Robust Paramagnon and Acoustic Plasmon in a Photo-excited Electron-doped Cuprate Superconductor

TL;DR

This study uses time-resolved resonant inelastic X-ray scattering to simultaneously observe and analyze the ultrafast dynamics of spin and charge excitations in an electron-doped cuprate superconductor under non-equilibrium conditions, revealing their intertwined behavior.

Contribution

It provides the first direct measurement of coupled spin and charge collective excitations in a photo-excited cuprate using trRIXS, highlighting their synchronized dynamics.

Findings

Paramagnon dispersion near zone center is modified without changing bandwidth.

Acoustic plasmon energy and spectral weight decrease after pumping.

Spin and charge dynamics are tightly coupled on femtosecond timescales.

Abstract

Characterizing the spin and charge degrees of freedom in high-temperature superconducting cuprates under non-equilibrium conditions provides new insights into their electronic correlations. However, their collective dynamics have been largely unexplored due to experimental challenges. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to simultaneously track the collective spin (paramagnon) and charge (acoustic plasmon) dynamics in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, though the bandwidth remained unchanged, suggesting no significant alteration to spin exchange interactions. Simultaneously, in the charge sector, the acoustic plasmon's energy and spectral weight decreased, suggesting a light-induced redistribution of charge carriers. The variations of both the paramagnon and the plasmon were locked in time, demonstrating a robust intertwining between the spin and charge degrees of freedom on a femtosecond timescale, even in this non-equilibrium state.