L. H. Ho

Zeeman splitting in ballistic hole quantum wires

We have studied the Zeeman splitting in ballistic hole quantum wires formed in a (311)A quantum well by surface gate confinement. Transport measurements clearly show lifting of the spin degeneracy and crossings of the subbands when an in-plane magnetic field B is applied parallel to the wire. When B is oriented perpendicular to the wire, no spin splitting is discernible up to B = 8.8 T. The observed large Zeeman splitting anisotropy in our hole quantum wires demonstrates the importance of quantum confinement for spin splitting in nanostructures with strong spin-orbit coupling.

Ballistic transport in induced one-dimensional hole systems

The authors have fabricated and studied a ballistic one-dimensional p-type quantum wire using an undoped AlGaAs∕GaAs heterostructure. The absence of modulation doping eliminates remote ionized impurity scattering and allows high mobilities to be achieved over a wide range of hole densities and, in particular, at very low densities where carrier-carrier interactions are strongest. The device exhibits clear quantized conductance plateaus with highly stable gate characteristics. These devices provide opportunities for studying spin-orbit coupling and interaction effects in mesoscopic hole systems in the strong interaction regime where rs>10.

0.7 structure and zero bias anomaly in ballistic hole quantum wires

We study the anomalous conductance plateau around G=0.7(2e2/h) and the zero bias anomaly in ballistic hole quantum wires with respect to in-plane magnetic fields applied parallel B parallel and perpendicular B perpendicular to the quantum wire. As seen in electron quantum wires, the magnetic fields shift the 0.7 structure down to G=0.5(2e2/h) and simultaneously quench the zero bias anomaly. However, these effects are strongly dependent on the orientation of the magnetic field, owing to the highly anisotropic effective Landé g-factor g* in hole quantum wires. Our results highlight the fundamental role that spin plays in both the 0.7 structure and zero bias anomaly.

The interplay between one-dimensional confinement and two-dimensional crystallographic anisotropy effects in ballistic hole quantum wires

In this paper, we study the Zeeman spin-splitting in hole quantum wires aligned along the and crystallographic axes of a high-mobility (311)A undoped AlGaAs/GaAs heterostructure. We obtained measurements of the effective g-factor g* as a function of the wire width for in-plane magnetic fields aligned both parallel and perpendicular to the wire axis. We interpret our data in terms of a qualitative model that involves the interplay of two effects—1D confinement and 2D crystalline anisotropy.

Effect of screening long-range Coulomb interactions on the metallic behavior in two-dimensional hole systems

We have developed a technique utilizing a double quantum well heterostructure that allows us to study the effect of a nearby ground plane on the metallic behavior in a GaAs two-dimensional hole system (2DHS) in a single sample and measurement cooldown, thereby maintaining a constant disorder potential. In contrast to recent measurements of the effect of ground-plane screening of the long-range Coulomb interaction in the insulating regime, we find surprisingly little effect on the metallic behavior when we change the distance between the 2DHS and the nearby ground plane.

Ground-plane screening of Coulomb interactions in two-dimensional systems: How effectively can one two-dimensional system screen interactions in another

The use of a nearby metallic ground-plane to limit the range of the Coulomb interactions between carriers is a useful approach in studying the physics of two-dimensional (2D) systems. This approach has been used to study Wigner crystallization of electrons on the surface of liquid helium, and most recently, the insulating and metallic states of semiconductor-based two-dimensional systems. In this paper, we perform calculations of the screening effect of one 2D system on another and show that a 2D system is at least as effective as a metal in screening Coulomb interactions. We also show that the recent observation of the reduced effect of the ground-plane when the 2D system is in the metallic regime is due to intralayer screening.

The 0.7 anomaly in one-dimensional hole quantum wires

In this paper we study the anomalous 0.7 structure in high quality ballistic one-dimensional hole systems. Hole systems are of interest because of their large effective mass, strong spin?orbit coupling, as well as having spin?3/2 compared to spin?1/2 for electrons. We observe remarkably clean conductance quantization in a variety of different samples, and a strong feature at ~0.7 ? 2e2/h, which shows a similar temperature and density dependence to the 0.7 feature observed in electron systems. In contrast to the case for electrons, the strong spin?orbit coupling results in an anisotropic Zeeman splitting, which we use to probe the 0.7 feature and the associated zero-bias anomaly. Our results indicate that the 0.7 feature and the zero-bias anomaly are related, and both are suppressed by spin polarization. These results place valuable constraints on models of the microscopic origins of the 0.7 feature.

Electrometry using the quantum Hall effect in a bilayer two-dimensional electron system

We discuss the development of a sensitive electrometer that utilizes a two-dimensional electron gas (2DEG) in the quantum Hall regime. As a demonstration, we measure the evolution of the Landau levels in a second, nearby 2DEG as the applied perpendicular magnetic field is changed, and extract an effective mass for electrons in GaAs that agrees within experimental error with previous measurements.

Quantum transport in one-dimensional GaAs hole systems

In many advanced semiconductor devices, the physical dimensions are sufficiently small that quantum physics becomes important in determining the device behaviour. A celebrated example is the quantum wire, where in the absence of scattering the conductance is quantised in units of 2e²/h. Although electron quantum wires have been studied extensively for almost two decades, the development of hole quantum wires has been a significant challenge, limiting studies of hole-based devices. Here we review our recent work on hole quantum wires, and show how they can be used to probe the spin properties of hole systems. The ability to fabricate ballistic quantum wires, and control their spin properties using electrical gate biases, may have implications for future spintronic devices.

Origin of the hysteresis in bilayer two-dimensional systems in the quantum Hall regime

The hysteresis observed in the magnetoresistance of bilayer 2D systems in the quantum Hall regime is generally attributed to the long time constant for charge transfer between the 2D systems due to the very low conductivity of the quantum Hall bulk states. We report electrometry measurements of a bilayer 2D system that demonstrate that the hysteresis is instead due to non-equilibrium induced current. This finding is consistent with magnetometry and electrometry measurements of single 2D systems, and has important ramifications for understanding hysteresis in bilayer 2D systems.