It is important to understand the phase daigram of cuprate high-temperature superconductors to clarify the mechanism of high-temperature superconductivity and also to design new superconducting materials. We investigate the ground state of the electronic models for high-temperature cuprates and clarify the phase diagram. We employ the optimization variational Monte Carlo method based on improved wave functions that take account of inter-site electron correlation and can be improved systematically. In our method the ground-state energy is lowered greatly, which gives the best estimate of the ground-state energy for the Hubbard model and also the three-band d-p model. The many-body effect plays an important role as an origin of superconductivity. We show the phase diagram by varying the Coulomb repulsive interaction U,the band parameters t', the level difference between d and p electrons, and the electron density.

Keywords: high-temperature superconductivity, strongly correlated electron systems, optimization Monte Carlo method, phase diagram