Ferroelectricity and superconductivity. Competing or cooperating phenomena

The discovery of high temperature superconductivity in layered systems has evoked a heated discussion about the microscopic electron (hole) pairing mechanism. In particular, the proximity of these systems to magnetic phases has been taken as evidence for a purely electronic mechanism. However, structural anomalies and unconventional isotope effects suggest that the lattice plays a crucial role and must be incorporated in modeling the mechanism. Here we show that polarizability effects and Jahn-Teller centers are of special importance, since these enable dynamical charge transfer, interband interactions, polaron and bipolaron formation. In this respect the layered superconductors approach ferroelectrics, where dynamical covalency is known to trigger the structural instability. (orig.)