Resonant cooling of the nuclear spin system of a semiconductor by spin-polarized charge carriers under pumping with helicity-modulated polarized light is considered theoretically. It is shown that in the case of strong Knight field of charge carriers, exceeding local fields of the dipole-dipole interaction of nuclear spins, the Overhauser field arising as a result of resonant cooling can considerably modify the overall shape of magnetic-field dependences of charge carrier spin polarization, experimentally observed as the Hanle effect.

The coherent spin dynamics of electrons are investigated for CsPbI3 perovskite nanocrystals in a glass matrix using time-resolved Faraday ellipticity. In nanocrystals with a diameter of about 11 nm, the Larmor precession frequency has a linear dependence on the magnetic field corresponding to the electron Land & eacute; g factor of 2.07. We find a finite Larmor precession frequency at zero magnetic field, corresponding to the electron spin splitting of 0.8 mu eV. This splitting is explained by the hyperfine interaction with nuclear spin fluctuations. Our model analysis shows that the hyperfine interaction for the conduction-band electrons is contributed both by the p orbitals of the lead atoms and by the s orbitals of the iodine atoms, with the leading contribution to the hyperfine field fluctuations coming from iodine. This fact agrees well with the 9% iodine contribution to the Bloch amplitude of the conduction band, obtained by density functional theory calculations. From these findings, the atomic hyperfine constant for the 5s orbital of iodine is evaluated as 190 mu eV.

In this paper we present experimental study of electron -nuclear spin dynamics in GaAs bulk layers doped with Mn ions at temperature 4.2 K. The electron spin dynamics is experimentally investigated by measuring the degree of polarization of photoluminescence in a transverse magnetic field (Hanle effect) and the recovery of the electron spin polarization in a longitudinal magnetic field (polarization recovery curve). To study nuclear spin dynamics, we use two -stage experimental protocol including optical cooling of nuclear spin system and measuring change of the polarisation degree of photoluminescence in different transverse magnetic fields. We show dependence of electron spin relaxation times on excitation power for three samples with different concentrations of shallow donors and acceptors. Electron spin relaxation times have been obtained as at the exciton transition as at the deep acceptor Mn transition. Also we show dependence of nuclear spin -lattice relaxation times T1 on value of external transverse magnetic field.

Optical orientation of carrier spins by circularly polarized light is the basic concept and tool of spin physics in semiconductors. We study the optical orientation of electrons and holes in a crystal of the FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ lead halide perovskite by means of polarized photoluminescence, time-resolved differential reflectivity, and time-resolved Kerr rotation. At the cryogenic temperature of 1.6 K the optical orientation degree measured for continuous-wave excitaton reaches 6% for localized electrons and 2\% for localized holes. Their contributions are distinguished from each other and from exciton optical orientation through the pronounced Hanle effect in transverse magnetic fields and the polarization recovery effect in longitudinal magnetic fields. The optical orientation degree is highly stable against detuning of the laser photon energy from the band gap by up to 0.25 eV, showing then a gradual decrease for detunings up to 0.9 eV. This evidences the inefficiency of spin relaxation mechanisms for free carriers during their energy relaxation. Spin relaxation for localized electrons and holes is provided by the hyperfine interaction with the nuclear spins. Dynamic polarization of nuclear spins is demonstrated by the Overhauser field reaching 4 mT acting on the electrons and $-76$ mT acting on the holes. This confirms the specifics of lead halide perovskite semiconductors, where the hole hyperfine interaction with the nuclei considerably exceeds that of the electron.

Nuclear magnetic resonance (NMR) is particularly relevant for studies of internuclear spin coupling at zero and ultra-low fields (ZULF), where spin-spin interactions dominate over Zeeman ones. Here we report on ZULF NMR in CdTe. In this semiconductor all magnetic isotopes have spin $I = 1/2$, so that internuclear interactions are never overshadowed by quadrupole effects. Our experiments rely on warm-up spectroscopy, a technique that combines optical pumping, additional cooling via adiabatic demagnetisation, and detection of the oscillating magnetic field-induced warm-up of the nuclear spin system via Hanle effect. We show that NMR spectra exhibit a rich fine structure, consistent with the low abundance of magnetic isotopes in CdTe, their zero quadrupole moments, as well as direct and indirect interactions between them. A model assuming that the electromagnetic radiation is absorbed by nuclear spin clusters composed of up to 5 magnetic isotopes allows us to reproduce the shape of a major part of the measured spectra.

