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 Tc for superconductivity as well as the fast decrease of Tc 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.