Pseudogap Phenomena and Phase Diagram in the 2-Band Hubbard Model

Abstract

High-Tc superconducting materials (HTSC) have anomalous properties such as the pseudo-gap or spin-gap etc., in Hall coefficient, 1/T1T and the density of states etc. First including effects of strong on-site repulsion between d-electrons at Cu-sites, we obtain quasi-particles with super-exchange interaction Js, whose band width tends to zero, i.e., the system goes to insulator, as the hole-doping rate tends to zero. The quasi-particles correspond to Zhang-Rice singlet states. Js larger than the band width combined with the 2-dimensional character of the system induces strong antiferro-magnetic (AF) and superconducting (SC) fluctuations. We treat effects of the AF ones in the FLEX approximation and those of the SC ones in the self-consistent t-matrix approximation to show that both fluctuations in the under-doped region start to increase at T0 as T decreases from T>>Tc, the AF ones dominate the SC ones at T>Tsg, while SC ones dominate at T<Tsg. This cross-over of the fluctuations causes the anomalous phenomena in the under-doped region. We also obtain the phase diagram of HTSC consistent to one observed in experiments.