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

Based on the X-operator approach and a 1/N expansion, we present results fo r the phase diagram and in particular, the incommensurate flux phase of a g eneralized 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 inco mmensurate flux and a superconducting phase, both having order parameters w ith d-wave symmetry. A charge-density wave ground state is found only at ra ther low dopings far away from the superconducting phase. The evolution of the single-particle gap and the associated destruction of a part of the Fer mi surface are studied by means of the dispersion and density of electronic states and the optical conductivity. (C) 1999 Elsevier Science B.V. All ri ghts reserved.