The real mechanism of ferromagnetism in diluted magnetic semiconductors (DMSs), in particular in GaMnAs, is not yet fully understood. The well-known Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, mediated by itinerant charge carriers, is accepted to be responsible for the ferromagnetism in GaMnAs in its metallic state. At the same time, GaMnAs can be realized in an insulating state when there are no mobile carriers, and holes are localized at acceptors. At not too low Mn concentration, the overlap of acceptor -wave -function tails can be sufficient, and Mn d shells can be aligned by means of indirect mechanism mediated by localized holes. An increase in the content of Mn ions leads to their incorporation into interstitial positions, which in turn leads to self -compensation, i.e., the number of localized holes per paramagnetic center decreases. This means that the pairwise interaction of d shells can be mediated by two holes as well as single hole. In order to study the microscopic mechanism of indirect exchange, the energy and spin structure of molecule -like complexes with one and two charge carriers, localized by field of two paramagnetic ions, is investigated. It is shown that at short interionic distances (high concentration of magnetic component) the established mechanisms of indirect exchange resemble the well-known mechanisms of double- and superexchange with ferro- and antiferromagnetic alignment of magnetic moments, respectively. However, these results that can be found perturbatively, are significantly modified when the interionic distance R increases. Our calculations show that the magnetic ordering in insulating DMS crystal significantly depends not only on the concentration of magnetic component but on the compensation degree as well: cases of low or total compensation correspond to the absence of any magnetic ordering, whereas, the pronounced ferromagnetism is realized at a compensation degree close to a half. The simple estimate for the Curie temperature is derived in the limit of half -compensation.

Nanowires (NWs) of III-V semiconductor materials have been of interest to researchers for the last two decades. Knowledge of the subband spectrum of charge carriers in NWs and NW-based structures is very important for current applications. The electronic subband spectrum in NWs is currently known in detail, while for holes it is found with significant simplifications. One or more of the following crucial features are usually neglected: the real NW cross section shape, the crystal orientation of the NW, an accounting for the real anisotropic Hamiltonian of the bulk host material, and contributions that are due to the lack of an inversion center in the crystal lattice. Here we present a detailed calculation of hole subbands in GaAs NWs with the [111] orientation with a zinc blende crystal lattice, taking into account all the above four features. The spectrum of hole subbands based on the 4 x 4 Luttinger Hamiltonian is numerically calculated taking into account two main contributions arising from the lack of inversion symmetry (the Td point group) in the lattice of the host crystal. Accounting for these contributions leads to the appearance of spin splitting only for some subbands, in accordance with symmetry considerations. However, a significant rearrangement also occurs in the spectrum of nonsplit subbands. The hole densities are visualized, and it is shown that the contribution of terms with Td symmetry significantly changes the structure of the multicomponent wave function. Thus, taking into account the lack of an inversion center is essential for the spectrum of hole subbands and wave functions in GaAs NWs. This can be more pronounced for NWs of III-V materials constituted by heavy elements, such as InSb, where spin-orbit interaction is stronger. The effect of a transverse electric field leading to so-called Rashba spin splitting is considered as well.

A theoretical explanation is proposed for the shape of the long-wavelength edge of the luminescence line, which is caused by the recombination of a free electron and a hole of a neutral acceptor. The formation of complexes, in which a single hole is localized by the field of two attracting ions (\(A_{2}^{ - }\) complexes) and the subsequent recombination of holes in such complexes with electrons of the conduction band are considered. The Coulomb repulsion in the final state after recombination and the dispersion of the complexes in terms of the interionic distance provide an extended long-wavelength tail of the luminescence line, comparable in magnitude to the ionization energy of a single acceptor.

