  The electron spin dynamics in an optically excited narrow quantum well is
studied, where the electron spins precess in a k-dependent magnetic field while
the electrons scatter at localized impurities. For the resulting spin decay,
which is commonly known as the D'yakonov-Perel' mechansim, analytical
expressions in the strong- and weak-scattering limits are available. It is
found by the numerical solution of quantum kinetic equations in a broad range
of parameters that, in situations that are typically relevant for ultrafast
optical experiments, the dynamics of the total spin polarization significantly
deviates from the pertinent analytical results. This is attributed to the broad
spectral width of the optically excited spin-polarized electron distribution,
which gives rise to a spin dephasing due to inhomogeneous broadening.
Furthermore, it is found that the decay of the spin polarization need not be
exponential. The notion of a spin decay time becomes ambiguous and different
definitions of spin decay times can lead to different outcomes. The long-term
dynamics of the decay of the spin polarization is even dominated by an
algebraically decaying component. These findings highlight the importance of
the effects of the broad spectral distribution of optically excited carriers in
ultrashort magneto-optical experiments.
