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Martensitic transformation (MT), magnetic properties, and magnetocaloric effect (MCE) in Heusler-type Ni47Mn40Sn13−x Cd x (x= 0, 0.75, 1, 1.25 at. %) metamagnetic shape memory alloys (MetaMSMAs) are investigated, both experimentally and theoretically, as a function of doping with Cd. Ab-initio computations reveal that the ferromagnetic (FM) configuration is energetically more favorable in the cubic phase than the antiferromagnetic (AFM) state in undoped and doped alloys as well. Moreover, it is revealed that the alloys in the ground state exhibit a tetragonal structure confirming the existence of MT, in agreement with the experiments. It was indicated, both in theory and practice, that a reduction of the unit cell volume and an increase of the MT temperature as a function of the Cd doping. Indirect estimations of MCE in the vicinity of MT were carried out by using thermomagnetization curves measured under different magnetic fields up to 5 T. The results demonstrated that the doped alloys exhibit enhanced values of the inverse MCE comparable with those of Ni-Mn-based MetaMSMAs. Maximum magnetic entropy change in a field change of 2 T increases from 3.0 J .k g − 1 K − 1 for the undoped alloy to 3.4 and 5.0 J .k g − 1 K − 1 for the alloys doped with 0.75 and 1 at.% of Cd, respectively. The inverse and conventional MCE were explored by direct measurements of the adiabatic temperature change under the magnetic field change of 1.96 T. The Cd doping increased the maximum of inverse MCE by nearly 78% from 0.9 K to 1.6 K for the undoped and doped alloys, respectively. The results depicted that Cd doping can effectively tailor the structural, magnetic, and MCE properties of the Ni–Mn–Sn MetaMSMAs.

The influence of the W-doping on the martensitic transformation, magnetic properties and exchange bias (EB) effect in the Ni47Mn40Sn13−x W x (x = 0, 0.5, 1, 1.25 at.%) magnetic shape memory alloys has been investigated. It is found that the W-doping causes a simultaneous reduction of both the ferromagnetic (FM) exchange coupling and enhancement of the magnetic anisotropy, leading to a decrease of the magnetic moment of the low-temperature phase and to a higher attainable EB. The magnetic memory measurements reveal the presence of a glassy magnetic ground state, which can significantly impact the reduction of magnetization and enhancement of EB in the studied bulk alloys. It is argued that the glassy magnetic ground state originates from the partial magnetic disorder resulting from the correlation between the antiferromagnetic and FM states. The results demonstrate that the doping by W instead of Sn is an efficient tool to tailor the EB effect in the Ni–Mn–Sn-based Heusler alloys, whereby they are promising for spintronic applications.

The photoluminescence of a nonequilibrium polariton condensate in cylindrical and rectangular micropillars etched on the surface of a high-Q GaAs microcavity is investigated in magnetic fields of up to 12 T. The measurements are carried out under different levels of nonresonant optical pumping with nanosecond laser pulses for a wide range of cavity detuning. As far as nonresonant excitation produces a high density of excitons in a reservoir, it should be expected that the exciton–polariton interaction, which depends on the pump level, has a considerable effect on the Zeeman splitting and polarization of the condensate. However, measurements of the Zeeman splitting and polarization in high magnetic fields demonstrate that only minor changes take place up to the highest available pump levels. This means that, in the case under study, the effect of exciton–polariton interaction on the polariton system is insignificant. At the same time, the data obtained provide an estimate for the exciton density in the reservoir. In contrast to cylindrical micropillars, the photoluminescence of the condensate in rectangular micropillars consists of two perpendicularly linearly polarized lines which retain a high degree of linear polarization even in a field as high as 12 T. The Zeeman splitting in this case is nearly independent of the pump power. The degrees of both circular and linear polarization change with pump power, but these changes are noticeably smaller than the ones predicted theoretically. This indicates that the system of polaritons in micropillars deviates considerably from thermodynamic equilibrium.

