Spin current swapping and Hanle spin Hall effect in a two-dimensional electron gas

We analyze the effect known as ``spin current swapping'' (SCS) due to electron-impurity scattering in a uniform spin-polarized two-dimensional electron gas. In this effect a primary spin current ${J}_{i}^{a}$ (lower index for spatial direction, upper index for spin direction) generates a secondary spin current ${J}_{a}^{i}$ if $i\ensuremath{\ne}a$, or ${J}_{j}^{j}$, with $j\ensuremath{\ne}i$, if $i=a$. Contrary to naive expectation, the homogeneous spin current associated with the uniform drift of the spin polarization in the electron gas does not generate a swapped spin current by the SCS mechanism. Nevertheless, a swapped spin current will be generated, if a magnetic field is present, by a completely different mechanism, namely, the precession of the spin Hall spin current in the magnetic field. We refer to this second mechanism as Hanle spin Hall effect, and we notice that it can be observed in an experiment in which a homogeneous drift current is passed through a uniformly magnetized electron gas. In contrast to this, we show that an unambiguous observation of SCS requires inhomogeneous spin currents, such as those that are associated with spin diffusion in a metal, and no magnetic field. An experimental setup for the observation of the SCS is therefore proposed.