{"id":"476f09a6-f3ab-4366-91dd-f816852ce3e0","arxiv_id":"2607.21808","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":8.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Nonzero second-order thermoelectric responses (nonlinear Seebeck, Nernst, and mixed-directional) are observed in WTe2 and TaIrTe4 at room temperature, governed by crystal symmetry.","lead":"This paper reports the first observation of intrinsic nonlinear thermoelectric effects—voltages that do not reverse when the heat bias is flipped—in thin flakes of the semimetals WTe2 and TaIrTe4, persisting up to room temperature. The result shows that low crystal symmetry alone can produce higher-order heat-to-electricity conversion without magnetic fields or engineered structures.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Symmetrization does not cleanly isolate the NLN signal: its raw 4ω does not reverse with heater swap, and the 2ω normalization channel is a non-reversing parasitic Seebeck, so a ~40× residual background can masquerade as S^(2)_baa.","rationale":"The reader's weakest assumption is exactly the symmetrization procedure that removes the temperature-dependent linear Seebeck background. My read agrees, and the SI provides even sharper evidence: for the NLN geometry the 2ω signal used for normalization does not reverse with heater switching and therefore does not measure the gradient component that drives the claimed S^(2)_baa; the raw 4ω signal also fails to reverse, which the authors themselves identify as the ambiguous case. The central claim requires all three tensor components, so a weakness in the NLN extraction is load-bearing. The paper is honest about many limitations and includes useful controls, but the symmetrization control is not sufficient for the NLN geometry. I do not change the verdict because the paper is already CONDITIONAL; my concern reinforces the condition rather than moving it to reject, since the NLS claim and several independent checks (symmetry-forbidden devices, frequency dependence, dc-offset analysis) provide partial support. The requested concrete test — a direct residual analysis or a temperature-profile measurement — would settle whether the NLN and NLMT components survive the background subtraction.","tokens_in":32035,"tokens_out":8517,"duration_ms":91981,"concrete_test":"For the TaIrTe4 NLN device at 292.5 K, take the raw V4(H1) and V4(H2) data from Fig. S7d and apply the authors' 2ω-based normalization. Then separately compute the non-reversing background using the measured 2ω parasitic ∇bT, the independently measured S_bb(T), and the heater R(T) calibration, and subtract it. If the residual is not <10% of the reported S^(2)_baa signal, the NLN coefficient is not established. A stronger version: fabricate a device with an additional resistance thermometer line between the two heaters to directly measure the in-plane temperature profile for each heater, and verify ΔT_H1/|∇T|_H1 = ΔT_H2/|∇T|_H2 to the few-percent level required by the 40× background ratio.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing step is the extraction of intrinsic S^(2) from a 4ω background that the authors state is ~40× larger (SI Sec. VI B, Fig. S6e,f). The symmetrization procedure cancels the ∂S^(1)/∂T·ΔT·∇T term only if, after 2ω normalization, the two heaters produce equal ΔT as well as equal |∇T|. The 2ω normalization constrains only |∇T|; it does not constrain ΔT. In the NLN geometry this is worse: the reported 2ω signal does not reverse sign when the active heater is swapped (Fig. S7c), so it is dominated by a parasitic longitudinal Seebeck from ∇bT, not by the ∇aT that drives S^(2)_baa. The raw 4ω also does not reverse sign (Fig. S7d) — exactly the case the SI warns 'does not allow one to unambiguously attribute' the signal to intrinsic nonlinear thermoelectric effects. The paper's own SI flags this failure mode, then addresses it by asserting ∇aT ~ 10×∇bT and by relying on estimated ∂S/∂T and heater-resistance backgrounds. But the residual after such an estimate is not a direct symmetrization; a few-percent error in the ΔT/∇T ratio or in the parasitic-gradient correction is comparable to the reported intrinsic signal. Thus the central claim that all three symmetry-allowed components are observed is not equally secure: the NLS result is more robust, but NLN (and possibly NLMT) rests on a model-subtracted residual.