We present a general unifying theory for spin polarization decay due to theinterplay of spin precession and momentum scattering that is applicable to bothspin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify andcharacterize a wide range of qualitatively different regimes. For strongmomentum scattering or slow spin precession we recover the D'yakonov-Perelresult, according to which the spin relaxation time is inversely proportionalto the momentum relaxation time. On the other hand, we find that, in theballistic regime the carrier spin polarization shows a very differentqualitative behavior. In systems with isotropic spin splitting the spinpolarization can oscillate indefinitely, while in systems with anisotropic spinsplitting the spin polarization is reduced by spin dephasing, which isnon-exponential and may result in an incomplete decay of the spin polarization.For weak momentum scattering or fast spin precession, the oscillations ornon-exponential spin dephasing are modulated by an exponential envelopeproportional to the momentum relaxation time. Nevertheless, even in this casein certain systems a fraction of the spin polarization may survive at longtimes. Finally it is shown that, despite the qualitatively different nature ofspin precession in the valence band, spin polarization decay in spin-3/2 holesystems has many similarities to its counterpart in spin-1/2 electron systems.
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