The Coulomb gap: The view of an experimenter

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.