Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Planckian scattering and parallel conduction channels in an iron chalcogenide superconductor

Nature Physics (2026) Cite this article

Subjects

Abstract

The linear-in-temperature resistivity of cuprate superconductors, which extends in some samples from the superconducting critical temperature to the melting temperature, remains unexplained. Although seemingly simple, this temperature dependence is incompatible with the conventional theory of metals, which dictates that the scattering rate should be quadratic in temperature if electron–electron scattering dominates. Understanding the origin of this temperature dependence and its connection to superconductivity may provide crucial information that helps to understand the superconducting mechanism. Here we show the presence of two conduction channels in the normal state of the iron-based superconductor FeTe1−xSex that add in parallel. One is the broad one in frequency with weak temperature dependence, whereas the other is sharper and has a scattering rate that goes as the Planckian-limited rate that is linear in temperature. This behaviour occurs in two samples, one with almost equal amounts of Se and Te that is believed to be a topological superconductor and the other that is more overdoped. By analysing the spectral weight of the superconducting condensate using our time-domain terahertz spectroscopy measurements, we show that it is mainly drawn from the channel that undergoes Planckian scattering.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on SpringerLink
  • Instant access to the full article PDF.

USD 39.95

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: THz conductivity of the x = 0.45 and x = 0.35 samples.
The alternative text for this image may have been generated using AI.
Fig. 2: Fits to the THz conductivity of the x = 0.45 sample at various temperatures.
The alternative text for this image may have been generated using AI.
Fig. 3: Fit parameters along with d.c. resistivity.
The alternative text for this image may have been generated using AI.
Fig. 4: Superfluid density, three ways.
The alternative text for this image may have been generated using AI.

Similar content being viewed by others

Data availability

The data that support the findings of this study are available via Zenodo at https://doi.org/10.5281/zenodo.18662533 (ref. 32). Source data are provided with this paper.

References

  1. Hsu, F.-C. et al. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl Acad. Sci. USA 105, 14262–14264 (2008).

    Article  ADS  Google Scholar 

  2. Her, J. L. et al. Anisotropy in the upper critical field of FeSe and FeSe0.33Te0.67 single crystals. Supercond. Sci. Technol. 28, 045013 (2015).

    Article  ADS  Google Scholar 

  3. Li, S. et al. First-order magnetic and structural phase transitions in Fe1+ySexTe1−x. Phys. Rev. B 79, 054503 (2009).

    Article  ADS  Google Scholar 

  4. Bao, W. et al. Tunable (δπ, δπ)-type antiferromagnetic order in α-Fe(Te,Se) superconductors. Phys. Rev. Lett. 102, 247001 (2009).

    Article  ADS  Google Scholar 

  5. Kreisel, A., Hirschfeld, P. J. & Andersen, B. M. On the remarkable superconductivity of FeSe and its close cousins. Symmetry 12, 1402 (2020).

  6. Yin, J.-X. et al. Observation of a robust zero-energy bound state in iron-based superconductor Fe(Te,Se). Nat. Phys. 11, 543–546 (2015).

    Article  Google Scholar 

  7. Zhang, P. et al. Observation of topological superconductivity on the surface of an iron-based superconductor. Science 360, 182–186 (2018).

    Article  ADS  Google Scholar 

  8. Wang, D. et al. Evidence for Majorana bound states in an iron-based superconductor. Science 362, 333–335 (2018).

    Article  ADS  Google Scholar 

  9. Machida, T. et al. Zero-energy vortex bound state in the superconducting topological surface state of Fe(Se,Te). Nat. Mater. 18, 811–815 (2019).

    Article  ADS  Google Scholar 

  10. Zaki, N., Gu, G., Tsvelik, A., Wu, C. & Johnson, P. D. Time-reversal symmetry breaking in the Fe-chalcogenide superconductors. Proc. Natl Acad. Sci. USA 118, e2007241118 (2021).

    Article  Google Scholar 

  11. Farhang, C. et al. Revealing the origin of time-reversal symmetry breaking in Fe-chalcogenide superconductor FeTe1−xSex. Phys. Rev. Lett. 130, 046702 (2023).

