K. Vakili

Valley susceptibility of an interacting two-dimensional electron system.

We report direct measurements of the valley susceptibility, the change of valley population in response to an applied symmetry-breaking strain, in an AlAs two-dimensional electron system. As the two-dimensional density is reduced, the valley susceptibility dramatically increases relative to its band value, reflecting the systems strong electron-electron interaction. The increase has a remarkable resemblance to the enhancement of the spin susceptibility and establishes the analogy between the spin and valley degrees of freedom.

Low-temperature, in situ tunable, uniaxial stress measurements in semiconductors using a piezoelectric actuator

We demonstrate the use of a piezoelectric actuator to apply, at low temperatures, uniaxial stress in the plane of a two-dimensional electron system confined to a modulation-doped AlAs quantum well. Via the application of stress, which can be tuned in situ and continuously, we control the energies and occupations of the conduction-band minima and the electronic properties of the electron system. We also report measurements of the longitudinal and transverse strain versus bias for the actuator at 300, 77, and 4.2 K. A pronounced hysteresis is observed at 300 and 77 K, while at 4.2 K, strain is nearly linear and shows very little hysteresis with the applied bias.

Two-dimensional electrons occupying multiple valleys in AlAs

Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the X-points of the Brillouin zone. These valleys have large effective mass and g -factor compared to the standard GaAs electrons, and are also highly anisotropic. With proper choice of well width and by applying symmetry-breaking strain in the plane, one can control the occupation of different valleys thus rendering a system with tuneable effective mass, g -factor, Fermi contour anisotropy, and valley degeneracy. Here we review some of the rich physics that this system has allowed us to explore. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin susceptibility of two-dimensional electrons in narrow AlAs quantum wells.

We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45 A wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band valley, rendering a system similar to two-dimensional electrons in Si-MOSFETs but with only one valley occupied. We observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased, following closely the prediction of quantum Monte Carlo calculations and continuing at finite values through the metal-insulator transition.

Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom.

We report measurements of the spin susceptibility, chi proportional, variant g(v)g*m*, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one ellipsoidal conduction-band valley to another (g(v) is the valley degeneracy, and m* and g* are the electron effective mass and g factor). At a given density, when the two valleys are equally populated (g(v)=2), the measured g*m* is smaller than when only one valley is occupied (g(v)=1). This observation counters the common assumption that a two-valley two-dimensional system is effectively more dilute than a single-valley system because of its smaller Fermi energy.

Spin-Dependent Resistivity at Transitions between Integer Quantum Hall States

The longitudinal resistivity at transitions between integer quantum Hall states in two-dimensional electrons confined to AlAs quantum wells is found to depend on the spin orientation of the partially filled Landau level in which the Fermi energy resides. The resistivity can be enhanced by an order of magnitude as the spin orientation of this energy level is aligned with the majority spin. We discuss possible causes and suggest a new explanation for the spikelike features observed at the edges of quantum Hall minima near Landau level crossings.

Spin–valley phase diagram of the two-dimensional metal–insulator transition

The metallic behaviour of the resistivity observed at low temperatures in low-disorder, dilute, two-dimensional (2D) carrier systems is of considerable interest as it defies the scaling theory of localization in two dimensions1. Although the origin of the metallic behaviour remains unknown and controversial, there is widespread evidence that the spin degree of the freedom plays a crucial role. Here, we directly probe the role of another discrete electronic degree of freedom, namely the valley polarization. Using symmetry-breaking strain together with an in-plane magnetic field to tune the valley and spin polarizations of an AlAs 2D electron system at fixed density, we map out a spin–valley phase diagram for its metal–insulator transition. The insulating phase occurs in the quadrant where the system is sufficiently spin and valley polarized. This observation establishes the equivalent roles of spin and valley degrees of freedom in the 2D metal–insulator transition.

Giant low-temperature piezoresistance effect in AlAs two-dimensional electrons

We present piezoresistance measurements in modulation doped AlAs quantum wells where the two-dimensional electron system occupies two conduction band valleys with elliptical Fermi contours. Our data demonstrate that, at low temperatures, the strain gauge factor (the fractional change in resistance divided by the sample’s fractional length change) in this system exceeds 10 000. Moreover, in the presence of a moderate magnetic field perpendicular to the plane of the two-dimensional system, gauge factors up to 56 000 can be achieved. The piezoresistance data can be explained qualitatively by a simple model that takes into account intervalley charge transfer.

Effective mass suppression upon complete spin-polarization in an isotropic two-dimensional electron system

We measure the effective mass

Realization of an Interacting Two-Valley AlAs Bilayer System

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