{"id":"acd78c0c-064f-4a6a-a66e-be8891e9fddc","arxiv_id":"2506.22240","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"A 104-nm-thick YIG film coherently couples to a YBCO superconducting resonator with collective coupling g/2π ≈ 230 MHz, and the temperature dependence is reproduced by a two-fluid model based on YBCO's penetration depth.","lead":"Researchers coupled a 104-nanometer-thick YIG magnetic film to a high-temperature superconducting YBCO microwave resonator and measured a photon-magnon coupling of about 230 MHz. The result shows that very thin magnetic films can still interact strongly with superconducting circuits, which matters for miniaturized hybrid quantum devices.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The model's quantitative success is anchored by an internally inconsistent participating-spin number Ns and an admitted free geometric factor r, so the reproduction of the 230 MHz coupling is not an independent confirmation.","rationale":"The reader's weakest assumption was that Ns is unsubstantiated and, together with the free parameter r, sets the absolute coupling magnitude. My read agrees and sharpens this: the paper itself contains a numerical inconsistency for Ns (Section 4 quotes 1.49×10^13, while Section 5's Vm = Ns/ρ = 7×10^-14 m^3 with ρ = 2.1×10^28 m^-3 implies 1.49×10^15). This is a concrete internal contradiction, not merely a missing derivation, and it directly affects the model's claimed quantitative reproduction of the g/2π = 230 MHz splitting. The experimental evidence for coherent coupling—the avoided crossing and its temperature dependence—appears solid and is not invalidated by this concern. The paper is an incremental but plausible experimental report; the conditional verdict remains appropriate. I set verdict_should_be to UNCHANGED because the reader already reached CONDITIONAL and my concern reinforces that position without moving it to ACCEPT or REJECT.","tokens_in":13528,"tokens_out":15953,"duration_ms":174651,"concrete_test":"Recompute Ns from the stated mode volume and spin density: take Vm = 7×10^-14 m^3 and ρ = 2.1×10^28 m^-3. If the product is 1.49×10^15 (not 1.49×10^13), then the quoted Ns in Section 4 is inconsistent by a factor of 100. Next, recompute g = g_s sqrt(2 sFe Ns) using Eq. 10 with the authors' own parameters and compare with the measured g/2π = 230 MHz. If the predicted and measured couplings disagree by a factor near 10, the model's absolute scale is forced by Ns and r. A complementary experimental check would be to vary the YIG lateral size at fixed thickness and verify that the extracted g scales as sqrt(area), which would directly determine the participating spin number and validate or refute the assumed Ns.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing claim that the temperature evolution of the polariton branches is reproduced by the penetration-depth model rests on two parameters that set the absolute scale: Ns and r. Ns = 1.49×10^13 is introduced in Section 4 with no derivation, and Section 5 then states Vm = Ns/ρ = 7×10^-14 m^3 with ρ = 2.1×10^28 m^-3. Those numbers are mutually inconsistent: Vm × ρ gives Ns ≈ 1.49×10^15, a factor of 100 larger than the quoted Ns. Since g = g_s sqrt(2 sFe Ns), this factor of 100 changes the predicted collective coupling by a factor of 10. Moreover, the Discussion explicitly concedes that r = 0.45 is a free fitting parameter used to adjust the absolute magnitude of δsc. Therefore the model's ability to reproduce the observed 230 MHz splitting is not a parameter-free success: either Ns is a typo, or it is effectively tuned in tandem with r to match the data. The experimental observation of an avoided crossing is not in question, but the central modeling claim that the penetration depth quantitatively accounts for the spectrum is weakened by this internal inconsistency and by the absence of an independent determination of the participating spin number.