We analyze the photoluminescence of a nonequilibrium polariton condensate in cylindrical micropillars etched on the surface of a high-Q GaAs microcavity in a wide range of detunings in a magnetic field up to 12 T for various levels of nonresonant laser pumping by nanosecond pulses. With such a method of excitation, a considerable effect of the interaction of the reservoir of photoexcited excitons with the condensate on the Zeeman splitting of the polariton condensate levels can be expected, which can lead to a decrease in its value and even to sign reversal. However, the measurements of photoluminescence in a wide range of optical excitation densities show that Zeeman splitting weakly depends on the optical pumping (its variation does not exceed 15% of the splitting in a field of 12 T). The estimation of the exciton density in the reservoir based on these data gives a value lower than 108 cm–2. In addition, a noticeable decrease (by a factor of about 1.8) in the polariton condensation threshold in a magnetic field is detected.

In planar GaAs microcavities in a magnetic field up to 5 T perpendicular to the structure growth plane, under conditions of resonant pulsed pumping to a point close to the inflection point of the lower dispersion curve, Zeeman splitting of the spin sublevels of the polariton condensate is observed. This is accompanied by a significant change in the degree of circular polarization and the second-order correlator g2(0). It is found that the correlator is different for the spin sublevels of the polariton condensate, split in a magnetic field. In particular, correlator measurements indicate different condensation thresholds for the spin sublevels. The correlator values initially differing in terms of the absence of a field increase, reach a maximum, and then decrease and become equal for different polarizations in a field of 5 T.

The mechanisms of the nonradiative recombination of excitons in neutral and charged quantum dots based on II–VI semimagnetic semiconductors are investigated. It is shown that, along with the dipole–dipole and direct-exchange mechanisms, there is one more mechanism referred to as the indirect-exchange mechanism and related to sp–d mixing. The selection rules for nonradiative recombination by exchange mechanisms are subsequently derived. The dependence of the efficiency of all recombination mechanisms on the quantum-dot size is studied. The experimentally observed growth in the intracenter photoluminescence intensity with decreasing size of dots and nanocrystals is accounted for. Methods for experimental determination of the contributions of different mechanisms to nonradiative recombination are discussed.

The electron paramagnetic resonance spectra of KTaO3: Mn nanocrystalline powders in the temperature range from 77 to 620 K have been measured and studied for the first time. The change observed in the spectra has been investigated as a function of the doping level. The doping regions in which Mn2+ ions are individual paramagnetic impurities have been established, as well as the regions where the dipole-dipole and exchange interactions of these ions begin to occur. The spin-Hamiltonian constants for the spectrum of non-interacting individual Mn2+ ions have been determined as follows: g = 2.0022, D = 0.0170 cm−1, and A = 85 × 10−4 cm−1. A significant decrease in the axial constant D in the KTaO3: Mn nanopowder, as compared to the single crystal, has been explained by the remoteness of the charge compensator from the paramagnetic ion and by the influence of the surface of the nanoparticle. It has been assumed that the Mn2+ ions are located near the surface and do not penetrate deep into the crystallites.

Photoluminescence measurements on individual CdSe / ZnSe / ZnMnSe quantum dots in a magnetic field up to 11 T both parallel and perpendicular to the sample growth plane at 1.6 K reveal a qualitative difference between samples with different strength of the sp-d exchange interaction controlled by means of varying the thickness of the nonmagnetic ZnSe layer. The observed difference is related with the dependence of the non-radiative Mn assisted recombination and the spin relaxation on magnetic field and the strength of the exchange interaction.

