Surface Josephson plasma waves in a high-temperature superconductor

TL;DR

This paper demonstrates the generation and detection of surface Josephson plasma waves in a high-temperature superconductor, revealing their potential for non-invasive surface and interface studies in advanced heterostructures.

Contribution

It reports the first observation of surface-confined Josephson plasmons in an ultrathin high-temperature superconductor using near-field THz probing.

Findings

Enhanced near-field signal below critical temperature

Visualization of surface Josephson plasmons with high spatial resolution

Potential applications in studying superconductivity and heterostructures

Abstract

Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide information about surface and interface properties of hetero-structures. In ultra-thin metallic films these plasmonic excitations are heavily damped. Superconductivity is predicted to reduce dissipation allowing detection of these resonances. Emerging low-loss interface Cooper-pair waves have been studied before, however, the observation of surface-confined Josephson plasmons has remained elusive. Here, we report on generation and coupling to these excitations in an ultrathin single-crystal film of high-temperature superconductor La1.85Sr0.15CuO4. The film becomes brighter than Au below the critical temperature when probed with sub-gap THz photons. We show that the enhanced signal in the superconducting state, which can be visualized with a spatial resolution better than {\lambda}/3,000, originates from near-field coupling of light to surface Josephson plasmons. Our results open a path towards non-invasive investigation of enhanced superconductivity in artificial multilayers, buried interface states in topological hetero-structures, and non-linear phenomena in Josephson devices.