Electron–Electron Interaction and Isotope Effects in Superconducting Metal–Hydrogen (Deuterium) Systems Like PdH x (D x )

Using various experimental and theoretical data the origin of the inverse isotope effect (IIE) for the superconducting transition temperature T c of the PdH(D) system is analyzed on the basis of a solution of the Berk‐Schrieffer equations. It is found that the IIE can be described at weak or intermediate anharmonicity of the electron–optical phonon interaction β = (λ   2 D /   2 H−1≈to 0.14, standard or slightly enhanced values of Coulomb pseudopotential μ', and weak spin fluctuations λ sp ≈ 0.05 to 0.1. The importance of the electron‐electron interaction in determining the superconducting properties of non‐stoichiometric PdH x (D x ) and some other hydrides (deuterides) is emphasized.