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REVIEW 4 major objections 5 minor 34 references

Planckian scattering and parallel conduction channels in the iron chalcogenide superconductors FeTe$_{1-x}$Se$_x$

T0 review · 4 major / 5 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read In FeTe1−xSex films, a slow conduction channel scatters at the Planckian rate ~3kT/h and supplies the superfluid condensate.

desk verdict Solid THz study finding a hidden Planckian channel in FTS, but the central decomposition is not tested against alternatives; deserves serious refereeing. read the letter →

arxiv 2505.00623 v1 pith:GPGNBWOL submitted 2025-05-01 cond-mat.supr-con cond-mat.str-el

Ralph Romero III , Hee Taek Yi , Seongshik Oh , N. P. Armitage This is my paper · ORCID
classification cond-mat.supr-concond-mat.str-el
keywords ironchalcogenidesuperconductorsFeTe1-xSexPlanckianscatteringterahertzspectroscopyparallelconductionchannelssuperfluiddensitynon-Fermiliquidlinear-in-Tresistivity
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

This paper reports terahertz conductivity measurements on two FeTe1−xSex films and decomposes the spectra into two parallel conduction channels. One channel is broad in frequency and nearly temperature-independent; the other is a sharper Drude channel whose scattering rate grows linearly with temperature, Γ ≈ 3 kT/h, the Planckian limit. By comparing the spectral weight of the Drude term with the superfluid density from the Ferrell–Glover–Tinkham sum rule, the authors show that the superconducting condensate draws almost entirely from this Planckian-scattered channel. The finding ties the anomalous linear-in-T scattering to the pairing condensate in an iron-chalcogenide superconductor.

What carries the argument

The central object is the additive two-channel conductivity model $\tilde{\sigma}(\nu)=S/(\Gamma-i\nu)+\sigma_0+\frac{\pi}{2}S_\delta\delta(\nu=0)+iS_\delta/\nu-i\epsilon_0(\epsilon_\infty-1)\nu$. The Drude term supplies the slow channel, with spectral weight $S$ and scattering rate $\Gamma$; the frequency-independent $\sigma_0$ encodes the fast channel with $\Gamma\gg$ the measured range. Combining the parallel channels gives $\sigma_{DC}=\sigma_0+S/\Gamma$, which with $\Gamma=\alpha kT/h$ yields $\rho_{DC}=1/(a+b/T)$, matching DC data from 200 K to 25 K. The superfluid weight $S_\delta$ is computed via the FGT sum rule $S_\delta=S_N-S_U$ and compared with $S$ measured just above the transition.

What would settle it

A concrete falsifier would be a re-analysis of the same raw σ(ν) data using a generalized Drude model with a frequency-dependent scattering rate 1/τ(ω) but no constant offset. If such a fit reproduces the spectra at all temperatures as well as the two-channel model does, then the claim that there are two parallel channels, and hence the assignment of the condensate to a Planckian channel, would be called into question. Alternatively, extending the measurement window to higher frequencies where σ0's own Drude roll-off would appear (if it has a finite scattering rate) would directly confirm or rule out the fast channel.

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Extended reading notes

Core claim

The authors find that the measured THz conductivity of FeTe0.55Se0.45 and FeTe0.65Se0.35 films cannot be captured by a single Drude response across the whole temperature range. They fit the spectra with a model consisting of a Drude term, a constant (frequency-independent) offset σ0, a superfluid delta-function term, and a lattice polarizability ϵ∞=4, constrained by DC resistivity. The Drude scattering rate extracted from fits between 20 and 40 K is linear in temperature, Γ ≈ 3.0 kT/h for the optimally doped sample and Γ ≈ 2.4 kT/h for the overdoped sample, i.e., of the Planckian form. The constant offset represents a fast relaxation channel with scattering rate far above the measurement window. The superfluid density, obtained three independent ways (two-fluid fits, σ2ν extrapolation, and the FGT sum rule), saturates at a level that just matches the normal-state spectral weight of the Drude channel, showing that the charge carriers undergoing Planckian scattering are the principal contributors to the condensate, while the fast channel stays largely unaffected by the transition.