Real semiconductors usually contain both donor and acceptor impurities. A finite compensation degree leads to the possibility of the single carrier to be bounded by two impurity centers. For such an electron complex (near a simple conduction band), the spectral problem resembles the hydrogen molecule ion problem (up to renormalization of the effective mass and dielectric screening). In p-type semiconductors, the spectral problem for a single hole in the field of two attracting centers (A-2 complex) is more complicated due to the complex structure of the valence band. Here such a problem is presented for the case of the hole bounded at two shallow acceptors close to the I'8 valence band edge (AIIIBV or group IV semiconductors). The multicomponent envelope functions are used to develop quantum chemistrylike approach (molecular orbital method). The variational approach is applied to calculate the level structure of the complex. The states of the complex are classified by the total angular momentum projection onto the intercenter axis and by the parity with respect to the intercenter permutation. The ground state has a total angular momentum projection of +/- 1/2 and a wave function that is symmetric with respect to the intercenter permutation. The energy levels are found as a function of the intercenter distance. A possible manifestation of A-2 complexes in acceptor-related luminescence is discussed.

The diversity of various manganese types and its complexes in the Mn-doped A(III)B(V) semiconductor structures leads to a number of intriguing phenomena. Here we show that the interplay between the ordinary substitutional Mn acceptors and interstitial Mn donors as well as donor-acceptor dimers could result in a reversal of electron magnetization. In our all-optical scheme the impurity-to-band excitation via the Mn dimers results in direct orientation of the ionized Mn-donor d shell. A photoexcited electron is then captured by the interstitial Mn and the electron spin becomes parallel to the optically oriented d shell. That produces, in the low excitation regime, the spin-reversal electron magnetization. As the excitation intensity increases the capture by donors is saturated and the polarization of delocalized electrons restores the normal average spin in accordance with the selection rules. A possibility of the experimental observation of the electron spin reversal by means of polarized photoluminescence is discussed.

The recent advances in nanowire (NW) growth technology have made possible the growth of more complex structures such as core-multi-shell (CMS) NWs. We propose the approach for calculation of electron subbands in cylindrical CMS NWs within the simple effective mass approximation. Numerical results are presented for GaAs/Al0.3Ga0.7As radial heterostructure with AlGaAs-core and 4 alternate GaAs and AlGaAs shells. The influence of an effective mass difference in heterolayers is discussed.

A new model of a quantum ring defined inside a nanowire is proposed. The one-particle Hamiltonian for electron in [111]-oriented nanowire quantum ring is constructed taking into account both Rashba and Dresselhaus spin-orbit coupling. The energy levels as a function of magnetic field are found using the exact numerical diagonalization. The persistent currents (both charge and spin) are calculated. The specificity of spin-orbit coupling and arising anticrossings in energy spectrum lead to unusual features in persistent current behavior. The variation of magnetic field or carrier concentration by means of gates can lead to pure spin persistent current with the charge current being zero.

Two-dimensional (2D) semiconductor structures of materials without inversion center (e.g. zinc-blende AIIIBV) possess the zero-field conduction band spin-splitting (Dresselhaus term), which is linear and cubic in wavevector k, that arises from cubic in k splitting in bulk material. At low carrier concentration the cubic term is usually negligible. However, if we will be interested in the following dimensional quantization (in 2D plane) and the character width in this direction is comparable with the width of 2D-structure, then we have to take into account k3-terms as well (even at low concentrations), that after quantization leads to comparable contribution that arises from k-linear term. We propose the general procedure for derivation of Dresselhaus spin-splitting Hamiltonian applicable for any curvilinear 1D-structures. The simple examples for the cases of quantum wire (QWr) and quantum ring (QR) defined in usual [001]-grown 2D-structure are presented.

Experimental and theoretical piezospectroscopic investigation of A + centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A + center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.

AbstractExperimental and theoretical piezospectroscopic investigation of A^+ centers in GaAs/AlGaAs quantum wells doped with beryllium is presented. Spectra of linearly polarized photoluminescence are studied experimentally depending on applied uniaxial pressure. A model of the A^+ center in the quantum well in the presence of uniaxial deformation in the plane of the quantum well has been constructed. Analytical expressions for the level energy, optical transition intensities, and polarization ratio have been obtained. In the framework of the proposed theory, the experimentally observed change in the polarization ratio depending on pressure and the shift of the line maximum towards short waves are explained.

