  We find that the Rashba spin splitting is intrinsically a nonlinear function
of the momentum, and the linear Rasha model may overestimate it significantly,
especially in narrow-gap materials. A nonlinear Rashba model is proposed, which
is in good agreement with the numerical results from the eight-band kp theory.
Using this model, we find pronounced suppression of the D'yakonov-Perel' spin
relaxation rate at large electron densities, and a non-monotonic dependence of
the resonance peak position of electron spin lifetime on the electron density
in [111]-oriented quantum wells, both in qualitative disagreement with the
predictions of the linear Rashba model.
