Lattice and electronic instabilities in oxides

The rich variety of physical properties observed in perovskite related oxides has been the subject of intensive research for many decades. The discovery of high temperature superconductivity in oxide cuprates has increased the interest in this field even more. Microscopic models for these complex systems mostly deal with strong electronic correlations and rarely address effects arising from the interplay between electronic and lattice degrees of freedom. It will be shown here that these latter effects are of crucial importance in understanding the physical properties of these systems. Especially unconventional electron-phonon interactions are introduced which arise from nonlinear polarizability effects of the oxygen ion O^2-.