Load-bearing premise

The assumption that the measured spectra are exactly the sum of a Drude term, a constant σ0, and superfluid terms—so the two channels are truly independent—is load-bearing; if a single frequency-dependent scattering rate could reproduce the same data, the extracted Planckian scaling would be an artifact of the fit.

Editorial extensions

If this is right

  • The linear-in-T scattering rate of the slow channel implies that the normal-state resistivity of FTS is non-Fermi-liquid, even though the total resistivity looks curved; the T-linear component is hidden beneath a temperature-independent offset.
  • The match between the condensate spectral weight and the Drude channel weight implies that the same degrees of freedom producing Planckian scattering are the ones that pair to form the superfluid.
  • The fast channel, represented by σ0, is essentially unaffected by superconductivity and does not contribute to the condensate, pointing to two separate conduction populations.
  • The observed superfluid density ratio Sδ/SN of ~0.53 (x=0.45) and ~0.30 (x=0.35) is consistent with the nematic-fluctuation-mediated superconductivity prediction with impurity scattering.
  • The presence of in-gap spectral weight at low temperatures is reminiscent of cold-spot excitations predicted for a nematic quantum critical point.

Reading between the lines

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

  • If the two-channel picture is correct, then the linear-in-T scattering rate is a property of only a subset of carriers; a single generalized-Drude analysis that assumes a common scattering rate for all carriers would average the two channels and might mistakenly conclude the material is a conventional Fermi liquid at low T.
  • The identification of the slow channel with the condensate suggests a testable prediction: in FTS samples where the Planckian channel's spectral weight is suppressed (e.g., by disorder), the superfluid density should decrease proportionally.
  • The parallel-channel decomposition could be extended to other iron-based superconductors and to the cuprates; looking for a σ0-like fast channel in their THz spectra may reveal whether the Planckian component is generically paired with the condensate across strange metals.
  • A lower-frequency probe, such as microwave or muon spin rotation, could directly resolve the fast channel's own scattering rate and its temperature dependence, sharpening the claim that it is temperature-independent.
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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

4 major / 5 minor

Summary. The manuscript reports time-domain terahertz spectroscopy (TDTS) measurements from 0.2 to 3 THz on two FeTe1−xSex thin films with x = 0.45 and x = 0.35. The authors fit the complex conductivity with a model consisting of a Drude term, a frequency-independent offset σ0, a superfluid delta function, and a lattice background. From these fits they extract a Drude scattering rate Γ that grows linearly with temperature between 20 K and 40 K, with slopes α ≈ 3.02 (x = 0.45) and α ≈ 2.42 (x = 0.35) in units of kT/h. They determine the superfluid density via three methods—two-fluid fits, quadratic σ2ν extrapolation, and a Ferrell-Glover-Tinkham sum rule—and show that the DC resistivity is well described by ρ = 1/(a + b/T). The central claim is that the THz response contains two parallel conduction channels: a fast-relaxing channel giving the frequency-independent σ0 and a slower Drude channel whose scattering rate is Planckian, Γ ≈ 3 kT/h. The authors further claim that the superconducting condensate spectral weight is drawn mainly from the Planckian channel rather than from the fast background channel.

Significance. If the proposed decomposition is unique, this work would provide a striking observation of a resolved Planckian scattering channel in an iron-chalcogenide superconductor and a direct correlation between the spectral weight of that channel and the superfluid density. The paper has several strengths: it uses phase-sensitive TDTS to obtain both components of σ without Kramers-Kronig processing; it checks the optical model against measured DC resistivity; and it obtains Sδ by three independent methods whose temperature dependences are broadly consistent. However, the significance is currently conditional on model uniqueness. Because the spectral window is only 0.2–3 THz, a Drude term with a scattering rate moderately above 3 THz closely mimics a real constant with a small imaginary part, so the distinction between a true frequency-independent offset and a broad, finite-width conduction channel is not demonstrated. The paper provides no alternative fits, no statistical model comparison, and no uncertainty estimates for the extracted parameters, all of which are load-bearing for the Planckian and condensate-weight claims.

