{"id":"8f0bf295-d074-4498-be8a-d31978ac8a3e","arxiv_id":"2502.08928","paper_version":2,"verdict":"ACCEPT","confidence":"HIGH","novelty_score":6.0,"correctness_risk":"low","formal_verification":"none","parameter_count":2,"one_line_summary":"The apparent nonreciprocal transport seen in bulk FeSe is an artifact of Joule heating at current contacts acting through the thermoelectric effect, not an intrinsic material property.","lead":"Measurements of second-harmonic resistance in bulk FeSe crystals initially looked like nonreciprocal transport, but the signal turned out to come from Joule heating at the electrical contacts combined with FeSe's large thermoelectric effect. The paper is a cautionary result: apparent directional transport can arise from contacts rather than the material, and thermoelectric effects could be engineered to create superconducting diodes.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"No significant objection identified: thermal-artifact case is internally consistent and well controlled.","rationale":"The reader's verdict is ACCEPT with high confidence, and I agree. The reader's weakest_assumption is the unmeasured temperature gradient and thermoelectric coefficients. I view this as a genuine quantitative uncertainty but not as a load-bearing threat to the central claim, because the qualitative conclusion is anchored by multiple independent controls: superfluid helium suppression, contact reversal sign change, contact-resistance correlation, and frequency-dependent phase delay. The estimate in Section V is a plausibility argument, not the sole pillar; even conservative literature values for FeSe are sufficient, so a factor-of-several reduction in Delta-T or S would not overturn the mechanism assignment. Thus I would keep the ACCEPT verdict and note that direct thermometry would be a valuable confirmation but is not necessary for the paper's main message. A non-finding is the honest outcome here: the evidence for a contact-heating/thermoelectric artifact is strong and internally consistent.","tokens_in":12987,"tokens_out":12071,"duration_ms":126467,"concrete_test":"Perform a dedicated thermometry and thermoelectric characterization on the same type of crystal and contact configuration: mount a small thermometer (e.g., a chip resistor or AuFe-Chromel thermocouple) near a high-resistance current contact, measure the local temperature rise at I = 3 mA and 30 K in the same environment that produced the large R2, and separately measure the Seebeck and Nernst coefficients of the crystal. Compute the expected R2 from the measured Delta-T and S/ν; if the predicted magnitude and sign match the observed R2 within a factor of a few, the thermoelectric attribution is quantitatively confirmed.","verdict_should_be":"UNCHANGED","load_bearing_attack":"After close reading, no load-bearing concern disrupts the central claim that the observed second-harmonic resistance in bulk FeSe arises from joule heating at a current contact and the thermoelectric effect. The strongest evidence is not the order-of-magnitude estimate but the controlled phenomenology: R2 is almost zero when the sample is immersed in superfluid helium, jumps at the lambda point, grows when the sample is no longer surrounded by liquid helium, shows a frequency-dependent phase delay consistent with a thermal time constant, reverses sign when current and voltage contacts are exchanged, and is drastically reduced when the high-resistance contact is removed from the current path (samples #1 and #4). These observations would be very difficult to explain by any intrinsic bulk nonreciprocal transport, especially because FeSe is centrosymmetric. The quantitative estimate in Section V indeed assumes a 1 K local temperature difference and uses literature thermoelectric coefficients, but the assumption is not fragile: even the lower literature values (Seebeck of order -10 uV/K, Nernst of order 0.9 uV/K/T) exceed what is needed for the observed R2, and the estimated average sample temperature rise (0.06-0.23 K) makes local hot spots of order 1 K plausible. The absence of direct thermometry and same-sample Seebeck/Nernst measurements is a limitation but not a load-bearing one; the qualitative conclusion stands on the controls.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports low-frequency ac first- and second-harmonic resistance measurements and dc I-V measurements on bulk FeSe single crystals, together with contact-resistance characterization and field-angle/current-frequency variation. The authors first observe second-harmonic resistances that mimic nonreciprocal transport, with both a symmetric part and an antisymmetric part linear in field and current. They then present a series of controls — near-zero R2 in superfluid helium, a jump at the lambda point, an increase when the sample is no longer surrounded by liquid helium, a frequency-dependent phase delay, sign reversal when current and voltage contacts are exchanged, and a correlation between R2 and contact resistance — and conclude that the apparent nonreciprocal transport is not intrinsic to the bulk crystal but arises from Joule heating at a current contact combined with the thermoelectric effect. The paper explicitly supports the interpretation of the zero-field FeSe superconducting diode effect in the recent preprint of Nagata et al. as thermoelectric in origin.","tokens_in":13171,"tokens_out":8050,"duration_ms":94501,"significance":"If correct, the paper is an important cautionary result: