Nonconventional behavior of the one-band Hubbard Hamiltonian in two dimensions

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Abstract
The wave-function renormalizaton constant Z has been calculated for the one-band Hubbard model in the square lattice. Near half-filling the Hamiltonian has been solved on finite clusters (up to 16×16) by means of the unrestricted Hartree-Fock approximation, whereas at high dilutions (two electrons) exact calculations have been carried out. The region of the parameter space where the system shows nonconventional behavior (Z=0) is close to half-filling. For a given value of the Coulomb repulsion, Z vanishes at low dopings, and become finite as doping is increased. For very large U the constant Z remains finite for all dopings, excluding half-filling. These results are cast into a plausible phase diagram.