Structural and energetic aspects of defects in high-Tc oxides from atomistic lattice simulations

Abstract

This paper reports recent atomistic lattice calculations of specific aspects of point defects related to high-T_c superconductivity. They are Jahn–Teller effects in La₂CuO₄, the defect chemistry of potential electron superconductors and the dielectric contribution to bipolaron formation in La₂CuO₄ and Nd₂CuO₄. It is shown that Jahn–Teller effects can be introduced empirically into defect calculations and that in the case of La₂CuO₄ these effects favour polaron formation by 0.2–0.4 eV. From an examination of the stability of holes and defect electrons in a range of ternary cuprates it is shown why La₂CuO₄ leads only to hole conductors and Nd₂CuO₄ and Pr₂CuO₄ only to electron conductors. Calculations show important differences between the interaction of Cu and O holes in La₂CuO₄. In both cases a confining potential is found for interplanar small polaron pairs and an attractive interaction for intraplanar O hole pairs in the small-polaron limit. The interaction of defect electrons in Nd₂CuO₄ shows neither of these characteristics.