Momentum and doping dependence of the electron spectrum and ($\pi$,0) feature in copper oxide materials

The momentum and doping dependence of the electron spectrum of copper oxide materials in the underdoped regime is studied within the t-J model. It is shown that the pseudogap opens near ($\pi$,0) point in the Brillouin zone, and the electron dispersion exhibits the flat band around ($\pi$,0), which are consistent with the experiments.