Low-temperature electron-spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from 10 14 cm Ϫ3 to 5ϫ10 17 cm Ϫ3 . A peculiarity related to the metal-to-insulator transition is observed in the dependence of the spin lifetime on doping near n D ϭ2ϫ10 16 cm Ϫ3 . In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interaction

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

The spin-orbit interaction in semiconductors is shown to result in an anisotropic contribution into the exchange Hamiltonian of a pair of localized conduction-band electrons. The anisotropic exchange interaction exists in semiconductor structures which are not symmetric with respect to spatial inversion, for instance in bulk zinc-blend semiconductors. The interaction has both symmetric and antisymmetric parts with respect to permutation of spin components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the strongest one. It contributes significantly into spin relaxation of localized electrons; in particular, it governs low-temperature spin relaxation in n-GaAs with the donor concentration near 10 16 cm -3 . The interaction must be allowed for in designing spintronic devices, especially spin-based quantum computers, where it may be a major source of decoherence and errors.

The mechanisms that determine spin relaxation times of localized electrons in impurity bands of n-type semiconductors are considered theoretically and compared with available experimental data. The relaxation time of the non-equilibrium angular momentum is shown to be limited either by hyperfine interaction, or by spinorbit interaction in course of exchange-induced spin diffusion. The energy relaxation time in the spin system is governed by phonon-assisted hops within pairs of donors with an optimal distance of about 4 Bohr radii. The spin correlation time of the donor-bound electron is determined either by exchange interaction with other localized electrons, or by spin-flip scattering of free conduction-band electrons. A possibility of optical cooling of the spin system of localized electrons is discussed. 1) Introduction.Strange though it may seem, 40 years of research (since the pioneering work by G.Lampel [1]) on optical orientation of electron and nuclear spins have not filled all the major blank spaces in this very interesting area of physics. Answers to many questions of primary importance are being approached just now. One of these actively developed fields with a long history is the problem of spin memorynot only in nanostructures brought forth by sophisticated novel technologies, but also in bulk semiconductor crystals. A part of this broad field, the physics of relaxation processes in the spin system of interacting localized electrons in non-magnetic n-type semiconductors, is the subject of this paper. It consists of an extended theoretical introduction -partly based on published results, partly original -followed by a survey of relevant experiments. The consideration is limited to direct-gap semiconductors, like GaAs, which are mainly studied in the experiments on optical orientation.Aiming at bringing together experiment and theory, I will concentrate at bulk crystals where localizing potentials and the concentration of localization centres are determined by doping and can be easily calculated. The concentrations and temperatures considered are those low enough for electrons to remain localized. 2) Relaxation time scales in spin systems.Non-equilibrium spin polarization created by an external pumping (for instance, by circularly polarized light) persists during some characteristic time after switching off the pump. This time is often called the spin memory time. It should be, however, noted that relaxation processes in spin systems are not, generally, characterized by a single time scale. Depending on the experimental conditions, the observed "spin memory time" can be determined by different relaxation processes.These may include, for instance, relaxation of the components of the vector of total angular momentum (the spin relaxation proper) or of the energy of the spin system. The relaxation of a K.V.Kavokin, Spin relaxation of localized electrons in n-type semiconductors page 2 of 30 component of the angular momentum along a certain axis requires only that the spin interactions lack symmetry with re...