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports microwave transmission measurements of a 104-nm-thick YIG film placed on a YBCO coplanar waveguide and on a half-wavelength resonator. The authors observe a polariton splitting of about 460 MHz (g/2π ≈ 230 MHz) below 50 K and track the temperature evolution of the polariton branches between 10 and 85 K. They interpret the temperature dependence using a model in which the YBCO penetration depth (Eq. 8) enters both the resonator frequency shift (Eq. 7) and the collective coupling (Eq. 10), and they compare the extracted coupling with previous results on thicker YIG films.","tokens_in":13776,"tokens_out":5092,"duration_ms":49745,"significance":"If the experimental result and its interpretation are sound, the paper provides a useful data point showing that sub-micrometer YIG films can achieve strong magnon-photon coupling with high-Tc superconducting resonators, extending the thickness-dependence comparison in Refs. [9, 30]. The broadband spin-wave spectroscopy, including the identification of ω0 and the first PSSW mode ω1 using the Kalinikos-Slavin model, is careful and the temperature dependence of ω0 is consistent with the known YIG magnetization. However, the quantitative success of the coupling model is not an independent test: it depends on two adjustable parameters (r in Eq. 7 and N_s in Section 4), one of which is internally inconsistent with the stated mode volume in Section 5. The central experimental observation of an avoided crossing is solid, but the modeling claim that the penetration depth quantitatively accounts for the polariton spectrum requires substantial clarification and correction.","major_comments":[{"comment":"The text states V_m = N_s/ρ = 7×10^-14 m^3 with N_s = 1.49×10^13 and ρ = 2.1×10^28 m^-3. These numbers are mutually inconsistent: the quotient N_s/ρ equals 7.1×10^-16 m^3, a factor of 100 smaller than the quoted V_m. Since the collective coupling scales as sqrt(N_s) (Section 4), a factor of 100 in N_s changes the predicted coupling by a factor of 10. This is a load-bearing inconsistency because the claimed reproduction of the 230 MHz splitting in Fig. 3 relies on the numerical value of N_s. The authors must correct the typo, or if N_s is deliberately chosen as an effective spin number, they must justify the factor-of-100 reduction relative to the mode-volume estimate.","section":"Section 5 (Discussion), mode volume statement"},{"comment":"The model reproduction of the polariton branches is not parameter-free. The geometric factor r = 0.45 in Eq. 7 is explicitly described in the Discussion as a free fitting parameter used to adjust the absolute value of δ_sc. Likewise, N_s = 1.49×10^13 is introduced without derivation and is assumed constant over temperature. With both r and N_s adjustable, the absolute magnitude of the computed splitting can be tuned to match the observed 230 MHz, so the agreement in Fig. 3 does not constitute an independent confirmation of the penetration-depth mechanism. An independent estimate of the participating spin number, for example from the overlap volume of the resonator mode and the YIG film, is needed to make the quantitative claim meaningful.","section":"Section 4 and Discussion, free parameters r and N_s"},{"comment":"The temperature dependence of the model is partly generated from the same data it is meant to reproduce. ω_c(T) is fitted at each temperature as a free parameter in Eq. 5; the same ω_c(T) values are then used in Eq. 10 to compute g(T), and Eq. 9 is fitted to ω_c(T) to obtain λ_L(0) and T_c. Consequently, the statement that the penetration-depth model 'accounts for the evolution of the polaritonic spectrum' is not a prediction tested against independent data but a re-description of the fitted resonator frequency. To support the claim, the authors should clearly state which quantities are independently measured (e.g., a separate measurement of the bare resonator frequency as a function of T, or a direct λ_L(T) measurement) and which are fitted.","section":"Section 4, temperature dependence and model circularity"}],"minor_comments":[{"comment":"The symbol h appears in the equation for g_s but is not defined; the text mentions b_vac as the vacuum magnetic field. Please clarify the notation and check the dimensional consistency of the expression.","section":"Section 4, Eq. (10)"},{"comment":"The sentence 'Being d = 104 nm and k_y d = 0.03' could be more explicit: the authors should state the value of k_y used in the calculation (they mention k_y ≈ 3×10^5 rad/m in the preceding paragraph, which gives k_y d ≈ 0.031) and clarify that this justifies the proximity to ω_FMR.","section":"Section 3, Eq. (2) discussion"},{"comment":"The