Low-temperature (T = 1.6 K) photoluminescence (PL) of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) with different magnitudes of the sp-d exchange interaction between the magnetic impurity ions and charge carriers has been studied in a magnetic field up to 12 T applied in the Faraday and Voigt geometry. The magnitude of the interaction was controlled by changing the fraction (ηe, h) of the squared wave function of charge carriers in the semimagnetic barrier by means of variation of the nonmagnetic (ZnSe) layer thickness. It is established that the sp-d exchange interaction leads to a change in the sign of the effective hole g factor even for ηe, h ∼ 5%, while further increase in the interaction magnitude is accompanied by a rapid growth in the magnitude of spin splitting for both electrons and holes. The quantum yield of PL exhibits a significant decrease due to nonradiative Auger recombination with the excitation of Mn ions only for ηe, h ∼ 12%, while the rate of the holes spin relaxation starts growing only for still higher ηe, h values. In a strong magnetic field perpendicular to the sample plane, the alignment of Mn spins leads to suppression of the Auger recombination only in the excited spin state. For a small rate of the hole spin relaxation, this leads to a rather unusual result: the emission from an excited trion state predominates in strong magnetic fields.

We compare magneto-photoluminescence spectra of individual semimagnetic quantum dots (QDs) with different strengths of exchange interaction between a localized exciton and surrounding magnetic ions. The spectra of individual QDs with strong magnetic exchange interaction show a single line corresponding to a ground-state exciton. In QDs with weak interaction, exciton and biexciton spectra are identified, both showing a fine structure due to electron-hole exchange interaction. The anticrossing of dark and bright exciton states in low-symmetry QDs is demonstrated in magnetic fields in the Faraday geometry.

Self-assembled Cd(Mn)Se/Zn(Mn)Se quantum dots have been investigated by means of spatially and time-resolved magneto-optical spectroscopy. In such quasi zero-dimensional diluted magnetic semiconductors, the exchange interaction couples the spins of optically generated charge carriers with localized magnetic ion spins. We demonstrate that this can be used on the one hand to monitor nanoscale magnetization with a resolution of

Photoluminescence of ZnMnSe/ZnSSe multiple quantum wells under a bandgap continuous wave and fs-pulsed excitations is measured in magnetic fields up to 10 T in Faraday geometry at temperatures within the range of 1.6–20 K. The measurements reveal two dominant lines in the spectra and LO-phonon replicas of the lower-energy line. The photoluminescence and time-resolved studies show dramatically different behaviour of the lines. Analysis of their properties reveals that they correspond to recombination of indirect localized excitons and indirect acceptor-bound excitons (A0X). Crossing of exciton and A0X lines because of the difference in magnitudes of their Zeeman shifts is observed. Analysis of LO-phonon replicas of photoluminescence lines provides additional evidence for strong carrier localization bound to A0X. A model of phonon-assisted recombination of indirect acceptor-bound excitons is proposed. The fitting of photoluminescence lines with this model gives the Huang–Rhys factor S 0.25 for A0X and the hole localization size ah 30 A. Contrary to expectations the exciton magnetic polaron effect is hardly observed in these structures.

M. D. and A. V. K. acknowledge support from the Russian Ministry of Science and Education (Contract No. 11.G34.31.0067). A. V. C. acknowledges support from the Russian Foundation for Basic Research. We observe a strong variation of the Zeeman splitting of exciton polaritons in microcavities when switching between the linear regime, the polariton lasing, and photon lasing regimes. In the polariton lasing regime the sign of Zeeman splitting changes compared to the linear regime, while in the photon lasing regime the splitting vanishes. We additionally observe an increase of the diamagnetic shift in the polariton lasing regime. These effects are explained in terms of the nonequilibrium "spin Meissner effect." Publisher PDF

Polariton emission has been investigated in a GaAs based microcavity under various excitation conditions which excite predominately hot free excitons and carriers (He–Ne laser excitation), localized excitons (Ti–sapphire laser excitation), or mixed system. Strong differences have been found both in the intensity, the energy distribution, and the temperature dependence of polariton emission for the different excitation conditions. The relaxation into the low polariton branch due to consecutive scattering via polariton states in the bottleneck region and via the states of localized excitons has been found ineffective. Instead, we found that the highly effective filling of low polariton states occurs via relaxation of mobile excitons delocalized due to either thermal activation or interaction with hot carriers.

