Effect of nickel and zinc substitutions on the electronic charge-density redistribution in aYBa2Cu3O7superconductor

It is shown, on the basis of our electronic structure calculations, that the nature of the perturbations created by Ni and Zn impurities in a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ superconductor is quite different although both impurities are divalent. While the charge-density redistribution due to Ni in its neighborhood is weak and well localized, the one due to Zn is very strong and extended. This indicates that the mechanisms of the degradation of $T_c$ by these two impurities are quite different. The extended nature of the perturbation due to Zn substitution results in strong electron scatterings not only from Zn but also from the neighboring atomic sites. The hole carrier density, which is determined by the distribution of the electronic charge, is as a result also modified locally and becomes nonuniform in the neighborhood of the Zn impurity, although there is no overall change in the carrier density. In contrast, in the case of Ni substitution it is primarily the magnetic moments at the Ni site which may be considered as the primary source for the deterioration of the superconducting transition temperature.