Colloquia

The standard model of electronic structure in solids is founded on the notion that electrons inevitably delocalize. In contrast, strong Coulomb repulsion in certain transition-metal oxide compounds can cause electron localization, resulting in the so-called "Mott-insulator" state. Cuprate superconductors consist of electronically-doped Mott insulators. Much of the continuing controversy over how to understand the cuprates concerns the issue of whether one can apply more or less conventional concepts of delocalized electrons, or whether radical new concepts are necessary. I will present experimental evidence, especially from neutron scattering, that the competition been kinetic and Coulomb energies leads to spatial inhomogeneities of charge carriers and antiferromagnetic correlations. It is possible that dynamic inhomogeneities are essential to achieving superconductivity at high temperature.