Semiconductor spintronics is based on the controlled generation of localized spin densities. Finite spindensities in semiconductors have traditionally [1] been generated by external magnetic fields, by circularly polarized light sources, or by spin injection from spin-aligning materials, such as ferromagnets. Recently there has been considerable interest [2] in an alternate strategy in which edge spin densities are generated electrically via the spin Hall effect (SHE) [3, 4], i.e., in a planar device by the current of spins oriented perpendicular to the plane that is generated by and flows perpendicular to an electric field. The SHE has traditionally been thought of as a consequence of spin-dependent chirality in impurity scattering that occurs in systems with spin-orbit (SO) coupling [5, 6]. Recently it has been recognized that the SHE also has an intrinsic contribution due to SO coupling in a perfect crystal [7, 8]. In this work, we study SHE induced edge spin accumulation in a two-dimensional hole gas (2DHG) with strong SO interactions. The 2D hole layer is a part of a p-n junction light-emitting diode with a specially designed coplanar geometry which allows an angle-resolved polarization detection at opposite edges of the 2D hole system. In equilibrium the angular momenta of the spin-orbit split heavy-hole states lie in the plane of the 2D layer. When an electric field is applied across the hole channel, a nonzero out-of-plane component of the angular momentum is detected whose sign depends on the sign of the electric field and is opposite for the two edges. Microscopic quantum transport calculations show only a weak effect of disorder, suggesting that the clean limit spin-Hall conductance description (intrinsic spin-Hall effect) might apply to our system.
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