[111] heterostructures for spin devices

The symmetry properties of [111] quantum wells (QWs) and superlattices, and in particular the fact that the symmetry is not reduced when bulk inversion asymmetry (BIA) is supplemented with structural inversion asymmetry [SIA (Rashba)], have important consequences for spin dynamics and transport. We compute the effective spin Hamiltonians for [111] and [110] structures, and show that for the [111] case the splitting can be made to vanish to lowest order when BIA and SIA effects are of equal strength. As a consequence, the D'yakonov Perel' spin relaxation mechanism is suppressed for all spin components. This effect forms the base for an improved version of the Datta-Das and a recently proposed family of spin transistors. For [110]-grown QWs, the effect of SIA is to augment the spin relaxation rate of the component perpendicular to the well. We derive analytical expressions for the spin lifetime tensor and its proper axes, and see that they are dependent on the relative magnitude of the BIA- and SIA-induced splittings.