T. Gokmen

Transference of transport anisotropy to composite fermions

It is widely expected that the properties of composite fermions should be independent of those of the electrons and flux quanta from which they emerge. Measurements of anisotropic transport in AlAs quantum wells suggest this is not the case.

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.

Ferromagnetic fractional quantum Hall states in a valley-degenerate two-dimensional electron system.

We study a two-dimensional electron system where the electrons occupy two conduction band valleys with anisotropic Fermi contours and strain-tunable occupation. We observe persistent quantum Hall states at filling factors nu=1/3 and 5/3 even at zero strain when the two valleys are degenerate. This is reminiscent of the quantum Hall ferromagnet formed at nu=1 in the same system at zero strain. In the absence of a theory for a system with anisotropic valleys, we compare the energy gaps measured at nu=1/3 and 5/3 to the available theory developed for single-valley, two-spin systems, and find that the gaps and their rates of rise with strain are much smaller than predicted.

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

We measure the effective mass

Density dependence of valley polarization energy for composite fermions

({m}^{\ensuremath{\ast}})

Evidence for developing fractional quantum Hall states at even denominator 1/2 and 1/4 fillings in asymmetric wide quantum wells.

of interacting two-dimensional electrons confined to a 4.5-nm-wide AlAs quantum well. The electrons in this well occupy a single out-of-plane conduction-band valley with an isotropic in-plane Fermi contour. When the electrons are partially spin polarized,

Contrast between spin and valley degrees of freedom

{m}^{\ensuremath{\ast}}

Spin susceptibility of interacting two-dimensional electrons with anisotropic effective mass

is larger than its band value and increases as the density is reduced. However, as the system is driven to full spin-polarization via the application of a strong parallel magnetic field,

Characterization of the anomalous giant piezoresistance in AlAs two-dimensional electrons with anti-dot lattices

{m}^{\ensuremath{\ast}}

Anomalous giant piezoresistance in AlAs 2D electron systems with antidot lattices.

is suppressed down to values near or even below the band mass. Our results are consistent with the previously reported measurements on wide AlAs quantum wells where the electrons occupy an in-plane valley with an anisotropic Fermi contour and effective mass, and suggest that the effective mass suppression upon complete spin-polarization is a genuine property of interacting two-dimensional electrons.