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In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure.

The photoluminescence and excitation of luminescence spectra of a series of samples containing two CdTe layers D1 and D2 with a nominal thickness of 1.5 and 4 monolayers in a ZnTe matrix were studied. The samples differ in the width of the ZnTe spacer separating the D1 and D2 inserts and constituting 15, 25, 35, 45 and 55 monolayers (samples nos. 1–5, respectively). ZnTe layers are grown on a GaAs substrate by the standard molecular beam epitaxy mode, while CdTe inserts are grown in atomic layer epitaxy mode. It is shown that when the barrier thickness is less than 25 monolayers electronic states in the quantum wells formed by D1 and D2 layers are tunnel-coupled, and only one band is recorded in the emission spectrum, caused by the recombination of excitons in the deep quantum well. At larger thicknesses of the spacer, two bands I1 and I2 are observed in the luminescence spectrum, which are associated with the recombination of excitons in shallow (D1) and deep (D2) wells, respectively. It was found that the intensity ratio I1/I2 significantly depends on the energy and intensity of the excitation. Possible origin of these dependences are discussed.

Voltage control of ferromagnetism on the nanometer scale is highly appealing for the development of novel electronic devices with low power consumption, high operation speed, reliable reversibility and compatibility with semiconductor technology. Hybrid structures based on the assembly of ferromagnetic and semiconducting building blocks are expected to show magnetic order as a ferromagnet and to be electrically tunable as a semiconductor. Here, we demonstrate the electrical control of the exchange coupling in a hybrid consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well, separated by a thin non-magnetic (Cd,Mg)Te barrier. The electric field controls the phononic ac Stark effect—the indirect exchange mechanism that is mediated by elliptically polarized phonons emitted from the ferromagnet. The effective magnetic field of the exchange interaction reaches up to 2.5 Tesla and can be turned on and off by application of 1V bias across the heterostructure., Manipulating magnetic orders by electric fields promises energy efficient spintronic applications. Here the authors demonstrate electrical control of s-p exchange interaction in a ferromagnet-semiconductor quantum well structure due to the phononic ac Stark effect.

The effective $g$ factor of holes is measured in modulation-doped ZnSe/(Zn,Mg)(S,Se) quantum wells and from surface-state $p$-doped CdTe/(Cd,Mg)Te quantum wells by time-resolved pump-probe Kerr rotation. The measurements are performed at a temperature of 1.7 K and in magnetic fields up to 5 T applied in the Voigt geometry with orientation perpendicular to the quantum-well growth axis. The absolute value of the in-plane hole $g$ factor increases with growing magnetic field in both studied heterostructures. A theoretical model is developed that considers the influence of magnetic field and interface mixing of heavy-hole and light-hole states on the $g$ factor. The model results are in good agreement with the experimental data.

The photoluminescence (PL) and PL excitation (PLE) spectra of CdTe/ZnTe asymmetric double quantum well (QW) structures are studied on a series of samples containing two CdTe layers with nominal thicknesses of 2 and 4 monolayers (ML) in the ZnTe matrix. The samples differ in the thickness of the ZnTe spacer between CdTe QWs which is 45, 65 and 75 ML. It has been found that at above-barrier excitation the PL from a shallow QW at sufficiently weak excitation intensities is determined by recombination of hot excitons. It is shown that under these conditions, when PL is excited by lasers with different wavelengths, the ratio of the PL intensities from shallow and deep QWs decreases exponentially with an increase of the initial kinetic energy of hot excitons. It is found that energy relaxation of hot excitons with LO phonon emission determine the shape of the PLE spectrum of shallow QW in the range of exciton kinetic energies up to more than 20 LO phonons above ZnTe bandgap. We have shown that the results obtained are well described by the model of charge and energy transfer between QWs.

