Dynamic cluster quantum Monte Carlo calculations for a doped two-dimensional extended Hubbard model are used to study the stability and dynamics of
d-wave pairing when a nearest-neighbor Coulomb repulsion
V is present in addition to the on-site Coulomb repulsion
U. We find that
d-wave pairing and the superconducting transition temperature
Tc are only weakly suppressed as long as
V does not exceed
U/2. This stability is traced to the strongly retarded nature of pairing that allows the
d-wave pairs to minimize the repulsive effect of
V. When
V approaches
U/2, large momentum charge fluctuations are found to become important and to give rise to a more rapid suppression of
d-wave pairing and
Tc than for smaller
V.
The phase diagram of the t-J-V model is discussed using a 1/N expansion in terms of X operators. It is shown that a flux phase of d-wave symmetry is stabilized by the Coulomb interaction V at intermediate dopings and competes with d-wave superconductivity. Since the flux wave instability is stronger than the superconducting one optimal doping is essentially determined by the onset of the flux phase. Below optimal doping the flux phase coexists with superconductivity at low and exists as a pseudo gap phase at higher temperatures. It is also found that the flux phase boundary is much less sensitive to impurity scattering than the boundary for superconductivity in agreement with experiments in Zn doped La-214 and (Y,Ca)-123.