    Article  ADS  Google Scholar 

  12. Roppongi, M. et al. Topology meets time-reversal symmetry breaking in FeSe1-xTex superconductors. Nat. Commun. 16, 6573 (2025).

    Article  ADS  Google Scholar 

  13. Sato, M. & Ando, Y. Topological superconductors: a review. Rep. Progr. Phys. 80, 076501 (2017).

    Article  ADS  Google Scholar 

  14. Mukasa, K. et al. Enhanced superconducting pairing strength near a pure nematic quantum critical point. Phys. Rev. X 13, 011032 (2023).

    Google Scholar 

  15. Yin, Z. P., Haule, K. & Kotliar, G. Kinetic frustration and the nature of the magnetic and paramagnetic states in iron pnictides and iron chalcogenides. Nat. Mater. 10, 932–935 (2011).

    Article  ADS  Google Scholar 

  16. Legros, A. et al. Universal T-linear resistivity and Planckian dissipation in overdoped cuprates. Nat. Phys. 15, 142–147 (2019).

    Article  Google Scholar 

  17. Homes, C. C. et al. Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45. J. Phys. Chem. Solids 72, 505–510 (2011).

    Article  ADS  Google Scholar 

  18. Guo, H., Patel, A. A., Esterlis, I. & Sachdev, S. Large-N theory of critical Fermi surfaces. II. conductivity. Phys. Rev. B 106, 115151 (2022).

    Article  ADS  Google Scholar 

  19. Cheng, B. et al. Anomalous gap-edge dissipation in disordered superconductors on the brink of localization. Phys. Rev. B 93, 180511 (2016).

    Article  ADS  Google Scholar 

  20. Homes, C. C., Dai, Y. M., Wen, J. S., Xu, Z. J. & Gu, G. D. FeTe0.55Se0.45: a multiband superconductor in the clean and dirty limit. Phys. Rev. B 91, 144503 (2015).

    Article  ADS  Google Scholar 

  21. Cooper, R. A. et al. Anomalous criticality in the electrical resistivity of La2−xSrxCuO4. Science 323, 603–607 (2009).

    Article  ADS  Google Scholar 

  22. van Heumen, E. et al. Strange metal electrodynamics across the phase diagram of Bi2−xPbxSr2−yLayCuO6+δ cuprates. Phys. Rev. B 106, 054515 (2022).

    Article  ADS  Google Scholar 

  23. Clayhold, J. A. et al. Constraints on models of electrical transport in optimally doped La2−xSrxCuO4 from measurements of radiation-induced defect resistance. J. Supercond. Nov. Magn. 23, 339–342 (2010).

    Article  Google Scholar 

  24. Tagay, Z. et al. BCS d-wave behavior in the terahertz electrodynamic response of electron-doped cuprate superconductors. Phys. Rev. B 104, 064501 (2021).

    Article  ADS  Google Scholar 

  25. Mahmood, F., He, X., Božović, I. & Armitage, N. P. Locating the missing superconducting electrons in the overdoped cuprates La2−xSrxCuO4. Phys. Rev. Lett. 122, 027003 (2019).

    Article  ADS  Google Scholar 

  26. Wang, Y. et al. Separated transport relaxation scales and interband scattering in thin films of SrRuO3, CaRuO3, and Sr2RuO4. Phys. Rev. B 103, 205109 (2021).

    Article  ADS  Google Scholar 

  27. Tinkham, M. & Ferrell, R. A. Determination of the superconducting skin depth from the energy gap and sum rule. Phys. Rev. Lett. 2, 331–333 (1959).

    Article  ADS  Google Scholar 

  28. Ferrell, R. A. & Glover, R. E. Conductivity of superconducting films: a sum rule. Phys. Rev. 109, 1398–1399 (1958).

    Article  ADS  Google Scholar 

  29. Tinkham, M. Introduction to Superconductivity 2nd edn (Dover Publications, 2004).

  30. Islam, K. R. & Chubukov, A. Unconventional superconductivity mediated by nematic fluctuations in a multi-orbital system – application to doped FeSe. Phys. Rev. B https://doi.org/10.1103/PhysRevB.111.094503 (2025).

