Coherent propagation of spin helices in a quantum-well confined electron gas

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in "format" a high mobility n-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 107cms-1 in an applied field of 70Vcm-1. © 2012 American Physical Society.