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports the observation of intrinsic second-order thermoelectric responses in exfoliated thin T_d-WTe2 and TaIrTe4 using harmonic detection. A 2ω linear Seebeck/Nernst response and a 4ω response quadratic in |∇T| are measured in three device geometries corresponding to the three symmetry-allowed components: nonlinear Seebeck S^(2)_bbb, nonlinear Nernst S^(2)_baa, and nonlinear mixed-directional S^(2)_aab. The 4ω response is symmetrized with respect to reversing the thermal gradient by swapping the active heater electrode. The authors also report the temperature dependence, a scaling analysis S^(2)=Aσ²+C to assign skew-scattering vs Berry-curvature contributions, null tests in one forbidden orientation, and frequency/DC-offset/capacitive-coupling controls. The central claim is that intrinsic nonlinear thermoelectricity persists to room temperature in zero magnetic field.","tokens_in":32323,"tokens_out":12491,"duration_ms":131513,"significance":"If established, this is a significant advance: it extends nonlinear Hall physics to thermal transport and shows that reduced crystal symmetry alone can produce nonreciprocal thermoelectric response at room temperature. The paper's strengths include the clear |∇T|² scaling of the 4ω signals, a null result in a forbidden configuration, an unusually transparent SI that quantifies the temperature-dependent linear background, and a broad set of control experiments (frequency dependence, dc-offset exclusion, harmonic-content checks). The main risk is the extraction of S^(2)_baa in the NLN configuration, where the symmetrization normalization channel does not track the gradient that drives the intrinsic response; this issue is flagged in the SI itself and needs an independent calibration before the 'all symmetry-allowed components' claim is fully secure.","major_comments":[{"comment":"In the NLN geometry, the symmetrization of SI Sec. VI B does not satisfy its own precondition. The 2ω normalization signal (Fig. S7c) is V_b=-S^(1)_bb L∇bT, a parasitic longitudinal Seebeck that does not reverse on heater swap; it calibrates only |∇bT|, not the |∇aT| that drives S^(2)_baa or the ΔT difference between heaters. The raw 4ω also does not reverse (Fig. S7d), the case the SI states cannot be unambiguously attributed to intrinsic nonlinearity. The subsequent defense (∇aT≈10∇bT; distinct T-dependence) is an estimate. With the temperature-induced background ~40× larger than the intrinsic signal (Fig. S6e,f), a few-percent heater asymmetry yields a residual of the size of the reported S^(2)_baa. An independent in-plane temperature profile or a phase-shift dual-heater measurement is required.","section":"SI Sec. VI B; Fig. S7"},{"comment":"The main text states that no detectable response is found in symmetry-forbidden configurations, but SI Sec. VII shows a null result only for the S^(2)_abb orientation; for S^(2)_bab the geometry is contaminated by allowed NLS/NLN terms and a pronounced 4ω response is observed (Eq. S19, Fig. S11). The wording should be qualified to avoid overstating the set of forbidden components experimentally tested.","section":"Main text 'Results and Discussion'; SI Sec. VII"},{"comment":"The microscopic assignment of the NLS/NLN coefficients from S^(2)_ijj = Aσ²_ii + C is underdetermined by the data shown. SI Sec. XI admits that a linear-in-σ scaling also gives a reasonable description, and the linear term is excluded by invoking theory (Ref. [25]) that intrinsic τ-linear terms cannot contribute to the NLS. With the limited low-temperature window and two-parameter fits, this does not uniquely 'demonstrate' the skew-scattering/Berry-curvature decomposition; the abstract and conclusions should be softened to 'consistent with' unless additional data or an independent mechanism test is provided.","section":"Main text scaling analysis; SI Sec. XI"}],"minor_comments":[{"comment":"The sentence introducing the Nernst calibration (V_b = ν_ba B_c L∇aT) is confusing because the NLN measurements are at zero field; clarify whether this is a separate finite-field calibration and how it is used to determine the zero-field ∇aT.","section":"SI Sec. VI B"},{"comment":"The caption says 'second-harmonic voltage (c) and the fourth-harmonic voltage (f)'; panel (f) is the symmetrized fourth-harmonic, so the raw fourth-harmonic is likely panel (d). Please correct the panel callouts.","section":"Fig. S6 caption"},{"comment":"There are several typographical issues in the SI, e.g., 'T emperature', 'Frquency', 'dunction', 'resitance'. These should be corrected.","section":"SI headings and text"}],"recommendation":"major_revision","confidential_remarks":"The NLN extraction is the main risk. The SI itself identifies the failure mode of a non-reversing 4ω response, and the paper's response to that concern is an estimate rather than a direct measurement. I am not recommending reject because the issue is local and fixable with additional calibration or re-analysis, and the NLS/NLMT observations appear substantially more robust. The mechanistic scaling interpretation should be tempered in the abstract and conclusions."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The paper reports something genuinely new: second-order thermoelectric responses in zero magnetic field in thin WTe2 and TaIrTe4, all three mirror-allowed in-plane components (NLS, NLN, NLMT), persisting to room temperature. That is a first, and the qualitative case is stronger than most first-claims: the 4ω response scales as |∇T|², the symmetry-forbidden geometry shows nothing, and the controls (frequency dependence, capacitive coupling, dc offset, heater resistance) are handled carefully. The dual-heater gradient reversal is a sensible adaptation of the established trick. The SI is unusually honest — it explicitly flags the failure mode the stress-test worries about, admits the coefficients are lower bounds, and acknowledges the sign ambiguity. The symmetry framework and the forbidden-configuration null are solid.