The nuclear spin systems in CdTe/(Cd,Zn)Te and CdTe/(Cd,Mg)Te quantum wells (QW) are studied using a multistage technique combining optical pumping and Hanle effect-based detection. The samples demonstrate drastically different nuclear spin dynamics in zero and weak magnetic fields. In CdTe/(Cd,Zn)Te, the nuclear spin relaxation time is found to strongly increase with the magnetic field, growing from 3 s in zero field to tens of seconds in a field of 25 G. In CdTe/(Cd,Mg)Te the relaxation is an order of magnitude slower, and it is field-independent up to at least 70 G. The differences are attributed to the nuclear spin relaxation being mediated by different kinds of resident electrons in these QWs. In CdTe/(Cd,Mg)Te, a residual electron gas trapped in the QW largely determines the relaxation dynamics. In CdTe/(Cd,Zn)Te, the fast relaxation in zero field is due to interaction with localized donor-bound electrons. Nuclear spin diffusion barriers form around neutral donors when the external magnetic field exceeds the local nuclear field, which is about $B_L\approx $0.4 G in CdTe. This inhibits nuclear spin diffusion towards the donors, slowing down relaxation. These findings are supported by theoretical modeling. In particular, we show that the formation of the diffusion barrier is made possible by several features specific to CdTe: (i) the large donor binding energy (about 10 meV), (ii) the low abundance of magnetic isotopes (only $\approx$30% of nuclei have nonzero spin), and (iii) the absence of nuclear quadrupole interactions between nuclei. The two latter properties are also favorable to nuclear spin cooling via optical pumping followed by adiabatic demagnetization. Under non-optimized conditions we have reached sub-microkelvin nuclear spin temperatures in both samples, lower than all previous results obtained in GaAs.

In tunnel-coupled quantum wells of different widths, it was found that the spin dynamics arising from resonant pulsed optical pumping of an exciton in the narrow well includes the dynamics of electron spin polarization in the wide well, although the electron level in the wide well is 55 meV lower than the electron level in the narrow well. An analysis of the obtained results showed that the observed effect is caused by the exchange interaction of spin-polarized electrons in the wide well with exciton states in the narrow well.

In this work, we measured the correlator spectrum of optically cooled nuclear spin system of a bulk n-GaAs crystal in a zero magnetic field. The resulting spectrum is described by two contours, which can be interpreted as the participation of nuclear spins in two types of interactions. We analyzed the measured spectrum in the model for classical magnetic moments in the GaAs lattice. The analysis showed that the main contribution to the high-frequency part of the spectrum is due to the dipole-dipole interaction, and that to the low-frequency part is due to the quadrupole interaction.

We propose a method of cooling nuclear spin systems of solid-state nanostructures by applying a time-dependent magnetic field synchronized with spin fluctuations. Optical spin noise spectroscopy is considered a method of fluctuation control. Depending on the mutual orientation of the oscillating magnetic field and the probe light beam, cooling might be either provided by dynamic spin polarization in an external static field or result from population transfer between spin levels without build-up of a net magnetic moment ("true cooling").

In this work, we measured the correlator spectrum of optically cooled nuclear spin system of a bulk n-GaAs crystal in a zero magnetic field. The resulting spectrum is described by two contours, which can be interpreted as the participation of nuclear spins in two types of interactions. We analyzed the measured spectrum in the model for classical magnetic moments in the GaAs lattice. The analysis showed that the main contribution to the high-frequency part of the spectrum is due to the dipole-dipole interaction, and that to the low-frequency part is due to the quadrupole interaction.

We study the formation of exciton-polariton condensates in potlike traps created by optical pumping in a planar microcavity with embedded quantum wells. The trap is formed by a repulsive reservoir of incoherent excitons excited by a ring-shaped nonresonant laser beam. Polariton condensates confined in a trapping potential are subject to spatial confinement leading to energy quantization. We reveal experimentally the discrete spectrum of polariton eigenstates in an optical trap that can be characterized by a pair of quantum numbers, azimuthal and radial quantum numbers, that correspond to the number of nodes of a condensate wave function in the corresponding directions. The occupation numbers of the eigenstates of a polariton condensate are determined by the overlap integral of the condensate wave function and the exciton reservoir spatial density distribution. The nonresonant pumping scheme enables engineering the shape and size of the trap, that allows to selectively excite specific superpositions of the eigenstates of a polariton condensate in each experiment. We demonstrate both single-and multiple-mode polariton lasing in an optical trap.

In tunnel-coupled quantum wells of different widths, it was found that the spin dynamics arising from resonant pulsed optical pumping of an exciton in the narrow well includes the dynamics of electron spin polarization in the wide well, although the electron level in the wide well is 55 meV lower than the electron level in the narrow well. An analysis of the obtained results showed that the observed effect is caused by the exchange interaction of spin-polarized electrons in the wide well with exciton states in the narrow well. Keywords: semiconductor nanostructures, exchange interaction, Kerr effect.

At low lattice temperatures the nuclear spins in a solid form a closed thermodynamic system that is well isolated from the lattice. Thermodynamic properties of the nuclear spin system are characterized by the local field of spin-spin interactions, which determines its heat capacity and the minimal achievable nuclear spin temperature in demagnetization experiments. We report the results of measurement of the local field for the nuclear spin system in GaAs, which is a model material for semiconductor spintronics. The choice of the structure, a weakly doped GaAs epitaxial layer with weak residual deformations, and of the measurement method, the adiabatic demagnetization of optically cooled nuclear spins, allowed us to refine the value of nuclear spin-spin local field, which turned out to be two times less than one previously obtained. Our experimental results are supported by calculations, which take into account dipole-dipole and indirect (pseudodipolar and exchange) nuclear spin interactions as well as quadrupole splitting of nuclear spins in the vicinity of charged impurity centers.