major comments (4)
  1. [Model equation after Fig. 2; fits in Fig. 2] The additive decomposition σ~(ν) = S/(Γ−iν) + σ0 + (π/2)Sδ δ(ν=0) + iSδ/ν − iϵ0(ϵ∞−1)ν is the load-bearing assumption of the paper, but the authors do not test whether the 0.2–3 THz data require this form over alternative models. A Drude term with a scattering rate only moderately above the upper edge of the window (for example, 5–10 THz) approximates a real constant with a small imaginary part, and that imaginary part can be absorbed into the other terms, so Γ(T), σ0(T), and S(T) are not pinned uniquely. I request fits with (i) two finite-width Drude terms and no offset, (ii) a generalized Drude model with frequency-dependent 1/τ(ω), and (iii) a multiband Drude sum with band weights constrained by independent measurements, together with a comparison of residuals or an information criterion. The claimed linearity of Γ(T) and the assignment of condensate weight to the 'slow' channel should then be shown to be robust across these competing models.
  2. [Fig. 3 and Fig. 4] The extracted parameters α, Γ0, S, σ0(T), and Sδ are reported as point values without uncertainties or goodness-of-fit statistics. This is especially problematic for the central comparison in Fig. 4b,d, where the claim that Sδ at low temperature 'just overshoots' S at Tc0+2K is made without error bars on either quantity. The Planckian slope α is also a fit parameter correlated with Γ0 and with the choice of the 20–40 K fitting interval. Please propagate uncertainties from the complex-conductivity fits, including systematic uncertainties in film thickness and substrate index, and report confidence intervals for α, Γ0, and Sδ.
  3. [DC resistivity section ('This perspective of parallel conduction channels')] The authors present the successful fit of the measured DC resistivity to ρ = 1/(a+b/T) as evidence for parallel conduction channels, but this test is not independent: a and b are derived from the same fitted S and Γ, and any optical model that gives σDC = σ0 + S/Γ with Γ ∝ T will reproduce the same DC functional form. To make the DC constraint meaningful, the authors should show that the measured ρ(T) is reproduced by the optical parameters without re-fitting, and that alternative optical decompositions fail this extrapolation test.
  4. [Fig. 3a,c and 'Notably a clear linear dependence...' paragraph] The Planckian scaling is extracted from a linear fit over only 20–40 K, although Γ is plotted from 50 K down to the measurement limit. With a narrow fitted interval and no residual analysis or comparison with alternative forms (T^2, T/(T+Θ), or saturating behavior), the claim that the scattering rate is linear in T over the measurable range is not fully established. Please show the complete Γ(T) dependence, mark the fit range, and report residuals or an F-test against competing temperature dependences.
minor comments (5)
  1. [Methods, Eq. (1)] The phrase 'the thickness difference between the the substrate the sample was grown on' contains a duplicated 'the'; it should read 'between the substrate...'.
  2. [Paragraph after Fig. 4] The sentence 'this spectral weight is mainly drawn from of channel that displays the NFL Planckian behavior' contains a typo; it should likely read 'drawn from the channel that displays...'.
  3. [Paragraph on dirty s-wave fit] The text says 'the fitting parameter Tc0 for the x = 0.45 sample needed to be 18K and for the x = 0.45, Tc0 = 16K'; the second doping value should presumably be x = 0.35, not x = 0.45.
  4. [General notation] The model equation after Fig. 2 is not numbered, although it is central to the paper; numbering it and referring to it explicitly in the discussion would improve readability.
  5. [Fig. 4 caption] The caption refers to 'the spectral weight of the Drude peak, S, at 2K above the onset of superconducting fluctuations', while the text uses 'Tc0 + 2K'; please clarify whether these are the same temperature and define both symbols consistently.

Circularity Check

2 steps flagged · score 4.0 of 10

Planckian slope is a genuine fit, but the DC 'recovery' and the assignment of condensate weight to the Planckian channel reuse the same two-channel model by construction.

