K. Das Gupta

Anomalous Coulomb Drag in Electron-Hole Bilayers

We report Coulomb drag measurements on GaAs-AlGaAs electron-hole bilayers. The two layers are separated by a 10 or 25 nm barrier. Below T approximately 1 K we find two features that a Fermi-liquid picture cannot explain. First, the drag on the hole layer shows an upturn, which may be followed by a downturn. Second, the effect is either absent or much weaker in the electron layer, even though the measurements are within the linear response regime. Correlated phases have been anticipated in these, but surprisingly, the experimental results appear to contradict Onsagers reciprocity theorem.

Zero-bias anomaly in quantum wires

We use quantum wires fabricated on undoped GaAs/AlGaAs heterostructures in which the average impurity separation is greater than the device size to compare the behavior of the zero-bias anomaly against predictions from Kondo and spin-polarization models. Both theories display shortcomings, the most dramatic of which is the linear electron-density dependence of the zero-bias anomaly spin splitting at fixed magnetic field

Fabrication and characterization of ambipolar devices on an undoped AlGaAs/GaAs heterostructure

B

Low temperature transport in undoped mesoscopic structures

and the suppression of the Zeeman effect at pinch off.

Distinguishing impurity concentrations in GaAs and AlGaAs using very shallow undoped heterostructures

We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are, instead, induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional electrons (μpeak = 4 × 106 cm2/Vs) and holes (μpeak = 0.8 × 106 cm2/Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory.

Fabrication of closely spaced, independently contacted electron-hole bilayers in GaAs-AlGaAs heterostructures

Undoped GaAs/AlGaAs heterostructures in which carriers are attracted from the Ohmic contacts by a voltage bias on an insulated top gate allows higher mobilities to be obtained at lower electron densities than is possible with modulation doped heterostructures. However a two level gating scheme and an Ohmic contacting process that maximizes lateral diffusion are necessary to fully exploit the advantages of the undoped system for fabricating lower dimensional mesoscopic structures. Ionized background impurities (at low densities) and interface roughness (at high densities) are found to be the dominant sources of scattering. An approximate length scale set by the number of impurities the interfacial wave function intersects is observed in the magnetoconductance of two-dimensional mesoscopic regions.

Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well

We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.

Experimental Progress towards Probing the Ground State of an Electron-Hole Bilayer by Low-Temperature Transport

We describe a technique to fabricate closely spaced electron-hole bilayers in GaAs-AlGaAs heterostructures. Our technique incorporates a novel method for making shallow contacts to a low density (

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device

10^{5}{\rm cm}^{2}{\rm V}^{-1}{\rm s}^{-1}

Critical currents and vortex-unbinding transitions in quench-condensed ultrathin films of bismuth and tin

and an electron mobility