Planckian scattering and parallel conduction channels in an iron chalcogenide superconductor

Nature Physics (2026)Cite this article 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 Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any timeSubscribe to this journal Receive 12 print issues and online access $259.00 per yearonly $21.58 per issueBuy this articleUSD 39.95Prices may be subject to local taxes which are calculated during checkoutThe 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.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 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 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 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 Kreisel, A., Hirschfeld, P. J. & Andersen, B. M. On the remarkable superconductivity of FeSe and its close cousins. Symmetry 12, 1402 (2020).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 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 Wang, D. et al. Evidence for Majorana bound states in an iron-based superconductor. Science 362, 333–335 (2018).Article ADS Google Scholar 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 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 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 Roppongi, M. et al. Topology meets time-reversal symmetry breaking in FeSe1-xTex superconductors. Nat. Commun. 16, 6573 (2025).Article ADS Google Scholar Sato, M. & Ando, Y. Topological superconductors: a review. Rep. Progr. Phys. 80, 076501 (2017).Article ADS Google Scholar Mukasa, K. et al. Enhanced superconducting pairing strength near a pure nematic quantum critical point. Phys. Rev. X 13, 011032 (2023).

Google Scholar 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 Legros, A. et al. Universal T-linear resistivity and Planckian dissipation in overdoped cuprates. Nat. Phys. 15, 142–147 (2019).Article Google Scholar 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 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 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 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 Cooper, R. A. et al. Anomalous criticality in the electrical resistivity of La2−xSrxCuO4. Science 323, 603–607 (2009).Article ADS Google Scholar 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 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 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 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 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 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 Ferrell, R. A. & Glover, R. E. Conductivity of superconducting films: a sum rule. Phys. Rev. 109, 1398–1399 (1958).Article ADS Google Scholar Tinkham, M. Introduction to Superconductivity 2nd edn (Dover Publications, 2004).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).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 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 referencesWe 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.William H. Miller III Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USARalph Romero III & N. P. ArmitageDepartment of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ, USAHee Taek Yi & Seongshik OhSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarR.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.Correspondence to Ralph Romero III or N. P. Armitage.The authors declare no competing interests.Nature Physics thanks the anonymous reviewers for their contribution to the peer review of this work.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.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.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.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.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.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.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.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.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.(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.Real and imaginary parts of the conductivity as a function of frequency for all measured temperatures.Conductivities and relevant fits for the x = 0.45 sample.Parameters extracted from the fits in Fig. 2, the linear fits to scattering rates and d.c. resistivities.σ2ν and corresponding fits as well as the three measures of superfluid densities.d.c. resistivities and fit using our derived functional form.Conductivities and relevant fits for the x = 0.35 sample.S/Γ, σ0 and 1/ρd.c. values.Real part of the conductivity for 6, 10 and 20 K (same quantities from refs. 17,20).Data necessary to reproduce all plots in Extended Data Fig. 5.Data necessary to reproduce all plots in Extended Data Fig. 6.Data necessary to reproduce all plots in Extended Data Fig. 7.Data necessary to reproduce all plots in Extended Data Fig. 8.Fits to the real and imaginary parts of the conductivity using the two Lorentzian models from ref. 20.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 permissionsRomero 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/s41567-026-03266-8Download citationReceived: 12 May 2025Accepted: 25 March 2026Published: 23 April 2026Version of record: 23 April 2026DOI: https://doi.org/10.1038/s41567-026-03266-8Anyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative