Dielectric function near a polarization catastrophe

The approach to the metal insulator transition from the insulating phase is presented when viewed as a divergence of the dielectric constant (polarization catastrophe). Results of optical measurements are compared with theory within the framework of a scaling picture of the transition. 

In addition, recent developments, both theoretical and experimental, in regard to the role played by the low frequency hopping modes, characteristic of the disordered system, in the description of the transition are described.