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The Goldstone-Feynman diagrammatic technique is adopted to study the interaction between mixed-valent rare-earth impurities in BCS superconductors. The influence of cerium-impurity coupling together with isolated cerium-impurity upon superconductivity of ${\mathrm{Th}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$ system is worked out. In comparison with magnetic rare-earth impurities, depression of superconductivity due to impurity interaction of nonmagnetic mixed-valent rare-earth dopants is much smaller.

The YBa2Cu3O7−xCu3O7−x superlattices are treated as superconductor‐insulator multilayers, where CuO2 planes in the YBa2Cu3O7−x layers are taken as two‐dimensional (2D) superconductors and CuO2 planes in the PrBa2Cu3O7−x layer are described by the 2D Mott–Hubbard insulator with large Coulomb repulsion. We obtain good Tc vs number of PrBa2Cu3O7−x unit cells in a single layer by using only one adjustable parameter and without assuming charge‐carrier‐transfer effect.