Enhancement of Cu-O vibrational modes and superconductivity inYBa2Cu3O7−δstudied by x-ray photoelectron spectroscopy and high-resolution electron-energy-loss spectroscopy

We find peaks around 60 meV (∼484 ${\mathrm{cm}}^{-1\mathrm{}}$) in high-resolution electron-energy-loss spectroscopy spectra for ortho- and tetra-$\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. It is found that the peak intensity for superconductors (ortho-Y-Ba-Cu-O) is strongly enhanced as compared with nonsuperconductors (tetra-Y-Ba-Cu-O). From these results, it is suggested that this enhancement is due to the strong coupling of phonons with charge fluctuations.