NOVEL 1 N EXPANSION FOR SELF-ENERGY AND CORRELATION-FUNCTIONS OF THE HUBBARD-MODEL/

The novel slave-free (i.e. without slave bosons and fermions) approach to the problem of strong correlations is used in studying the Hubbard (t-J) model, where the self-energy is expressed in terms of charge an d spin vertex functions of Hubbard operators. A systematic 1/N expansi on (N is the number of spin components) of the self-energy is carried out. In O(1) it gives the same quasiparticle spectrum as the slave bos on theory. The next order O(1/N) of the self-energy Sigma(k, omega) is calculated exactly by solving analytically the integral equations for the charge and spin vertices. As an example for the feasibility of th e proposed method explicit results for Sigma(k, omega) are given in th e case J = 0. Dealing only with observable quantities our method is tr ansparent and easily applicable to models which exclude double occupan cy on lattice sites, such as the Anderson model and the Emery model fo r high temperature superconductors.