SINGLE-PARTICLE SPECTRAL DENSITY OF THE HUBBARD-MODEL

We calculate the single-particle spectral function for the Hubbard mod el within the framework of a projection technique equivalent to the tw o-pole approximation. We show that the two-pole approximation can be w ell understood as an average characterization of the upper and the low er Hubbard bands, rather than describing a dispersive quasiparticle. B y comparing with numerical spectra of finite Hubbard rings and of a 4 x 4 cluster [P. W. Leung et al., Phys. Rev. B 46, 11 779 (1992)], we s how that the present approximation is capable of reproducing essential properties of the single-particle spectral function. In particular; t he two-pole spectrum is characterized by a direct gap, in agreement wi th the exact spectrum. We emphasize the role of local antiferromagneti c correlations.