We study the magnetic-field dependence of the longitudinal spin relaxation time ${T}_{1}$ of donor-bound electrons placed in the middle of an 8-nm CdTe quantum well with different doping concentrations in the range from $1\ifmmode\times\else\texttimes\fi{}{10}^{9}$ to $2.9\ifmmode\times\else\texttimes\fi{}{10}^{11}$ $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ and at low temperature. We use an extended photoinduced Faraday rotation technique, which expands the usual domain of the measured decays from tens of ns to $\ensuremath{\mu}\mathrm{s}$. As in high-purity bulk semiconductors, a maximum relaxation time of around ${T}_{1}\ensuremath{\sim}10\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ is observed for a residually doped sample at low magnetic field of $B=0.08\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. For higher doping concentrations, the magnetic-field dependence of ${T}_{1}$ shows a nonmonotonic behavior: first a rapid increase, followed by a plateau or a decrease of ${T}_{1}$. The fast increase of ${T}_{1}$ at low magnetic fields is explained by the inhibition of the mechanisms identified at zero field---hyperfine and anisotropic exchange interactions---while the behavior at high magnetic field can be succesfully explained by a mechanism proposed by Lyubinskiy and associated to electron hops [I. S. Lyubinskiy, JETP Lett. 88, 814 (2008)]. A good agreement between experiment and theory is found for samples below the metal-insulator transition, when Dresselhaus terms of spin-orbit coupling are considered to be the dominant ones in the Hamiltonian describing the system.

Heterostructures of the CdTe/ZnTe type with ultrathin quantum wells are studied. Lines of light and heavy excitons bound to the CdTe layers are observed in the spectra. It is found that the photoluminescence intensity of the light exciton is comparable to the luminescence intensity of the heavy exciton. The temperature shifts of these lines are different, and the lines are intersected at a temperature of 65 K. The estimates of the energy and wave functions of the exciton states have made it possible to clarify parameters of some bands and excitons in such structures.

We report the observation of tens of minor lines of the combinational spin-flip Raman scattering in a CdTe:Mn quantum well by means of the high-resolution optical spectroscopy. Classification of this manifold leads to four characteristic values of energy, that correspond to four different types of pair clusters of Mn ions: the nearest, second, third, etc., neighbors. All the four energies show up in a single experiment with a very high precision, providing experimental grounds for a deeper understanding of the $d\ensuremath{-}d$ exchange interactions in a diluted magnetic semiconductor and demonstrating the capacity of the employed method. The major (nearest-neighbor) exchange constant ${J}_{1}=6.15\ifmmode\pm\else\textpm\fi{}0.05$ K was found to consent with its previously reported value. Other detected characteristic energies are as follows ($\ifmmode\pm\else\textpm\fi{}0.05$ K): ${J}_{(2)}=1.80$ K, ${J}_{(3)}=1.39$ K, ${J}_{(4)}=0.81$ K.

We use the two-pulse spin-dependent photon echo technique to study the in-plane hole spin anisotropy in a 20~nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te single quantum well by exciting the donor-bound exciton resonance. We take advantage of the photon echo sensitivity to the relative phase of the electron and hole spin precession and study various interactions contributing to the hole in-plane spin properties. The main contribution is found to arise from the crystal cubic symmetry described by the Luttinger parameter $q=0.095$, which is substantially larger than the one theoretically expected for CdTe or found in other quantum well structures. Another contribution is induced by the strain within the quantum well. These two contributions manifest as different harmonics of the spin precession frequencies in the photon echo experiment, when strength and orientation of the Voigt magnetic field are varied. The magnitude of the effective in-plane hole $g$ factor is found to vary in the range $|\tilde{g_h}|$=0.125--0.160 in the well plane., 7 figures

The cyclotron and magnetoplasmon resonances were studied at 2 K in grating metamaterials fabricated on wafers with one or two modulation doped CdTe/CdMgTe quantum wells. The gratings (with the period varied between 2 &mu, m and 8 &mu, m) were prepared with an electron beam lithography either by etching or by evaporation of Au. The gratings were studied with an atomic force microscope which revealed a correlation between the depth and width of etched grooves at a constant time of etching. The sharpest resonances observed are due to excitation of magnetoplasmon in the case of Au gratings on a wafer with one quantum well. Etched samples with two quantum wells showed the strongest tuneability of magnetoplasmon resonances with the period of the grating and illumination with white light. We showed that the samples studied can be used as resonant or quasi-resonant terahertz detectors tuneable with magnetic field and white light.

