Critical covalence and superconductivity in Ln2−xCexCuO4

The variations of superconductive properties with x of the n-type Ln_2−xCe_xCuO₄ (Ln = La_0.5Nd_0.5, Nd, or Gd) systems have been investigated. As the size of Ln³⁺ decreases, (i) the solubility limit x of Ce decreases, (ii) the value of x at which a transition from antiferromagnetic semiconductor to superconductor occurs increases, and (iii) the width Δx of the superconductive region decreases. The decreasing solubility of Ce with decreasing size of Ln³⁺ is due to decreasing tensile strain in the CuO₂ sheets. The progressive shift of the semiconductor to superconductor transition to higher x values with decreasing size of Ln³⁺ is explained on the basis of increasing electrostatic Madelung energy E_M caused by decreasing Cu---O bond length. A larger E_M means a larger charge transfer gap Δ and a smaller covalent-mixing parameter λ and bandwidth W; so a decreasing size of Ln³⁺ necessitates a higher level of Ce-doping in order to achieve a critical covalence essential for superconductivity to occur.