Effects of the Backward Scattering in Two-Dimensional Electron System

Effects of the backward scattering with large momentum transfer are examined in two-dimensional electron system with a special emphasis on electrons around ($\pi$,0), (0,$\pi$). The phase diagram is shown in the plane of temperature $T$ and hole doping $\delta$ in the mean field approximation and it is found that d-wave superconductivity, antiferromagnetism and $\pi$-triplet pair can coexist near half filling.