  1. fitted input called prediction [Discussion following Fig. 3 / Extended Data Fig. 1 (DC-resistivity section)]
    "one notes the qualitative temperature dependence of the DC conductivity, σDC = 1/ρ, can be recovered when adding σ0 to a σDrude(ν→ 0) that goes like S/Γ (Extended Data Fig. 3). ... From our fits we know Γ is linearly proportional to T, thus σDC = σ0 + S/Γ = σ0 + S/αkT/h which can be taken to be a + b/T. ... The fits (Extended Data Fig. 1) using this remarkably simple function match the data well from 200K to 25K."

    The formula σDC = σ0 + S/Γ is exactly the ν→0 limit of the fit model σ̃(ν) = S/(Γ−iν) + σ0 + ... used to extract Γ(T), S(T), and σ0(T). Therefore the reported agreement of ρDC = 1/(a+b/T) with measured DC resistivity does not test a new prediction; it re-plots the same fitted parameters. The abstract states the analysis was 'constrained with DC resistivity', so the DC agreement is at best a consistency check and at worst a re-fitting of the input. It does not independently confirm the two-channel decomposition because any model that fits the THz spectra at all temperatures with the same parameters would produce this 'match' automatically.

  2. self definitional [Fig. 4 discussion ('Superfluid density, three ways')]
    "In Fig. 4 b) and d), the spectral weight at Tc0 + 2K of the Drude peak is plotted as the horizontal dashed line. For both samples we find that the low temperature value of Sδ just overshoots this value of the spectral weight. This indicates that the charge carriers that undergo Planckian scattering are the principal participants in the condensate."

    In the underlying model, the condensate is represented by a separate delta-function term π/2 Sδδ(ν=0) and the incoherent channel σ0 is forced to be temperature-independent through the transition ('σ0 has little if any temperature dependence through the transition'). The FGT sum rule Sδ = SN − SU then removes only spectral weight from the fitted Drude term when σ0 is unchanged, so Sδ ≈ S_Drude(Tc0+2K) is close to an identity, not a measured coincidence. The conclusion that the 'Planckian channel' supplies the condensate therefore reads back the model's imposed split: the fast channel was never allowed to lose weight. A different model that permits σ0 to partially condense, or a two-Drude decomposition with different channel assignments, could change this attribution.

full rationale

The central empirical content — a linear-in-T scattering rate Γ ≈ (2.4–3)kT/h — is a genuine fit to the complex THz conductivity, not a parameter renamed from the input. The superfluid density Sδ is also measured by three mutually independent methods (two-fluid fits, σ2ν extrapolation, FGT sum rule), so parts of the paper are self-contained. No load-bearing step is justified by a self-citation: refs. [20–22] and [29] support the parallel-channel idea and the nematic-fluctuation comparison, but the Planckian slope and the Sδ values do not come from those papers. However, two claims are circular in the mild sense of reusing the paper's own model. First, the DC-resistivity 'recovery' is the ν→0 limit of the same fit function, so the agreement with ρDC is a consistency check, not an independent prediction. Second, the assertion that the condensate is drawn from the Planckian channel follows from the model's built-in assumption that only the Drude term loses spectral weight while σ0 stays constant across Tc; the comparison of Sδ to the Drude weight at Tc0+2K is thus close to a tautology under the fitted model. The paper does not test alternative decompositions (generalized Drude with frequency-dependent scattering, or two finite-width Drudes without a constant offset), so the two-channel separation itself is an assumed ansatz rather than a derived result. This is a model-identification risk more than definitional circularity, but it lowers the independence of the headline claims. Overall score 4: partial circularity via reuse of the fitted model, while the raw Γ(T) and Sδ measurements retain independent content.

