Femtosecond Spectroscopy Studies on YBa₂Cu₃O_7-x and PrBa₂Cu₃O_7-x Epitaxial Thin Films

We report the results of femtosecond spectroscopy studies on YBa₂Cu₃O_7-x (x=0.1, 0.4, 0.8) and PrBa₂Cu₃O_7-x (x=0.l) epitaxial thin films at room temperature. The transient reflectivity signals ΔR(t) in the samples are consistent with a band filling model. The relaxation times of ΔR(t) in YBa₂Cu₃O_7-x increase from less than 100 fs to more than 1ps with increasing x and with substitution of Pr for Y. These results reveal that in high T_c superconductors decrease of carrier concentration slows down the carrier-phonon coupling.