Femtosecond spectroscopy studies of ultrafast dynamics in PrxY1-xBa2Cu3O7 and YBa2Cu3O7-delta thin films at different temperatures

We have systematically measured the transient reflectivity (Delta R/R) in P r-doped YBCO (PrxY1-xBa2Cu3O7) and partially oxygen deficient YBCO (YBa2Cu3 O7-delta) thin films at temperatures ranging from 300 K to 12 K by using th e femtosecond pump-probe technique. At room temperature, the electron-phono n coupling strength and the position of Fermi-level as a function of Pr dop ing concentration and oxygen content have been studied. Measurements perfor med at low temperatures indicate that the magnitude, polarity, and relaxati on time of the transient reflectivity is strongly dependent on the photon e nergy and pumping intensity of the optical excitation, the ambient temperat ure, and the doping level of the superconducting thin films. Moreover, the temperature dependence of the Fermi-level position was also observed.