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

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.

Phys. Rev. Research 7, 023175 (2025)