EXCITATIONS AND RESPONSE FUNCTIONS OF THE DOPED 2-DIMENSIONAL HUBBARD-MODEL - A RANDOM-PHASE-APPROXIMATION ANALYSIS

We calculate the excitation spectrum and the response functions of the textures induced by doping in the half-filled Hubbard model. Localized states appear within the antiferromagnetic gap. The spin-wave spectrum is distorted, and shifted towards lower energies; its total strength is reduced. The antiferromagnetic peak at (pi,pi) is broadened and moves to higher energies. Charge excitations appear at low energies and mix with the spin waves. Their polarization induces screened interactions that can overcome the initial, repulsive, ones. The general features of the results cannot be described in conventional-Fermi-liquid or, alternatively, insulator terms.