Ultrafast charge-transfer dynamics in Ca$_2$CuO$_2$Cl$_2$ from time-resolved optical reflectivity

TL;DR

This study uses time-resolved optical reflectivity to explore ultrafast charge-transfer gap dynamics in Ca$_2$CuO$_2$Cl$_2$, revealing persistent red shifts, electron screening effects, and phonon coupling, enhancing understanding of correlated materials.

Contribution

It provides new insights into the ultrafast charge-transfer dynamics and electron-phonon interactions in a parent cuprate, highlighting the complex interplay of degrees of freedom.

Findings

Persistent photoinduced red shift of the CTG lasts up to 1000 ps.

Maximum CTG shift involves Hubbard U renormalization from screening.

Coherent oscillations at ~20 GHz linked to phonon coupling.

Abstract

We employ time-resolved optical reflectivity to investigate the ultrafast dynamics of the charge-transfer gap (CTG) in a parent cuprate compound Ca$_2$CuO$_2$Cl$_2$ (CCOC). We observe a persistent photoinduced red shift of the CTG that lasts up to 1000 ps. The red shift during the slow decay after 10 ps can be well modeled by the localized picture, whereas its maximum value at ~0.9 ps involves additional contribution from the renormalization of the Hubbard U due to screening effect from delocalized electrons. Furthermore, coupling between the mid-gap absorption and a slow acoustic phonon launches coherent oscillations below the CTG, observed as a ~20 GHz modulation with a dispersion independent of the pump fluence. These results demonstrate the tunning of the CTG by light, unveil complex interplay between multiple degrees of freedom, and contribute to a deeper understanding of superconductivity and correlated materials.