Role of Cu 4s Band in Cu 2p-XPS of Nd_2-xCe_xCuO₄

The role of the Cu 4s band in the Cu 2p-XPS of Nd_2-xCe_xCuO₄ is investigated on the basis of the impurity Anderson model. The model consists of the impurity Cu 3d shell and the conduction and valence bands formed from the O 2p and Cu 4s bands under the tight-binding approximation. Because of the symmetry of the crystal, the 3d states couple weakly to the Cu 4s band so that the effect of the 4s band on the Cu 2/7-XPS is very small in general. However, in the case where the 4s band is lower by a few eV in energy than that estimated by the first-principle calculation, the 4s state is important in the electron doping process in Nd_2-xCe_xCuO₄ because the doped electron enters this state.