Pseudogap phenomena and phase diagram in the two-band Hubbard model

High-T-C superconducting materials (HTSC) have anomalous properties such as the pseudogap in the Hall coefficient. 1/T1T and the density of states etc. First, including the effects of strong on-site repulsion between d-electrons at Cu-sites. we obtain quasi-particles with superexchange interaction J(s), whose band-width tends to reach zero. i.e., the system goes to the insulator, as delta tends to reach zero. The quasi-particles correspond to Zhang nice singlet states. J(s) larger than the band width combined with the two-dimensional characteristic of the system induces strong antiferromagnetic fluctuations (AFF) and superconducting fluctuations ISC:F). We treat effects of the AFF in the FLEX approximation and those of the SCF in the self-consistent t-matrix approximation to show that both fluctuations in the underdoped region start to increase at T-0 as T decreases from T much greater than T-C. and that the AFF dominate the SCF at T > T-sg while the SCF dominate at T < T-sg. This cross-over of the fluctuations causes the anomalous phenomena in the underdoped region. We also obtain the T - delta phase diagram of HTSC consistent with the one observed in experiments.