A change between a material behaving as an electrical conductor and as an electrical insulator. The distinction concerns the limiting electronic response as temperature tends to zero; finite thermally activated electrical conductivity alone does not establish a metallic state. A Peierls transition opens a band gap through a change in the crystal structure.
The approximate donor metal-insulator threshold is often written . It estimates when overlap and screening destroy isolated donor localization. The numerical coefficient is empirical and not a universal exact result; disorder, compensation and anisotropy can shift the threshold.[ref]
The Mott criterion: So simple and yet so complex by Alexander Pergament and others. Abstract and effective-radius scaling
