Competition of anharmonic electron-phonon and electron-electron interaction in ferroelectric and superconducting oxides

Abstract

An extended version of the polarizability model for displacive type ferroelectrics combines the local anharmonic electron-phonon interaction with an anharmonic electron-electron interaction. Two types of phase transitions are attributed to the respective double-well potentials which either yield a polar ferroelectric state (electron-phonon) or a frozen in shell-shell configuration (electron-electron) which - in a quantum mechanical analogue - can be identified as a paired electron (hole) state. The lattice dynamics of BaPb0.75Bi0.15O3 are calculated within this model and compared to experimental data. The isotope effect on both transition temperatures is investigated using the self-consistent phonon approximation. The transition temperature to the ferroelectric state increases with increasing mass while an insignificant decrease is observed for the paired electron state. The transformation of the classical Hamiltonian to an extended Peierls-Hubbard type Hamiltonian admits the determination of the parameter regimes where pairing of electrons (holes) occurs and thus possibly superconductivity may be observed.