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The physics of low-dimensional electron systems continues to be an active field of research; using two examples we show how double-layer electron gases can extend these investigations. First, after reviewing the properties of 2D–2D tunnelling, we present equilibrium tunnelling measurements between a two-dimensional electron gas (2DEG) and an array of independent and identical 1D wires. Using a parallel magnetic field full information about the wires can be obtained, their 1D energies and wave functions, and hence the functional form of the confining potential. In the second case, the Coulomb drag (where a voltage is induced in one layer when a current is passed through the other) is measured between two separately contacted and electrically isolated 2DEGs. At temperatures comparable to the Fermi temperature, the measured transresistivity shows a plasmon enhancement. A comparison between theory and experiment shows the importance of many-body corrections to the coupled plasmon modes.

Electron–electron interactions between two closely spaced electron gases can be directly measured by the drag voltage induced in one layer when a current is driven through the other. We have used such drag measurements of GaAs/AlGaAs double quantum wells to probe both the electronic excitations within each layer and the interlayer Coulomb interaction. Two contrasting regimes have been explored. First, at high temperatures we measure an enhanced drag due to the “antiscreening” of the coupled plasmon modes – a result which is in good agreement with theoretical predictions. Moreover, we show the importance of many-body corrections to the plasmon modes, enabling us to test a local-field description of the correlations. Second, drag measurements in the quantum Hall regime show more pronounced structure than the Shubnikov–de Haas oscillations; the drag is more sensitive to spin splitting at high filling factors and enhances the features seen at low filling factors. We show that the localised states away from the Landu level centre influence the drag.

We present measurements in a perpendicular magnetic field of the frictional drag between two closely spaced, but electrically isolated, two-dimensional electron gases. At high temperatures, when the Landau level structure is smeared out, the transresistivity shows a magnetic field dependence and an approximately linear temperature dependence. As the temperature is lowered below 20 K, the transresistivity shows structure reflecting the formation of Landau levels and, in general, the drag is more sensitive to the spin splitting of the Landau levels than the Shubnikov - de Haas oscillations. When the Fermi energy lies at the centre of a Landau level, the drag can be enhanced by two orders of magnitude over the zero field signal. In contrast, when lies between Landau levels in the quantum Hall regime, the drag signal tends to zero. We have also measured the transverse voltage in the drag layer and find no evidence for a Hall transresistance.

We have studied electron transport in two closely spaced, electrically isolated, two-dimensional electron gases (2DEGs) using GaAsAl/GaAs double quantum wells. A frictional drage voltage due to interlayer electron-electron (e-e) scattering is induced in one layer when a current is passed through the other. We present new experimental data based on a detailed carrier-density dependence of these e-e interactions. We have shown, in particular, that the interlayer scattering rate is a function of the relative electron densities in the layers, with a maximum scattering rate when the two carrier densities are matched.

At temperatures comparable to the Fermi temperature, we have measured a plasmon enhanced Coulomb drag in a GaAs/AlGaAs double quantum well electron system. This measurement provides a probe of the many-body corrections to the coupled plasmon modes, and we present a detailed comparison between experiment and theory testing the validity of local field theories. Using a perpendicular magnetic field to raise the magnetoplasmon energy we can induce a crossover to single-particle Coulomb scattering., Comment: plain tex, 4 pages, 3 figures, to be published in PRL