We present here a brief overview of current-induced spin polarization in bulk semiconductors and semiconductor structures of various dimension. The role of band structure and spin relaxation processes is discussed. The related phenomena, such as spin Hall effect, inverse spin Hall effect and other are discussed. Our recent results in this field are presented as well.

The paper reports optical orientation experiments performed in the narrow GaAs/AlGaAs quantum wells doped with Mn. We experimentally demonstrate a control over the spin polarization by means of the optical orientation via the impurity-to-band excitation and observe a sign inversion of the luminescence polarization depending on the pump power. The $g$ factor of a hole localized on the Mn acceptor in the quantum well was also found to be considerably modified from its bulk value due to the quantum confinement effect. This finding shows the importance of the local environment on magnetic properties of the dopants in semiconductor nanostructures.

We carry out microphotoluminescence measurements of an acceptor-bound exciton (A(0)X) recombination in the applied magnetic field with a single impurity resolution. In order to describe the obtained spectra we develop a theoretical model taking into account a quantum well (QW) confinement, an electron-hole and hole-hole exchange interaction. By means of fitting the measured data with the model we are able to study the fine structure of individual acceptors inside the QW. The good agreement between our experiments and the model indicates that we observe single acceptors in a pure two-dimensional environment whose states are unstrained in the QW plain.

The orientation of spins of charge carriers has been studied during the passage of an electric current through a quasi-one-dimensional system. It has been established that the effect disappears in a strictly one-dimensional case, while in the case of the occupation of a large number of subbands and the possibility of intersubband scattering, its value is comparable with that of the two-dimensional case studied earlier and can be discovered experimentally.

We constructed the Hamiltonian of spin–orbit splitting for carriers of a tubular electron gas in InAs nanowires. The spectral problem is solved using an exact numerical diagonalization. It is shown that the contribution of k-linear Dresselhaus-like spin–orbit (SO) coupling leads to renormalization of the so-called SO-gaps and appearance of anticrossings in subband spectrum. These features can be detected in ballistic transport.

A key concept in the field of semiconductor spintronics is an electric field control of spins via the spin-orbit coupling (SOC) and the SOC strength governs efficiency of this control. We propose a new approach that allows the experimental determination of the Rashba SOC strength for ballistic InAs nanowires. The energy spectrum and ballistic transport of carriers through the nanowire with surface two-dimensional electron gas (2DEG) in a homogeneous magnetic field are studied. A general formula for the linear-response one-dimensional ballistic thermopower is derived in the case of complex subband structure. The ballistic conductance and the thermopower are shown to reveal specific features due to strong SOC that allows us to propose a method for the SOC strength determination.

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.

The electron transport through the parabolic quantum wire placed in longitudinal magnetic field in the presence of the system of short-range impurities inside the wire is investigated. Using approach based on the zero-range potential theory we obtained an exact formula for the transmission coefficient of the electron through the wire that allows to calculate such the transport characteristics as the conductance and differential thermopower. The dependencies of conductance and thermopower on the chemical potential and magnetic field are investigated. The effect of elastic scattering due to short-range impurities on low-temperature conductance and thermopower is studied. It was shown that the character of the electron transport essentially depends on the position of the every scattering center. The presence even isolated impurity leads to destruction of conductance quantization. In some cases it is possible that thermopower can change the sign in dependence on chemical potential and magnetic field.

A dc electric current due to a flow of ultrasonic phonons through a quantum wire containing an isolated point impurity is analyzed theoretically. An explicit expression for the acoustoelectric (AE) current is obtained and its dependence on the chemical potential of the electron gas and the induction of the external magnetic field is investigated. It is shown that a point impurity may lead to a partial or complete destruction of the steplike structure of the AE current as a function of the chemical potential.

The behavior of an acoustoelectric current through a three-dimensional quantum microconstriction placed in a longitudinal uniform magnetic field is studied theoretically in the ballistic transport regime. The oscillation periods of the acoustoelectric current are studied in detail as functions of the chemical potential and the magnetic field induction. The temperature effect is taken into account. It is shown that the acoustoelectric current as a function of the chemical potential can exhibit a steplike behavior. The limits for the existence of a steplike structure are determined.