Interacting Anderson insulators: The intrinsic electron glass

The out-of-equilibrium transport properties of Anderson insulators exhibit many glassy features such as slow relaxation, slow approach to a steady state, ageing, and other memory effects. These non-equilibrium features were theoretically anticipated and are believed to result from the interplay between disorder and interactions. Here we review the field-effect experiments that establish the generic nature of electron glasses. It is pointed out that the common feature to all systems that exhibit the characteristic electron-glass properties is their relatively high carrier concentration. Experiments performed as a function of the carrier concentration indicate that below a critical value, the relaxation time becomes very short. It is suggested that this may be the reason why lightly doped semiconductors do not exhibit intrinsic electron-glass effects. The possible connection between the Coulomb gap and the memory dip is discussed.