The Hubbard model with on-site attractive interaction (negative-U Hubbard model) is studied by means of the composite operator method in the static approximation for the case of half filling. At zero temperature, when the strength of the attractive interaction equals the band width, the system exhibits a phase transition to a pair state, where all the electrons are locally paired. The temperature T_p which controls the cross over to the pair state is calculated as a function of U. The double occupancy and the spin magnetic susceptibility are calculated for various values of U and T. For strong attractive interaction is strongly depressed, and tends to zero as T->T_P.Comments:Refs. 9, Tabs. 0, Figs. 4.
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