{"id":"c5350164-41d2-49e0-9ef1-7c32dc9788ad","arxiv_id":"1908.05807","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"In CrSiTe3/Pt and CrGeTe3/Pt, the longitudinal spin Seebeck signal persists above the Curie temperature, attributed to exchange-driven interlayer spin transport.","lead":"This paper reports the spin Seebeck effect, a voltage generated from a temperature difference in a magnetic material, in two layered 2D magnetic insulators. The effect survives above the temperature where the material stops being magnetic, which suggests spins move between layers and could be useful for thermoelectric devices.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The central claim that the paramagnetic LSSE signal is carried by Jc-mediated interlayer transport rests on an untested inference from the signal's disappearance temperature; neither the experiment nor the supporting simulation directly probes interlayer transport.","rationale":"The paper reports a well-posed experiment with credible magnetization and transport data, including careful surface preparation and a plausible temperature dependence of the LSSE signal below and above TC. I agree with the reader that the central claim is conditional rather than established. The weakest point is the indirect inference that the disappearance temperature of the paramagnetic signal is set by the interlayer exchange Jc. The reader's weakest assumption overlaps with this, but my concern is narrower and more specific: the paper provides no direct measurement or simulation of interlayer transport. The atomistic spin-dynamics calculation is a partial strength because it reproduces the magnetization and estimates the interface spin-pumping contribution, but it explicitly omits transport. The paper's footnote [40] honestly acknowledges the normal-Nernst ambiguity at high temperatures, which narrows the regime of certainty but does not fully resolve the bulk-versus-interface question. A thickness-dependence experiment would be a decisive, feasible check. Since the reader already assigned CONDITIONAL and my concern reinforces that conditionality, the verdict remains unchanged.","tokens_in":9854,"tokens_out":8335,"duration_ms":90388,"concrete_test":"Measure the normalized LSSE signal S in exfoliated CrSiTe3/CrGeTe3 single crystals of varying thickness Lz (e.g., 10, 40, 100, and 200 µm) with identical Pt contacts, ΔT, and H in the paramagnetic regime. If the signal is volume-generated via interlayer exchange transport, S should change systematically with Lz (rising with Lz up to the spin diffusion length and then saturating); if it is purely interfacial spin pumping, S should be thickness-independent. This test directly interrogates the proposed Jc-mediated bulk transport without relying on the neutron-scattering extrapolation.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper attributes the LSSE signal above TC to exchange-dominated interlayer transport of in-plane paramagnetic moments, and interprets the disappearance near 50 K (CrSiTe3) and 90 K (CrGeTe3) as the loss of out-of-plane exchange transport (Jc). This inference is load-bearing because the argument decoupling bulk transport from interfacial spin pumping assumes that if interface spin pumping from in-plane correlations were responsible, the signal would persist to 300 K. That assumption is unverified: a temperature-dependent spin-mixing conductance, or a temperature-dependent paramagnetic spin diffusion length, could suppress the signal without a sharp change in Jc. The supporting atomistic spin-dynamics calculation does not model interlayer transport at all; it computes local spin pumping in the absence of transport and then interprets the discrepancy with experiment as evidence that out-of-plane transport has vanished. No interlayer transport coefficient, spin diffusion length, or thickness dependence is measured. The disappearance temperature is therefore consistent with the proposed mechanism, but it is not a direct test of it. This is the weakest point on which the central claim rests.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"Ito et al. report longitudinal spin Seebeck effect (LSSE) measurements on Pt-capped single crystals of the layered ferromagnetic insulators CrSiTe3 and CrGeTe3. They observe an antisymmetric thermovoltage that saturates with magnetic field in a way that tracks the magnetization, and, unlike in YIG, the signal persists above the Curie temperature: it disappears near 50 K for CrSiTe3 and near 90 K for CrGeTe3 at low fields, while higher fields extend the signal to higher temperatures. An atomistic spin-dynamics simulation for CrSiTe3, using magnetic parameters from the independent neutron-scattering study of Ref. [22], computes the local spin-pumping contribution in the absence of interlayer spin transport; this calculation predicts substantial paramagnetic spin pumping at temperatures where the experimental signal has vanished. The authors attribute the discrepancy to the loss of out-of-plane, exchange-mediated