second-harmonic resistance measurements, including antisymmetric-in-field components, are not by themselves a reliable diagnostic of broken space-inversion symmetry or nonreciprocal transport. The manuscript's central claim is supported by multiple independent controls rather than by a single fitting procedure, which is a genuine strength: the superfluid-helium experiment, the contact-swapping sign reversal, the frequency-dependent phase delay, and the cross-sample correlation with contact resistance each point to a thermal/contact artifact. The paper also provides a concrete protocol for distinguishing such artifacts in future experiments and connects its result to the independent zero-field superconducting diode report in FeSe flakes. The main limitation is that the quantitative Section V estimate assumes a local temperature difference and uses literature thermoelectric coefficients rather than measuring them on the same crystals; this affects the magnitude of the proposed mechanism but not the qualitative artifact conclusion.","major_comments":[],"minor_comments":[{"comment":"The order-of-magnitude estimate that explains the observed R2 through the thermoelectric effect assumes a local temperature difference of about 1 K between the voltage contacts and uses Seebeck and Nernst coefficients from the literature, in particular ref. [10], rather than from direct measurements on the same crystals. The authors should state more explicitly that this is a plausibility estimate; the qualitative conclusion that the signal is a thermal-contact artifact is already firmly established by the superfluid-helium, contact-reversal, and frequency-response controls, but the specific thermoelectric mechanism is not directly metrologically confirmed.","section":"Section V"},{"comment":"The interpretation of the jump at T = 4.2 K relies on the sample 'becoming no longer surrounded by liquid helium'; the text should clarify the precise thermal environment change (for example, liquid level falling below the sample versus a transition to exchange-gas cooling), since this distinction is central to the heat-transfer argument.","section":"Section IV.A, Fig. 7"},{"comment":"The frequency dependence of V2^Ly and V2^Lx is presented qualitatively as evidence of a thermal phase delay. A fit to a simple thermal time-constant model, even with a single effective time constant, would make the argument more quantitative and would strengthen the identification of the second-harmonic response with Joule-heating-induced temperature oscillations.","section":"Section IV.A, Fig. 8"},{"comment":"The statement that sign reversal of Ra2 upon exchanging current and voltage contacts is 'difficult to explain if the second-harmonic resistance was intrinsic to sample bulk' is plausible but is not supported by a formal reciprocity or symmetry argument. The authors could add a short remark or reference explaining why intrinsic bulk second-harmonic response would be expected to survive current/voltage exchange in this geometry.","section":"Section IV.A"},{"comment":"The early data are described as compatible with the polar-structure expression R = R0(1 + beta B^2 + gamma I·(P x B)), but FeSe is centrosymmetric. It would be helpful to state explicitly in this section that this compatibility is only formal and does not imply that FeSe is polar; the later thermal-artifact discussion already makes this clear, but the early framing could mislead a reader.","section":"Section III, Eq. (2)"}],"recommendation":"minor_revision","confidential_remarks":"This is a solid experimental artifact study with a well-supported negative conclusion. The shared authorship with the thermoelectric-coefficient reference [10] is not a circularity problem because that reference is used only as a plausibility input for an order-of-magnitude estimate; the central evidence is the controlled phenomenology. The manuscript fits the journal's scope and is likely to be a useful reference for anyone using second-harmonic resistance to search for nonreciprocal transport. The minor comments are about framing and additional quantitative support; they do not affect the main conclusion."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The one thing to know: this paper shows that the second-harmonic resistance in bulk FeSe is not a bulk nonreciprocal effect but a contact-heating artifact. The evidence is convincing and the paper is worth reading for anyone who measures nonlinear transport. It extends the flake result of Nagata et al. to bulk crystals with a set of controls that go beyond the prior work: the signal vanishes below the superfluid lambda point, reverses sign when current and voltage contacts are swapped, correlates with contact resistance, and shows a frequency-dependent phase delay consistent with a thermal response. The cleanest diagnostic is the sign reversal of the antisymmetric part, which would be very hard to explain by any intrinsic bulk mechanism.\n\nWhat the paper does well: it presents the data honestly, first showing results that look like genuine nonreciprocal transport, then systematically dismantling that interpretation. The ac/dc consistency checks and the estimates of average temperature rise are careful. The order-of-magnitude estimate in the discussion is appropriately rough and does not overclaim; the authors make clear that the local temperature gradient is not measured directly.