Awschalom Group

Spin Hall Effect in Two-dimensional Electron Gases

The observations of the spin Hall effect and current-induced spin polarization in bulk semiconductors demonstrated that electric currents could be used to generate electron spin polarization in non-magnetic semiconductors without applied magnetic fields. Previous work also showed that strain resulted in an internal magnetic field that could act on electron spins. This internal field is due to spin-orbit coupling and is related to a breaking of inversion symmetry in the environment of electron.

Advances in materials science have made it possible to grow semiconductors layer-by-layer with high precision in the growth rate and composition, through techniques such as molecular-beam epitaxy. By varying the composition of the material, one can design a quantum well, in which electrons are confined along one dimension, and create a two-dimensional electron gas. We grew our sample on a (110) gallium arsenide substrate in order to form a (110) quantum well.

We fabricate our samples using a chemical wet etch to define a mesa along the [001] crystal direction and anneal evaporated metal to form electrical contacts to the two-dimensional electron gas. We mount these structures in a flow cryostat and measure the electron spin polarization using Kerr rotation. A microscope objective provides a ~1 micron spot size and allows us to spatially resolve electron spins. When we look near the edges of the opposite sides of the semiconductor channel, we observe electron spin polarization of opposite sign. This spin accumulation is due to the spin Hall effect. Unlike previous measurements of the spin Hall effect in bulk gallium arsenide, we notice that there appear to be 2 peaks at each edge.

We also made channels along different in-plane crystal directions and observed an out-of-plane current-induced spin polarization. The internal magnetic field produced by spin-orbit coupling depends on the direction in which the electrons are moving, and the current-induced spin polarization that we observe confirms this. When we spatially resolve the electron spin polarization near the edges of a channel that is fabricated along the [110] direction, we see the spin Hall effect as well as the spatially-uniform current-induced spin polarization. The spin accumulation on the right-hand side also seems to exhibit two peaks that are ~2 microns apart, just as in the [001] channel.

Although we do not observe an in-plane component of the current-induced spin polarization, it is possible to determine the in-plane component of the internal magnetic field. We measure the Kerr rotation of optically-injected electron spins as a function of applied magnetic field and voltage. The internal field causes the signal to shift away from zero.

Our measurement yields a value for the internal magnetic field which corresponds to a spatial spin precession period of 3.5 microns, which is close to the ~2 micron distance observed between the spin Hall peaks and suggests that the spacing between the spin Hall peaks could be due to spin precession.

To learn more about our studies, please refer to "Spatial imaging of the spin Hall effect and current-induced polarization in two-dimensional electron gases," V. Sih, R. C. Myers, Y. K. Kato, W. H. Lau, A. C. Gossard and D. D. Awschalom, Nature Physics 1, 31 (2005).