Nature of Doped Electrons in Nd_2−xCe_xCuO₄: Electronic Band Structures of Nd₂CuO₄ Type M₂CuO₄ (M=La, Ce and Th)

抄録

One-electron energy band structures for La₂CuO₄, Ce₂CuO₄ and Th₂CuO₄ with the Nd₂CuO₄ type crystal structure were calculated by the self-consistent APW method with the local density approximation. The bonding-antibonding splitting between the Cu 3d(x²−y²) and O 2p states on the CuO₂ plane makes a pair of sub-bands. In La₂CuO₄, the Fermi energy cuts the middle of antibonding band resulting in the half-filled band. By replacing La with tetravalent Th, the Cu 3d states raise due to the increase in the number of Cu 3d electron but the bonding band made from Th 6d(3z²−r²) state falls down. In Ce₂CuO₄, the Fermi energy cuts the bottom of 4f bands. To raise the 4f states, a correction is introduced to the Ce potential and then Ce becomes nearly tetravalent resulting in similar band structure to that of Th₂CuO₄. By considering the strong correlation in the Cu 3d(x²−y²) state, the doped electrons in Nd_2−xM_xCuO₄ (M=Ce, Th) are thought to be injected into the M d(3z²−r²) bonding band.