NONADIABATIC AND NONLOCAL ELECTRON-PHONON INTERACTION AND PHONON-PLASMON MIXING IN THE HIGH-TEMPERATURE SUPERCONDUCTORS

Within the framework of an electronic density response approach, recen tly worked out to describe screening, lattice dynamics, and the electr on-phonon interaction in the high-temperature superconductors, it is s hown, using La2CuO4 as an example, that the already large (nonlocal) e lectron-phonon coupling as calculated in the adiabatic approximation i s further strongly enhanced for certain phonons in the high-temperatur e superconductors (HTSC's) in the nonadiabatic regime where dynamical screening comes into play. At the same time, a low-lying plasma mode o f the electronic system appears for wave vectors in the (0,0,1) direct ion which mixes strongly with the phonons of appropriate symmetry and leads in addition to the phonons to very large changes of the crystal potential. Such a coexistence of phononlike and plasmonlike modes in t he same frequency range is argued to be a specific feature of the HTSC 's related to their layered structure and weak screening of the strong ionic forces in the c direction. It leads to a significant increase o f the retarded attractive and provides, at least in part, an understan ding of the reduction of the oxygen isotope effect in these materials, which is difficult to explain for a pure (harmonic) electron-phonon s uperconductor.