Thermodynamics and transport properties of interacting systems with   localized electrons
  The review of the modern results in the theoretical and experimental study of the localized interacting electrons is given. After theoretical prediction of the Coulomb gap and the new temperature law in the variable range hopping conduction many efforts have been made to discover it experimentally. The most important of them are attempts to demonstrate experimentally that the long range Coulomb interaction is not screened in this system and that it is the origin of the gap. The next stage begins when the recent experiments have discovered glassy properties of the interacting system with localized electrons. This review is concentrated on the origin of the glassy properties of this system and it connects the glassy properties with another interesting phenomenon - insulator-metal transition. 
A. L. Efros