Assumptions & free parameters 5 free parameters · 6 assumptions · 0 invented entities

The central claim rests on the assumed additive conductivity model, the parallel-conductance ansatz, the FGT sum rule, and on fitted parameters alpha, Gamma0, sigma0, S, and Tc0. The paper introduces no new physical entities; the two channels are phenomenological components of a conductivity model rather than new entities.

free parameters (5)
  • alpha (Planckian slope) = 3.02 (x=0.45); 2.42 (x=0.35)
    Fitted slope of Gamma(T) over 20-40 K; the Planckian claim is quantified by this fitted slope, not derived from theory.
  • Gamma0 (scattering-rate intercept) = 10 GHz (x=0.45); <10 GHz (x=0.35)
    Intercept in the linear fit Gamma = Gamma0 + alpha kT/h.
  • sigma0(T) residual conductivity offset = Temperature-dependent fitted values
    The fast parallel channel is a fitted constant offset in sigma1 at each temperature; its weak temperature dependence is part of the two-channel conclusion.
  • S (Drude spectral weight) = Temperature-dependent values from fits
    The spectral weight of the slow Drude channel; comparing S to Sdelta establishes the condensate-origin claim.
  • Tc0 (BCS fit parameter) = 18 K (x=0.45); 16 K (x=0.35)
    Fitted transition temperature used to overlay the dirty-BCS superfluid density curve; differs from measured Tc values of 13.2 K and 11.8 K, so it is an adjustable shape parameter.
assumptions (6)
  • domain assumption Additive conductivity model with Drude, constant offset, and superfluid contributions is the correct representation.
    Introduced in the model equation for sigma~(nu); alternative decompositions are not tested, so the two-channel interpretation depends on this assumption.
  • domain assumption Conduction channels combine as parallel conductances (sigmaDC = sigma0 + S/Gamma).
    The derivation of rho = 1/(a+b/T) and the parallel-channels language rely on this ansatz, supported only by cuprate precedents rather than derived for FTS.
  • domain assumption Ferrell-Glover-Tinkham sum rule can be applied using the measured 0.2-3 THz window.
    The paper computes Sdelta = SN - SU from the measured range and assumes high-frequency spectral weight is unchanged across Tc.
  • domain assumption epsilon_infinity = 4.
    Fixed from earlier optical results in Ref 17, not determined in this experiment.
  • domain assumption Dirty BCS s-wave form describes the superfluid-density temperature dependence.
    Used as a guide with fitted Tc0 values; the paper itself notes deviations may indicate strong fluctuations, so this form is not established for this material.
  • standard math Fresnel thin-film optics applies to the layered film/substrate system.
    Used in Eq. (1) to convert the transmission ratio to sigma~(omega); standard but requires the substrate index and film thickness.

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Cite this review

Pith. "Pith review of Planckian scattering and parallel conduction channels in the iron chalcogenide superconductors FeTe$_{1-x}$Se$_x$." pith.science (2026). https://pith.science/paper/GPGNBWOL

@misc{pith2026250500623,
  author       = {Pith},
  title        = {Pith review of: Planckian scattering and parallel conduction channels in the iron chalcogenide superconductors FeTe$_1-x$Se$_x$},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GPGNBWOL}},
  note         = {Machine review of arXiv:2505.00623}
}
abstract

The remarkable linear in temperature resistivity of the cuprate superconductors, which extends in some samples from $T_c$ to the melting temperature, remains unexplained. Although seemingly simple, this temperature dependence is incompatible with the conventional theory of metals that dictates that the scattering rate, $1/\tau$, should be quadratic in temperature if electron-electron scattering dominates. Understanding the origin of this temperature dependence and its connection to superconductivity may provide the key to pick the lock of high-temperature superconductivity. Using time-domain terahertz spectroscopy (TDTS) we elucidate the low temperature conducting behavior of two FeTe$_{1-x}$Se$_x$ (FTS) samples, one with almost equal amounts of Se and Te that is believed to be a topological superconductor, and one that is more overdoped. Constrained with DC resistivity, we find two conduction channels that add in parallel, a broad one in frequency with weak temperature dependence and a sharper one whose scattering rate goes as the Planckian limited rate, $\sim kT/h$. Through analysis of its spectral weight we show the superconducting condensate is mainly drawn from the channel that undergoes this Planckian scattering.

Figures

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Figure 1
Figure 1. FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
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Figure 2. FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
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Figure 3. FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
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Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]

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    We would like to thank A. Chubukov and P. Cole- man for encouraging discussions

Pith tools

Reviewed August 16, 2026 · model on record in the stance chip above.