Electron-phonon interactions in ferroelectrics and superconductors

The lattice instability versus a transverse optic mode observed in ferroelectrics is a direct consequence of a crucial electron-phonon interaction consisting of an attractive harmonic term and a stabilizing anharmonic repulsive fourth-order term. In conventional superconductors BCS-theory has proven that the electron-(hole)-pairing mechanism is phonon mediated. For high-Tc superconductors no consensus is yet achieved in explaining this phenomenon, but it is argued in the following that electron-phonon interactions play an important role here too if the complexity of the structure and Fermi surface are seriously considered. Opposite to ferroelectrics it is found that the attractive harmonic terms need no repulsive counterpart as the effective interactions are small. This finding leads to the immediate conclusion that in spite of various common features of ferroelectrics and superconductors, both are mutually exclusive phenomena.

Keywords:

• electron-phonon-interaction
• superconductivity
• ferroelectricity