Local lattice distortions in YBa2Cu3Oy probed by XAS: Critical fluctuation, pseudogap opening and stripe ordering

Local lattice distortions in YBa2Cu3Oy single crystals are studied by polar ized x-ray absorption spectroscopy (XAS) for E/lab and E//c orientations be tween 300 K and 20 K. From the Fourier transform analysis of the Cu K-EXAFS oscillations, we find that the out-of-plane Cu-O correlation shows an anom alous temperature dependence of radial distribution below T-1=240-250 K, in dicating an increase of lattice fluctuations, while no anomalies are found for the Cu-Ba correlation. On further lowering temperature, the Fourier tra nsfom shows that the lattice fluctuation are reduced below T(2)similar toT* =140-150 K associated with pseudogap opening. Below T*, the radial distribu tion of oxygen atoms became asymmetric, indicating that the distorted domai n is formed. A strong electron-lattice coupling is indicated by the anomalo us local lattice distortions at the cross-over temperatures characterizing various electronic transitions in the title system.