Luyi Yang, J. D. Koralek, J. Orenstein, D. R. Tibbetts, J. L. Reno, M. P. Lilly
We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in 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 10$^7$ cm s$^{-1}$ in an applied field of 70 V cm$^{-1}$.
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http://arxiv.org/abs/1212.3033
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