CHARGE-DENSITY WAVES AND SUPERCONDUCTIVITY AS AN ALTERNATIVE TO PHASE-SEPARATION IN THE INFINITE-U HUBBARD-HOLSTEIN MODEL

We investigate the density instabilities present in the infinite-U Hub bard-Holstein model both at zero and finite momenta as well as the occ urrence of Cooper instabilities with a specific emphasis on the role o f long-range Coulomb forces. In carrying out this analysis special att ention is devoted to the effects of the strong local e-e interaction o n the e-ph coupling and particularly to both the static and dynamic sc reening processes dressing this coupling. We also clarify under which conditions in strongly correlated electron systems a weak additional i nteraction, e.g., a phonon-mediated attraction, can give rise to a cha rge instability. In the presence of long-range Coulomb forces, the fru strated phase separation leads to the formation of incommensurate char ge density waves. These instabilities, in turn, lead to strong residua l scattering processes between quasiparticles and to superconductivity , thus providing an interesting clue to the interpretation of the phys ics of the copper oxides.