dx2−y2-Density Wave and dx2−y2-wave Superconducting Gap on the Extended Hubbard Model on a Square Lattice: Approach to Finite Temperature Properties

We investigate the finite temperature properties of the extended Hubbard model with a nearest-neighbor Coulomb repulsion on the square lattice. Solving the self-consistent equations obtained using our mean-field theory [J. Phys. Soc. Jpn. 88, 084706 (2019)] numerically, we obtain three-dimensional phase diagrams of the dx2−y2-density wave (DDW) and dx2−y2-wave superconducting (DSC) order parameters according to temperature and hole concentrations. The obtained phase diagrams show three distinctive regions: underdoped, optimal, and overdoped. Within the underdoped region, the ratios of the order parameters at absolute zero temperature to the critical temperatures are in approximate agreement with the experimental results of cuprate high Tc superconductors. The optimal region exhibits a unique reentrant phenomenon regarding the hole concentration. In the overdoped region, the DDW disappears and the DSC becomes normal.