The coherent spin dynamics of electrons in tunnel-coupled CdTe and (Cd,Mn)Te quantum wells (QWs) is studied by time-resolved pump-probe Kerr rotation. The coupled QWs have different thicknesses; the narrow one is doped by ${\mathrm{Mn}}^{2+}$ magnetic ions. A short range proximity effect between them is observed: the Zeeman splitting of electrons in the wide QW is given in addition to the intrinsic electron $g$ factor by the exchange interaction with the ${\mathrm{Mn}}^{2+}$ ions mediated by electron tunneling into the narrow QW. The exchange interaction strength scales with the ${\mathrm{Cd}}_{0.88}{\mathrm{Mg}}_{0.12}\mathrm{Te}$ barrier thickness separating the QWs. The Kerr rotation signal measured on the wide QW shows two close frequencies of electron spin Larmor precession in a transverse magnetic field. These components have very different spin dephasing times, 50 ps and 1 ns. The two frequencies originate from electrons in the wide QW being either part of an exciton or being resident. The proximity effect of the exciton electron is smaller due to the binding by Coulomb interaction, which decreases the tunneling to the narrow well. The experimental data are in good agreement with model calculations.

Magneto-optical phenomena such as the Faraday and Kerr effects play a central role in controlling the polarization and intensity of optical fields propagating through a medium. Intensity effects in which the direction of light emission depends on the orientation of the external magnetic field are of particular interest, as they can be harnessed for routing light. Effects known so far for accomplishing such routing all control light emission along the axis parallel to the magnetic field. Here we report a new class of emission phenomena where directionality is established perpendicular to the externally applied magnetic field for light sources located in the vicinity of a surface. As a proof of principle for this effect, which we call transverse magnetic routing of light emission, we demonstrate the routing of emission for excitons in a diluted-magnetic-semiconductor quantum well. In hybrid plasmonic semiconductor structures, we observe significantly enhanced directionality of up to 60%.

The magnetization properties of a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure are investigated by several techniques. Exploiting the proximity effect between acceptor bound holes and magnetic ions we detect the magnetization curves by measuring the circular polarization of photoluminescence in an out-of-plane magnetic field. We show that magnetization originates from interfacial ferromagnet on Co-CdMgTe interface and the proximity effect is caused by magnetization of interfacial Co-CdMgTe ferromagnetic layer whose magnetic properties are very different from Co.

Light emission from ZnMnTe/ZnMgTe structures with the quantum wells ZnTe containing monolayer manganese inclusions and quantum wells Zn0.45Mn0.15Te were investigated under the different excitation conditions. The heavy-hole exciton σ+ and σ– magnetic components show the unusual behavior concerning their energy shifts and intensities. It is possible to explain the Zeeman splitting of the heavy exciton emission band if the following factors are taken into account. Firstly, ZnMnTe/ZnMgTe quantum well structures are of the type II due to the strains as concerns to the electron and heavy hole. Secondly, the electron and hole magnetic sublevels population is far from equilibrium due to the fast energy transfer from the electron- hole system to the 3d-shells of magnetic ions.

The optical properties of the Cd0.4Mn0.4Te/Cd0.5Mg0.5Te heterostructure with quantum wells of 7 monolayers in thickness are studied. The temperature dependences of the integral intensity of luminescence of the barrier layer and quantum well and intracenter luminescence of manganese ions are determined, and the luminescence excitation spectra are measured. Numerous components corresponding to LO phonons of the CdTe, MgTe, and MnTe types are observed in the Raman scattering spectra.

We study the quantum beats in the polarization of the photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the $\sigma^+$ and $\sigma^-$ circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole $g$ factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te quantum well. From these quantum beats we obtain the in-plane heavy hole $g$ factor $g_h=-0.143\pm0.005$., Comment: 4 pages, 4 figures

International audience; The behavior of the spin relaxation time of electrons bound to donors immersed in the middle of a CdTe quantum well was measured in the range of temperatures 10-80 K, by using picosecond pump-probe Kerr rotation. Different doping concentrations spanning from isolated donors up to a concentration beyond the metal-insulator transition were considered at a fixed magnetic field; at very low temperature, in insulating regime all electrons are bound to donors but for one high concentration we had to consider that a fraction of electrons is in conduction states. By increasing the temperature, the number of conduction electrons increases. The experimental temperature dependences were explained by invoking spin exchange between electron spins localized on donors and the spin of electrons promoted to conduction states. A good agreement between experiment and theory was found and allowed us to conclude that, while the spin of localized electrons undergoes the effect of both hyperfine and anisotropic exchange interactions, the D'yakonov-Perel' mechanism governs the spin relaxation of the conduction electrons for the whole range of the studied doping concentrations. Moreover, we identified the scattering mechanisms possibly undergone by the conduction electrons at low temperature.

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that independent of the particular spin relaxation mechanism, hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei., 11 pages, 8 figures

Double CdTe/CdMgTe quantum wells modulation doped with Iodine were etched to fabricate grids with a period of $8 \mu \mathrm{m}$ and a geometrical aspect ratio ($\delta$) between 0.2 and 0.7. Etching removed only the well closer to the sample's surface. Transmission of terahertz radiation as a function of magnetic field (B) up to 10 T was measured at 2 K. The spectra show a single magnetoplasmon resonance (MPR) which position in B depends on $\delta$. A huge amplitude of MPR, comparable to that of the cyclotron resonance, allows us to propose such structures as tunable plasmonic terahertz detectors.

Low-temperature spin relaxation time of electrons localized on iodine donors placed in the middle of a 8-nm CdTe quantum well is studied by using the photoinduced Faraday rotation technique, for donor concentrations in the range from $1\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ to $3.6\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$. A maximum relaxation time of the order of 20 ns is found at a doping concentration near the metal-insulator transition. Adapting the theoretical description developed for bulk GaAs [R. I. Dzhioev et al., Phys. Rev. B 66, 245204 (2002)] to a quantum well, we have fitted the experimental results taking as a parameter the spin-orbit constant in CdTe. A satisfactory fit is obtained with ${\ensuremath{\alpha}}_{so}=0.079\ifmmode\pm\else\textpm\fi{}0.011,$ which is also very close to the calculated theoretical value. A comparison of the experimental spin relaxation times measured previously in bulk CdTe to those predicted by the theory is done. Finally, we compare the predicted spin relaxation times in bulk CdTe to those predicted in one of the most studied bulk semiconductor materials: GaAs, and in (less studied) ZnSe.

The exciton luminescence of CdTe based quantum well structures with the embedded ultrathin MnTe layers is studied. It appears that the energy position and width of the exciton emission bands as well as their lifetime and temperature behavior depend strongly on the real profiles of manganese distribution in the direction of the structure growth. The relative contributions of free and localized states to the low temperature exciton luminescence show that the manganese incorporation is favorable to smooth the CdTe/Cd1−xMgxTe interfaces.

To study the energy transfer (ET) between the Mn2+ intra-center luminescence (IL) state and the band state in CdTe quantum wells (QWs), we studied the temperature dependences of the photoluminescence (PL) spectra of the CdTe multi-QWs sandwiched by ultrathin MnTe layers and found an anomalous thermal variation of their PL intensities. With elevating temperature, the PL intensities of the thinner QWs having 4 and 8 monolayers (MLs) remain at rather high temperatures than 100 K despite the PL of the thicker QWs disappears rapidly. In the thinner QWs, the PL bands appear in an overlap region with the Mn2+ IL-band and energy transfer is expected between the IL-state and the band states in the thinner QWs. The anomalous thermal variations of the PL intensities can be explained by considering the ET and the role of Mn2+ IL-state as reservoir states against the thermal quenching processes.

Heterostructures with atomically thin double quantum wells based on CdTe/ZnTe are investigated by optical spectroscopy (photoluminescence and reflectivity methods) as a function of temperature and density of excitation. Heavy and light exciton luminescence lines are observed with comparable intensities and different temperature behaviors (they cross each other at about 65° K). All these features agree with a complete calculation that takes into account both a very small chemical band offset for such monolayer CdTe inclusions in the ZnTe matrix (namely, 2%), and, consequently, the importance of Coulombic interaction in these double quantum wells.

Grating metamaterials were fabricated with electron beam lithography on CdTe/CdMgTe modulation doped structures with two non-interacting quantum wells. Two types of samples were studied: with etched gratings and with gratings formed by deposition of Au stripes. The polarization properties at THz frequencies of the gratings were determined at room temperature. It was shown that Au gratings formed a linear polarizer, while etched gratings did not polarize THz radiation. Transmission of circularly polarized THz radiation at low temperatures through a sample with no grating showed a strongly circularly polarized cyclotron resonance transition. Transmission of this radiation through a sample with an etched grating showed a magnetoplasmon transition that was almost perfectly linearly polarized. We concluded that magnetoplasmons in metamaterials with etched gratings are linearly polarized excitations, possibly with a small contribution of a circular component. This work opens the possibility of the detailed study of the polarization of magnetoplasmons, which has not been explored in the past.

