Some problems of principle in the discovery of the Coulomb gap and associated experimental investigations in doped semiconductors are considered. Under certain conditions the gap exists in the low-energy electron excitation spectrum in the insulator state of a doped semiconductor (and other disordered systems) as a result of the electron-electron interaction, The Coulomb gap is thus closely related to low-temperature electron transport properties, for example, in the variable-range hopping regime, and the insulator-metal transition phenomenon. The Coulomb gap collapses just at the critical point of the transition. reflecting a divergence of the dielectric constant. Far from the transition at strong and small compensations, the gap observed is described by the Efros-Shklovskii single-electron model; at moderate compensations it is anomalously narrowed probably by multiple-electron correlations in hopping. Thus the Coulomb interaction turned out to be very important for formation of the insulator state, especially at high electron densities (so-called 'Coulomb glass') including the pretransition range, where this state disappears toward the critical point. [References: 34]
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