interlayer spin transport above these temperatures, concluding that the paramagnetic LSSE is carried by in-plane short-range ferromagnetic correlations transported along the c axis via the interlayer exchange Jc.","tokens_in":10112,"tokens_out":5279,"duration_ms":50509,"significance":"The experimental core is valuable: clean single crystals, a well-defined antisymmetric LSSE voltage whose field dependence tracks M, and a clear demonstration that the signal persists above TC, in contrast to conventional YIG. The proposed mechanism—exchange-dominated interlayer transport of paramagnetic moments reinforced by short-range ferromagnetic correlations and Zeeman effects—would be a new route for spin-current generation in quasi-two-dimensional magnetic insulators. The numerical support uses independently published exchange parameters rather than fits to the LSSE data, and the paper honestly records the normal-Nernst ambiguity in footnote [40]. The weakness is the logical bridge from the observations to the interlayer-transport mechanism: as detailed in the major comments, the disappearance temperature is consistent with the proposal but is not a direct test of it, and the supporting simulation does not model interlayer transport.","major_comments":[{"comment":"The central claim that the paramagnetic LSSE signal is carried by Jc-mediated interlayer transport is inferred from the temperature at which the signal disappears, but neither the experiment nor the simulation directly probes interlayer transport. The atomistic spin-dynamics calculation in Fig. 4(e) computes local spin pumping without any interlayer transport, and the disagreement with experiment is interpreted as evidence that out-of-plane transport has vanished; however, no interlayer transport coefficient, spin diffusion length, or sample-thickness dependence is measured. A temperature-dependent spin-mixing conductance or a short paramagnetic spin diffusion length could produce the same disappearance without a sharp change in Jc. I request a direct test—for example, the LSSE signal versus crystal thickness, a nonlocal spin-transport measurement, or an independent calculation of the interlayer spin conductivity—or a reformulated conclusion that presents the interlayer-transport scenario as one consistent explanation rather than the demonstrated mechanism.","section":"Fig. 4(e) and the paragraph following it"},{"comment":"The paper uses high-field data above the temperatures at which it states the S(H) signal becomes linear in H. Footnote [40] says that above 65 K for CrSiTe3/Pt and 85 K for CrGeTe3/Pt the H-linear normal Nernst contribution cannot be distinguished from the LSSE, yet the 87-kOe curves in Figs. 4(a) and 4(c) are used to argue that S survives to roughly 80 K and that Zeeman-enhanced spin polarization is responsible. Because the 87-kOe CrSiTe3 data extend above the linearity threshold, the quantitative high-field enhancement claims are not supported unless the normal Nernst contribution is measured and subtracted. Please either perform that subtraction or restrict the high-field mechanistic discussion to temperatures below the stated threshold.","section":"Figs. 3(a), 3(c), 4(a), 4(c), and footnote [40]"},{"comment":"The decoupling argument assumes that if interfacial spin pumping from in-plane correlations were the source, the LSSE signal 'would appear until 300 K'. This assumption is not established: the neutron-scattering correlations of Ref. [22] are bulk properties, and the spin-mixing conductance at the Pt interface, as well as the relevant correlation time, may be strongly temperature dependent even while the bulk in-plane susceptibility remains large. As written, the disappearance near 50 K and 90 K is consistent with loss of interlayer transport, but it does not exclude an interface-controlled or diffusion-length-controlled suppression. Please state this limitation explicitly or provide a quantitative estimate of the interfacial spin-pumping contribution versus temperature.","section":"Text at Fig. 5 and the 'paramagnetic phase' discussion"},{"comment":"The numerical support is presented only for CrSiTe3, using the exchange couplings of Ref. [22], while the conclusions are extended to CrGeTe3/Pt without independent verification of its magnetic correlation anisotropy. CrGeTe3 has a substantially different Curie temperature and, presumably, a different Jab/Jc ratio, so the claim that the same interlayer-transport mechanism governs its paramagnetic LSSE is an assumption. The text should either state this assumption prominently or support the CrGeTe3 case with a calculation or with a measurement of its in-plane versus out-of-plane correlation temperatures.","section":"Abstract and the 'To confirm' paragraph"}],"minor_comments":[{"comment":"'Arrott-Noaks' should be spelled 'Arrott-Noakes', matching the standard literature.","section":"Eq. (1)"},{"comment":"The title contains a typo: 'Spin-currnt-driven thermoelectric coating' should be 'Spin-current-driven thermoelectric coating'.","section":"Reference [2]"},{"comment":"'maintained at theses temperatures and then slowly cooled' should read 'maintained at these temperatures and then slowly cooled'.","section":"Crystal-growth paragraph"},{"comment":"The definition S = (V_LSSE/ΔT)(Lz/Ly) would benefit from an explicit statement of the sign convention for V_LSSE relative to the field and magnetization directions.","section":"Measurement definition of S"},{"comment":"The sentence beginning 'The reason this is not seen in our experimental measurements...' is the key interpretive step; consider labeling it explicitly as an inference from the simulation rather than a direct experimental conclusion.","section":"Sentence after Fig. 4(e)"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Straight to the point: this is the first longitudinal spin Seebeck measurement on the layered van der Waals ferromagnets CrSiTe3 and CrGeTe3, and the first report of LSSE surviving above TC in this class. The core data look clean: the antisymmetric voltage tracks the magnetization in the ordered phase, and above TC it persists to roughly 50 K in CrSiTe3 and 90 K in CrGeTe3 before flattening. That is a new observation worth taking seriously.\n\nThe paper earns credit for not stopping at the measurement. Using atomistic spin dynamics with exchange parameters from an independent neutron-scattering study (Williams et al.), the authors show that pure interface spin pumping from the in-plane correlations would produce a large signal far above the temperature where their signal dies. The contrast is the basis for their claim that interlayer transport along the c axis, mediated by the weak out-of-plane exchange Jc, is what actually feeds nonequilibrium magnons to the Pt interface. That is a clever way to decouple interfacial pumping from bulk transport, and it fits the data.\n\nThe soft spot is that the mechanism is an inference from a disappearance temperature, not a direct measurement. The simulation is explicitly a no-transport baseline; it does not compute a spin diffusion length or an interlayer conductivity. A temperature-dependent spin-mixing conductance at the Pt interface, or a paramagnetic spin diffusion length that collapses with temperature, could also remove the signal without Jc being the culprit. There is no thickness series, no direct measurement of spin propagation along c, and no control for the normal Nernst background except the footnote that limits the clean regime. For CrGeTe3, the in-plane correlation picture is borrowed from CrSiTe3 without independent verification. These are real gaps, but they do not undermine the basic observation, which is solid.\n\nWho is this for: spin caloritronics, 2D magnetism, and van der Waals heterostructure people. It deserves a serious referee. I would recommend sending it out, with the expectation that the referee asks for a few control measurements (Pt thickness dependence, a Nernst-subtraction or YIG comparison) and a slightly more cautious statement of the transport mechanism.","headline":"First LSSE in a van der Waals ferromagnet, with a clever but indirect case that interlayer exchange transport, not interface pumping, governs the signal above TC.","tokens_in":10617,"tokens_out":3733,"would_cite":true,"duration_ms":36480,"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":"The paper reports that the longitudinal spin Seebeck effect in CrSiTe3 and CrGeTe3 persists above the magnetic ordering temperatures and attributes the paramagnetic signal to interlayer exchange transport of in-plane correlated moments.","keywords":["spin Seebeck effect","longitudinal spin Seebeck effect","CrSiTe3","CrGeTe3","van der Waals magnets","interlayer spin transport","paramagnetic spin pumping","quasi-two-dimensional ferromagnet"],"falsifier":"Measure the LSSE on the same Pt/CrSiTe3 interface with the interlayer exchange path removed, for example on a single exfoliated CrSiTe3 layer or on a stack in which a nonmagnetic spacer interrupts the out-of-plane coupling; if a paramagnetic signal of comparable size still appears above $T_C$, the central claim that interlayer transport is essential is wrong.","tokens_in":9666,"feed_emoji":"🧲","tokens_out":13587,"duration_ms":114933,"temperature":0.7,"pith_summary":"This paper studies the longitudinal spin Seebeck effect (LSSE), the generation of a spin current by a temperature gradient, in the layered ferromagnetic insulators CrSiTe3 and CrGeTe3 capped with platinum, with heat flowing across the Cr layers. It finds that the LSSE voltage, which in a standard magnet such as yttrium iron garnet vanishes at the magnetic ordering temperature, persists well above the Curie temperatures of both compounds. The authors argue that this paramagnetic signal is not caused by interfacial spin pumping alone, but by spin transport along the c axis: short-range ferromagnetic correlations within the Cr honeycomb layers are carried between layers by the weaker out-of-plane exchange coupling, reinforced by Zeeman alignment under high magnetic fields. The result matters because