# Photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$ probed by time-resolved terahertz spectroscopy

## TL;DR

This study investigates the ultrafast optical response of underdoped YBa₂Cu₃O₆₊ₓ, revealing transient changes in superconducting and normal states induced by laser pulses, and clarifying the mechanisms behind photo-induced phenomena.

## Contribution

It provides a detailed analysis of photo-induced nonequilibrium effects in underdoped cuprates, distinguishing between different excitation mechanisms and their impact on superconductivity and normal state properties.

## Key findings

- Transient suppression and reappearance of Josephson plasma edge after photo-excitation.
- Normal state shows enhanced or altered reflectance and conductivity with pump pulses.
- Phonon pumping is excluded as the primary mechanism for observed effects.

## Abstract

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We find that upon photo-excitations, Josephson plasma edge in superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at frequency lower than the static plasma edge within short time. In normal state, an enhancement or weaker edge-like shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and mid-infrared excitations and exclude phonon pumping as a scenario for the photo-induced effects above. We further elaborate the transient responses in normal state are unlikely to be explained by photo-induced superconductivity.