Possible influences of ionic charges, radii, softness and magnetic impurities on the superconductivity in cuprates

Cuprate model cluster calculations are performed to explore possible sensitive parameters to the stability of a cluster pair state. Effects of replacing Ca2+ by {Mg2+; Na+; K+; Sc3+; Cu+; Cu2+; Zn2+} and Cu2+ by {Mg2+; Ni2+; Zn2+} in a Ca2Cu4O4 cluster are investigated. Ionic charge is found to stabilize the pair state, while the ionic radius determines the distance to the CuO2 plane which will break the pair state. Magnetic impurities such as Ni2+ are found to reduce the pair stability. The non-magnetic Mg2+ and Zn2+ stabilize the pair state. Soft ions in the calcium position also stabilize the pair state.