COULOMB VERSUS MADELUNG GAP - ORDERING IN A SYSTEM OF POINT CHARGES

Spatial correlations of impurity charges in a mixed valence regime are studied with the use of Monte Carlo simulations. The influence of var ious, kinds of disorder on the one-particle density of states is deter mined. A continuous transitions from a ''soft'' Coulomb gap (density o f states vanishing only at the chemical potential) to a ''hard'' gap ( with a finite range of energies with vanishing density of states) is f ound, driven by decreasing amount of built-in disorder in the system. The ''hard'' Coulomb gap resembles the Madelung gap, found in crystall ine arrangements of charges. The similarity reflects the fact that bot h the Coulomb and Madelung gaps are manifestations of the same phenome non, resulting from ordering.of the positions of point charges, the on ly difference being the range of correlations.