\n\nThe soft spots are real but not load-bearing. There is no direct thermometry on the measured crystals, and the quantitative estimate assumes a 1 K local temperature difference and literature values for the Seebeck and Nernst coefficients. The paper acknowledges this. The use of ref. [10], which shares authors, as a source for large thermoelectric coefficients is a minor concern, but the conclusion does not rest on that coefficient value; it rests on the controlled phenomenology. The sample count is small (three usable crystals), yet the internal logic is strong and the controls are varied enough to make the artifact interpretation convincing.\n\nThis paper is for experimentalists working on nonreciprocal transport, superconducting diodes, and any field where second-harmonic resistance is used as a symmetry diagnostic. It deserves a serious referee and, after minor revision, publication. The main revision I would ask for is a clearer statement of the unmeasured quantities and, if possible, the raw data behind the key figures.","headline":"A careful experimental demonstration that apparent nonreciprocal transport in bulk FeSe is a joule-heating/thermoelectric artifact; strong controls make the qualitative conclusion robust.","tokens_in":13743,"tokens_out":1603,"would_cite":true,"duration_ms":17733,"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":"Bulk FeSe's second-harmonic resistance, a signal often read as nonreciprocal transport, is argued to be a Joule-heating artifact at the current contacts acting through FeSe's thermoelectric response.","keywords":["nonreciprocal transport","second-harmonic resistance","Joule heating","thermoelectric effect","FeSe","superconducting diode","Nernst effect","contact resistance"],"falsifier":"Directly measure the temperature difference between the two voltage contacts while 3 mA of alternating current flows, and measure the Seebeck and Nernst coefficients of the same crystal. If the contact-to-contact gradient is far below the assumed 1 K, or the coefficients are much smaller, the thermoelectric explanation cannot account for the observed $R_2$ magnitude.","tokens_in":12762,"feed_emoji":"⚡","tokens_out":8893,"duration_ms":83478,"temperature":0.7,"pith_summary":"The authors report that the sizable second-harmonic resistance they measured in bulk FeSe crystals, which at first looks like nonreciprocal charge transport (resistance differing for $+I$ and $-I$), is not intrinsic to the crystal. The signal tracks the quality of the electrical contacts: it is large when a current contact has high resistance, nearly vanishes when contacts are good, flips sign when current and voltage contacts are swapped, and disappears when the sample is immersed in superfluid helium. They conclude that Joule heating at a current contact creates a temperature gradient, and FeSe's large thermoelectric response turns that into an $I^2$ voltage that masquerades as nonreciprocal transport. If correct, the result matters because it validates the thermoelectric explanation previously proposed for the zero-field superconducting diode effect in FeSe flakes, and it warns that second-harmonic resistance diagnoses of broken inversion symmetry must first exclude contact heating.","feed_headline":"FeSe nonreciprocal signal traced to contact Joule heating","feed_subtitle":"Swapping current and voltage contacts flips the second-harmonic signal, matching a thermoelectric artifact.","key_machinery":"The analytical core is the expansion $V = R_1I + R_2I^2 + R_3I^3$ and its decomposition into field-symmetric and antisymmetric parts. The load-bearing object is the second-harmonic resistance $R_2$: because a current contact dissipates Joule power proportional to $I^2$, the local temperature oscillates at twice the drive frequency, and FeSe's Seebeck and Nernst effects convert the resulting temperature gradient into a voltage at the same $I^2$ harmonic. The paper's diagnostics are contact reversal, which changes the temperature-gradient direction and hence the sign of $R_2$; immersion in superfluid helium, which suppresses the gradient; and the frequency-dependent phase lag of the lock-in signal, which follows the expected thermal-response behavior.","core_discovery":"The paper's central claim is that the nonzero second-harmonic resistance $R_2$ observed in bulk FeSe does not come from a genuine nonreciprocal transport effect. The authors show that $R_2$ (both the magnetic-field-symmetric part $R^s_2$ and the antisymmetric part $R^a_2$) correlates with contact resistance and sample heating: it was large for contact configurations containing a roughly 3 to 5 ohm contact, small or absent for low-resistance configurations, changed sign when current and voltage leads were exchanged, and collapsed below the superfluid helium $\\lambda$ point where heat exchange is strongest. They attribute the effect to Joule heating at the current contact producing a temperature gradient, which FeSe's large Seebeck and Nernst coefficients convert into a voltage proportional to $I^2$; the frequency-dependent phase lag of the second-harmonic signal matches this thermal picture. This supports the interpretation that the zero-field superconducting diode effect in FeSe flakes described in ref. [6] is thermoelectric rather than intrinsic.","pith_inferences":["Beyond the paper: the same contact-heating pathway should produce apparent nonreciprocal signatures in any material with large Seebeck and Nernst coefficients, so published second-harmonic data on small-Fermi-energy semimetals may need re-examination even when contact reversal was not performed.","Beyond the