Elementary Remarks on High Temperature Superconductors
These materials are based on metal-oxides, including perovskites and certain atoms with incomplete inner shells, e.g., rare earths. It should be noted that more than 90% of all ferroelectrics are oxides, based on 0--. In these cases the 0-- electrons are displaced arising from mode softening. In our case the mode is not softened, but a polar oscillation ω of the 0-- electrons should exist. Its frequency would be expected to be higher than that of an ionic vibration.