  A spin relaxation mechanism is proposed based on a second-order spin-flip
intersubband spin-orbit coupling together with the spin-conserving scattering.
The corresponding spin relaxation time is calculated via the Fermi golden rule.
It is shown that this mechanism is important in symmetric GaAs (110) quantum
wells with high impurity density. The dependences of the spin relaxation time
on electron density, temperature and well width are studied with the underlying
physics analyzed.