it identifies interlayer exchange as the controlling factor for spin-current generation in layered two-dimensional magnets, and because it offers a way to probe spin transport in heterostructures made from such materials.","feed_headline":"Spin Seebeck signal outlives magnetic order in layered Cr magnets","feed_subtitle":"Above the Curie temperature, the signal traces interlayer exchange transport, not just interface pumping.","key_machinery":"The load-bearing mechanism is the anisotropy between two exchange couplings in these quasi-two-dimensional magnets: a strong in-plane coupling $J_{ab}\\sim 15$ K that sustains short-range ferromagnetic correlations in the Cr honeycomb layers up to room temperature, and a much weaker out-of-plane coupling $J_c$, more than five times smaller, that becomes ineffective above about 50 K in CrSiTe3. The paper's argument is carried by the direction of the measurement: with the temperature gradient along the c axis, the LSSE requires spin current to traverse the Cr layers, so the out-of-plane exchange acts as the conduit for the in-plane correlated moments. The numerical support is an atomistic spin-dynamics calculation of the interfacial spin-pumping amplitude based on the CrSiTe3 Hamiltonian, which isolates what the interface alone would contribute and shows that it would not reproduce the observed disappearance of the signal, leaving interlayer exchange transport as the essential ingredient.","core_discovery":"On its own terms, the paper establishes that the longitudinal spin Seebeck effect in CrSiTe3/Pt and CrGeTe3/Pt is observable not only in the ferromagnetic phase but also in the paramagnetic phase above the Curie temperature, up to roughly 50 K for CrSiTe3 and 90 K for CrGeTe3 at low fields, and to higher temperatures under applied fields up to 87 kOe. The central claim is that these high-temperature signals arise from exchange-dominated interlayer transport of in-plane paramagnetic moments: strong in-plane ferromagnetic correlations persist to much higher temperatures, and the out-of-plane exchange coupling carries these correlated moments along the temperature gradient to the Pt interface, where they are detected as a spin current. An atomistic spin-dynamics calculation of the pure interfacial spin-pumping contribution, using the magnetic parameters of CrSiTe3, shows that such pumping alone would remain sizeable far above the Curie temperature and would scale with magnetization; the authors take the experimental disappearance of the signal around 50/90 K as evidence that interlayer transport, not interface pumping, controls the response in the paramagnetic regime.","pith_inferences":["The paper does not itself compute the interlayer transport; an explicit calculation of the out-of-plane spin current carried by short-range correlated moments could turn the qualitative mechanism into a quantitative prediction of the paramagnetic signal's magnitude.","If the mechanism is general, single-layer measurements of the same compounds should show no comparably sized LSSE above the ordering temperature, because the out-of-plane conduit is missing; this is a testable consequence of the paper's picture, not something the authors report.","A complementary probe would be to vary the interlayer spacing or insert a nonmagnetic spacer layer between Cr layers: the paper's picture predicts the paramagnetic LSSE should weaken as out-of-plane exchange is reduced, even though in-plane correlations are unchanged."],"forward_implications":["If the mechanism is right, the LSSE in a layered two-dimensional magnet provides a transport-based readout of interlayer exchange coupling: the temperature at which the signal disappears should track the scale of $J_c$, not the bulk magnetization.","In-plane ferromagnetic correlations alone are not enough to generate a spin Seebeck signal; without an active out-of-plane exchange path, the interface sees an equilibrium magnon distribution and the voltage vanishes.","Strong applied fields extend the measurable spin Seebeck response to temperatures well above the zero-field Curie temperature, because the Zeeman energy polarizes the paramagnetic moments and makes the interlayer transport visible.","The same bilayer geometry can be used to study spin transport through other layered two-dimensional magnetic insulators and through heterostructures combining them with topological insulators.","The paramagnetic LSSE in these compounds is a bulk transport phenomenon distinct from the interfacial spin-pumping contribution, so measurements of its temperature and field dependence can separate the two in a steady state."],"supporting_citations":[{"why":"Supplies the anisotropic-exchange Hamiltonian and the neutron-scattering evidence that in-plane ferromagnetic correlations in CrSiTe3 persist to 300 K while out-of-plane correlations die out near 50 K; this is the physical picture the paramagnetic-signal interpretation rests on.","marker":"[22]"},{"why":"Establishes intrinsic ferromagnetism in two-dimensional van der Waals