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We have preformed a series of transient reflectivity(△R/R) measurements on oxygen-controlled YBCO thin films at different temperatures. A home-made femtosecond pump-probe system including a passively mode-lock Ti:sapphire laser and a continuous transfer cold finger cryostat were used for these measurement.
The △R/R curves versus delay time of the oxygen-rich YBa2Cu3Ox (x=6.95) film for temperature ranging from 120K to 20Kwere different from those reports previously. The difference may due to the fact that they used different photon energy (~2eV) for the measurements and the drastically changes of density of states near Fermi level caused by pump beam could be probed. We also find the relaxation time has an abrupt change and become longer at superconducting state for oxygen-rich film while the relaxation time increases slightly with decreasing temperature for oxygen-deficiency (x=6.0) film.
Additionally, we also measured the transient differential reflectivity of PBCO thin film. The sign of △R/R at room temperature is positive but it has a tendency to become negative at 240K and finally becomes negative below 220K. The change in sign if △R/R may due to the width of charge-transfer gap decreases when the temperature decreases.
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