Competition between hole superconductivity and ferroelectricity in oxides

Abstract

The relationship between ferroelectricity and high-temperature superconductivity is investigated. The proposed approach combines an extension of Hirsch's description of the oxygen hole superconductivity and the coupling to an anharmonic lattice. Using standard variational principles, the superconducting order parameter and the static lattice distortion are calculated for certain sets of model parameters. Comparison of the free energies of different possible phases shows to what extent the anharmonic lattice influences superconductivity. This depends mainly on the modulation of the hopping interaction which couples the superconducting subsystem (holes) to the lattice. In particular, the phase boundaries are obtained for different hole concentrations. There is a parameter regime where superconductivity, and ferroelectricity coexist. However, both order parameters (the energy gap and the lattice distortion) are reduced as compared to the pure phases. In the paraelectric regime the anharmonic lattice instability supports the superconducting state.