ABSTRACT
Figure 3.1 Winners of the Nobel Prize in physics in 1977: P. Anderson (a) and N. Mott (b). From the point of view of physics, this can be explained as follows. The density of electronic states at the conduction and valence bands is high. It means that the states at the same energy are separated by very thin barriers. Electrons can tunnel through these barriers under the influence of concentration gradient or weak electrical field (Fig. 3.2a). If the density of states decreases, the states of the same energy become well separated in the lattice site, so that the tunneling probability between these sites is zero (Fig. 3.1b). The states become localized. Electrons in these states cannot diffuse (Anderson’s definition) or take part in electrical conductivity (Mott’s definition). Only over-barrier electron transitions are possible in this case.
