Nature and Scaling Properties of the Intermediate Phase of the Impurity Band Metal-Insulator Transition

The transport properties of semiconductor impurity bands are almost universal near the transition, and they exhibit power-law behavior over composition ranges 100 times larger than are found for magnetic critical transitions. This behavior can be explained by the existence of an intermediate phase. Several explanations for this new phase are contrasted and compared. The new phase may have been observed near the stiffness transition in network glasses, and near optimal doping in cuprate high-temperature superconductors.