ORIGIN OF PHONON ANOMALIES IN LA2CUO4

The experimentally observed softening of the oxygen breathing mode at the X point, O-X(B), and of a second CuO bond stretching mode of Delta (1) symmetry at the wave vector q = (0.5,0,0) in the doped metallic ph ase of La-Cu-O is investigated and shown to be driven by long-range, n onlocal electron-phonon interaction effects of monopole charge-redistr ibution type. The amplification of softening for both modes by about 1 THz, recently found in inelastic neutron scattering for an optimally doped La1.85Sr0.15CuO4 probe as compared with an underdoped La1.9Sr0.1 CuO4 crystal is explained by the growing importance of the more extend ed orbitals in the charge response in particular at the Cu ion. Such a n enlarged contribution of the more extended orbitals leads to a reduc tion of the dominant self-interaction, U, of the electrons mainly at t he Cu and allows for an enhancement of the monopole charge redistribut ions whose magnitude determines the softening of the phonon modes. The existence of a specific mixture of localized and more extended states at the Fermi energy is crucial for the appearance of the strong nonlo cal electron-phonon coupling of ionic origin and for the reinforcement of the phonon mediated part of pairing in the high-temperature superc onductors.