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

TL;DR

This study investigates how memory effects in ac hopping conductance in the quantum Hall regime relate to DX$^-$ centers, revealing dependencies on cooling rate and illumination, and suggesting defect-related origins.

Contribution

It demonstrates the influence of cooling conditions and illumination on ac conductance and electron density, highlighting the role of DX$^-$ centers in these memory effects.

Findings

Conductance and electron density depend on cooling rate.

Infrared illumination alters conductance and density.

Universal correlation between conductance and density regardless of change method.

Abstract

Using simultaneous measurements of the attenuation and velocity of surface acoustic waves propagating along GaAs/Al$_{0.3}$Ga$_{0.7}$As 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, $\sigma (\omega)$, and the sheet electron density, $n_s$, 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 $\sigma (\omega)$ and $n_s$. Remarkably, the correlation between $\sigma (\omega)$ and $n_s$ is \emph{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.