{"id":"8f6a232b-90ed-4a8d-8190-6dcfec6ed8d6","arxiv_id":"2607.27392","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":7,"one_line_summary":"YBCO coplanar-waveguide resonators show a low-temperature microwave loss and frequency shift that do not fit the two-level-system model, with loss growing logarithmically in temperature.","lead":"Microwave resonators made from the high-temperature superconductor YBCO lose energy in a way that the standard two-level-system model cannot explain. The measurements provide a low-temperature benchmark for high-Tc resonators and point to a new, unidentified loss channel.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Posited paramagnetic mechanism for the low-T frequency upturn is supported only by a two-parameter fit to the same data it explains; no independent measurement rules out alternative reactive mechanisms, leaving the paper's main positive interpretation speculative.","rationale":"I read the paper in good faith and agree with the reader's assessment. The strongest negative claim—that the conventional TLS framework alone cannot explain the observed power-independent, non-saturating low-temperature response—is convincingly supported by the data and analysis. The failure of the TLS frequency-shift model to reproduce the expected dip and curvature, combined with the absence of power dependence over a range spanning many orders of magnitude in photon number, makes a strong case that TLS is not the dominant low-temperature mechanism. However, the paper's positive proposal—a paramagnetic contribution from local moments or Andreev bound states—rests entirely on fitting c and θ in a phenomenological μ_r(T) to the same data it explains. This is a valid concern because the fit cannot distinguish the two candidate mechanisms, and there is no independent evidence for a paramagnetic susceptibility. The authors are appropriately cautious in describing the interpretation as 'consistent with' rather than definitive, but the central conclusion of the abstract includes this paramagnetic picture as the main alternative to TLS. If that picture is wrong, the paper's claim of having shown a \"paramagnetic response\" is unsupported, even though the underlying data and the negative TLS result would stand. The concrete test I propose is a thinner-film experiment, which the authors themselves suggest in the Discussion and which directly distinguishes the bulk paramagnetic mechanism from the surface Andreev mechanism. This is a real but not disqualifying weakness: the paper's negative result is robust, and the positive interpretation is explicitly tentative. The reader's CONDITIONAL verdict is appropriate, and I do not see a reason to change it.","tokens_in":13189,"tokens_out":13963,"duration_ms":156331,"concrete_test":"Fabricate identical CPW resonators from YBCO films of significantly lower thickness (e.g., 50 nm and 100 nm, compared to the present 210 nm) while keeping the same growth and patterning conditions. In the thin-film limit d ≪ λ_ab, Eq. 6 reduces to L'_k ≃ G μ0 λ_ab²/d, which is independent of μ_r; thus a bulk local-moment paramagnetic contribution to the frequency shift should vanish, whereas a surface Andreev bound-state contribution (which enters through a surface-current response) should persist. If the low-T frequency upturn remains essentially unchanged in thin films, the μ_r = 1 + c/(T+θ) interpretation is ruled out; if it disappears, the local-moment picture is favored. Either outcome would resolve the ambiguity the authors themselves flag in the Discussion.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central negative claim—that conventional TLS alone cannot describe the low-temperature response—is well supported by the power-independent Q_i, the absence of the TLS-expected dip/plateau on the hf/k_B scale, and the failed TLS fits. The load-bearing weakness lies in the positive interpretation of the frequency upturn as a paramagnetic response. Section III introduces μ_r = 1 + c/(T+θ) in Eq. 6, with c and θ fit to the very Δf_r/f_r data they are meant to explain. The authors explicitly state that localized moments and Andreev bound states yield nearly indistinguishable 1/(T+θ) and 1/T corrections, so the fit cannot discriminate between the two. More importantly, no independent measurement—no magnetometry, no field dependence, no control of surface Andreev states—verifies that a paramagnetic susceptibility is present in these films. If the true reactive mechanism is something else (e.g., a temperature-dependent dielectric response of the CeO₂ buffer, a nonlinear Meissner effect with a different functional form, or a surface impedance effect), the specific conclusion \"paramagnetic response\" collapses, even though the data and the negative TLS claim might remain intact. The paper itself acknowledges the loss channel is unresolved, so the frequency-shift interpretation is the only proposed microscopic explanation; without it, the paper's contribution is reduced to an anomaly without a mechanistic account.