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Imaging a terahertz superfluid plasmon in a two-dimensional superconductor

Nature (2026)Cite this article

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Abstract

The superconducting gap defines the fundamental energy scale for the emergence of dissipationless transport and collective phenomena in a superconductor1,2,3. In layered high-temperature cuprate superconductors, in which the Cooper pairs are confined to weakly coupled two-dimensional (2D) copper–oxygen (CuO2) planes4,5, terahertz (THz) spectroscopy at subgap millielectronvolt (meV) energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers6,7,8,9. However, within the CuO2 planes, the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation2,6,9,10. Here we present spectroscopic evidence of a below-gap, 2D superfluid plasmon in few-layer Bi2Sr2CaCu2O8+x and spatially resolve its deeply subdiffractive THz electrodynamics. By placing the superconductor in the near field of a spintronic THz emitter, we reveal this distinct resonance—absent in bulk samples and observed only in the superconducting phase—and determine its plasmonic nature by mapping the geometric anisotropy and dispersion. Crucially, these measurements offer a direct view of the momentum-dependent and frequency-dependent superconducting transition in two dimensions.

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Fig. 1: THz microspectroscopy of few-layer Bi2Sr2CaCu2O8+x.
Fig. 2: Temperature dependence of the THz conductivity.
Fig. 3: THz hyperspectral imaging of Bi2Sr2CaCu2O8+x.
Fig. 4: Scale dependence of the superconducting transition.

Data availability

Datasets collected and/or analysed during the current study are available in the Supplementary Information and from the corresponding author on request. Source data are provided with this paper.

Code availability

The codes used to generate the data for this study are available from the corresponding author on request.

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Acknowledgements

We acknowledge the support from the U.S. Department of Energy, Materials Science and Engineering Division, Office of Basic Energy Sciences (BES DMSE) (data taking, analysis and manuscript writing) and the EPiQS Initiative grant GBMF9459 (instrumentation) of the Gordon and Betty Moore Foundation. The theory work was supported in part by the European Research Council (ERC-2024-SyG-101167294; UnMySt), the Cluster of Excellence: Advanced Imaging of Matter (AIM) and Grupos Consolidados UPV/EHU (IT1249-19). We also acknowledge support from the Max Planck–New York City Center for Non-Equilibrium Quantum Phenomena. The Flatiron Institute is a division of the Simons Foundation. P.K. and X.C. acknowledge support from AFOSR (FA9550-25-1-0019). The work at BNL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, contract no. DOE-SC0012704. A.v.H. acknowledges support from the Alexander von Humboldt Foundation. C.J.A. acknowledges support from the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program. E.V.B. acknowledges support from the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 101106809. T.T. acknowledges support by the National Science Scholarship by the Agency for Science, Technology and Research (A*STAR), Singapore. M.Y. acknowledges support from the National Science Foundation MPS-Ascend Postdoctoral Research Fellowship under grant no. 2402151. K.T. acknowledges support from the NSF GRFP fellowship. Fabrication was carried out with the use of MIT.nano.

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Authors and Affiliations

  1. Massachusetts Institute of Technology, Cambridge, MA, USA

    A. von Hoegen, T. Tai, C. J. Allington, M. Yeung, J. Pettine, A. E. Kossak, B. Lee, G. S. D. Beach & N. Gedik

  2. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany

    M. H. Michael, E. Viñas Boström & A. Rubio

  3. Max Planck Institute for the Physics of Complex Systems, Dresden, Germany

    M. H. Michael

  4. Harvard University, Cambridge, MA, USA

    X. Cui, K. Torres & P. Kim

  5. Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, NY, USA

    G. D. Gu

  6. Center for Computational Quantum Physics (CCQ), Flatiron Institute, New York, NY, USA

    A. Rubio

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  1. A. von Hoegen
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Contributions

A.v.H., C.J.A. and N.G. conceived the project. A.v.H. led the experimental effort. A.v.H., T.T. and C.J.A. built the THz microscopy set-up. T.T. developed the measurement software. A.v.H. and T.T., with support from C.J.A., performed the THz microscopy measurements. A.v.H. and T.T., with support from C.J.A., analysed the experimental data. B.L. and A.E.K., supervised by G.S.D.B., fabricated the spintronic emitter. M.Y., with support from A.v.H., designed and deposited the distributed Bragg reflectors. M.Y. and J.P. fabricated the metallic marker structures used for the calibration measurements. X.C. and K.T., supervised by P.K., prepared the BSCCO samples. G.D.G. provided the BSCCO single crystals. J.P. carried out the atomic force microscopy characterization. M.H.M., supported by E.V.B. and supervised by A.R., developed the theoretical description of the experiment. A.v.H., with support from T.T., wrote the manuscript, with input from all authors. The project was supervised by N.G.

Corresponding author

Correspondence to N. Gedik.

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Supplementary Sections 1–23 including Supplementary Figs. 1–32 and further references – see Contents for details.

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von Hoegen, A., Tai, T., Allington, C.J. et al. Imaging a terahertz superfluid plasmon in a two-dimensional superconductor. Nature (2026). https://doi.org/10.1038/s41586-025-10082-2

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