The Coulomb quasigap and the metal-insulator transition in granular systems

The temperature dependence of DC conductivity in granular systems is analysed in terms of the Coulomb quasigap. It is pointed out that whereas the stability criterion proposed by Efros and Shklovskii (1975) gives BE² as the upper bound for the quasigap density of states rho (E), in granulars the functional dependence of rho (E) does not necessarily follow that of the E ²-bound. Instead, the detailed shape of rho (E) is a function of the grain size distribution. The authors show that sigma approximately exp(-A/T ¹2/) behaviour observed experimentally could arise from a density of states which is a combination of an impurity contribution and the Coulomb quasigap for the grains. The existence of a second quasigap is conjectured.