In this paper, we present an overview of the possibilities of nuclear spin warm-up spectroscopy method. Nuclear spin warm-up spectroscopy method is based on optical cooling and subsequent warming up of nuclear spins by oscillating magnetic field. Changes of nuclear spin temperature before and after applying of oscillating magnetic field are determined from the degree of photoluminescence polarization. This method is applied to studying the properties of the cooled nuclear spin system in bulk n-GaAs crystals. Using warm-up spectroscopy, we can investigate such thermodynamical characteristics of cooled nuclear spins as local fields, absorption coefficients and fluctuations spectral density (correlator spectrum). In particular, such experimental opportunities accompanied by theoretical interpretations allows us to investigate and control the presence of quadrupole interactions in structures. Furthermore, the nuclear spin fluctuations are reflected in the correlator spectrum, which can be recalculated from absorption coefficients. Measurements of the nuclear spin correlator are important experimental opportunity because fluctuations of nuclear spins are one of the main sources of electron spin decoherence in n-GaAs.

Heat capacity of the nuclear-spin system (NSS) in GaAs-based microstructures has been shown to be much greater than expected from dipolar coupling between nuclei, thus limiting the efficiency of NSS cooling by adiabatic demagnetization. It was suggested that quadrupole interaction induced by some small residual strain could provide this additional reservoir for the heat storage. We check and validate this hypothesis by combining nuclear-spin relaxation measurements with adiabatic remagnetization and nuclear magnetic resonance experiments, using electron spin-noise spectroscopy as a unique tool for detection of nuclear magnetization. Our results confirm and quantify the role of the quadrupole splitting in the heat storage within NSS and provide additional insight into the fundamental, but still actively debated relation between a mechanical strain and the resulting electric field gradients in GaAs.

We discuss the implications of a small indium content (3%) in a GaAs epilayer on the electron and nuclear spin relaxation due to enhanced quadrupolar effects induced by the strain. Using the weakly perturbative spin noise spectroscopy, we study the electron spin relaxation dynamics without explicit excitation. The observed temperature dependence indicates the presence of localized states, which have an increased interaction with the surrounding nuclear spins. Time-resolved spin noise spectroscopy is then applied to study the relaxation dynamics of the optically pumped nuclear spin system. It shows a multi-exponential decay with time components, ranging from several seconds to hundreds of seconds. Further, we provide a measurement of the local magnetic field acting between the nuclear spins and discover a strong contribution of quadrupole effects. Finally, we apply the nuclear spin diffusion model, that allows us to estimate the concentration of the localized carrier states and to determine the nuclear spin diffusion constant characteristic for this system.

A trion magnetic polaron formed by the exchange interaction of a positively charged exciton (trion) with localized spins of Mn2+ ions is found experimentally in a 4-nm-wide Cd0.98Mn0.02Te/Cd0.78Mn0.02Mg0.2Te quantum well containing resident holes. The experiment is performed at a temperature of 1.6 K using resonant excitation of the trion with circularly polarized light. The trion is formed from a resident hole, which is in a hole magnetic polaron state, and a photogenerated electron-hole pair. The dynamical evolution from the hole magnetic polaron to the trion magnetic polaron is accompanied by a spin flip of the electron, which results in negative circular polarization of the photoluminescence. The degree of circular polarization reaches -8% at zero magnetic field and strongly decreases in transverse magnetic fields exceeding 0.2 T. Our model considerations show that different localization sizes of the resident and photogenerated holes and the resulting difference in their exchange interaction with the Mn2+ spins maintains Mn spin polarization. The resulting exchange field of Mn acting on the electron provides a robust spin polarization of the trion magnetic polaron. We evaluate the electron exchange energy in the T+MP to be 0.19 meV, and the T+MP binding energy to be about 0.5-1 meV.

The behavior of the electron spin relaxation time in bulk GaAs layers doped with Mn was studied experimentally. We investigated the dependence of the degree of circular polarization of photoluminescence in transverse and longitudinal magnetic fields. An increase in the relaxation time of the electron spin is found from 25 ns at low pumping power to 400 ns at the threshold pumping power. The effect of resonant cooling of the nuclear spin system by optically-oriented electron spins is demonstrated.

The possibility of realization of magnetoreception in vertebrates with chains of magnetite nanocrystals (magnetosomes) attached to hair cells of the inner ear is evaluated. To this end, statistical mechanics is applied to analyze fluctuations of stereocilia bundles. Correlation functions of fluctuations of the bundle position and of the number of open mechanoreceptor channels are derived. The sensitivity threshold of the hair cell to applied forces is calculated. Its comparison with the force couple exerted by the magnetosome in the geomagnetic field suggests that a compass magnetoreceptor can be realized with ~ 100 specifically adapted hair cells. To the opposite, no viable magnetic map receptor is possible within this system.