Driving the persistent spin helix and electron-hole density waves with an electric field in n-GaAs quantum wells

We use transient spin grating spectroscopy to study the persistent spin helix (PSH) state of a 2D electron gas in the presence of an in-plane electric field parallel to the wavevector of the spin helix. By directly measuring its phase, we can measure the PSH displacement with 10 nm spatial and sub-picosecond time resolution. We demonstrate that the phase velocity of the PSH crosses zero at a nonzero wavevector. The data indicate that spin Coulomb drag may play a role in the spin wave drifting process. We also study the displacement of electron-hole density waves (EHDW's) as a function of electric field. Although charge neutral, the EHDW is found to drift, but with with a velocity that is much smaller than that of the surrounding electrons. We speculate that the drift is caused by a Coulomb drag interaction between the Fermi sea and the EHDW.