\n\nThe soft spots are concentrated in one place, and the stress-test note is right that it is load-bearing. The symmetrization meant to kill the ∂S/∂T·ΔT·∇T background requires the 2ω normalization to track the relevant gradient component for each heater. In the NLN geometry it does not: the 2ω signal there is a non-reversing parasitic Seebeck from ∇bT, and the raw 4ω also fails to reverse on heater swap — the SI's own warning case. The authors rescue the claim by estimating ∇aT ≈ 10∇bT and subtracting the parasitic background. That could be right, but a few percent error in a ~40× background is the same order as the reported intrinsic signal. The NLN coefficient — and possibly NLMT, which has the same parasitic-gradient sensitivity — is thus an estimate rather than a direct observation. The NLS result, where the 2ω channel does what it should, is on firmer ground. Add the acknowledged COMSOL overestimate (magnitudes are lower bounds), the two-parameter Aσ²+C mechanism fit over a narrow window with an alternative scaling that also fits, and no raw data: the microscopic attribution is suggestive, not probative.\n\nThis is for anyone working on nonlinear Hall or thermoelectric transport in low-symmetry materials, and for experimentalists who want a template for harmonic thermal measurements. It deserves a serious referee; the referee should push hardest on the NLN separation, ideally asking for an independent measurement of the in-plane temperature profile or the raw data. My verdict matches the reader's: the qualitative first-observation claim is likely right; the quantitative NLN value is not yet nailed down.","headline":"Genuinely new — first room-temperature intrinsic nonlinear thermoelectric effects with all three symmetry-allowed tensor components — but the NLN signal is a model-subtracted residual and the symmetrization is the load-bearing seam.","tokens_in":32942,"tokens_out":8476,"would_cite":true,"duration_ms":80925,"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":"This paper reports the first observation of intrinsic nonlinear thermoelectric effects—nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional—in zero magnetic field in thin WTe2 and TaIrTe4, persisting to room temperature.","keywords":["nonlinear thermoelectric effects","nonlinear Seebeck effect","nonlinear Nernst effect","second-order thermoelectric tensor","Berry curvature dipole","skew scattering","WTe2","TaIrTe4"],"falsifier":"Measure the actual in-plane temperature profile in the device, for example with scanning thermal microscopy or local resistive thermometers patterned along the flake, and check that the temperature difference between the two heater configurations is antisymmetric to within the claimed precision; a measured asymmetric component would directly contaminate the symmetrized 4ω signal. Alternatively, a control measurement on a centrosymmetric material with identical heater geometry should show a vanishing symmetrized 4ω response.","tokens_in":1482,"feed_emoji":"🔥","tokens_out":1971,"duration_ms":67604,"temperature":0.7,"pith_summary":"This paper reports that intrinsic nonlinear thermoelectric effects—responses that do not reverse when the driving temperature gradient is reversed—can arise purely from low crystal symmetry and persist to room temperature. In thin flakes of the type-II Weyl semimetals WTe2 and TaIrTe4, where only a single mirror plane remains, all three symmetry-allowed components of the second-order thermoelectric tensor are observed at zero magnetic field: the nonlinear Seebeck, nonlinear Nernst, and nonlinear mixed-directional effects. The key is a heater-reversal symmetrization that separates the intrinsic fourth-harmonic signal from the far larger temperature-dependent linear Seebeck background. Temperature- and conductivity-scaling analysis indicates skew scattering dominates the nonlinear Seebeck effect, while the nonlinear Nernst effect contains both Berry-curvature-dipole and skew-scattering contributions. If correct, this establishes nonlinear thermoelectricity as a symmetry-governed, material-intrinsic route to thermal-to-electrical rectification beyond engineered structures.","feed_headline":"Nonlinear thermoelectric effects observed at room temperature","feed_subtitle":"Symmetry alone lets two semimetals rectify heat into voltage with no magnetic field or engineered asymmetry.","key_machinery":"The central object is the third-rank second-order thermoelectric tensor S^(2)_ijk linking electric field to products of temperature-gradient components. The carrying mechanism is harmonic detection: Joule heating at frequency ω creates a temperature gradient at 2ω, so linear thermoelectric responses appear at 2ω and intrinsic