Percolative nanostructure and superconductivity in the mixed crystalline cuprate Nd1 - xPrxBa2Cu3O7 - δ

The Pr-substituted cuprate Nd_{1 - x}Pr_xBa₂Cu₃O_{7 - δ} (NdPr-123) has been investigated with particular attention on the relationship between the solid solution structure and the superconducting properties concerning the resistivity ρ and diamagnetic susceptibility χ. The compound NdPr-123 is regarded as a mixed crystalline solid solution that consists of unit-cells of Nd-123 and Pr-123, where a percolative nanostructure is constructed. The properties ρ and χ are largely characterized by such structural inhomogeneity. Several characteristic points (x₁, x₂, x₃) are observed in χ as a function of x. Occurrence of these characteristic points is well explained in terms of a nonclassical percolation process, where the Nd-123 cell adjacent to the Pr-123 cells is converted into a different cell, thereby creating virtual percolation thresholds that appear as the characteristic points. The effective Pr-concentration x^* is newly considered in reference to the cell-conversion. The concept of x^* will provide clues for understanding various issues on the Pr-substituted cuprates. In addition, as an alternative case, a thoroughly different type of effective Pr-concentration has also been discussed in reference to the irregular substitution of Pr at Ba sites, which may happen to take place in RPr-123 (R = rare earth including Nd).