We study optical pumping of resident electron spins under resonant excitation of trions in $n$-type CdTe/(Cd,Mg)Te quantum wells subject to a transverse magnetic field. In contrast to the comprehensively used time-resolved pump-probe techniques with polarimetric detection, we exploit here a single beam configuration in which the time-integrated intensity of the excitation laser light transmitted through the quantum wells is detected. The transmitted intensity reflects the bleaching of light absorption due to optical pumping of the resident electron spins and can be used to evaluate the Larmor precession frequency of the optically oriented carriers and their spin relaxation time. Application of the magnetic field leads to depolarization of the electron spin ensemble so that the Hanle effect is observed. Excitation with a periodic sequence of laser pulses leads to optical pumping in the rotating frame if the Larmor precession frequency is synchronized with the pulse repetition rate. This is manifested by the appearance of Hanle curves every 3.36 or 44.2 mT for pulse repetition rates of 75.8 or 999 MHz, respectively. From the experimental data we evaluate the $g$ factor of $|g|=1.61$ and the spin relaxation time of 14 ns for the optically pumped resident electrons, in agreement with previous time-resolved pump-probe studies.

It is reported an enhancement of the transverse magneto-optical Kerr effect (TMOKE) in the vicinity of the excitonic and hybrid plasmon-exciton modes. Such modes are excited in quantum well in the bare diluted semiconductor thin films and in the plasmonic semiconductor nanostructures. Two-order enhancement of the TMOKE is observed in bare semiconductor films in the spectral region of the excitonic resonances. In plasmonic semiconductor nanostructures with quantum wells we also observe the hybridization of the TMOKE spectrum due to excitation of plasmon-exciton modes.

Time-resolved Kerr-rotation microscopy explores the influence of optical doping on the persistent spin helix in a [001]-grown CdTe quantum well at cryogenic temperatures. Electron spin diffusion dynamics reveal a momentum-dependent effective magnetic field providing SU(2) spin-rotation symmetry, consistent with kinetic theory. The Dresselhaus and Rashba spin-orbit coupling parameters are extracted independently from rotating the spin helix with external magnetic fields applied parallel and perpendicular to the effective magnetic field. Most importantly, a non-uniform spatiotemporal precession pattern is observed. The kinetic theory framework of spin diffusion allows for modeling of this finding by incorporating the photocarrier density into the Rashba ($\alpha$) and the Dresselhaus ($\beta_3$) parameters. Corresponding calculations are further validated by an excitation-density dependent measurement. This work shows universality of the persistent spin helix by its observation in a II-VI compound and the ability to fine-tune it by optical doping., Comment: 5 pages, 4 figures, journal submission

In typical conditions of the observation of multiple paramagnetic Raman resonance in model heterostructures containing manganese ions, we observed a never-reported series of Raman peaks, showing up in a narrow range of applied magnetic fields. These weak ``fractional'' peaks are located close to the middle between the strong ``integer'' Mn spin-flip peaks, and they reveal a remarkably weak dependence of the intensity on the peak number, in a pronounced contrast with the ``integer'' series. We discuss conditions to observe the ``fractional'' series as well as pathways for further exploration of the effect.

The luminescence, luminescence excitation, reflection, and Raman scattering spectra of planar ZnMnTe/ZnMgTe nanostructures with ZnMnTe quantum wells of different widths and elemental composition have been studied. Based on the optical measurements, the energy levels of the nanostructures have been found. It has been shown that the intracenter luminescence excitation spectrum of manganese ions contains a detailed information on the electronic spectrum of the nanostructures. The factors affecting the band widths of exciton luminescence of the quantum wells has been considered. The Raman scattering spectra depends considerably on the parameters of the nanostructure and the relation between the energies of incident photons and intrinsic resonances of the nanostructure.

