  In the absence of an external field, the Rashba spin-orbit interaction (SOI)
in a two-dimensional electron gas in a semiconductor quantum well arises
entirely from the screened electrostatic potential of ionized donors. We adjust
the wave functions of a quantum well so that electrons occupying the first
(lowest) subband conserve their spin projection along the growth axis (Sz),
while the electrons occupying the second subband precess due to Rashba SOI.
Such a specially designed quantum well may be used as a spin relaxation
trigger: electrons conserve Sz when the applied voltage (or current) is lower
than a certain threshold V*; higher voltage switches on the Dyakonov-Perel spin
relaxation.
