Current Research Projects

The research in our group is centered on the physics of semiconductors, using GaAs/AlAs heterostructures as our primary tool. We fabricate our own heterostructures via molecular beam epitaxy, process them into measuring devices via optical and e-beam lithography, and measure their response in high magnetic fields and at low temperatures.

Very clean (low-disorder) quantum-confined carrier systems, and measurements of their electronic transport properties, are a means to an end in semiconductor physics. The systems we study, namely high-quality two-dimensional electron and hole systems in selectively doped GaAs/AlAs heterojunction structures and quantum wells, are among the cleanest carrier systems available. In these structures, the mobile carriers are spatially separated from the dopant (impurity) atoms to minimize scattering. As a result, the mean-free-path of carriers at low temperatures reaches several microns, so that ballistic and phase-coherent transport can be studied. Such structures also provide a crucial and important test bed for new many-body physics, since the dominant interaction at low temperatures is the repulsion between the electrons themselves.

Some of our current projects are described below.

Spin interference in GaAs 2D holes - Observation of Berry's Phase via Aharonov-Bohm Measurements

AFM picture of an Aharonov-Bohm ring

An important and, at times, mysterious concept in modern physics, is the phase factor that a quantum mechanical wave function acquires upon a cyclic evolution. This phase factor can lead to interference phenomena which are experimentally observable. We have been studying this quantum effect by measuring the Aharonov-Bohm oscillations in high-mobility GaAs two-dimensional hole systems grown by molecular beam epitaxy (MBE). The most recent result of our work has been published in Phys. Rev. Lett. 88, 146801 (2002), and also featured by Physics Portal in the journal Nature on March 26, 2002 (Report links: Physics Portal*,PDF, JPEG).

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Valley Occupation Control in AlAs using In-plane Strain

In wide AlAs quantum wells, electrons occupy two valleys in the conduction band: ellipsoids centered at the XX and XY points in the Brillouin zone. By applying uniaxial stress to the sample, the deformation potential lifts the degeneracy of these two valleys, resulting in a transfer of charge from one valley to another. Since the valleys are ellipsoids, the effective mass of electrons varies with direction, and it is possible to observe this charge transfer as a change in the resistance of the sample. With a large enough applied strain, it is possible to tune an AlAs quantum well from a two valley to a single valley system. Appl. Phys. Lett. 85, 3766 (2004).

Dependence of resistance on valley occupation due to strain

Asymmetric Double Quantum Wells in AlAs

Double quantum wells are interesting in their own right. By providing an additional degree of freedom to the two-dimensional system, a variety of unique states can be observed. However, the barrier between wells must be thin, on the order of the electron-electron separation within the well, and hence tunneling can be a problem. Fortunately, in AlAs the location of the conduction band minimum depends on the well width, due to strain effects. This can be exploited by creating a structure in which the electrons reside in different parts of the Brillouin zone, dramatically reducing the tunneling between layers. Phys. Rev. Lett. 92, 186404 (2004).

Diagram of AlAs/GaAs heterostructure showing strain-determined valley occupation

Properties of GaAs Bilayer 2D Holes

For some time now, experimentalists have been seeking a Bose-Einstein condensate of excitons. In ordinary semiconductors the exciton lifetime is too short to allow a condensate to form. However, in GaAs bilayer 2D systems in high magnetic fields, electrons in one layer can form excitons with holes in the other layer, with long recombination times due to the barrier between layers. Thus, a condensate is possible. Probably the clearest example of such condensation is the dissipationless current flow observed in GaAs 2D electron and hole bilayers.

Data showing current flow in bilayer and counterflow geometries