ENHANCED ELECTRON-PHONON COUPLING AND PHONON-PLASMON MIXING IN THE HIGH-TEMPERATURE SUPERCONDUCTORS

It is shown, using La2CuO4 as an example, that the already large (nonl ocal) electron-phonon coupling as calculated in the adiabatic approxim ation is further strongly enhanced for certain phonons in the nonadiab atic regime, where dynamical screening comes into play. At the same ti me, a low-lying plasmon mode of the electronic system appears for wave vectors in the (0,0,1) direction which mites strongly with the phonon s of appropriate symmetry and leads in addition to the phonons to very large changes of the crystal potential. Such a coexistence of phononl ike and plasmonlike modes in the same frequency range is argued to be a specific feature of the high-temperature superconductors(HTSC's) rel ated to their layered structure and weak screening of the strong ionic forces in c direction. It leads to a significant increase of the reta rded attractive interaction.