Interaction phenomena in 2D electrons confined toAlAs quantum wells

Mansour Shayegan

Princeton University, USA

Two-dimensional (2D) electrons in AlAs quantum wells occupy multiple conduction-band minima (or valleys) at the X point of the Brillouin zone. These valleys have large effective mass and g-factor compared to the standard GaAs electrons, and are also highly anisotropic. The system is rather unique in that, with proper choice of well width and by applying in situ 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 with single, double, or triple valley degeneracy. By adding a magnetic field, we obtain a system which allows us to explore very rich physics ensuing from the valley and spin degrees of freedom in a strongly interacting system.
In this presentation, I will highlight some of our latest results on 2D electrons confined to wide (thickness > 5nm) AlAs quantum wells where the electrons reside in two in-plane valleys whose occupation can be controlled via the application of strain. Our studies include spin susceptibility and valley susceptibility (dependence of valley population on applied strain) measurements which show strong enhancements of both of these susceptibilities at low densities, attesting to the dominant role that interaction plays. I will then present the results of our effective mass measurements, via analyzing the temperature dependence of the Shubnikov-de Haas oscillations, in this system. When the 2D electrons occupy only one conduction band valley and are partially spin-polarized, the effective mass is larger than its band value and increases as the density is reduced, consistent with previous results in various 2D carrier systems. This is the expected behavior for a 2D system of interacting Fermions. An unexpected trend emerges, however, as the 2D electron system is fully spin polarized by subjecting it to a strong parallel magnetic field: the effective mass falls below the band value and tends to decrease with decreasing density! Evidently the mass renormalization is dramatically modified when the system is spin polarized. I will also discuss the fate of this mass suppression when the second valley is occupied, as well as its relation to the 2D metal-insulator transition problem.