{"id":"c9f8e709-d1e7-43fc-9be6-9ee4d646eadd","arxiv_id":"2502.06962","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"In Fe/MgO/V/MgO/Fe/Co spin valves, switching from antiparallel to parallel magnetic alignment multiplies the cryogenic thermoelectric voltage by more than ten, reaching estimated Seebeck coefficients around 100 microvolts per kelvin at 0.3 K.","lead":"The paper measures thermoelectric voltages in superconducting spin valves made of common materials and finds the signal grows more than tenfold when the two magnetic layers are switched from antiparallel to parallel. This makes magnetic control of cryogenic heat-to-electricity conversion look practical for quantum devices.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The ~100 µV/K Seebeck value depends on an unmeasured model-derived ΔT, and the Supplemental 1D thermal calculation appears internally inconsistent with its own Table I (k_eq stated as 11.57 W/mK vs ≈3.6 W/mK from Σd_i/k_i); a direct ΔT measurement is needed before the quantitative claim is settled.","rationale":"The paper's central claim has two parts: a large magnetic modulation of the thermoelectric voltage, and an absolute Seebeck coefficient of about 100 µV/K. The modulation part is supported by raw ΔV data and by the disappearance of the signal above Tc = 4.7 K; it does not depend on the thermal calibration and is credible. The absolute conversion factor, however, depends entirely on the ratio ΔV/ΔT. The reader correctly identified the unmeasured, model-derived ΔT as the weakest assumption, and my re-derivation of Supplemental Section II indicates a concrete internal inconsistency: with the tabulated thicknesses and conductivities, the series thermal resistance is about 33.7×10^-9 m²K/W, giving k_eq ≈ 3.6 W/mK, not 11.57 W/mK. If the series sum is right, the reported ΔT is too small by roughly a factor of 3.3 and S_P falls below 100 µV/K; if the stated k_eq is right, the tabulated values or the MgO tuning must be corrected. The micromagnetic and Usadel modeling is qualitative and uses several fitted parameters, so it does not independently pin down the absolute Seebeck value. These considerations keep the verdict at CONDITIONAL: the spin-valve modulation is promising, but the quantitative claim should not be taken at face value until ΔT is directly measured or the thermal model is corrected and validated.","tokens_in":24666,"tokens_out":11600,"duration_ms":101571,"concrete_test":"Recompute ΔT and k_eq directly from Eq. (7) and Table I for V_LED = 7.1 V (I_LED = 100 mA), with the stated q from Eq. (9); check whether k_eq = 11.57 W/mK is reproduced. If not, propagate the corrected k_eq through Fig. S3(c) and S4. Independently, fabricate an identical Fe/MgO/V/MgO/Fe/Co stack with a local thermometer integrated on the top electrode (e.g., a second tunnel junction or a thin-film superconducting transition-edge sensor) and measure ΔT at the same LED powers, reporting S_P = ΔV/ΔT with an uncertainty budget. If the measured or recomputed ΔT is larger than the reported value by a factor of about 3, the headline Seebeck value must be revised downward; the >10x magnetic modulation claim would be unaffected.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The quantitative centerpiece of the paper is the parallel-state Seebeck coefficient of about 100 µV/K, reported as ΔV/ΔT. ΔT is not measured; it comes from a 1D stationary heat-diffusion model (Supplemental Section II) that combines a rescaled room-temperature LED I-V curve, the manufacturer radiation pattern, a geometry factor A_contact/A_sample, and literature thermal conductivities. Recomputing the series thermal resistance from the values in Table I (Fe 70, MgO 0.1, V 10, Co 100 W/mK, top MgO = 4×0.1) gives Σ d_i/k_i ≈ 33.7×10^-9 m²K/W and hence k_eq ≈ 3.6 W/mK for L = 119.6 nm, not the stated 11.57 W/mK. If the series sum is correct, the modeled ΔT is underpredicted by roughly a factor of 3.3, and S_P at T = 0.3 K would be about 30-40 µV/K rather than 100 µV/K; if the stated k_eq is correct, then Table I or the tuned MgO conductivities have to be revised. Either way the absolute conversion factor and the 'exceeding 100 µV/K' conclusion are not yet quantitatively supported. The magnetic modulation of raw ΔV by more than an order of magnitude and its disappearance above Tc remain credible qualitative observations.