Pseudogap and strong correlation effects in the d–p model

The superconducting (SC) state and the anomalous metallic phase of the high-Tc superconducting materials (HTSC) exist in the vicinity of the Mott insulator. In the metal near the Mott insulator, doped holes create new state inside the charge transfer gap called as in-gap state. The bandwidth of the coherent component of the in-gap state is approximately proportional to the hole doping rate δ in the underdoped region, and the quasi-particles of the in-gap state interact with each other through the superexchange interaction Js, which has been explicitly shown in the d–p model with the help of the slave-boson technique and the 1/N-expansion theory. Thus the bandwidth is comparable or smaller than Js in the underdoped region. This situation induces strong antiferromagnetic (AF) fluctuations and SC fluctuations with the help of the quasi-2-dimension effect. We treat the AF fluctuations in the fluctuation-exchange (FLEX) approximation and the SC fluctuations in the self-consistent t-matrix approximation, and calculate the SC transition temperature Tc, the AF transition temperature TN, the NMR relaxation rate 1/T1. We obtain the results, which yield a unified view accounting for essential features of the phase diagram observed in HTSC. The narrow in-gap state and large Js are the key factors for describing the features of the phase diagram in the underdoped region.