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Apparent nonreciprocal transport in FeSe bulk crystals

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Pith's one-line read 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.

desk verdict A careful experimental demonstration that apparent nonreciprocal transport in bulk FeSe is a joule-heating/thermoelectric artifact; strong controls make the qualitative conclusion robust. read the letter →

arxiv 2502.08928 v2 pith:MOIDQCZD submitted 2025-02-13 cond-mat.supr-con cond-mat.mtrl-sci

Taichi Terashima , Shinya Uji , Yuji Matsuda , Takasada Shibauchi , Shigeru Kasahara This is my paper · ORCID
classification cond-mat.supr-concond-mat.mtrl-sci
keywords nonreciprocaltransportsecond-harmonicresistanceJouleheatingthermoelectriceffectFeSesuperconductingdiodeNernstcontact
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

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.

What carries the argument

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.

What would settle it

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.

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Extended reading notes

Core claim

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.

Load-bearing premise

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.

Editorial extensions

If this is right

  • 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.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • 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.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 5 minor

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.

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.

minor comments (5)
  1. [Section V] 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.
  2. [Section IV.A, Fig. 7] 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.
  3. [Section IV.A, Fig. 8] 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.
  4. [Section IV.A] 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.
  5. [Section III, Eq. (2)] 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.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the thermal-artifact conclusion rests on internal controls; the only same-author citation is a non-load-bearing magnitude check.

full rationale

The paper's central claim is that the observed second-harmonic resistance in bulk FeSe is not genuine nonreciprocal transport but results from Joule heating at a current contact acting through the thermoelectric effect. This conclusion is established by controlled phenomenology rather than by any derivation that assumes the result: R2 is almost zero 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 do not depend on the quantitative thermoelectric estimate in Section V. The only same-author citation is ref. [10], used to argue that large thermoelectric coefficients are plausible for FeSe. But that citation is not load-bearing: the paper also cites non-overlapping refs. [19] and [20], whose more moderate values (Seebeck of order -10 uV/K, Nernst of order 0.9 uV/K/T) already exceed the order-of-magnitude requirements estimated from the observed R2 (0.2 uV/K for the Seebeck contribution, and a small fraction of the Nernst voltage). The logical chain is therefore not circular: an external empirical magnitude is compared with a consistency estimate, and the central 'artifact' conclusion stands independently of that estimate. The unmeasured local temperature gradient is an assumption in the quantitative plausibility check, but it is not a fitted parameter renamed as a prediction, and it does not make the conclusion equivalent to its inputs. No self-citation chain, uniqueness argument, or ansatz-smuggling step is load-bearing. The paper is self-contained against its own controls, and the minor self-citation does not affect the central result.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central claim rests on two unmeasured quantitative inputs: a local temperature difference of about 1 K between voltage contacts and large thermoelectric coefficients from literature. Both are explicitly assumed in Section V for an order-of-magnitude estimate. The qualitative conclusion is supported by non-quantitative controls, so these inputs do not constitute fitted parameters in a predictive model.

free parameters (2)
  • Local temperature difference between voltage contacts = 1 K (assumed, not measured)
    Used in Section V to estimate the Seebeck voltage needed to explain R2; no thermometry was performed on the measured crystals.
  • Nernst coefficient = 2 µV/K/T (order-of-magnitude assumption)
    Used in Section V to estimate the Nernst contribution to Ra2; the value is not directly measured for these samples.
assumptions (4)
  • domain assumption The measured crystals have thermoelectric coefficients of the order of those reported in ref. [10] (large Seebeck and Nernst coefficients).
    Section V: the order-of-magnitude estimate relies on literature values; the present samples were not directly characterized for S and nu.
  • domain assumption A local temperature difference of order 1 K develops between the voltage contacts under measurement conditions.
    Section V: assumed to make estimates; no direct temperature-gradient measurement was made.
  • standard math The I-V response can be expanded as V = R1 I + R2 I^2 + R3 I^3.
    Section III: standard Taylor expansion used for harmonic analysis; not derived in this paper.
  • standard math Rikken et al.'s symmetry framework for nonreciprocal transport (Eqs. 1 and 2) is valid background.
    Section III: used to interpret Ra2; the paper does not re-derive the framework.

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Cite this review

Pith. "Pith review of Apparent nonreciprocal transport in FeSe bulk crystals." pith.science (2026). https://pith.science/paper/MOIDQCZD

@misc{pith2026250208928,
  author       = {Pith},
  title        = {Pith review of: Apparent nonreciprocal transport in FeSe bulk crystals},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MOIDQCZD}},
  note         = {Machine review of arXiv:2502.08928}
}
abstract

We performed low-frequency ac first- and second-harmonic resistance measurements and dc $I-V$ measurements on bulk FeSe crystals in a temperature range between 1.8 and 150 K and in magnetic field up to 14 T. We observed considerable second-harmonic resistance, indicative of nonreciprocal charge transport, in some samples. By examining correlation between contact resistances and second-harmonic signals, we concluded that the second-harmonic resistance was not due to the genuine nonreciprocal transport effect but was caused by joule heating at a current contact through the thermoelectric effect. Our conclusion is consistent with a recent preprint (Nagata \textit{et al.}, arXiv:2409.01715), in which the authors reported a zero-field superconducting diode effect in devices fabricated with FeSe flakes and attributed it to the thermoelectric effect.

Figures

Figures reproduced from arXiv: 2502.08928 by the authors.

Figure 1
Figure 1. FIG. 1. FeSe samples mounted on a rotation platform at [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. Ac resistance versus temperature measurements on [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Second-harmonic resistance versus magnetic field at [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Dependence of [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 7
Figure 7. Figure 7: FIG. 7. First- and second-harmonic resistance ( [PITH_FULL_IMAGE:figures/full_fig_p005_7.png]
Figure 10
Figure 10. Figure 10: FIG. 10. (a) First- and (b) second-harmonic resistance as a [PITH_FULL_IMAGE:figures/full_fig_p006_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. Dc [PITH_FULL_IMAGE:figures/full_fig_p007_11.png]

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