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports thermoelectric voltage measurements in epitaxial Fe/MgO/V/MgO/Fe/Co spin-valve junctions, finding a decrease of more than an order of magnitude in the zero-bias thermovoltage when the ferromagnetic electrodes switch from parallel to antiparallel alignment, with the signal disappearing above the superconducting critical temperature Tc = 4.7 K. The authors estimate the temperature difference across the junction from a one-dimensional heat-diffusion model and, using that estimate, report a parallel-state Seebeck coefficient of about 100 microvolts per kelvin at 0.3 K. The experiment is supplemented by micromagnetic MuMax3 simulations and self-consistent Keldysh-Usadel Green's function calculations, which reproduce the qualitative field dependence of both the magnetoresistance and the thermovoltage.","tokens_in":25065,"tokens_out":5452,"duration_ms":48315,"significance":"If the quantitative calibration is corrected, the experiment would demonstrate a strong, magnetically switchable cryogenic thermoelectric effect in a material stack based on commonly used elements. The raw Delta-V(H) data and its disappearance above Tc are compelling and indicate a real physical effect. The theoretical framework is well established, and the combination of non-equilibrium Keldysh-Usadel theory with micromagnetic simulations is a strength of the paper. However, the absolute Seebeck value is currently tied to an unmeasured and internally inconsistent thermal model, so the headline claim of a Seebeck coefficient exceeding 100 microvolts per kelvin is not yet established.","major_comments":[{"comment":"The stated value k_eq = 11.57 W/(m.K) is inconsistent with the layer conductivities and thicknesses listed in Table I. From the tabulated values one obtains sum(d_i/k_i) = 45/70 + 2.3/0.1 + 40/10 + 2.3/0.4 + 10/70 + 20/100 nm/(W/m.K) ≈ 33.7 x 10^-9 m^2.K/W, and with L = 119.6 nm this gives k_eq ≈ 3.6 W/(m.K), not 11.57 W/(m.K). Since Delta-T = qL/k_eq, the modeled Delta-T values in Fig. S3(c) are underestimated by about a factor of 3.3 if the series sum is correct. Consequently the parallel-state Seebeck coefficient would be roughly 30-40 microvolts per kelvin rather than about 100 microvolts per kelvin, and the claim of exceeding 100 microvolts per kelvin would not be supported. Conversely, if the stated k_eq was actually used in the calculations, then Table I or the tuned MgO conductivity must be revised. The authors should either measure Delta-T directly or present a corrected k_eq, state the resulting range of S, and adjust the abstract and conclusions accordingly.","section":"Supplemental Section II, Eq. (7), Table I"},{"comment":"The quantitative claims, including the parallel-state Seebeck coefficient and the factor-of-ten modulation, are presented without error bars, multi-device statistics, or repeated thermal-cycling data. The raw Delta-V(H) trace shows a clear effect, but the absence of any uncertainty estimate makes the absolute calibration impossible to assess. The authors should state how many nominally identical devices were measured, show representative reproducibility, and quantify the uncertainty associated with the V_OFF background subtraction that defines Delta-V.","section":"Experimental results, Fig. 2 and Methods"},{"comment":"The comparison between theory and experiment is only qualitative, and the theory contains several adjustable parameters: the spin-mixing conductance G_phi/G_0 = 2.75, the Dynes broadening delta/Delta_0 = 0.01, the RKKY coupling -0.5 mJ/m^2, the interface-to-bulk conductance G_0/G = 1/3.5, the polarization p = 0.8, the micromagnetic defect concentrations, and the top-MgO thermal-conductivity multiplier. This is not circular, because the experiment is independent of the theory, but the quoted agreement does not provide quantitative support for the magnitude of the thermoelectric effect. A sensitivity analysis over the fitted parameters is needed to show which qualitative features, such as the sign of the magnetoresistance, the overshoot, and the P/AP ratio, are robust to parameter choices.","section":"Thermoelectricity via non-equilibrium quasiclassical theory, Figs. 4-5 and Supplemental Section IV"}],"minor_comments":[{"comment":"The statement that the large Seebeck coefficient 'remains large and more than an order of magnitude greater than in the AP configuration as the base temperature is increased' is a theoretical prediction from Fig. S7, not an experimental observation; the wording in the main text should be clarified.","section":"Discussion and Supplemental Section V"},{"comment":"Only an upward magnetic-field sweep is shown for the thermovoltage; presenting both sweep directions would clarify whether the sharp AP-to-P transition and overshoot are hysteretic.","section":"Fig. 2(a)"},{"comment":"The phrase 'commercially available materials' should be made precise: the elemental materials are commercially available, but the MBE-grown epitaxial stacks described here are not off-the-shelf products; a phrase such as 'based on commonly used materials' would be more