On the Magnetic and Metal-Insulator Transitions in a Two-Dimensional Hubbard Model

抄録

Antiferromagnetic and metal to insulator transitions in the ground state of two-dimensional half-filledand nearly half-filled Hubbard models with the nearest neighbor and the second nearest neighbor transfers (-t and t^′, respectively) are studied by using the Green's function approach with the self-energy up to the second order in the coupling constant U.For a choice of the parameter value t^′/t=0.2the half-filled model undergoes the first order phase transition between paramagnetic metallic (PM) phase and antiferromagnetic insulator (AFI) phase at U/t=4.0 while the first order transition between PM and antiferromagnetic metallic (AFM) phases takes place for nearlyhalf-filled band. The one-electron spectral density is calculated for the PM, AFM, and AFI phases near the phase boundaries. In the PM phase the quasi-particle dispersion indicates the bandnarrowing from the Hartree-Fock results and the flat band behavior as wasindicated by previous stadies and was observed in high-T_c cuprates. The quasi-particle dispersion in the AFM phase is seen to be close to the Hartree-Fock results.