FEMTOSECOND REFLECTIVITY OF 60 K Y-BA-CU-O THIN-FILMS

We report systematic studies of the femtosecond transient reflectivity at 2 eV in partially oxygen-depleted Y-Ba-Cu-0 thin films exhibiting the resistive superconducting transition at 60 K. Our measurements, performed at room temperature, reveal that in 60 K films, the Fermi level in the Cu-O2 plane lies approximately 2 eV above the filled copper d9/d10 band. By mapping the transient optical response across a 1 cm2 film, we show that the position of the Fermi level is very sensitive to the apparent local variations of the film oxygen content, which gives rise to dramatically different transient optical responses. We also observe a strong transient reflectivity dependence on intensity, which can be attributed to the shift of the Fermi level caused by nonequilibrium hole heating.