Role of the electron-phonon interaction in the strongly correlated cuprate superconductors

Using high-resolution angle-resolved photoemission data in conjunction with that from neutron and other probes, we show that electron-phonon coupling is strong in cuprate superconductors and that it plays an important role in pairing. In addition to the strong electron correlation, the inclusion of phonons provides a theoretical framework explaining many important phenomena that cannot be understood by a strongly correlated electronic model alone. In particular, it is indispensable to explain the difference among materials. The phonons with the wavenumber around the (0, q(y)) and (q(x), 0) axes create d-wave pairing while that near (pi, pi) are pair breaking. Therefore the half-breathing mode of the oxygen motions helps d-wave superconductivity.