  Spin-orbit coupling provides a versatile tool to generate and to manipulate
the spin degree of freedom in low-dimensional semiconductor structures. The
spin Hall effect, where an electrical current drives a transverse spin current
and causes a nonequilibrium spin accumulation observed near the sample
boundary, the spin-galvanic effect, where a nonequilibrium spin polarization
drives an electric current, or the reverse process, in which an electrical
current generates a nonequilibrium spin polarization, are all consequences of
spin-orbit coupling. In order to observe a spin Hall effect a bias driven
current is an essential prerequisite. The spin separation is caused via
spin-orbit coupling either by Mott scattering (extrinsic spin Hall effect) or
by Rashba or Dresselhaus spin splitting of the band structure (intrinsic spin
Hall effect). Here we provide evidence for an elementary effect causing spin
separation which is fundamentally different from that of the spin Hall effect.
In contrast to the spin Hall effect it does not require an electric current to
flow: It is spin separation achieved by spin-dependent scattering of electrons
in media with suitable symmetry. We show that by free carrier (Drude)
absorption of terahertz radiation spin separation is achieved in a wide range
of temperatures from liquid helium up to room temperature. Moreover the
experimental results give evidence that simple electron gas heating by any
means is already sufficient to yield spin separation due to spin-dependent
energy relaxation processes of nonequilibrium carriers.
