A study of the attractive Hubbard model by means of the COM is reported in Refs. [76,77,78]. In particular the following properties have been computed:
The COM results have been compared with the ones obtained by:
The Hubbard model with on-site attractive interaction has been studied by means of the Composite Operator Method. At zero temperature, when the strength of the attractive interaction equals the bandwidth, the system exhibits a phase transition to a pair state, where all the electrons are locally paired . The temperature which controls the crossover to the pair state is calculated as a function of . Some properties, as the double occupancy and the spin magnetic susceptibility , show a good agreement with numerical data. For strong attractive interaction the static uniform susceptibility is strongly depressed; increases by increasing and tends to zero as . Analytical and numerical calculations show that there is a critical value of the interaction, , where the system exhibits a metal-insulator transition. At zero temperature it is found that , where is the bandwidth .