Energy spectra for the Hubbard andt-Jmodels

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Abstract
Using symmetry considerations and exact diagonalization techniques, we examine the quantum numbers of the states that define the antiferromagnetic and superconducting gaps in the multiple-particle spectrum of the repulsive and attractive two-dimensional Hubbard models. Studying the behavior of the lowest-energy levels as a function of the chemical potential, we analyze all possible dopings in 10- and 16-site clusters for many values of U/t. The possibility of superconductivity in the repulsive Hubbard and in the t-J models is discussed.