Abstract
AbstractA frequency‐ and momentum‐renormalization‐group acceleration together with an analytical approach is used to obtain the retarded Green's function in the self‐consistent and conserving fluctuation‐exchange (FLEX) approximation for the two‐dimensional Hubbard model in the normal state and in the superconducting state. The analytical expressions for this approach are given. For band‐fillings near half filling the self‐energy in the normal state exhibits Fermi‐liquid behaviour for, low temperatures and frequencies near the chemical potential, if the momentum is chosen near the Fermi‐surface. However due to the presence of large many body effects the observed Fermi‐liquid region near the chemical potential and near the Fermi‐surface is very small. Results for the single particle density of states, the occupation number and the spectral function are presented. The superconducting state with symmetry is obtained for U = 2 to U = 6 and a (U, n)‐phase diagram for the transition temperature Tc is presented. A maximum Tc/t of 0.0275 is obtained for U = 6 near half filling.
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