accurate.","section":"Abstract and Introduction"},{"comment":"The factor-of-four increase in the top MgO thermal conductivity is a significant tuning parameter that directly affects k_eq and hence Delta-T; its basis and uncertainty should be discussed in more detail.","section":"Supplemental Section II, Table I"},{"comment":"The notation '1.76/T/T_c' is ambiguous; the dimensionless temperature ratio should be written explicitly, for example as (1.76 k_B T / Delta_0) or with parentheses.","section":"Supplemental Eq. (22)"}],"recommendation":"major_revision","confidential_remarks":"The qualitative spin-valve thermoelectric effect appears credible and is worth publishing once the quantitative thermal calibration is fixed. The internal inconsistency in the supplemental thermal model is serious enough that the current 'exceeding 100 microvolts per kelvin' claim should not stand as written. I would encourage the editor to require either a direct Delta-T measurement or a fully corrected and sensitivity-analyzed thermal model before acceptance."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The thing to know: the qualitative result is likely real—an order-of-magnitude magnetic modulation of the thermovoltage in an epitaxial Fe/MgO/V/MgO/Fe/Co spin valve, with the signal disappearing above Tc. The quantitative headline—Seebeck coefficient 'about 100 µV/K'—is not supported as stated, because the Supplemental's thermal model contains an internal arithmetic inconsistency that overestimates S by roughly a factor of three.\n\nWhat is actually new and good: this is a clean materials extension to a commercially viable F/S/F stack with V (Tc=4.7 K) and MgO barriers. The raw ΔV(H) traces show a >10x change between AP and P states, reproduce on field sweeps, and vanish above Tc—a strong sanity check. The negative MR and the overshoot are interesting, and the micromagnetic simulations capture the switching behavior qualitatively. The Keldysh-Usadel modeling is standard and honestly labeled as qualitative; the observation itself is not circular, since the experiment stands independently of the theory.\n\nThe soft spots, in proportion: First, the ΔT estimate. In Supplemental Section II, the stated k_eq = 11.57 W/mK is inconsistent with the Table I values: summing d_i/k_i for the six layers gives k_eq ≈ 3.5–3.6 W/mK. That means their ΔT is too small by a factor ~3.3 and the reported S is too large by the same factor. The true value, under their own assumptions, would be ~30–40 µV/K, not ~100–125. If they intended to tune the top MgO conductivity differently, they need to show it; as written, the conversion factor is not quantitatively anchored. Second, there are no error bars or multi-device statistics, and the LED-based ΔT relies on a rescaled room-temperature I–V curve and manufacturer radiation pattern—understandable, but not a measurement. Third, the theoretical 'agreement' uses several fitted parameters (G_phi/G0 = 2.75, Dynes δ/Δ0 = 0.01, RKKY coupling, defect fractions) and is qualitative; that is acceptable for a support, not for a confirmation.\n\nBottom line: the paper deserves a serious referee. The qualitative spin-valve thermoelectric effect is a genuine step for cryogenic thermoelectric devices, and the absolute S is a fixable problem. I'd send it to review, but with a strong request to correct the thermal calculation, add uncertainty estimates, or measure ΔT directly. A reader in superconducting spintronics would get value from the experiment and the modeling, and the discussion of the ΔT issue is exactly what a reading group should chew on.","headline":"Solid qualitative spin-valve thermoelectric effect, but the headline 100 µV/K value rests on a ΔT estimate that is internally inconsistent by ~3x.","tokens_in":25615,"tokens_out":4267,"would_cite":true,"duration_ms":35374,"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":"In a superconducting spin valve made of iron, magnesium oxide, and vanadium, switching the magnetic alignment changes the thermoelectric voltage by more than an order of magnitude, with a parallel-state Seebeck coefficient near 100…","keywords":["superconducting spin valve","thermoelectricity","Seebeck coefficient","vanadium superconductor","ferromagnet/superconductor junctions","cryogenic thermoelectric generator","micromagnetic simulation","spin-dependent transport"],"falsifier":"Embedding thin-film thermometers on both sides of the vanadium layer would measure the temperature difference directly and settle whether the reported parallel-state Seebeck coefficient near 100 microvolts per kelvin is accurate; a complementary check would be to replace the superconducting vanadium with a normal-metal spacer, which should eliminate the large magnetic modulation