Analysis of the dynamics of noncentral ions in superconductors

The temperature-Green's-function method and the diagram technique are used to analyze the dynamics of noncentral ions, i.e., ions moving in a two-well potential, in superconductors. The screening by conduction electrons hardens the vibrations of the noncentral ions, while the superconducting correlations soften these vibrations, by weakening the screening effect. Two mechanisms are considered in a description of the effect of superconducting correlations on the ion dynamics. One is the BCS model. The other is the local hole-pair superconductivity model. A simple expression is derived for the temperature dependence of the mean square displacement of the noncentral ions in both the normal and superconducting phases