PREPARATION AND ELECTRONIC-PROPERTIES OF YBA2CU3OX FILMS WITH CONTROLLED OXYGEN STOICHIOMETRIES

We describe a novel technique capable of controlling the oxygen conten t of YBa2Cu3Ox (YBCO) films in a precise and reversible manner. The te mperature dependence of resistivity and the distinct two-plateau behav ior in critical temperature T-CO versus oxygen content plot of these f ilms are consistent with those observed in the bulk and single crystal s of YBCO. The O 1s and Cu 2p absorption spectra of these films were m easured by polarization-dependent X-ray absorption spectroscopy (XAS). The intensity variations of the pre-edge peaks as a function of oxyge n content are discussed. We also used these films to systematically st udy the electron-phonon coupling strength and the position of Fermi le vel by using a femtosecond pump-probe technique. A clear sign-reversal of the transient reflectivity, which was consistently explained by th e thermomodulation model, was observed. Both of these optical measurem ents support the idea that the electronic structure of YBCO cuprates i s based on the charge transfer model with hybridization between the Cu and O sites.