The metal-nonmetal transition: a global perspective

A wide range of condensed matter systems traverse the metal–nonmetal transition. These include doped semiconductors, metal–ammonia solutions, metal clusters, metal alloys, transition metal oxides, and superconducting cuprates. Certain simple criteria, such as those due to Herzfeld and Mott, have been highly successful in explaining the metallicity of materials. In this article, we demonstrate the amazing effectiveness of these criteria and examine them in the light of recent experimental findings. We then discuss the limitations in our understanding of the phenomenon of the metal–nonmetal transition