Squeezed Josephson plasmons in driven YBa$_2$Cu$_3$O$_{6+x}$

TL;DR

This study uses advanced two-dimensional spectroscopy to reveal how driven phonons in YBa$_2$Cu$_3$O$_{6+x}$ amplify Josephson plasmons, creating a squeezed state that may influence photo-induced superconductivity.

Contribution

It demonstrates the coherent coupling of phonons and Josephson plasmons using two-dimensional spectroscopy, revealing the formation of a squeezed state in a high-temperature superconductor.

Findings

Driven zone-center phonons amplify Josephson plasma polaritons.

A squeezed state of interlayer phase fluctuations is generated.

The results suggest a role for squeezed states in photo-induced superconductivity.

Abstract

The physics of driven collective modes in quantum materials underpin a number of striking non-equilibrium functional responses, which include enhanced magnetism, ferroelectricity and superconductivity. However, the coherent coupling between multiple modes at once are difficult to capture by single-pump probe (one-dimensional) spectroscopy, and often remain poorly understood. One example is phonon-mediated amplification of Josephson plasmons in YBa$_2$Cu$_3$O$_{6+x}$, in which at least three normal modes of the solid are coherently mixed as a source of enhanced superconductivity. Here, we go beyond previous pump-probe experiments in this system and acquire two-dimensional frequency maps using pairs of mutually delayed, carrier envelope phase stable mid-infrared pump pulses, combined with measurements of the time-modulated second-order nonlinear optical susceptibility. We find that the driven zone-center phonons amplify coherent pairs of opposite-momentum Josephson plasma polaritons, generating a squeezed state of interlayer phase fluctuations. The squeezed state is a potentially important ingredient in the microscopic physics of photo-induced superconductivity in this and other materials.