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DOE OSTI · 3378233

Imaging a terahertz superfluid plasmon in a two-dimensional superconductor

von Hoegen, A. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000259756350)·Tai, T. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000294785499)·Allington, C. J. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000201222815)·Yeung, M. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)]·Pettine, J. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)]·Michael, M. H. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Max Planck Institute for the Physics of Complex Systems, Dresden (Germany)]·Viñas Boström, E. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany)] (ORCID:0000000233184454)·Cui, X. [Harvard Univ., Cambridge, MA (United States)] (ORCID:0000000228242154)·Torres, K. [Harvard Univ., Cambridge, MA (United States)]·Kossak, A. E. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000192331445)·Lee, B. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)]·Beach, G. S. D. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000191587430)·Gu, Genda D. [Brookhaven National Laboratory (BNL), Upton, NY (United States)] (ORCID:0000000298863255)·Rubio, A. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Flatiron Institute, New York, NY (United States)] (ORCID:0000000320603151)·Kim, P. [Harvard Univ., Cambridge, MA (United States)] (ORCID:0000000282550086)·Gedik, N. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000263944987)
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Abstract

The superconducting gap defines the fundamental energy scale for the emergence of dissipationless transport and collective phenomena in a superconductor. In layered high-temperature cuprate superconductors, in which the Cooper pairs are confined to weakly coupled two-dimensional (2D) copper–oxygen (CuO 2 ) planes, terahertz (THz) spectroscopy at subgap millielectronvolt (meV) energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers. However, within the CuO 2 planes, the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation. Here, in this study, we present spectroscopic evidence of a below-gap, 2D superfluid plasmon in few-layer Bi 2 Sr 2 CaCu 2 O 8+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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75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITYFOS: Physical sciencesMesoscale and Nanoscale PhysicsStrongly Correlated ElectronsSuper-resolution microscopySuperconducting properties and materialsSuperconductivityTerahertz optics

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von Hoegen, A. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000259756350), Tai, T. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000294785499), Allington, C. J. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000201222815), Yeung, M. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)], Pettine, J. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)], Michael, M. H. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Max Planck Institute for the Physics of Complex Systems, Dresden (Germany)], Viñas Boström, E. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany)] (ORCID:0000000233184454), Cui, X. [Harvard Univ., Cambridge, MA (United States)] (ORCID:0000000228242154), Torres, K. [Harvard Univ., Cambridge, MA (United States)], Kossak, A. E. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000192331445), Lee, B. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)], Beach, G. S. D. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000191587430), Gu, Genda D. [Brookhaven National Laboratory (BNL), Upton, NY (United States)] (ORCID:0000000298863255), Rubio, A. [Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Flatiron Institute, New York, NY (United States)] (ORCID:0000000320603151), Kim, P. [Harvard Univ., Cambridge, MA (United States)] (ORCID:0000000282550086), Gedik, N. [Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)] (ORCID:0000000263944987). 2026-02-04. Imaging a terahertz superfluid plasmon in a two-dimensional superconductor. https://doi.org/10.1038/s41586-025-10082-2

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