The effect of Fe-doping on superconductivity of YBa2Cu3Oy

In the present paper, an approximate band-structure treatment based on the EHMO approach is suggested and used to calculate the electronic structures of the Fe-doped superconductors YBa2Cu3-xFexOy. The present treatment gives, indeed, average band structures and average densities of states as the doping fraction increases. From investigations of the influence of the Fe-doping at the Cu-site on their properties, it is shown that as the Fe-doping fraction x in YBa2Cu3-xFexOy is raised from 0.0 to 0.5, (i) the broad anisotropic bands arising from the 1D Cu-O chains and the 2D Cu-O planes are displaced and depart from the Fermi level E(f) toward the high-energy zone by degrees, while the total electronic densities of states near E(f) are drastically decreased; (ii) the band arising from the Cu-O chains doped by Fe is gradually separated from the broad anisotropic bands arising from the 2D Cu-O planes; (iii) at the doping fraction x = 0.5, the Fe-doping results in an energy gap (about 0.2 eV) near E(F); (iv) the oxygen content is not a predominant factor for the superconducting properties of the Fe-doped Y-Ba-Cu-O system; (v) the total densities of states at E(f), N(E(f)), and their decrease caused by the Fe-doping arise mainly from the 2D Cu-O planes, which implies the important role of the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system.