Photoluminescence studies of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) allow us to observe excitons and negatively charged trions in spectra recorded at liquid He temperatures. Exciton and trion lines equally split into a doublet of circularly polarized lines in a magnetic field B directed perpendicular to the sample plain. The ratio of intensities of lines in the doublet differs for excitons and trions. The difference can be explained if spin-dependent non-radiative recombination is taken into account. Contrary to expectations processes induced by the Coulomb interaction do not lead to such a recombination. The dominant mechanism of spin-dependent energy transfer from photoexcited QDs to Mn ions is related to the sp-d mixing. The efficiency of this mechanism substantially exceeds that induced by the Coulomb interaction. Suppression of the non-radiative recombination by a magnetic field in Faraday geometry leads to the increase in QDs PL intensity.

The field dependence of the magnetization of the isotropic ferromagnet, Ni 2 MnGa has been measured close to the Curie temperature of 363.0 (0.4) K and the critical index δ has been determined to be 4.77 (0.50). An analysis has been carried out to see if the index together with that of pure nickel could be described by the theory by Takahashi for a weakly ferromagnetic itinerant electron system or by conventional molecular field theory.

This paper reports on the first investigation made of luminescence of Ge/Ge1−xSix heterostructures at liquid-helium temperatures in a magnetic field of up to 14 T. The luminescence lines observed in the spectra are due to both free and impurity bound excitons in Ge layers. The diamagnetic shift of the quasi-two-dimensional exciton has been measured. From the experimental data the size of the exciton has been estimated to be 75–90 A.

Use of diamond grinding wheels with organic and ceramic bonds makes it possible to avoid the phenomenon of wheel glazing during fine grinding of molybdenum single crystals and to obtain a high quality ground surface with the minimum level of structural distortions. The nature of the orientation dependence of surface structural distortions is established by studying {001}, {110}, and {111} molybdenum single crystals. This is connected with features of plastic deformation during grinding.

Luminescence spectra of uniaxially and uniformly strained high-purity germanium crystals at liquid-helium temperatures in a magnetic field of up to 14 T have been investigated. In strongly strained Ge crystals, a new line has been detected on the low-energy side of the excitonline in magnetic fields higher than 4 T. Studies of this line’s characteristics as functions of pressure, temperature, and magnetic field have led us to conclude that its presence is due to recombination of electron-hole pairs in an electron-hole liquid. The experimental data suggest that the metallic electron-hole liquid is stabilized in a strong magnetic field. By approximating the shape of the newly detected line using the model of metallic electron-hole liquid, we have obtained the electron-hole liquid density n EHL(B) and Fermi energies E Fe,h of electrons and holes. The liquid binding energy o as a function of magnetic field has been estimated.

We report on pump-power-dependent emission features of a nonthermalized and interacting dynamical condensate of exciton polaritons. The system based on a planar AlAs/AlGaAs microcavity sample with twelve GaAs quantum wells in the active region is investigated comprehensively by measuring the energy-momentum dispersion characteristics, the spatial coherence, and the photon statistics under resonant, fs-pulsed optical excitation at high momentum. We observe a significant polariton-population-dependent modification of the emission signatures above the quantum degeneracy. The nonequilibrium polariton condensate is confirmed by its polaritonic energy-momentum dispersion as well as excitation-power-dependent coherence properties and its photon statistics, being different from that of an ideal coherent state. The polaritonic condensate is characterized by a spatial coherence length of up to 4 \ensuremath{\mu}m and a super-Poissonian photon statistic of the emitted light well above threshold. Results obtained in second-order photon autocorrelation measurements in momentum-space resolved spectroscopy indicate polariton repulsive interaction throughout the condensate and a spatial coherence length being shorter than the condensate extension of 20--30 \ensuremath{\mu}m.

An experimental study of the effect of foreign inclusions of both chemically active materials, capable of burning in an air atmosphere and inert oxide materials on the propagation of a combustion front over the surface of titanium powders was carried out. It has been established how the character and the rate of surface combustion Vc of titanium powders change upon their dilution, depending on diluent concentration, melting temperature, disperse composition, and chemical activity.