if superconducting spin splitting is the source.","tokens_in":24497,"feed_emoji":"🧲","tokens_out":9978,"duration_ms":80234,"temperature":0.7,"pith_summary":"The paper reports a magnetically switchable thermoelectric effect in superconducting spin valves built from iron, magnesium oxide, vanadium, and cobalt, materials that are commercially available and relatively easy to grow epitaxially. With the two ferromagnetic electrodes parallel, the junction converts a temperature difference of roughly 0.1–0.2 K into a thermovoltage corresponding to a Seebeck coefficient near 100 microvolts per kelvin at 0.3 K. Rotating the electrodes into the antiparallel configuration reduces the thermoelectric signal by more than an order of magnitude, which the authors identify as a large thermoelectric spin-valve effect. They reproduce the field dependence with a non-equilibrium quasiclassical Green's function calculation combined with micromagnetic simulations, and argue that this combination of large conversion factor, large magnetic modulation, and accessible materials meets the requirements for a practical tunable cryogenic thermoelectric generator.","feed_headline":"Magnetic switch swings thermoelectric output tenfold at 0.3 K","feed_subtitle":"Iron, magnesium oxide, and vanadium turn heat into voltage that a magnetic field can switch.","key_machinery":"The working mechanism is the combination of a spin-split superconducting density of states in the vanadium film and spin-selective tunneling through the two MgO barriers. The proximity of the ferromagnetic electrodes creates spin splitting in the superconductor, while the MgO barriers filter spins with effective polarization above 0.7–0.8, so thermally excited electrons and holes see an asymmetric density of states around the Fermi energy and a net thermovoltage is generated. The magnitude of this thermovoltage is controlled by the angle between the two interface magnetizations, which is why the magnetic state of the spin valve sets the thermoelectric output. The theoretical work carries the argument through a self-consistent non-equilibrium quasiclassical Green's function (Usadel) calculation of the current under a temperature gradient, with the field-dependent magnetization angle supplied by micromagnetic simulations.","core_discovery":"The central discovery is that the thermoelectric response of a fully epitaxial Fe/MgO/V/MgO/Fe/Co junction is dominated by the relative magnetic alignment of its two ferromagnetic electrodes. In the parallel state, the measured thermovoltage divided by the estimated temperature difference gives a Seebeck coefficient of about 100 microvolts per kelvin, and this signal is more than ten times larger than the antiparallel-state signal. The effect disappears above the superconducting critical temperature of the vanadium film (4.7 K), tying the thermovoltage to the superconducting state. The authors show that the mechanism is a spin-dependent electron-hole asymmetry in the spin-split superconductor combined with spin-dependent transmission through the MgO barriers, and that the magnetic-field dependence reflects the field-driven rotation and domain motion of the ferromagnetic layers.","pith_inferences":["An implication the authors leave implicit is that the same junction could serve as a probe of magnetic texture, since the thermovoltage responds to domain-wall formation and motion at the interfaces, not only to the average magnetization angle.","Because the thermovoltage grows with interface polarization and spin mixing, replacing vanadium with a higher-gap superconductor such as niobium could plausibly raise both the output and the operating temperature, provided the epitaxial Fe/MgO interfaces can be preserved.","The large modeled parallel/antiparallel ratio below two kelvin suggests these junctions could be integrated into superconducting-qubit refrigerator circuits as magnetically controlled heat valves, though the estimated temperature-drop uncertainty would need to be removed for quantitative engineering."],"forward_implications":["A cryogenic heat-to-voltage converter can be switched between high- and low-output states by an in-plane magnetic field of about 0.1–0.2 tesla, without changing the temperature gradient.","The effect stays magnetically switchable over a useful temperature range: the theory predicts the parallel/antiparallel Seebeck ratio remains above ten for base temperatures below about two kelvin.","Because the superconducting film is vanadium with a critical temperature of 4.7 K, the device operates in a standard helium-3 cryostat rather than requiring dilution temperatures.","The physical ingredients are general: an epitaxial spin-filtering barrier and a spin-split superconductor, so the