Acoustoelectric effect in systems with localized states

The response of the charge carriers of a semiconductor to a traveling acoustic wave is discussed. As is well known, such a wave drags free carriers and, as a result, a dc voltage across the sample appears (the acoustoelectric effect). We show that the same phenomenon exists for hopping transport of localized electrons. There are two hopping contributions to the voltage. One is due to a drag of charged carriers by the acoustic wave along infinite percolation paths. The incubation time for this contribution is the Maxwellian relaxation time, which depends on the conductivity. The other is due to a polarization of finite conducting clusters by the acoustic wave. This polarization induces a voltage pulse when the acoustic wave is switched on, and the characteristic decay time is again of the order of the Maxwellian time. The information one can obtain from the two hopping contributions in acoustic experiments is discussed.