| Popis: |
Results for the single-particle density of states and the conductivity were obtained for both the attractive-and repulsive-U Hubbard models. At half-filling the densities of states for both models are identical, but the gap for the attractive case arises from the formation of charge-density-wave and superconducting correlations, while for the repulsive-U Hubbard model the gap is the Mott-Hubbard gap and arises from the antiferromagnetic, Coulomb, correlations. Hubbard chains were studied by use of a generalization of Handscomb`s quantum Monte Carlo scheme. Monte Carlo calculations of the two-particle vertex of the 2D repulsive-U Hubbard model were carried out. Criteria for determining whether a system is insulating, metallic, or superconducting were investigated; it was found for lattice models (Hubbard, Holstein, etc.) that this is determined by the value of the current- function. |