reported coupling g/2π ≈ 230 MHz and the polariton splitting of approximately 460 MHz are given without uncertainty estimates; adding error bars (e.g., from the fit of Eq. 5) would strengthen the quantitative comparison with the model.","section":"General, error bars"}],"recommendation":"major_revision","confidential_remarks":"The experimental observation of strong coupling in a 104-nm-thick YIG film is credible and potentially interesting, but the modeling claims need careful revision. The internal inconsistency between N_s and V_m is a clear error that must be fixed before the paper can be accepted. Even after correction, the absence of an independent determination of N_s and the explicitly free parameter r mean that the model's agreement with the spectra is not a strong test of the penetration-depth mechanism. The authors should either provide an independent estimate of the participating spin number or recast the claim as a demonstration that a plausible choice of N_s can reproduce the observed scale. I recommend major revision rather than rejection because the central experimental result is sound and the modeling issues are fixable within the scope of the manuscript."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The one thing to know: this is a modest but genuine experimental step. The authors place a 104-nm-thick YIG film on a YBCO coplanar waveguide resonator and observe a clear polariton anticrossing with collective coupling g/2π ≈ 230 MHz. The avoided crossing is unambiguous, the coupling extraction is standard, and the temperature trend is qualitatively captured. If you care about scaling magnonic devices down to sub-micrometer YIG films, this is a useful data point.\n\nWhat is actually new: the thickness comparison. The coupling drops from about 1.1 GHz for a 5-µm film to 230 MHz for 104 nm, a factor of 5 in coupling for a factor of 50 in thickness. That is a meaningful scaling result, consistent with the microwave field being concentrated near the YBCO surface. The broadband spin-wave spectra are also analyzed cleanly with the Kalinikos-Slavin model, and the exchange constants are in line with prior work.\n\nThe soft spots are real, and they land on the paper's central modeling claim. The paper says the number of spins is N_s = 1.49 × 10^13, then in the Discussion writes V_m = N_s/ρ = 7 × 10^-14 m^3 with ρ = 2.1 × 10^28 m^-3. Those numbers are mutually inconsistent: V_m × ρ gives N_s ≈ 1.5 × 10^15, a factor of 100 larger. The collective coupling scales as sqrt(N_s), so this is a factor-of-10 discrepancy in the predicted g. The paper gives no derivation of N_s, and the Discussion explicitly says r = 0.45 is a free geometric factor used to adjust the absolute magnitude of δ_sc. So the reproduction of the observed 230 MHz splitting is not an independent test of the penetration-depth model; it is a consistency check that depends on tuned parameters. This is the paper's load-bearing weakness, and it needs to be fixed.\n\nThere is also a milder circularity: ω_c(T) is extracted from the same spectra that the model then reproduces. That is acceptable for a scaling argument, but it means the paper overstates \"reproduction\" as though it were prediction.\n\nWho is this for: people working on cavity magnonics with superconducting resonators, particularly those interested in thin-film YIG and miniaturization. The paper deserves a serious referee, not a desk reject, but the authors should be asked to correct the arithmetic, justify or derive N_s, and report error bars on the coupling. I would send it to review with a major-revision recommendation on the modeling section.","headline":"A useful new experimental data point—104-nm YIG coupled to a YBCO resonator at 230 MHz—but the quantitative modeling claim is undercut by an arithmetic slip in the spin number and a free geometric factor.","tokens_in":764,"tokens_out":1153,"would_cite":false,"duration_ms":38478,"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":"A 104-nm-thick YIG film on a YBCO resonator shows coherent magnon-photon coupling of about 230 MHz, with the temperature evolution following the superconductor's penetration depth.","keywords":["YIG","YBCO","cavity magnonics","magnon-photon hybridization","strong coupling","superconducting resonator","penetration depth","polaritons"],"falsifier":"Measure