Interplay of superconductivity with structural phases in a generalized t-J model

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 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 at zero temperature at the doping delta (c) similar to 0.14 for J/t = 0.3. For delta < <delta>(c) the flux phase coexists with superconductivity at low and exists as a single phase at higher temperatures. Due to the different origins of the two instabilities the flux phase boundary and especially delta (c) is much less sensitive to impuritiy scattering than the boundary for superconductivity in agreement with experiments in Zn doped La - 214 and (Y, Ca)- 123.