Ground-state study of a donor-acceptor model with finite charge-transfer energy

A donor-acceptor model with randomly distributed N donor and N acceptor sit es and with finite charge transfer energy Delta is investigated at zero tem perature by means of computer simulations on two-dimensional samples of dif ferent sizes, We demonstrate that the density of one-electron excitations g (epsilon) for accepters in the vicinity of the Fermi energy mu depends esse ntially on the size of the sample investigated and that g(epsilon) further away from mu exhibits a size-independent power-law with the non-universal e xponent decreasing with increasing Delta. Rigorous relations for mu and for the mapping of g(epsilon) for donors into g(epsilon) for accepters are als o given. (C) 2000 Elsevier Science B.V. All rights reserved.