Exchange-coupled complexes consisting of Mn ions and holes have been revealed by optically detected magnetic resonance in the narrowest quantum wells of asymmetrical multiple-quantum-well structures (CdMn)Te/(CdMg)Te. Calculations have been performed to estimate the parameters of the complexes (exchange interactions and hole g-factors) and simulate the spectra. The formation of such complexes implies the directional electron tunneling from narrow to larger wells, i.e., an electron cascade, which results in the creation of the excess hole concentration in the narrowest and intermediate-width quantum wells.

The exchange interaction between magnetic ions and charge carriers in semiconductors is considered to be a prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range $p\ensuremath{-}d$ exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10-nm-thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10 nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift ${\mathrm{\ensuremath{\Delta}}}_{S}$ on the external magnetic field $B$. We show that in a strong magnetic field, ${\mathrm{\ensuremath{\Delta}}}_{S}$ is a linear function of $B$ with an offset of ${\mathrm{\ensuremath{\Delta}}}_{pd}=50--100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{eV}$ at zero field from the FM induced effective exchange field. On the other hand, the $s\ensuremath{-}d$ exchange interaction between conduction band electrons and FM, as well as the $p\ensuremath{-}d$ contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure.

This paper reports on the observation of the magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field ($B$), applied normal to the quantum well plane. An electric current excited by terahertz laser radiation shows 1/$B$-periodic oscillations with amplitude much larger than the photocurrent at zero magnetic field. It is shown that the photocurrent generation is due to the combined action of a spatially periodic lateral potential and the spatially modulated radiation due to the near-field effects of light diffraction. The magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization. For (Cd,Mn)Te-based structures at low temperatures, a beating-like pattern of the oscillations is observed, caused by the interplay of Zeeman and Landau splitting. A theoretical analysis, considering the magnetic quantum ratchet effect in the framework of a semiclassical approach, describes quite well the experimental results.

We report on the observation and systematic study of polarization sensitive magnetic quantum ratchet effects induced by alternating electric fields in the terahertz frequency range. The effects are detected in (Cd,Mn)Te-based quantum well (QW) structures with inter-digitated dual-grating-gate (DGG) lateral superlattices. A dc electric current excited by cw terahertz laser radiation shows 1/B-periodic oscillations with an amplitude much larger than the photocurrent at zero magnetic field. Variation of gate voltages applied to individual grating gates of the DGG enables us to change the degree and the sign of the lateral asymmetry in a controllable way. The data reveal that the photocurrent reflects the degree of lateral asymmetry induced by different gate potentials. We show that the magnetic ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, which are sensitive to the corresponding polarization of the driving electromagnetic force. Theoretical analysis performed in the framework of semiclassical approach and taking into account Landau quantization describes the experimental results well.

Intra-center luminescence dynamics was investigated under low excitation density in Zn0.8Mn0.2Te/Zn0.59Mg0.41Te multiquantum wells both experimentally and by Monte Carlo simulation. A model developed earlier for bulk Cd1−xMnxTe is applied to the nanostructures with wells׳ width equal to 7 and 26 monolayers. Low temperature experimental transients showed build-up part. Besides time-resolved luminescence spectra of Mn2+-ions׳ band indicate that energy of the band maximum shifts in two time scale. The Monte Carlo simulation showed that fast shift derives from transition between the lowest intra-center states 4T2→4T1 within the same Mn2+-ion. The explanation is supported by calculations applied to bulk Cd0.4Mn0.6Te under similar excitation conditions. Mean relaxation rate of the 4T2→4T1-transition is estimated to be 5 µs−1. The slow relaxation is determined by energy transfer between Mn2+-ions, its rate being four orders of magnitude less than that of the transition. Calculations showed weak correlation between energies of the 4T1- and 4T2-states which causes the transition energy fluctuations. This results in the transition rate dispersion, which is taken into account by exponential dependence on the transition energy. Loss of the correlation is shown to be mainly originated from symmetry breakdown. Noticeable non-exponential decay in Mn2+-luminescence transients is described by stochasticity in decay rates.

In tunnel-coupled quantum wells (QWs) CdTe (20 nm wide) and CdMnTe (8 nm wide) separated by Cd0.88Mg0.12Te barrier with a thickness of L B = 5, 7, 9, and 11 monolayers (MLs), the dependence of electron g-factor on the barrier thickness and temperature is investi-gated by means of the pump-probe Kerr rotation technique. The renormalization of the electron g-factor occurs due to the s–d exchange interaction of electrons with manganese ions in a magnetic QW. The most change of the electron g-factor value Δg = 0.25 is registered at the least barrier width of 5 MLs and temperature T = 5 K. In this case, the penetration of the electron wave function from the nonmagnetic QW into the magnetic one is estimated to be 0.6%.