paper: the frequency-dependent phase lag of $R_2$ could be turned into a quantitative thermal diagnostic, since fitting the quadrature-to-in-phase crossover would give a local thermal time constant for the contact-sample system.","Beyond the paper: the paper's suggestion that thermoelectric gradients could be used deliberately to build superconducting diodes implies a testable device concept: pattern asymmetric contacts or a small heater on a superconductor with large thermoelectric response and measure the direction-dependent critical current."],"forward_implications":["Second-harmonic resistance in bulk FeSe should not be read as evidence for intrinsic nonreciprocal transport; contact quality and thermal anchoring control its magnitude and sign.","Reversing current and voltage contacts should flip the antisymmetric second-harmonic signal when the artifact dominates, as observed in two of the samples.","Measurements in superfluid helium or with low-resistance contacts suppress the artifact, so a nearly vanishing $R_2$ under those conditions is a practical test for intrinsic origin.","The zero-field superconducting diode effect reported in FeSe flakes is more plausibly thermoelectric in origin, consistent with the field-free diode signal seen in the bulk crystal.","Diagnoses of broken space-inversion symmetry based on second-harmonic resistance need to exclude thermoelectric contamination, especially in materials with small Fermi energy."],"supporting_citations":[{"why":"Reports the zero-field superconducting diode effect in FeSe flakes and attributes it to the thermoelectric effect; it is the motivating comparison the present result supports.","marker":"[6]"},{"why":"Supplies the large Seebeck and Nernst coefficients of FeSe used in the order-of-magnitude estimate.","marker":"[10]"},{"why":"Provides the symmetry-based framework for nonreciprocal transport that motivates the second-harmonic measurement.","marker":"[12]"},{"why":"Extends the framework to magnetoelectric anisotropy and grounds the field-symmetrization analysis.","marker":"[13]"},{"why":"Describes the thermal response of a contact that oscillates at twice the drive frequency when heated by an ac current.","marker":"[17]"},{"why":"Supplies the thermal-response formalism behind the observed frequency-dependent phase lag of the second-harmonic signal.","marker":"[18]"},{"why":"Reports the temperature dependence of the Seebeck coefficient in FeSe.","marker":"[19]"},{"why":"Reports the Nernst coefficient of FeSe.","marker":"[20]"}],"fun_headline_variants":["FeSe's apparent nonreciprocity is a thermoelectric artifact","Contact heating, not superconductivity, drives FeSe second harmonic","Thermoelectric heating explains FeSe's fake nonreciprocal signal","Joule heating at contacts creates FeSe's apparent diode effect","FeSe nonlinear transport is a thermal artifact from contacts"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The mechanism's size depends on an assumed local temperature difference of about 1 kelvin between the voltage contacts and on thermoelectric coefficients as large as the ones reported for FeSe in one cited study; neither quantity was directly measured on the crystals used here.","fun_headline_variants_meta":{"raw":{"variants":["FeSe's apparent nonreciprocity is a thermoelectric artifact","Contact heating, not superconductivity, drives FeSe second harmonic","Thermoelectric heating explains FeSe's fake nonreciprocal signal","Joule heating at contacts creates FeSe's apparent diode effect","FeSe nonlinear transport is a thermal artifact from contacts"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000721,"raw_usage":{"total_tokens":3211,"prompt_tokens":893,"completion_tokens":2318,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":509,"completion_tokens_details":{"reasoning_tokens":2232}},"tokens_in":509,"tokens_out":2318,"duration_ms":16710,"temperature":1.0,"reasoning_tokens":2232,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-07T23:11:08.173735+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Directly measure the temperature difference between the two voltage contacts while 3 mA of alternating current flows, and measure the Seebeck and Nernst coefficients of the same crystal. If the contact-to-contact gradient is far below the assumed 1 K, or the coefficients are much smaller, the thermoelectric explanation cannot account for the observed $R_2$ magnitude.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the temperature dependence of the Seebeck coefficient in FeSe."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the Nernst coefficient of FeSe."},{"cited_title":"Margadonna, Y","cited_arxiv_id":null,"evidence_quote":"Supplies the large Seebeck and Nernst coefficients of FeSe used in the order-of-magnitude estimate."},{"cited_title":"Kasahara, T","cited_arxiv_id":null,"evidence_quote":"Provides the symmetry-based framework for nonreciprocal transport that motivates the second-harmonic measurement."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Extends the framework to magnetoelectric anisotropy and grounds the field-symmetrization analysis."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Describes the thermal response of a contact that oscillates at twice the drive frequency when heated by an ac current."},{"cited_title":"Ideue, K","cited_arxiv_id":null,"evidence_quote":"Supplies the thermal-response formalism behind the observed frequency-dependent phase lag of the second-harmonic signal."}],"review_version":1}