  31. Matsuura, K. et al. Two superconducting states with broken time-reversal symmetry in FeTe1−xSex. Proc. Natl Acad. Sci. USA 120, e2208276120 (2023).

    Article  Google Scholar 

  32. Romero, R. III, Yi, H. T., Oh, S. & Armitage, N. P. Planckian scattering and parallel conduction channels in an iron chalcogenide superconductor. Zenodo https://doi.org/10.5281/zenodo.18662533 (2026).

Download references

Acknowledgements

We acknowledge support from the Army Research Office MURI program (W911NF2020166). R.R.III and N.P.A. also acknowledge support from the Gordon and Betty Moore Foundation EPiQS Initiative Grant GBMF-9454.

Author information

Authors and Affiliations

  1. William H. Miller III Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA

    Ralph Romero III & N. P. Armitage

  2. Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ, USA

    Hee Taek Yi & Seongshik Oh

Authors
  1. Ralph Romero III
    View author publications

    Search author on:PubMed Google Scholar

  2. Hee Taek Yi
    View author publications

    Search author on:PubMed Google Scholar

  3. Seongshik Oh
    View author publications

    Search author on:PubMed Google Scholar

  4. N. P. Armitage
    View author publications

    Search author on:PubMed Google Scholar

Contributions

R.R.III performed the TDTS and the corresponding data analysis. H.T.Y. and S.O. grew the thin films and performed the transport measurements. R.R.III and N.P.A. wrote the manuscript with input from all authors. N.P.A. initiated and supervised the project.

Corresponding authors

Correspondence to Ralph Romero III or N. P. Armitage.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Physics thanks the anonymous reviewers for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 DC resistivity from transport.

ρd.c. of both samples fit with the functional form derived in the main text. The top row shows the full temperature range while the bottom is zoomed in to 50 K. (a), (b) correspond to the x = 0.45 sample and (c), (d) to the x = 0.35 sample.

Extended Data Fig. 2 Fits to the THz conductivity of the x = 0.35.

Figure 2 from the main text for the x = 0.35 sample. a,b σ1, σ2 at a tem- perature (38 K) where a single Drude term suffices. c,d, A temperature (26 K) where a constant offset, σ0, must be in- cluded alongside the Drude term. e,f, A temperature, above Tc, (16 K) where the two fluid model must be invoked. g,h, The lowest temperatures where the conductivity becomes mainly sensitive to the condensate at ν = 0.

Extended Data Fig. 3 Hint of parallel conduction channels.

Experimental motivation for the starting point to derive functional form which fits d.c. data. (a) corresponds to the x = 0.45 sample and b) to the x = 0.35 sample. Error bars represent the 95% confidence interval (2 standard deviations) in the least squares fitting procedure. When not visible they are within the symbol.

Extended Data Fig. 4 Comparison of the optical conductivity of thin films to single crystals.

Comparison of the real part of the optical conductivity of the x = 0.45 thin film (solid lines) in this study and a x = 0.45 single crystal (dashed) measured with FTIR by C. Homes et al, reproduced from (17, 20) At temperatures above the transition (20 and 10 K) the frequency dependence and magnitude are remarkably similar. Below the transition the single crystals appear to show a stronger gap, however these frequencies are at the limit of the FTIR spectrometer. Overall, the qualitative agreement of the spectra give us confidence that we are in the bulk limit.

Extended Data Fig. 5 Lower bound of σ0 for the x = 0.45 sample.

Top row: Here we determine the σ0 which fits the 1 K data, σ0 = 1.0 ± 0.1 mΩ−1 cm−1. Fig. 2 (a)–(h), Fig. 3 (a) and (b), Fig. 4 (b), and Extended Data Fig. 3 (a) from the main text with fixed σ0 = 1.0 ± 0.1 mΩ−1 cm−1.

Extended Data Fig. 6 Upper bound of σ0 for the x = 0.45 sample.

Here we determine the upper bound of σ0 as when the fits visually do poorly, σ0 = 1.4 ± 0.1 mΩ−1 cm−1. Figure 2 (a)–(h), Fig. 3 (a) and (b), Fig. 4 (b), and Extended Data Fig. 3 (a) from the main text with fixed σ0 = 1.4 ± 0.1 mΩ−1 cm−1.