the YIG film's participating volume and the resonator mode volume independently, for instance by varying the film area or mapping the microwave field, and compute $g$ from the coupling formula without fitting $N_s$ or $r$; if the predicted polariton splitting deviates from roughly 460 MHz by more than the linewidth, the claim that penetration depth alone controls the temperature evolution would be contradicted.","tokens_in":13285,"feed_emoji":"🧲","tokens_out":9586,"duration_ms":88638,"temperature":0.7,"pith_summary":"This paper reports coherent coupling between a 104-nm-thick yttrium-iron-garnet (YIG) film and a microwave resonator patterned from a high-temperature superconducting YBCO film. A polariton splitting of about 460 MHz, corresponding to a collective coupling $g/2\\pi \\approx 230$ MHz, is observed at low temperature, and the two hybrid branches persist down to 10 K. The authors show that the temperature evolution of both branches is reproduced by a model in which the temperature-dependent penetration depth of YBCO shifts the resonator frequency and, through Meissner currents, the magnon frequency. The result matters because it shows that sub-micrometer magnetic films, not only millimeter spheres or multi-micrometer slabs, can reach the strong-coupling regime in superconducting magnonic circuits.","feed_headline":"A 104-nm-thick YIG film reaches 230 MHz magnon-photon coupling","feed_subtitle":"Sub-micrometer YIG films can couple coherently to YBCO microwaves, opening the way to thinner hybrid magnonic devices.","key_machinery":"The load-bearing object is the hybrid polariton dispersion $\\Omega_\\pm = \\frac{1}{\\sqrt{2}}\\sqrt{\\omega_c^2+\\omega_b^2\\pm\\sqrt{(\\omega_c^2-\\omega_b^2)^2+16\\omega_c\\omega_b g^2}}$, where $\\omega_c(T)$ is the resonator mode and $\\omega_b=\\omega_0+\\delta_{\\mathrm{sc}}$ is the magnon mode shifted by the superconductor. The temperature enters through a two-fluid penetration depth $\\lambda_L(T)=\\lambda_L(0)\\sqrt{1-(T/T_c)^p}$ with $p=4/3$, which determines the resonator inductance in Eq. (9) and the Meissner-current shift in Eq. (7). The magnon frequencies $\\omega_0$ and $\\omega_1$ come from the dipole-exchange spin-wave dispersion with the 104-nm film thickness setting the perpendicular standing-wave quantization. The collective coupling is written as $g=g_s\\sqrt{2 s_{\\mathrm{Fe}} N_s}$ with $s_{\\mathrm{Fe}}=5/2$ and a spin number $N_s=1.49\\times10^{13}$ that, together with a geometric factor $r=0.45$, sets the absolute size of the splitting.","core_discovery":"The paper's central claim is that a 104-nm-thick YIG film in direct contact with a YBCO coplanar-waveguide resonator forms coherent magnon-photon polaritons with collective coupling $g/2\\pi\\approx 230$ MHz. The same experiment shows only two well-separated spin-wave resonances, the uniform mode and the first perpendicular standing spin-wave mode, in contrast with the many closely spaced modes of thicker films. The authors account for the full temperature evolution of the polariton branches between 10 and 85 K with a simple model: the two-fluid penetration depth $\\lambda_L(T)$ enters the resonator frequency through its inductance and enters the magnon frequency through a spin-wave-induced Meissner-current shift $\\delta_{\\mathrm{sc}}$, and this single temperature dependence reproduces the observed spectra. Above $T_c$ the anticrossing disappears, which the authors attribute to the loss of superconducting screening.","pith_inferences":["If the absolute normalization rests on $N_s$ and $r$, the measured 230 MHz should be treated as a calibration-dependent estimate until the participating spin number is measured independently; the shape of the temperature dependence is the robust part of the claim.","A direct test would vary the YIG film area while keeping the resonator fixed: a true collective coupling should grow as the square root of area, while a fixed-$N_s$ fit would not.","The same penetration-depth mechanism could be exploited to tune or switch hybrid devices thermally near $T_c$, since both the cavity pull and the magnon shift respond to $\\lambda_L$.","The reduced mode density of a 104-nm film could make it a cleaner testbed for quantum magnonics at liquid-nitrogen temperatures than thicker slabs with many closely spaced modes."],"forward_implications":["Sub-micrometer YIG films can be used for strong magnon-photon coupling on high-$T_c$ superconducting circuits, extending the approach beyond the 5-$\\mu$m films used previously.","The temperature drift of the polariton branches below $T_c$ can be tracked with a single physical input, the YBCO penetration depth, instead of separate temperature-dependent fitting of the coupling.","Above $T_c$ the anticrossing disappears, so the hybridized response is tied to the superconducting state and vanishes with the Meissner screening.","Reducing the YIG thickness from 5 $\\mu$m to 104 nm (a factor of 50) lowers the coupling only from about 1.1 GHz to 0.23 GHz (a factor of about 5), indicating that the near-surface region of the film dominates the coupling."],"supporting_citations":[{"why":"supplies the polariton dispersion formula and the previous 5-µm YIG result that the thinner film is compared against.","marker":"[9]"},{"why":"provides the analysis protocol, the Meissner-current frequency shift, and the 1.1 GHz thicker-film coupling baseline.","marker":"[30]"},{"why":"supports the spin-wave-induced Meissner-current shift used to set $\\omega_b = \\omega_0 + \\delta_{\\mathrm{sc}}$.","marker":"[21]"},{"why":"gives the dipole-exchange spin-wave model used to describe the YIG mode spectrum.","marker":"[42]"},{"why":"supplies the explicit formulas for the uniform and first perpendicular standing spin-wave frequencies.","marker":"[43]"},{"why":"provides the temperature-dependent YIG magnetization used to reproduce the broadband resonance positions.","marker":"[45]"},{"why":"supplies the two-fluid penetration-depth form with $p=4/3$ for d-wave superconductors.","marker":"[46]"},{"why":"gives the resonator-frequency relation used to extract $\\lambda_L(0)$ and $T_c$ from $\\omega_c(T)$.","marker":"[48]"}],"fun_headline_variants":["104-nm YIG film hits 230 MHz magnon-photon coupling","Thin YIG film achieves 230 MHz coupling with YBCO","Sub-micron YIG film couples to YBCO at 230 MHz","Coherent 230 MHz coupling in 104-nm YIG on YBCO"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The quantitative reproduction of the split spectra depends on assuming exactly $N_s=1.49\\times10^{13}$ participating spins and on a free geometric factor $r=0.45$; neither number is measured independently, so if they are wrong the model's fit to the 230 MHz coupling is forced rather than predicted.","fun_headline_variants_meta":{"raw":{"variants":["104-nm YIG film hits 230 MHz magnon-photon coupling","Thin YIG film achieves 230 MHz coupling with YBCO","Sub-micron YIG film couples to YBCO at 230 MHz","Coherent 230 MHz coupling in 104-nm YIG on YBCO"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000479,"raw_usage":{"total_tokens":2333,"prompt_tokens":868,"completion_tokens":1465,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":484,"completion_tokens_details":{"reasoning_tokens":1380}},"tokens_in":484,"tokens_out":1465,"duration_ms":11462,"temperature":1.0,"reasoning_tokens":1380,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T22:08:12.264803+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the YIG film's participating volume and the resonator mode volume independently, for instance by varying the film area or mapping the microwave field, and compute $g$ from the coupling formula without fitting $N_s$ or $r$; if the predicted polariton splitting deviates from roughly 460 MHz by more than the linewidth, the claim that penetration depth alone controls the temperature evolution would be contradicted.","supporting_citations":[{"cited_title":"Ghirri, C","cited_arxiv_id":null,"evidence_quote":"provides the analysis protocol, the Meissner-current frequency shift, and the 1.1 GHz thicker-film coupling baseline."},{"cited_title":"Borst, P","cited_arxiv_id":null,"evidence_quote":"supports the spin-wave-induced Meissner-current shift used to set $\\omega_b = \\omega_0 + \\delta_{\\mathrm{sc}}$."},{"cited_title":"281–346.doi:10.1007/978-3-030-63210-6_ 6","cited_arxiv_id":null,"evidence_quote":"supplies the explicit formulas for the uniform and first perpendicular standing spin-wave frequencies."},{"cited_title":"Ghigo, D","cited_arxiv_id":null,"evidence_quote":"gives the resonator-frequency relation used to extract $\\lambda_L(0)$ and $T_c$ from $\\omega_c(T)$."}],"review_version":1}