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports measurements of coplanar-waveguide (CPW) resonators fabricated from YBCO thin films at temperatures from ~70 mK to 40 K. The authors find that the internal quality factor Q_i and the fractional frequency shift Δf_r/f_r both increase with temperature at low temperature, a behavior superficially resembling that of two-level-system (TLS) defects. However, they observe no microwave-power dependence of Q_i, and neither quantity saturates on the characteristic temperature scale h f_r / k_B. The high-temperature response is consistent with quasiparticle losses and the London penetration-depth variation of a d-wave superconductor. The authors show that a conventional TLS model cannot describe the low-temperature data and propose that the frequency upturn is consistent with a paramagnetic contribution from local moments or Andreev bound states, while the loss has an approximately logarithmic temperature dependence of unknown origin. The central negative claim—that TLS alone cannot explain the response—is supported by direct data features, whereas the positive paramagnetic interpretation is presented as one possible explanation with acknowledged ambiguity.","tokens_in":13564,"tokens_out":11448,"duration_ms":123133,"significance":"If the negative claim holds, the paper provides an important benchmark for millikelvin microwave performance of patterned YBCO resonators and a clear demonstration that the TLS framework, developed for conventional superconducting circuits, is insufficient for high-T_c devices. The absence of power dependence and the failure of TLS temperature scales are strong, direct empirical results. The paper also offers a plausible speculative mechanism for the low-temperature anomaly, but the positive interpretation is not uniquely established. The measurements are of good quality and the paper is clearly written. The significance would be enhanced if the paramagnetic model were tested against alternative functional forms or supported by independent measurements.","major_comments":[{"comment":"The paramagnetic interpretation of the frequency upturn rests on the ad hoc insertion of μ_r = 1 + c/(T+θ) into the kinetic-inductance formula. The parameters c and θ are fit to the same Δf_r/f_r data they are intended to explain, and the paper itself states that localized moments and Andreev bound states produce nearly indistinguishable corrections. No independent measurement (magnetization, field dependence, or control of surface Andreev states) verifies the presence of a paramagnetic response. To make the 'paramagnetic response' label more than a curve fit, the authors should compare Eq. (6) against alternative low-temperature reactive models (power-law, logarithmic, or a second kinetic-inductance term) and report appropriate goodness-of-fit or evidence metrics. Lacking this, the abstract and conclusions should be worded more cautiously, e.g., 'consistent with a paramagnetic-like cont","section":"§III, Eq. (6)"},{"comment":"The authors note that in the thin-film limit d ≪ λ_ab, the μ_r dependence in Eq. (6) cancels, leaving L'_k ≃ G μ_0 λ_ab^2/d. For the actual films, d = 210 nm and λ_ab(0) = 160–232 nm, so d/λ_ab ≈ 1.3–1.9, and the sensitivity to μ_r is suppressed relative to the thick-film case but not negligible. The paper does not quantify how strongly the fitted c and θ values modify L'_k for the measured d/λ_ab, nor whether the extracted paramagnetic susceptibility is physically plausible. A quantitative assessment of the magnitude and uncertainty of this correction is needed to support the paramagnetic model and to motivate the proposed thin-film differentiation experiment.","section":"§V, thin-film limit discussion"}],"minor_comments":[{"comment":"The argument that Kondo scattering is in the opposite direction is incorrect as stated. If 1/τ ∝ −ln T, then as T increases, 1/τ decreases and τ increases, so Q_i ∝ τ would increase with T, which is the observed trend. The sign is therefore consistent, not opposite. The conclusion that Kondo scattering is unlikely may still be correct, but it needs a different justification (e.g., the magnitude of the effect or the known τ ≈ 1 ps in YBCO).","section":"§IV, Kondo scattering paragraph"},{"comment":"The text says 'fit Δf_r/f_r to obtain a, c, θ, and n', but n is not defined in the model of Eqs. (3)–(5). This should be β or another clearly defined parameter.","section":"Appendix A"},{"comment":"The definition of Δf_r/f_r lacks an explicit reference temperature. The normalization constant C is described as a small offset but is not reported; please state the reference and include C in the fit table.","section":"§III, Eq. (3)"},{"comment":"For the TLS frequency-shift fit (Eq. 2) and TLS loss fit (Eq. 10), the text claims failure but does not provide fit parameters or goodness-of-fit statistics. Reporting reduced χ² or residuals would quantitatively support the central negative claim.","section":"§III–IV, TLS fits"},{"comment":"The statement that Q_i shows 'negligible dependence' on power would be strengthened by quoting the maximum relative variation over the measured power range (e.g., ≤ 2%) and the power range in dBm.","section":"§II, power dependence"}],"recommendation":"minor_revision","confidential_remarks":"The negative result regarding TLS is well supported and is the main strength of the paper. The positive paramagnetic interpretation is speculative but clearly flagged as such; the revision should focus on either softening the language or providing a more rigorous model comparison. There is no basis for rejection; the data are novel and relevant to the community."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"You should know: this is a careful mK study of patterned YBCO CPW resonators. The headline result is that the low-temperature frequency and quality-factor upturn, which superficially looks like TLS, does not behave like TLS: no power dependence and no saturation on the hf/k_B scale. That negative claim is the real contribution, and it is well supported by the data. The positive interpretation—a paramagnetic response from local moments or Andreev bound states—is plausible but under-supported, resting on a two-parameter fit.\n\nWhat is new: this is the first systematic mK characterization of patterned YBCO CPW resonators showing this anomalous upturn plus a logarithmic loss with no established explanation. The measurements look clean: multiple resonators on two chips, proper complex fitting of the transmission, and the high-temperature behavior fits the expected d-wave quasiparticle and penetration-depth trends. The extracted zero-temperature penetration depths are consistent across geometries on each chip, which is a good sanity check.\n\nThe stress-test concern is fair. The paramagnetic model enters through μ_r = 1 + c/(T+θ) inside the kinetic-inductance formula, and c and θ are fit to the very Δf_r/f_r data they then explain. No magnetometry, no field dependence, no thickness series. The paper itself acknowledges that localized moments and Andreev bound states give nearly identical corrections, so the fit cannot distinguish them. If the true reactive mechanism is something else—say, a temperature-dependent dielectric response from the CeO₂ buffer—the specific \"paramagnetic response\" conclusion collapses, even though the data and the TLS-negative claim survive. That is a real soft spot, but it is not concealed. The authors say the loss channel is unresolved and the frequency mechanism is presented as \"consistent with\" rather than proven.\n\nThe logarithmic loss fit is purely phenomenological. The paper correctly notes that Kondo scattering would give the wrong sign, so the origin is genuinely open. That is fine for a first report, but it means the paper's value is mostly in the empirical anomaly and the careful exclusion of standard TLS, not in a finished model.\n\nOn the citation pattern: it looks reasonable. They cite the relevant YBCO resonator work (Velluire-Pellat, Fohmann) and the classic penetration-depth and microwave-conductivity papers. No red flags.\n\nVerdict: I would accept this for peer review. A serious referee should push for follow-up experiments—field dependence, thinner films, possibly magnetometry—but the negative TLS result is important and the paper is honest about what it cannot determine. It deserves a place in the literature as a benchmark and a challenge to the community.