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

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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, σ(ω), 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 σ(ω) and nₛ. Remarkably, the correlation between σ(ω) 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.