Theoretical evidence forc-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 account. It is shown for La₂CuO₄ that the already large (nonlocal) adiabatic electron-phonon coupling is further strongly enhanced for certain phonons along the (0, 0, 1) direction. In this region of wave-vector space we also have a low-lying plasmon mode which mixes strongly with the phonons of appropriate symmetry leading to an anomalous renormalization for certain phonons. The coexistence of phononlike and plasmonlike resonances in the same frequency range is argued to be a specific feature of the layered high-temperature superconductors.