Photoluminescence and Raman scattering spectra in CdTe/ZnTe heterostructures and superlattices with narrow quantum wells (4.8–9.2 A) in a temperature range of 5–300 K have been measured. The temperature dependences of the intensity of exciton luminescence in isolated quantum wells have been studied, and the thermal activation energies associated with the effective barriers for electrons and holes have been determined. In CdTe/ZnTe heterostructures, the binding energies of an exciton with a heavy hole have been determined as functions of the quantum well width. The multiphonon Raman spectra that exhibit distinctive features, such as the weak intensity of nLO phonon lines of ZnTe (n 2), and the multiple participation in scattering of acoustic LA phonons with large wave vector, have been investigated. The results have been explained based on the concept of the relaxation of hot excitons over the exciton band.

The low-temperature luminescence of ZnMnTe/ZnMgTe and CdMnTe/CdMgTe quantum well structures with different quantum well widths and different Mn proportions is studied at optical-excitation power densities ranging from 104 to 106 W cm−2. Because of saturation of the lowest excited state 4 T 1 of the 3d shell of Mn2+ ions, transitions to higher states start to play an important role. As a result, the intracenter luminescence of Mn2+ ions deteriorates at high excitation levels. Simultaneously, the temperature-dependent saturation of the main exciton-emission band e1hh1 of the quantum wells occurs, and the band e2hh2 emerges. As the optical excitation is increased, the intracenter luminescence band of Mn2+ ions changes its shape. This effect is attributed to the faster saturation of the excited states of interface ions. For CdMnTe/CdMgTe structures, the effect of the quantum well width and Mn content on the relation between the emission intensities corresponding to excitons in quantum wells, excitons in barriers, and the 3d shell of Mn2+ ions is established.

Several experiments in nanowires detected signatures of Majorana fermions, building block for topologicaly protected quantum computer. Now the focus of research is shifting toward systems where non-Abelian statistics of excitations can be demonstrated. To achieve this goal we are developing a new dilute magnetic semiconductor-based platform where non-Abelian excitations can be created and manipulated in a two-dimensional plane, with support for Majorana and higher order non-Abelian excitations. Here we report development of heterostructures where spin polarization of a two-dimensional electron gas in a quantum Hall regime can be controlled locally by electrostatic gating. This is demonstrated via voltage induced shift of quantum Hall ferromagnetic transition in the CdTe quantum wells with engineered placement of paramagnetic Mn impurities. The structures can be used to form helical domain walls in integer quantum Hall regime which, coupled to an s-wave superconductor, are expected to support Majorana zero modes. These heterostructures can be used as a testbed to study gate-reconfigurable domain walls networks.

We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times. Fil: Grinberg, Patricio. Universite Paris Sorbonne - Paris Iv; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bernardot, F.. Universite Paris Diderot - Paris 7; Francia. Universite Paris Sorbonne - Paris Iv; Francia Fil: Eble, B.. Universite Paris Sorbonne - Paris Iv; Francia Fil: Karczewski, G.. Institute Of Physics Of The Polish Academy Of Sciences; Polonia Fil: Testelin, C.. Universite Paris Sorbonne - Paris Iv; Francia Fil: Chamarro, M.. Universite Paris Sorbonne - Paris Iv; Francia

Larmor's theorem holds for magnetic systems that are invariant under spin rotation. In the presence of spin-orbit coupling this invariance is lost and Larmor's theorem is broken: for systems of interacting electrons, this gives rise to a subtle interplay between the spin-orbit coupling acting on individual single-particle states and Coulomb many-body effects. We consider a quasi-two-dimensional, partially spin-polarized electron gas in a semiconductor quantum well in the presence of Rashba and Dresselhaus spin-orbit coupling. Using a linear-response approach based on time-dependent density-functional theory, we calculate the dispersions of spin-flip waves. We obtain analytic results for small wave vectors and up to second order in the Rashba and Dresselhaus coupling strengths $\alpha$ and $\beta$. Comparison with experimental data from inelastic light scattering allows us to extract $\alpha$ and $\beta$ as well as the spin-wave stiffness very accurately. We find significant deviations from the local density approximation for spin-dependent electron systems., Comment: 11 pages, 7 figures

