Electric field dependence of the spin relaxation anisotropy in (111) GaAs/AlGaAs quantum wells

Time-resolved optical spectroscopy experiments in (111)-oriented GaAs/AlGaAs quantum wells (QWs) show a strong electric field dependence of the conduction electron spin relaxation anisotropy. This results from the interplay between the Dresselhaus and Rashba spin splitting in this system with C3v symmetry. By varying the electric field applied perpendicular to the QW plane from 20 to 50 kV cm^-1 the anisotropy of the spin relaxation time parallel (τ^s∥) and perpendicular (τ^s) to the growth axis can be first canceled and eventually inversed with respect to the one usually observed in III-V zinc-blende QW (τ^s= 2τ^s∥). This dependence stems from the nonlinear contributions of the k-dependent conduction band spin splitting terms which begin to play the dominant spin relaxing role while the linear Dresselhaus terms are compensated by the Rashba ones through the applied bias. A spin density matrix model for the conduction band spin splitting including both linear and cubic terms of the Dresselhaus Hamiltonian is used which allows a quantitative description of the measured electric field dependence of the spin relaxation anisotropy. The existence of an isotropic point where the spin relaxation tensor reduces to a scalar is predicted and confirmed experimentally. The spin splitting compensation electric field and collision processes type in the QW can be likewise directly extracted from the model without complementary measurements.