design offers a path toward optimizing thermoelectric output through interface polarization, spin-mixing conductance, and superconductor length."],"supporting_citations":[{"why":"Predicted the spin-splitting-based thermoelectric effect in ferromagnet/superconductor junctions that this experiment exploits.","marker":"[14, 15]"},{"why":"Predicted complete magnetic control of the superconducting thermoelectric effect in ferromagnet/superconductor/ferromagnet structures, the effect measured here.","marker":"[25]"},{"why":"Reported an earlier observation of magnetic-state-dependent thermoelectricity in superconducting spin valves; this work extends that result to a larger modulation with different materials.","marker":"[26]"},{"why":"Supplemental Material that contains the heat-diffusion model for the temperature difference, micromagnetic settings, and the quasiclassical calculation details used to interpret the measurements.","marker":"[27]"},{"why":"Provides the nonequilibrium Green's function description of superconductors with a spin-splitting field used as the theoretical basis for the numerical model.","marker":"[2]"},{"why":"Supplies the open-source micromagnetic simulation engine used to compute the field-dependent angle between the ferromagnetic layers.","marker":"[41]"},{"why":"Gives the conductance-versus-angle formula used to calculate magnetoresistance curves from the simulated magnetization configurations.","marker":"[43]"},{"why":"Supplies material parameters for the vanadium and Fe/MgO layers, including coherence length and spin polarization, used in the simulations.","marker":"[29]"}],"fun_headline_variants":["Spin valve flips thermoelectric signal tenfold at 4 Kelvin","Magnetic switch swings thermoelectric output 10x in superconductor","100 µV/K thermoelectric effect that magnets can turn off","Superconducting spin valve: tenfold thermoelectric change by magnet","Fe/MgO/V junction delivers big switchable thermoelectric signal"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The temperature difference across the junction is estimated from a one-dimensional heat-diffusion model using tabulated thermal conductivities and a manufacturer's LED radiation pattern, rather than measured, and the reported Seebeck coefficient is obtained by dividing the measured voltage by that estimated value.","fun_headline_variants_meta":{"raw":{"variants":["Spin valve flips thermoelectric signal tenfold at 4 Kelvin","Magnetic switch swings thermoelectric output 10x in superconductor","100 µV/K thermoelectric effect that magnets can turn off","Superconducting spin valve: tenfold thermoelectric change by magnet","Fe/MgO/V junction delivers big switchable thermoelectric signal"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000263,"raw_usage":{"total_tokens":1622,"prompt_tokens":990,"completion_tokens":632,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":606,"completion_tokens_details":{"reasoning_tokens":551}},"tokens_in":606,"tokens_out":632,"duration_ms":5869,"temperature":1.0,"reasoning_tokens":551,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-08T14:12:19.802121+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Embedding thin-film thermometers on both sides of the vanadium layer would measure the temperature difference directly and settle whether the reported parallel-state Seebeck coefficient near 100 microvolts per kelvin is accurate; a complementary check would be to replace the superconducting vanadium with a normal-metal spacer, which should eliminate the large magnetic modulation if superconducting spin splitting is the source.","supporting_citations":[{"cited_title":"Complete magnetic control over the superconducting thermoelectric effect,","cited_arxiv_id":null,"evidence_quote":"Predicted complete magnetic control of the superconducting thermoelectric effect in ferromagnet/superconductor/ferromagnet structures, the effect measured here."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplemental Material that contains the heat-diffusion model for the temperature difference, micromagnetic settings, and the quasiclassical calculation details used to interpret the measurements."},{"cited_title":"The design and verification of MuMax3,","cited_arxiv_id":null,"evidence_quote":"Supplies the open-source micromagnetic simulation engine used to compute the field-dependent angle between the ferromagnetic layers."},{"cited_title":"Superconductivity-induced change in mag- netic anisotropy in epitaxial ferromagnet-superconductor hybrids with spin-orbit interaction,","cited_arxiv_id":null,"evidence_quote":"Supplies material parameters for the vanadium and Fe/MgO layers, including coherence length and spin polarization, used in the simulations."}],"review_version":1}