The Coulomb gap: the view of an experimenter

Some problems of principle in the discovery of the Coulomb gap and associat ed experimental investigations in doped semiconductors are considered. Unde r certain conditions the gap exists in the low-energy electron excitation s pectrum in the insulator state of a doped semiconductor (and other disorder ed systems) as a result of the electron-electron interaction, The Coulomb g ap is thus closely related to low-temperature electron transport properties , for example, in the variable-range hopping regime, and the insulator-meta l transition phenomenon. The Coulomb gap collapses just at the critical poi nt of the transition. reflecting a divergence of the dielectric constant. F ar from the transition at strong and small compensations, the gap observed is described by the Efros-Shklovskii single-electron model; at moderate com pensations it is anomalously narrowed probably by multiple-electron correla tions in hopping. Thus the Coulomb interaction turned out to be very import ant for formation of the insulator state, especially at high electron densi ties (so-called 'Coulomb glass') including the pretransition range, where t his state disappears toward the critical point.