THEORETICAL EVIDENCE FOR C-AXIS PHONON-PLASMON COUPLING AND PHONON RENORMALIZATION IN LA2CUO4

A suitable decomposition of the electronic density response and of the electron-phonon interaction into local and nonlocal contributions is extended to the nonadiabatic regime taking dynamical screening into ac count. It is shown for La2CuO4 that the already large (nonlocal) adiab atic electron-phonon coupling is further strongly enhanced for certain phonons along the Lambda similar to (0, 0, 1) direction. In this regi on of wave-vector space we also have a low-lying plasmon mode which mi xes strongly with the phonons of appropriate symmetry leading to an an omalous renormalization for certain phonons. The coexistence of phonon like and plasmonlike resonances in the same frequency range is argued to be a specific feature of the layered high-temperature superconducto rs.