METAL-INSULATOR TRANSITION.

The vast majority of naturally occurring elements on earth are characterized by having metallic behavior in the condensed state (i. e. either as solids or liquids). These materials possess itinerant electrons that produce the electrical and thermal transport properties and optical properties that we associate with metals. Most condensed matter scientists would explain the existence of itinerant electrons in the solid in terms of the electronic band structure, and we start with that approach here. Although elements that are insulators at normal pressures can be forced into the metallic phase by the application of extraordinarily high pressures, most experiments on the metal-insulator (MI) transition have involved heterogeneous systems that are much easier to study. We limit our discussion to the technologically important doped semiconductors and amorphous semiconductor-metal alloys, in which both disorder and Coulomb interactions between charges play important roles in altering the optical, magnetic, and transport properties near the transition.