Effect of Zn and Ni substitution on the local electronic structure of the YBa2Cu3O7 superconductor -: art. no. 214509

Embedded cluster calculations at the MP2 electron correlation level of pure and Zn- and Ni-doped YBa2Cu3O7(Y123) ceramics reveal great changes of the local charge distribution and crystal bonding in the vicinity of impurities. In the Zn-doped ceramics the obtained changes are stronger and more extended than in the Ni-doped ceramics. This can be one of explanations of the stronger depression of T-c due to Zn. The calculated increase of positive charge for both impurities, comparing with substituted Cu, indicates that the impurity atoms lose more negative charge than Cu. As a result, all nearest CuO2 units around impurity lose their holes: completely in the Zn case and partly in the Ni case. This leads to the breaking of the hole pairs in the vicinity of impurity and correlated with an increase of the coherence length in Zn- and Ni-doped Y123 crystals measured by Timomoto [Phys. Rev. B 60, 114 (1999)]. Both Zn and Ni impurities, the latter to a less extent, act as effective pair breakers due to the elimination of holes on adjacent CuO2 groups.