Extended Data Fig. 7 Lower bound of σ0 for the x = 0.35 sample.

Top row: Here we determine the σ0 which fits the 1 K data, σ0 = 1.7 ± 0.1 mΩ−1 cm−1. Extended Data Fig. 2 (a-h), Fig. 3 (c) and (d), Fig. 4 (d), and Extended Data Fig. 3 (b) from the main text with fixed σ0 = 1.7 ± 0.1 mΩ−1 cm−1.

Extended Data Fig. 8 Upper bound of σ0 for the x = 0.35 sample.

Here we determine the upper bound of σ0 as when the fits visually do poorly, σ0 = 2.1 ± 0.1 mΩ−1 cm−1. Extended Data Fig. 2 (a)–(h), Fig. 3 (c) and (d), Fig. 4 (d), and Extended Data Fig. 3 (b) from the main text with fixed σ0 = 2.1 ± 0.1 mΩ−1 cm−1.

Extended Data Fig. 9 Equivalence of ϵ∞ to two Lorentzians from (20) in low frequency limit.

(a), (b) are Fig. 2 (e), (f) from main text. (c), (d) σ1,2 at 16 K being fit with 2 Drude + 2 Lorentzian model from (20) with the Lorentzians centered at 51 THz (low freq) and 120 THz (high freq). The equivalence of the two methods can be quantified by comparing the fit parameters of the two models. For our model (at 16 K) the narrow Drude has a scattering rate and spectral weight of Γ ≈ 1.2 THz and S ≈ 1.9 mΩ−1 cm−1 THz where as the broad Drude (σ0) has a scattering rate and spectral weight orders of magnitude higher than the narrow Drude which we quantified by its DC intercept of σ0 ≈ 1 mΩ−1 cm−1. The fit parameters from the two Lorentzian fit are qualitatively equivalent. The narrow Drude from the Lorentzian fit is parameterized by ΓNarrow ≈ 0.9 THz and SNarrow ≈ 1.5 mΩ−1 cm−1 THz. While the broad Drude from the two Lorentzian fit is parameterized by a DC intercept of σ0 ≈ 1.2 mΩ−1 cm−1.

Source data

Source Data Fig. 1 (download XLSX )

Real and imaginary parts of the conductivity as a function of frequency for all measured temperatures.

Source Data Fig. 2 (download XLSX )

Conductivities and relevant fits for the x = 0.45 sample.

Source Data Fig. 3 (download XLSX )

Parameters extracted from the fits in Fig. 2, the linear fits to scattering rates and d.c. resistivities.

Source Data Fig. 4 (download XLSX )

σ2ν and corresponding fits as well as the three measures of superfluid densities.

Source Data Extended Data Fig. 1 (download XLSX )

d.c. resistivities and fit using our derived functional form.

Source Data Extended Data Fig. 2 (download XLSX )

Conductivities and relevant fits for the x = 0.35 sample.

Source Data Extended Data Fig. 3 (download XLSX )

S/Γ, σ0 and 1/ρd.c. values.

Source Data Extended Data Fig. 4 (download XLSX )

Real part of the conductivity for 6, 10 and 20 K (same quantities from refs. 17,20).

Source Data Extended Data Fig. 5 (download XLSX )

Data necessary to reproduce all plots in Extended Data Fig. 5.

Source Data Extended Data Fig. 6 (download XLSX )

Data necessary to reproduce all plots in Extended Data Fig. 6.

Source Data Extended Data Fig. 7 (download XLSX )

Data necessary to reproduce all plots in Extended Data Fig. 7.

Source Data Extended Data Fig. 8 (download XLSX )

Data necessary to reproduce all plots in Extended Data Fig. 8.

Source Data Extended Data Fig. 9 (download XLSX )

Fits to the real and imaginary parts of the conductivity using the two Lorentzian models from ref. 20.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Romero III, R., Yi, H.T., Oh, S. et al. Planckian scattering and parallel conduction channels in an iron chalcogenide superconductor. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03266-8

Download citation

  • Received:

  • Accepted:

  • Published:

  • Version of record:

  • DOI: https://doi.org/10.1038/s41567-026-03266-8

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing