WAVE-FUNCTION RENORMALIZATION CONSTANT FOR THE ONE-BAND HUBBARD HAMILTONIAN IN 2 DIMENSIONS

The wave-function renormalization constant Z has been calculated for the one-band Hubbard model on a square lattice. Near half-filling the Hamiltonian has been solved on finite clusters (up to 16 x 16) by means of the unrestricted Hartree-Fock (UHF) approximation. Some checks of the UHF results have been made on 4 x 4 clusters by means of the Lanczos method for U = infinity. At high dilutions (two electrons) exact calculations have been carried out. The ground-state wave functions and energies for the different cases here considered are discussed. The system shows nonconventional behavior (Z = 0) near half-filling. For a given value of U, Z vanishes at low dopings, and becomes finite as doping is increased. For very large U the value of Z remains finite for all dopings, excluding half-filling. These results are cast into a plausible phase diagram.