The isotope effect in ferroelectric and superconducting oxides

Abstract

A double-shell model with coupled double well potentials combines the ferroelectric instability due to anharmonic electron-phonon interaction with an electronic instability, i.e. anharmonic electron-electron coupling. The application of the self consistent phonon approximation either leads to a transition to a ferroelectric state or a transition to a paired electron (hole) state. Numerical investigations of an isotope effect on the respective transition temperatures T_F, T_s yield an increase of T_F for the ferroelectric state and a decrease of T_s for the paired electron state with increasing mass.