d^6$, $S=2$) ions with $g\\approx 4.8$ and $2.0$, and shows that the non-resonant signal is field-independent above $T_c$ but strongly nonlinear once vortices are present.","pith_inferences":["We infer that tracking the phase-jump field as a function of temperature, which the paper does not do, would yield a continuous $H_{c1}(T)$ curve and a direct test of whether the jump field stays proportional to $H_{c1}$ throughout the superconducting range.","We infer that the demagnetization correction $H_p\\simeq H_{c1}(d/w)^{0.5}$ is a simplified shape factor; measuring crystals with different aspect ratios would show whether the relation captures the geometry or needs revision.","We infer that a control experiment on a normal metal or a type-I superconductor in the same resonator would reveal whether the 180-degree phase flip is unique to vortex penetration or is partly a generic cavity response.","We infer that simultaneous magnetization and EPR phase measurements on the same crystal would give the cleanest quantitative check of the paper's identification of the phase jump with $H_{c1}$."],"forward_implications":["If the phase-jump assignment is correct, a single weak-field EPR sweep gives a contactless estimate of $H_{c1}$ for FeSe without electrical contacts.","The same non-resonant signal shape yields $T_c^{\\rm onset}\\approx 8$ K and a transition width of about 4 K, matching the resistive transition picture based on the two-fluid model.","Because the method senses the mixed state through vortex-induced microwave absorption, it could be extended to other iron-based superconductors where $H_{c1}$ is awkward to measure resistively.","The normal-state resonances at 1400 Oe and 3400 Oe provide an EPR fingerprint of Fe$^{2+}$ in FeSe, which may help monitor stoichiometry or doping in iron chalcogenides."],"supporting_citations":[{"why":"Supplies the standard EPR convention that the recorded signal is the first field derivative of absorbed microwave power, the basis for treating non-resonant signals as derivatives.","marker":"[1]"},{"why":"Establishes the earlier contactless use of EPR to determine Hc1 as the threshold for the mixed state in high-temperature superconductors.","marker":"[6]"},{"why":"Provides the independent magnetization value Hc1 about 25 Oe for a FeSe crystal, the comparison that anchors the paper's Hc1 about 27 Oe result.","marker":"[11]"},{"why":"Supplies the demagnetization relation Hp approximately Hc1(d/w)^0.5 used to convert the measured phase-jump field into Hc1.","marker":"[12]"},{"why":"Provides the two-fluid model used to explain why the EPR-derived transition shape has opposite sign to a four-probe resistive measurement.","marker":"[13]"},{"why":"Describes the type-II mixed state and Abrikosov vortex lattice, the physical picture behind attributing the 180-degree phase jump to vortex entry.","marker":"[14]"}],"fun_headline_variants":["EPR phase jump reveals FeSe's vortex-entry field","Non-resonant EPR traces FeSe's Tc and Hc1","FeSe's critical field found via EPR signal flip","EPR detects Fe2+ spins and FeSe's mixed state","Phase change in EPR signal locates FeSe's Hc1"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The $H_{c1}$ determination rests on the unproven interpretive step that the 180-degree phase jump of the non-resonant EPR signal is caused by vortex entry into the mixed state, rather than by some other resonator or sample effect.","fun_headline_variants_meta":{"raw":{"variants":["EPR phase jump reveals FeSe's vortex-entry field","Non-resonant EPR traces FeSe's Tc and Hc1","FeSe's critical field found via EPR signal flip","EPR detects Fe2+ spins and FeSe's mixed state","Phase change in EPR signal locates FeSe's Hc1"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00024,"raw_usage":{"total_tokens":1565,"prompt_tokens":1037,"completion_tokens":528,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":653,"completion_tokens_details":{"reasoning_tokens":438}},"tokens_in":653,"tokens_out":528,"duration_ms":5067,"temperature":1.0,"reasoning_tokens":438,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T04:16:35.850346+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A decisive test would be to record the EPR phase while monitoring the local magnetic induction on the same FeSe crystal; if the first vortex penetration, seen as the first deviation of local induction from the Meissner value, does not occur at the applied field of the 180-degree phase jump, the identification of that jump with $H_{c1}$ is wrong.","supporting_citations":[{"cited_title":"The usefulness of its use for studying bulk superconductors was questioned by the work of Azbel and Lifshitz [2]","cited_arxiv_id":null,"evidence_quote":"Supplies the standard EPR convention that the recorded signal is the first field derivative of absorbed microwave power, the basis for treating non-resonant signals as derivatives."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the earlier contactless use of EPR to determine Hc1 as the threshold for the mixed state in high-temperature superconductors."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the independent magnetization value Hc1 about 25 Oe for a FeSe crystal, the comparison that anchors the paper's Hc1 about 27 Oe result."},{"cited_title":"Khasanov, Yu.M","cited_arxiv_id":null,"evidence_quote":"Supplies the demagnetization relation Hp approximately Hc1(d/w)^0.5 used to convert the measured phase-jump field into Hc1."},{"cited_title":"Salakhutdinov, Y","cited_arxiv_id":null,"evidence_quote":"Provides the two-fluid model used to explain why the EPR-derived transition shape has opposite sign to a four-probe resistive measurement."},{"cited_title":"Aliev, Ya","cited_arxiv_id":null,"evidence_quote":"Describes the type-II mixed state and Abrikosov vortex lattice, the physical picture behind attributing the 180-degree phase jump to vortex entry."}],"review_version":1}