In a bulk GaAs crystal, an unusual magnetoresistance effect, which takes place when a spin-polarized current flows through the sample, was detected. Under conditions of optical pumping of electron spins, an external magnetic field directed along the electric current and perpendicular to the oriented spins decreases the resistance of the material. The phenomenon is due to the spin-dependent scattering of electrons by neutral donors. It was found that the sign of the magnetoresistance does not depend on the sign of the exciting light circular polarization, the effect is even with respect to the sign of the spin polarization of the carriers, which indicates a correlation between the spins of optically oriented free electrons and electrons localized on donors.

Exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment were studied. The negligible (less than 0.2 μeV) value of the anisotropic exchange interaction in these QDs prevents the mixing of the excitonic basis states and makes the formation of spin-polarized bright excitons possible under quasi-resonant, circularly polarized excitation. The recombination and spin dynamics of excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons was observed in the magnetic field, that eliminates the impact of nuclear spin fluctuations. A kinetic model which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics was developed to quantitatively describe the experimental data.

We report on the experimental and theoretical studies of optical alignment and optical orientation effects in an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. The dependences of three Stokes parameters on the magnetic field applied in the Faraday geometry are measured under continuous wave resonant excitation of the exciton photoluminescence. Theoretical model is developed to take into account both bright and dark exciton states in the case of strong electron and hole exchange interaction and random in-plane orientation of the nanoplatelets in ensemble. The data analysis allows us to estimate the time and energy parameters of the bright and dark excitons. The optical alignment effect enables identification of the exciton and trion contributions to the photoluminescence spectrum even in the absence of a clear spectral line resolution.

The dynamics of the photoluminescence negative circular polarization of the InP/(In,Ga)P quantum dots ensemble was studied. We find that in the time-resolved dependences of the polarization there are no oscillations in Voigt magnetic field. Also, with increasing field the polarization declines to zero. Such behavior is attributed to the peculiarities of the negatively charged exciton spin dynamics, particularly, to the fact that in the negatively charged exciton ground state the spin dynamics is governed by the heavy hole. We show that magnetic field depolarization of the photoluminescence occurs once the field of dynamically polarized nuclear spins acting on electron spins is surpassed.

The dynamics of the photoluminescence negative circular polarization of an ensemble of InP/(In,Ga)P quantum dots is studied. It was found that, the time-resolved polarization has no oscillations in the magnetic field in the Voigt geometry. At the same time, with increasing field, the polarization decreases to zero. This effect is explained by the specificities of the spin dynamics of a negatively charged exciton; in particular, by the fact that, in the ground state, its spin dynamics is determined by a heavy hole. It is shown that, in order that the photoluminescence be depolarized by a magnetic field, it is necessary to overcome the field of dynamically polarized nuclear spins acting on the electron spins.

The spin dynamics of positively (X+) and negatively (X-) charged excitons in InP/In0.48Ga0.52P quantum dots subject to magnetic field is studied. We find that a characteristic feature of the system under study is the presence of nuclear quadrupole interaction, which leads to stabilization of the nuclear and electron spins in a quantum dot in zero external magnetic field. In detail, the nuclear quadrupole interaction leads to pinning of the Overhauser field along the quadrupole axis, which is close to the growth axis of the heterostructure. The nuclear effects are observed only when resident electrons are confined in the quantum dots, i.e., for X- trion photoexcitation. The presence of X- and X+ trion contributions to the photoluminescence together with the quadrupole interaction significantly affects the dynamics of optical orientation in Voigt magnetic field. In the absence of dynamic nuclear spin polarization the time evolution of the photoluminescence polarization is fitted by a form which describes the electron spin relaxation in "frozen" nuclear field fluctuations. In relatively large external magnetic fields exceeding 60 mT good agreement between theory and experiment is achieved.

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

Photoluminescence (PL) negative circular polarization (NCP) dynamics of InP/InGaP quantum dots (QDs) was studied. Time resolved measurements of PL demonstrated that NCP vanishes, when transverse magnetic field is applied, while oscillations of polarization (that are typical for both low-dimensional and bulk materials) do not occur. Hole g-factor spread in the QD ensemble was supposed to be the most probable reason for such NCP magnetic field behavior. The dependence of NCP dynamics on the repetition period of excitation laser pulses was investigated. In case of fairly small repetition period (T = 13.3 ns) long living NCP (13.3 ns < t < 133 ns) was detected, what was ascribed to resident electron spin orientation, accumulated during many laser pulses. In that regime more than one luminescence polarization decay time exist.