D-WAVE SUPERCONDUCTIVITY IN A STRONGLY CORRELATED ELECTRON-PHONON SYSTEM

The problem of phonon-mediated and phonon-free contributions to d wave superconductivity in the two-dimensional Hubbard model has been addre ssed. Strong local correlations (U-->infinity limit) have been incorpo rated in the slave boson formulation. In order to consider electron-bo son and electron-phonon coupling on equal footing, fluctuations of the auxiliary boson fields over their mean-field value have been introduc ed. The magnitude of effective pairing interactions derived with the h elp of a canonical transformation indicates that within the weak-coupl ing theory, phonon-mediated pairing plays a marginal role and can be d ominated by a phonon-free pairing mechanism. On the other hand, in the strong-coupling limit (Eliashberg formulation), electron-phonon and e lectron-phonon-boson interactions can significantly enhance the superc onducting transition temperature. It is also shown that Coulomb correl ations together with the electron-phonon interaction can effectively s tabilize a d-wave superconducting state.