Mechanisms of nonradiative recombination of electron-hole complexes in Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations of Mn$^{2+}$ ions are analyzed within the framework of single electron model of deep {\it 3d}-levels in semiconductors. In addition to the mechanisms caused by Coulomb and exchange interactions, which are related because of the Pauli principle, another mechanism due to {\it sp-d} mixing is considered. It is shown that the Coulomb mechanism reduces to long-range dipole-dipole energy transfer from photoexcited quantum dots to Mn$^{2+}$ ions. The recombination due to the Coulomb mechanism is allowed for any states of Mn$^{2+}$ ions and {\it e-h} complexes. In contrast, short-range exchange and ${\it sp-d}$ recombinations are subject to spin selection rules, which are the result of strong {\it lh-hh} splitting of hole states in quantum dots. Estimates show that efficiency of the {\it sp-d} mechanism can considerably exceed that of the Coulomb mechanism. The phonon-assisted recombination and processes involving upper excited states of Mn$^{2+}$ ions are studied. The increase in PL intensity of an ensemble of quantum dots in a magnetic field perpendicular to the sample growth plane observed earlier is analyzed as a possible manifestation of the spin-dependent recombination., Comment: 14 pages, 2 figures

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral $\mathrm{CdSe}∕\mathrm{ZnSe}∕\mathrm{ZnMnSe}$ quantum dots (QDs) with various magnitudes of $sp\text{\ensuremath{-}}d$ exchange interaction has been investigated in magnetic fields $B$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnitude of the $sp\text{\ensuremath{-}}d$ interaction is varied by changing the penetration of the exciton wave function ${\ensuremath{\psi}}_{X}$ into the diluted magnetic semiconductor (DMS) barrier, $\ensuremath{\eta}$, by variation of the nonmagnetic $\mathrm{ZnSe}$ barrier layer thickness about a value of $1.75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The small penetration of ${\ensuremath{\psi}}_{X}$ into the DMS barrier allowed a decrease of the X and XX emission line broadening caused by magnetic fluctuations and resolution of the fine structure of X states in a single QD. In contrast to bulk DMSs, the exciton line broadening when $B$ is normal to the QD plane was found to be a nonmonotonic function of magnetic field: at high $B$ it is suppressed due to Mn ion spin alignment, whereas at low $B$ it is decreased due to the mixing of ${J}_{z}=+1$ and $\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{X}$ states in low-symmetry QDs by the electron-hole $(e\text{\ensuremath{-}}h)$ exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the $e\text{\ensuremath{-}}h$ exchange interaction at $B=0$ and (ii) strong enhancement of spin relaxation between ``bright'' $J=1$ exciton states in magnetic fields normal to the QD plane already at $\ensuremath{\eta}\ensuremath{\sim}1%$. The spin relaxation from bright to ``dark'' exciton states is negligible up to $\ensuremath{\eta}\ensuremath{\sim}4%$. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at $B=0$ already at $\ensuremath{\eta}\ensuremath{\sim}4%$.

Magnetoluminescence of $\mathrm{Cd}\mathrm{Mn}\mathrm{Se}\text{\ensuremath{-}}\mathrm{Zn}\mathrm{Se}$ disk-shaped quantum dots in a magnetic field up to $11\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is measured in the Faraday and Voigt geometries at liquid He temperatures and various levels of laser excitation. Within the range of $B=0--11\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ the intensity of the quantum dot photoluminescence increases strongly (up to two orders of magnitude) in the Faraday geometry and only slightly ($\ensuremath{\sim}1.5$ times) in the Voigt geometry. To explain the observed dependence on magnetic field direction the selection rules for the Auger recombination of excitons with excitation of Mn-ion are revised. The magnetic field dependence of the quantum yield of quantum dot exciton emission and its anisotropy are shown to appear due to the dependence of Auger recombination of uncharged excitons with excitation of Mn-ion on the spin of both exciton and Mn-ion. The anisotropy is provided by a quick spin relaxation of photoexcited excitons into the ground state which spin structure depends on the direction of the magnetic field.

Statistical fluctuations of the magnetization on a nanometer scale have been investigated with the use of magneto-optical spectroscopy of quasi-zero-dimensional excitonic magnetic polarons (EMP) in individual pancake-shaped CdSe/ZnMnSe quantum dots (QD's). The EMP emission line demonstrates a qualitatively different behavior in magnetic field B normal and parallel to the QD plane. In the first case, the alignment of the Mn spins in high magnetic fields results in a giant Zeeman shift and linewidth narrowing of almost one order of magnitude to values characteristic of nonmagnetic QD's. No similar effects are observed in the second case where the emission line demonstrates a weak blue shift and small linewidth variation. It is shown that the difference originates from the anisotropy of the hole exchange field in the QD's and can be described using the fundamental fluctuation-dissipation theorem and taking into account the anisotropy of the hole g factor. Both the longitudinal and transverse fluctuations of the magnetization are determined from the analysis of EMP emission lines recorded in the two directions of magnetic field B. We demonstrate that in high magnetic fields the longitudinal fluctuations decrease exponentially whereas the transverse ones decrease as $1/B.$

Very large changes in the population distributions of microcavity polaritons are reported under conditions of simultaneous resonant and nonresonant excitation. The strong relaxation bottleneck, which results in weak emission from the lower polariton branch for resonant excitation alone, is fully suppressed if additional nonresonant excitation is employed. By comparison with theory, the massive enhancement of the polariton relaxation is shown to arise from scattering of polaritons by free carriers, present in quantum wells due to residual doping, which are then heated by the nonresonant excitation.

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only $\ensuremath{\approx}100{\ensuremath{\mu}}_{B}$ and to resolve statistical changes on the order of a few ${\ensuremath{\mu}}_{B}$.

There has been a considerable progress in the photoluminescence (PL) properties investigations of SiGe/Si and Ge/Si structures of late. The PL investigations of Ge-Ge1-xSix structures with quantum wells in germanium layers have been started recently1,2. This work reports the studies of the exciton and magnetoexciton luminescence of Ge-Ge1-xSix heterostructures grown by CVD method on Ge [111] substrates.

The influence of exchange interaction between exciton and magnetic impurity spins on magnetic moment and its fluctuations in semimagnetic quantum dots has been investigated with the use of individual dot magnetophotoluminescence technique. In quantum dots with strong exchange interaction, both the polarization of magnetic ions and magnetic fluctuations in a dot are highly influenced by the formation of exciton magnetic polaron. In contrast, in the quantum dots with weak interaction, the localized exciton acts as a perfect noninvasive probe: it allows one to analyze the quantum-dot magnetic moment and its fluctuations with negligibly small influence on the dot magnetization. Numerical values of exchange magnetic field and magnetic polaron energy are obtained and analyzed for both types of dots.

The paper deals with the processes of particle and energy transport by localized large-scale drift vortices in the ocean and atmosphere. In the case of nondivergent (in the first order approximation) fluid motions the evolution equation of the rotor of a generalized momentum can be reduced to the Charney-Hasegawa-Mima equation. The same equations are used to describe plasma motions. Vortex solutions of the Charney-Hasegawa-Mima equations are shown to participate in transport processes.

Photoluminescence spectra of CdSe/ZnSe/ZnMnSe semimagnetic quantum dots were studies in Faraday and Voigt geometries in the magnetic field up to 11 T at T=1.6 K. Incorporation of the nonmagnetic ZnSe layer between CdSe quantum dot layer and semimagnetic barriers reduced the strength of sp‐d exchange interaction and hence broadening of emission lines that obscures an observation of the fine structure of electron‐hole complexes. This allows one to identify exciton, biexciton and trion lines in the spectra of quantum dots and study their fine structure. An account of both sp‐d and electron‐hole exchange interactions are required in order to describe the Zeeman splitting and polarization behavior of the spectra.