Zero-Bias Spin Separation in Quantum Wells

We show that in gyrotropic semiconductor structures spin-dependent asymmetry of electron scattering induce a pure spin current which, in contrast to the spin Hall effect, does not require an electric current to flow. The effect is observed in GaAs/AlGaAs quantum wells at free-carrier absorption of terahertz radiation in a wide range of temperatures from liquid helium up to room temperature. The results agree with the phenomenological description based on the symmetry. Experimental and theoretical analysis evidences unumbiguously that the observed photocurrents are spin-dependent and related to the gyrotropy of the low-dimensional structures. Microscopic theory of this effect based on asymmetry of photoexcitation and relaxation processes is developed being in a good agreement with experimental data.