crystals and in bilayer CrGeTe3, supporting the relevance of the quasi-2D magnetism studied here.","marker":"[17]"},{"why":"Provides the atomistic spin-dynamics method for computing the interfacial spin-pumping contribution, which the paper uses to show that interface pumping alone would not reproduce the data.","marker":"[4]"},{"why":"Supplies the quantum statistics for the thermal fields in the low-temperature spin-dynamics calculation, making the numerical spin-pumping result valid at the measured temperatures.","marker":"[42]"},{"why":"Gives the procedure for separating the longitudinal spin Seebeck signal from anomalous and normal Nernst voltages, which is what lets the authors assign the observed thermoelectric voltage to spin current.","marker":"[27]"},{"why":"Demonstrates the decoupling of interfacial spin pumping from bulk spin transport on ultrafast timescales, the prior result this paper extends to a steady-state thermal measurement.","marker":"[35]"},{"why":"Provides the magnon-driven theory of the spin Seebeck effect used to interpret the ferromagnetic-phase response and the role of the nonequilibrium magnon population at the interface.","marker":"[5]"}],"fun_headline_variants":["Spin Seebeck effect persists above Curie temperature in Cr magnets","Interlayer exchange drives spin Seebeck in paramagnetic CrSiTe3/CrGeTe3","Spin Seebeck in paramagnetic phase: exchange transport, not interface pumping","Spin Seebeck outlasts magnetic order in CrGeTe3 and CrSiTe3"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The argument rests on the neutron-scattering result that in CrSiTe3 the in-plane magnetic correlations survive to at least 300 K while the out-of-plane correlations vanish above about 50 K, and on applying that same picture to CrGeTe3; if the out-of-plane correlation scale is actually different, or if the signal's disappearance near 50/90 K comes from a temperature-dependent interface effect rather than loss of interlayer exchange, the attribution collapses.","fun_headline_variants_meta":{"raw":{"variants":["Spin Seebeck effect persists above Curie temperature in Cr magnets","Interlayer exchange drives spin Seebeck in paramagnetic CrSiTe3/CrGeTe3","Spin Seebeck in paramagnetic phase: exchange transport, not interface pumping","Spin Seebeck outlasts magnetic order in CrGeTe3 and CrSiTe3"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00095,"raw_usage":{"total_tokens":4041,"prompt_tokens":919,"completion_tokens":3122,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":535,"completion_tokens_details":{"reasoning_tokens":3032}},"tokens_in":535,"tokens_out":3122,"duration_ms":21486,"temperature":1.0,"reasoning_tokens":3032,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:04:09.160156+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the LSSE on the same Pt/CrSiTe3 interface with the interlayer exchange path removed, for example on a single exfoliated CrSiTe3 layer or on a stack in which a nonmagnetic spacer interrupts the out-of-plane coupling; if a paramagnetic signal of comparable size still appears above $T_C$, the central claim that interlayer transport is essential is wrong.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the anisotropic-exchange Hamiltonian and the neutron-scattering evidence that in-plane ferromagnetic correlations in CrSiTe3 persist to 300 K while out-of-plane correlations die out near 50 K; this is the physical picture the paramagnetic-signal interpretation rests on."},{"cited_title":"Leb´ egue, T","cited_arxiv_id":null,"evidence_quote":"Establishes intrinsic ferromagnetism in two-dimensional van der Waals crystals and in bilayer CrGeTe3, supporting the relevance of the quasi-2D magnetism studied here."},{"cited_title":"Barker and G","cited_arxiv_id":null,"evidence_quote":"Provides the atomistic spin-dynamics method for computing the interfacial spin-pumping contribution, which the paper uses to show that interface pumping alone would not reproduce the data."},{"cited_title":"3(a) and 3(c)]","cited_arxiv_id":null,"evidence_quote":"Supplies the quantum statistics for the thermal fields in the low-temperature spin-dynamics calculation, making the numerical spin-pumping result valid at the measured temperatures."},{"cited_title":"Carteaux, F","cited_arxiv_id":null,"evidence_quote":"Gives the procedure for separating the longitudinal spin Seebeck signal from anomalous and normal Nernst voltages, which is what lets the authors assign the observed thermoelectric voltage to spin current."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Demonstrates the decoupling of interfacial spin pumping from bulk spin transport on ultrafast timescales, the prior result this paper extends to a steady-state thermal measurement."},{"cited_title":"Spin Quantum Rectification Project","cited_arxiv_id":null,"evidence_quote":"Provides the magnon-driven theory of the spin Seebeck effect used to interpret the ferromagnetic-phase response and the role of the nonequilibrium magnon population at the interface."}],"review_version":1}