Competition of an incommensurate flux phase and superconductivity in a t-J model with Coulomb interactions

Based on the X-operator approach and a 1/N expansion, we present results for the phase diagram and in particular, the incommensurate flux phase of a generalized t-J model which also contains Coulomb interactions. It is found that the optimal T-c for superconductivity as well as the fast decrease of T-c in the underdoped regime is the result of a competition between an incommensurate flux and a superconducting phase, both having order parameters with d-wave symmetry. A charge-density wave ground state is found only at rather low dopings far away from the superconducting phase. The evolution of the single-particle gap and the associated destruction of a part of the Fermi surface are studied by means of the dispersion and density of electronic states and the optical conductivity.