Memory effects in ac hopping conductance in the quantum Hall effect regime: Possible manifestation of DX centers

Using simultaneous measurements of the attenuation and velocity of surface acoustic waves propagating along GaAs/Al0.3Ga0.7As heterostructures, complex ac conductance of the latters has been determined. In the magnetic fields corresponding to the middles of the Hall plateaus both the ac conductance, �(!), and the sheet electron density, ns, in the two-dimensional conducting layer turn out to be dependent on the samples’ cooling rate. As a result, the sample “remembers” the cooling conditions. The complex conductance is strongly dependent on an infrared illumination which also changes both �(!) and ns. Remarkably, the correlation between �(!) and ns is universal, i. e. it is independent of the way to change these quantities. The results are attributed to two-electron defects (so-called DX centers) located in the Si doped layer.