Electronic structures of superconductors YBa2Cu3?x M x O y (M=Sn,Ni)

Based on the EHMO approach, the energy band structures for superconductors YBa₂Cu_3–x Sn _x O _y (y>7) and YBa₂Cu_3–x Ni _x O_y (y<7) were calulated in the present paper. The influence of the cation doping at the Cu site in the unit cell and the oxygen content on their electronic structures was studied. The results showed that the cation doping at the Cu site resulted in the great decreases in the bandwidths of the broad anisotropic Cu-O bands and the densities of states. In YBa₂Cu_3–x Sn _x O _y , however, these decreases are compensated by the increase in the oxygen content caused by the Sn-doping, which results in a small change in the total densities of states. For YBa₂Cu_3–x Ni _x O _y , the effect of the doping on its electronic structures in dominant. The Ni-doping, therefore, results in a great change in the electronic structures. In addition, the study on the projected densities of states of the Ni-doped system revealed that the 2D Cu-O planes in the Y-Ba-Cu-O system played a dominant role in superconductivity.