It is theoretically predicted that large charge fluctuations are induced in the copper oxides by the interplay of the electron transfer between Cu- and O-ions and the Coulomb interaction. The interaction between the charge fluctuations and the LO phonons strongly enhances the spectral density of the vibration in the metallic samples. This theoretical prediction was confirmed by electron energy loss spectroscopy; the loss intensity at the vibration frequency in the superconducting samples of YBa2Cu3O7-δ is much stronger than that in the non-superconducting samples. The enhancement of the spectral intensity effectively strengthens the electron-phonon interaction and causes the enhancement of Tc.
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