Visualizing a Terahertz Superfluid Plasmon in a Two-Dimensional Superconductor

TL;DR

This paper reports the discovery and visualization of a two-dimensional superfluid plasmon in a layered high-temperature superconductor using near-field terahertz spectroscopy, revealing new insights into superfluid dynamics at finite momenta.

Contribution

It provides the first spectroscopic evidence of a below-gap superfluid plasmon in a 2D superconductor and maps its anisotropic dispersion, advancing understanding of superfluid phenomena at THz frequencies.

Findings

Observation of a below-gap superfluid plasmon in Bi2Sr2CaCu2O8+x

Spatially resolved its anisotropic dispersion and geometric properties

Demonstrated the superfluid plasmon's dependence on the superconducting phase

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, where the Cooper pairs are confined to weakly coupled two-dimensional copper-oxygen planes, terahertz (THz) spectroscopy at sub-gap millielectronvolt energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers. However, within the copper-oxygen planes the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation. Here, we present spectroscopic evidence of a below-gap, two-dimensional superfluid plasmon in few-layer Bi2Sr2CaCu2O8+x and spatially resolve its deeply sub-diffractive 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- and frequency dependent superconducting transition in two dimensions. These results establish a new platform for investigating superfluid phenomena at finite momenta and THz frequencies, highlighting the potential to engineer and visualize superconducting devices operating at ultrafast THz rates.