\n\nI would bring this to a reading group focused on microwave resonators or cuprate devices, and I would cite it if I were working on high-Tc superconducting circuits. Not a paradigm shift, but a solid piece of experimental work with a clear unresolved problem.","headline":"The TLS-negative result is solid and worth publishing; the paramagnetic interpretation is an honest but under-supported guess, and the paper should be reviewed rather than desk-rejected.","tokens_in":14067,"tokens_out":1695,"would_cite":true,"duration_ms":21224,"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":"YBCO microwave resonators show a low-temperature response that the two-level-system model cannot explain: power-independent loss, no saturation, and a frequency upturn consistent with a paramagnetic contribution.","keywords":["YBCO","coplanar waveguide resonator","two-level systems","paramagnetic response","kinetic inductance","Andreev bound states","microwave loss","d-wave superconductor"],"falsifier":"Measure the same film's microwave frequency shift under a small DC magnetic field, or perform a magnetization measurement at 70 mK–6 K: if the 1/(T+θ) term is a true local-moment response, both should show a field-dependent change that tracks c. Alternatively, fabricate resonators from a film with thickness d ≪ λ_ab, where the local-moment permeability correction cancels but a surface Andreev-bound-state response persists; if the low-temperature upturn survives, the local-moment version of the paramagnetic explanation is ruled out.","tokens_in":13083,"feed_emoji":"📡","tokens_out":4781,"duration_ms":47964,"temperature":0.7,"pith_summary":"The paper tries to establish that the millikelvin microwave response of patterned YBCO coplanar-waveguide resonators cannot be accounted for by the standard two-level-system (TLS) defect model used for conventional superconducting circuits. The internal quality factor shows no measurable dependence on microwave power, and neither the quality factor nor the fractional frequency shift saturates on the temperature scale set by the resonator frequency, as a TLS bath would require. The low-temperature frequency upturn is instead consistent with a paramagnetic reactive response—defect-induced local magnetic moments or surface Andreev bound states—added to the kinetic inductance. The low-temperature loss follows an approximately logarithmic temperature dependence whose microscopic origin is not identified. If correct, the work says that TLS-like signatures in cuprate resonators can be mimicked by electrodynamic effects intrinsic to the d-wave film, and it gives a quantitative loss benchmark for high-Tc microwave devices.","feed_headline":"YBCO resonator data escape the two-level-system model","feed_subtitle":"Frequency shift fits a paramagnetic response; loss grows logarithmically with temperature, origin still unknown.","key_machinery":"The central object is the kinetic inductance L'_k of the coplanar waveguide, whose temperature dependence converts material response into frequency shift via Δf_r/f_r. The load-bearing modification is Eq. 6, where the vacuum permeability is replaced by μ_0 μ_r with μ_r = 1 + c/(T+θ), a Curie-Weiss-like paramagnetic contribution; this single term is what lets the model fit the low-temperature frequency upturn. On the loss side, the standard TLS expression with its saturation scale h f_r/k_B is the model that the data reject, and Q_i = A + B ln(T/1 K) is the empirical replacement. The absence of microwave-power dependence in Q_i is the key experimental discriminator.","core_discovery":"On its own terms, the paper's discovery is that in YBCO thin-film coplanar-waveguide resonators between 70 mK and 6 K, both Q_i and Δf_r/f_r rise with temperature, yet this rise is not a TLS effect. The evidence: Q_i is power-independent up to −80 dBm, and the response continues well beyond h f_r/k_B, where a TLS bath would thermally saturate and produce a resolved dip near 100 mK. Adding a Curie-Weiss permeability correction μ_r = 1 + c/(T+θ) inside the kinetic inductance lets the model reproduce the frequency shift over 70 mK–40 K; the TLS-plus-quasiparticle model cannot. The authors deliberately stop short of identifying the microscopic source, since local moments and Andreev bound states","pith_inferences":["Editorial inference: if the same degrees of freedom cause both the frequency upturn and the logarithmic loss, then deliberately introducing controlled disorder into YBCO films, for example by ion irradiation, should scale c, θ, and the logarithmic slope together; one such measurement set would test the connection the authors leave implicit.","Editorial inference: the systematic difference between the two films—Cu-excess versus Y-excess growth—in both λ_ab(0) and the Curie-Weiss parameters hints that defect chemistry controls the moment density; comparing a series of stoichiometries could turn the phenomenological c and θ into a materials variable without needing magnetometry.","Editorial inference: the absence of power dependence does not eliminate Andreev bound states, since the drive used is far below the reported suppression power scale; a high-power sweep across the nonlinear Meissner regime would be a sharper falsifier than the temperature data alone.","Editorial inference: because the frequency-shift fits require a global λ_ab(0) per chip across different resonator geometries, the extracted parameters likely reflect the film rather than the circuit, so the same paramagnetic correction should appear in other YBCO geometries such as microstrip or 3D cavities."],"forward_implications":["If the response is not TLS-dominated, the usual TLS extraction procedure—checking power dependence and h f/k_B saturation—will misattribute low-temperature loss in YBCO devices to dielectric defects when the film itself is responsible.","The measured peak Q_i of roughly 8×10^3 to 1.7×10^4 near 6–8 K provides a concrete loss benchmark for whether YBCO resonators are usable in parametric amplifiers, detectors, and hybrid quantum circuits at millikelvin temperatures.","Because the paramagnetic term is fit to the frequency data alone, device modeling that uses the extracted λ_ab(0) (232±6 nm for chip 1, 160±9 nm for chip 2) and β values inherits the assumption that the paramagnetic interpretation is correct.","Thinner-film resonators should distinguish the two candidate mechanisms: a local-moment permeability correction cancels in the thin-film limit d ≪ λ_ab, whereas a surface Andreev-bound-state response persists.","Magnetic-field and stronger microwave-power studies would test whether the same low-temperature degrees of freedom produce the logarithmic loss."],"fun_headline_variants":["YBCO resonators defy TLS model with paramagnetic frequency shift","Paramagnetic response beats TLS in YBCO resonator data","Loss in YBCO resonators: logarithmic rise, microscopic origin unknown","TLS saturates? No: YBCO resonators keep rising in mK regime","YBCO microwave response goes beyond two-level-system framework"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The load-bearing premise is that a Curie-Weiss permeability term μ_r = 1 + c/(T+θ) slipped into the kinetic inductance formula is the right description of the low-temperature frequency shift; the parameters c and θ are fit to the very data they explain, with no independent magnetization, field-dependence, or surface-state measurement to confirm that a paramagnetic mechanism is present.","fun_headline_variants_meta":{"raw":{"variants":["YBCO resonators defy TLS model with paramagnetic frequency shift","Paramagnetic response beats TLS in YBCO resonator data","Loss in YBCO resonators: logarithmic rise, microscopic origin unknown","TLS saturates? No: YBCO resonators keep rising in mK regime","YBCO microwave response goes beyond two-level-system framework"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000341,"raw_usage":{"total_tokens":1750,"prompt_tokens":812,"completion_tokens":938,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":556,"completion_tokens_details":{"reasoning_tokens":846}},"tokens_in":556,"tokens_out":938,"duration_ms":9773,"temperature":1.0,"reasoning_tokens":846,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-01T08:08:17.834783+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the same film's microwave frequency shift under a small DC magnetic field, or perform a magnetization measurement at 70 mK–6 K: if the 1/(T+θ) term is a true local-moment response, both should show a field-dependent change that tracks c. Alternatively, fabricate resonators from a film with thickness d ≪ λ_ab, where the local-moment permeability correction cancels but a surface Andreev-bound-state response persists; if the low-temperature upturn survives, the local-moment version of the paramagnetic explanation is ruled out.","supporting_citations":[],"review_version":1}