A Different Spin Transistor A typical transistor consists of a source and a drain; the current that makes it to the drain is controlled by applying voltage to the third terminal, called the gate. In spin-based electronics, where spin current is used instead of charge, the source and the drain are ferromagnetic materials connected by a narrow semiconducting channel. This design, however, suffers from low efficiency. Betthausen et al. (p. 324 ; see the Perspective by Žutić and Lee ) combined homogeneous and helical magnetic fields to change the orientation of the spin on its way to the drain, preserving spin information over distances many times the spin mean free path. The transistor is “on” when the transport is adiabatic—i.e., slow enough for the spin to be able to adapt to the local magnetic field—and “off” otherwise.

The exchange interaction between excitons and manganese ions have been studied in wide quantum wells of Cd1-x Mnx Te with Cd1-y Mgy Te barriers. The widths L of the wells are sufficiently large for the excitons to be described by the centre of mass approximation, in which the translational wavevector is quantised according to Kz = Nπ/L. Excitonic transitions with different values of the quantisation index N can be resolved in the optical spectra. The giant Zeeman splittings induced by a magnetic field then enable the separate determination of the exchange energy for each value of the wavevector. We find that the quantity N0 (α – β), which describes the exchange splitting, is constant to within 3% over the range of wavevectors up to 4 × 106 cm–1 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-flip Raman spectroscopy (SFRS) reveals an intrinsic in-plane anisotropy of the conduction band spin splitting in dilute magnetic semiconductor quantum wells based on cadmium telluride, containing a twodimensional electron gas, and intended for studies of extrinsic anisotropies induced through nanomagnets. We consider various possible sources of this intrinsic anisotropy (interface, bulk or structural inversion asymmetries) and we ascribe the present results to an interface-related inversion asymmetry of the quantum wells

CdTe/MnTe/CdMgTe quantum-well structures with one or two monolayers of MnTe inserted at CdTe/CdMgTe interfaces were fabricated. The spectra of the excitonic luminescence from CdTe quantum wells and their variation with temperature indicate that introduction of ultrathin MnTe layers improves the interface quality. The effect of a magnetic field in the Faraday configuration on the spectral position of the exciton-emission peaks indicates that frustration of magnetic moments in one-monolayer MnTe insertions is weaker than in two-monolayer insertions. The effect of a magnetic field on the exciton localization can be explained in terms of the exciton wave-function shrinkage and obstruction of the photoexcited charge-carrier motion in the quantum well.

In this paper we demonstrate that the magnetic moment of the exciton increases due to its motion along the magnetic field, and the diamagnetic shift decreases. These effects were observed for a set of quantum well structures with different well widths and based on a variety of semiconductor compounds. The reason for these effects is mixing of the exciton’s center of mass motion and the internal motion of the electron and hole in the exciton. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The optical spectra of the CdTe/Cd0.7Mn0.3Te structure containing three CdTe quantum wells with nominal thicknesses of 16, 8, and 4 monolayers have been investigated. The temperature dependences of parameters of the exciton luminescence spectra (integrated intensity, full-width at half-maximum, position of the maximum, Stokes shift) for quantum wells with different thicknesses differ substantially. These differences are explained by a strong thickness dependence of the energy of Coulomb coupling in the exciton, the energy of localization of the exciton on bulges of the quantum well, and the degree of penetration of the exciton wave function into the barrier. At high excitation power densities, the emission contours of the quantum wells with thicknesses of 8 and 16 monolayers contain short-wavelength tails that correspond to optical transitions between excited quantum-well levels.

The effects of charge-density fluctuations on trion states and spin-density fluctuation on the exciton states in quantum well are considered in the limit of low doping by electrons. In both cases the quasi-two dimensionality of the systems is taken into account. The trion-chargedensity interaction is considered in the random phase approximation. The influence of the exchange electron-electron interaction on the excitons is considered using a simplified approach. It is shown that the broadening of the trion absorption band profile with free carrier concentration increase is essentially weaker than the broadening of the exciton absorption band. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)