nonlinear ones at 4ω. The essential trick is symmetrizing the 4ω response under swapping the active heater (reversing the in-plane gradient), which removes the much larger contribution from the temperature dependence of the linear Seebeck coefficient and of the heater resistance. A scaling relation S^(2)=Aσ²+C, with σ the longitudinal conductivity, separates skew-scatte","core_discovery":"The paper claims that in exfoliated thin flakes of T_d-WTe2 and TaIrTe4 the effective space group is Pm, with only the bc mirror plane. Under this symmetry, the in-plane second-order thermoelectric tensor has exactly three nonzero independent components: S^(2)_bbb, S^(2)_baa, and S^(2)_aab. Using dual-heater devices and lock-in detection, the authors observe all three at zero magnetic field, with clear |∇T|^2 scaling in the symmetrized fourth-harmonic voltage, and no detectable response in symmetry-forbidden orientations. The coefficients remain on the order of 10^-6 µV K^-2 cm up to room temperature. The authors further argue that the NLS coefficient scales as σ^2 with vanishing intercept,","pith_inferences":["An immediate testable extension would be gated devices: if the Berry-curvature-dipole term is present, tuning the Fermi level should change the intercept C of the σ² scaling while leaving the skew-scattering slope largely unchanged.","Because the temperature-dependent linear Seebeck background is roughly 40 times larger than the intrinsic signal, any fourth-harmonic thermoelectric measurement that does not report a simultaneous second-harmonic trace and heater-swap symmetrization should be treated with caution; this paper makes that point for its own data, but the implication extends to reinterpreting earlier reports.","If the intrinsic origin holds, the same symmetry rules predict vanishing nonlinear response in centrosymmetric controls and characteristic anisotropy patterns in other low-symmetry materials, offering a fast material-screening criterion.","The reported sign ambiguity of the coefficients under reversal of crystal axes could be resolved by combining polarized-Raman orientation with electrical Hall or piezoelectric response measurements, enabling quantitative sign comparison with theory."],"forward_implications":["If confirmed, nonlinear thermoelectric rectification becomes an intrinsic property of low-symmetry crystals, so it should appear in any material where inversion is broken and only a mirror plane survives.","The observed room-temperature magnitudes in simple exfoliated flakes suggest practical thermal sensors and energy harvesters that require no magnetic field or engineered asymmetry.","Because the nonlinear Seebeck effect is dominated by skew scattering, controlling disorder and scattering processes becomes a direct lever on its magnitude.","The nonlinear Nernst effect shares a symmetry origin with the nonlinear Hall effect, allowing thermoelectric and electrical nonlinear responses to be compared quantitatively in the same materials.","The method extends naturally to higher-order thermoelectric tensors, since deviations from the quadratic-in-∇T response appear at increased heater power."],"fun_headline_variants":["Semimetals rectify heat without magnets at 300 K","Nonlinear thermoelectric effect seen at room temp","Symmetry alone enables heat-to-voltage in semimetals","Zero-field nonlinear heat-to-charge conversion shown"],"cache_read_input_tokens":34048,"weakest_assumption_plain":"The result depends on the assumption that the two heaters create identical temperature patterns except for the direction of the heat flow, so swapping which heater is on cleanly cancels the large ordinary thermoelectric background; no measurement of the actual temperature profile inside the sample is reported.","fun_headline_variants_meta":{"raw":{"variants":["Semimetals rectify heat without magnets at 300 K","Nonlinear thermoelectric effect seen at room temp","Symmetry alone enables heat-to-voltage in semimetals","Zero-field nonlinear heat-to-charge conversion shown"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000134,"raw_usage":{"total_tokens":969,"prompt_tokens":731,"completion_tokens":238,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":475,"completion_tokens_details":{"reasoning_tokens":175}},"tokens_in":475,"tokens_out":238,"duration_ms":3192,"temperature":1.0,"reasoning_tokens":175,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-01T06:36:45.879173+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the actual in-plane temperature profile in the device, for example with scanning thermal microscopy or local resistive thermometers patterned along the flake, and check that the temperature difference between the two heater configurations is antisymmetric to within the claimed precision; a measured asymmetric component would directly contaminate the symmetrized 4ω signal. Alternatively, a control measurement on a centrosymmetric material with identical heater geometry should show a vanishing symmetrized 4ω response.","supporting_citations":[],"review_version":1}