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5 pages, 5 figures; International audience; We report on a two-dimensional (2D) V1–xPtxSe2 alloy that exhibits ferromagnetic order and Rashba spin–orbit coupling. Although ferromagnetism is absent in 1T-VSe2 because of the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe2 (10–50%) to form a homogeneous 2D alloy restores ferromagnetic order down to one monolayer of V0.65Pt0.35Se2. Moreover, the presence of platinum atoms gives rise to Rashba spin–orbit coupling in (V,Pt)Se2, providing an original platform to study the interplay between ferromagnetism and spin–orbit coupling in the 2D limit.

International audience; Two-dimensional materials (2D) arranged in hybrid van der Waals (vdW) heterostructures provide a route toward the assembly of 2D and conventional III–V semiconductors. Here{,} we report the structural and electronic properties of single layer WSe2 grown by molecular beam epitaxy on Se-terminated GaAs(111)B. Reflection high-energy electron diffraction images exhibit sharp streaky features indicative of a high-quality WSe2 layer produced via vdW epitaxy. This is confirmed by in-plane X-ray diffraction. The single layer of WSe2 and the absence of interdiffusion at the interface are confirmed by high resolution X-ray photoemission spectroscopy and high-resolution transmission microscopy. Angle-resolved photoemission investigation revealed a well-defined WSe2 band dispersion and a high p-doping coming from the charge transfer between the WSe2 monolayer and the Se-terminated GaAs substrate. By comparing our results with local and hybrid functionals theoretical calculation{,} we find that the top of the valence band of the experimental heterostructure is close to the calculations for free standing single layer WSe2. Our experiments demonstrate that the proximity of the Se-terminated GaAs substrate can significantly tune the electronic properties of WSe2. The valence band maximum (VBM{,} located at the K point of the Brillouin zone) presents an upshift of about 0.56 eV toward the Fermi level with respect to the VBM of the WSe2 on graphene layer{,} which is indicative of high p-type doping and a key feature for applications in nanoelectronics and optoelectronics.

PtSe2 is attracting considerable attention as a high mobility two-dimensional material with envisioned applications in microelectronics, photodetection and spintronics. The growth of high quality PtSe2 on insulating substrates with wafer-scale uniformity is a prerequisite for electronic transport investigations and practical use in devices. Here, we report the growth of highly oriented few-layers PtSe2 on ZnO(0001) by molecular beam epitaxy. The crystalline structure of the films is characterized with electron and x-ray diffraction, atomic force microscopy and transmission electron microscopy. The comparison with PtSe2 layers grown on graphene, sapphire, mica, SiO2 and Pt(111) shows that among insulating substrates, ZnO(0001) yields films of superior structural quality. Hall measurements performed on epitaxial ZnO/PtSe2 with five monolayers of PtSe2 show a clear semiconducting behaviour and a high mobility in excess of 200 cm2 V−1 s−1 at room temperature and up to 447 cm2 V−1 s−1 at low temperature.

International audience; We demonstrate radio-frequency tuning of the energy of individual CdTe/ZnTe quantum dots (QDs) by Surface Acoustic Waves (SAWs). Despite the very weak piezoelectric coefficient of ZnTe, SAW in the GHz range can be launched on a ZnTe surface using interdigitated transducers deposited on a c-axis oriented ZnO layer grown on ZnTe containing CdTe QDs. The photoluminescence (PL) of individual QDs is used as a nanometer-scale sensor of the acoustic strain field. The energy of QDs is modulated by SAW in the GHz range and leads to characteristic broadening of time-integrated PL spectra. The dynamic modulation of the QD PL energy can also be detected in the time domain using phase-locked time domain spectroscopy. This technique is in particular used for monitoring complex local acoustic fields resulting from the superposition of two or more SAW pulses in a cavity. Under magnetic field, the dynamic spectral tuning of a single QD by SAW can be used to generate single photons with alternating circular polarization controlled in the GHz range.

We demonstrate that the spin of a Cr atom in a quantum dot (QD) can be controlled optically and we discuss the main properties of this single spin system. The photoluminescence of individual Cr-doped QDs and their evolution in magnetic field reveal a large magnetic anisotropy of the Cr spin induced by local strain. This results in a splitting of the Cr spin states and in a thermalization on the lower energy states states S$_z$=0 and S$_z$=$\pm$1. The magneto-optical properties of Cr-doped QDs can be modelled by an effective spin Hamiltonian including the spin to strain coupling and the influence of the QD symmetry. We also show that a single Cr spin can be prepared by resonant optical pumping. Monitoring the intensity of the resonant fluorescence of the QD during this process permits to probe the dynamics of the optical initialization of the spin. Hole-Cr flip-flops induced by an interplay of the hole-Cr exchange interaction and the coupling with acoustic phonons are the main source of relaxation that explains the efficient resonant optical pumping. The Cr spin relaxation time is measured in the $\mu s$ range. We evidence that a Cr spin couples to non-equilibrium acoustic phonons generated during the optical excitation inside or near the QD). Finally we show that the energy of any spin state of an individual Cr atom can be independently tuned by a resonant single mode laser through the optical Stark effect. All these properties make Cr-doped QDs very promising for the development of hybrid spin-mechanical systems where a coherent mechanical driving of an individual spin in an oscillator is required., Comment: arXiv admin note: text overlap with arXiv:1606.04221, arXiv:1609.08829

International audience; We demonstrate the optical addressability of the spin of an individual chromium atom (Cr) embedded in a semiconductor quantum dot. The emission of Cr-doped quantum dots and their evolution in magnetic field reveal a large magnetic anisotropy of the Cr spin induced by local strain. This results in the zero field splitting of the 0, ±1, and ±2 Cr spin states and in a thermalization on the magnetic ground states 0 and ±1. The observed strong spin to strain coupling of Cr is of particular interest for the development of hybrid spin-mechanical devices where coherent mechanical driving of an individual spin in an oscillator is needed. The magneto-optical properties of Cr-doped quantum dots are modeled by a spin Hamiltonian including the sensitivity of the Cr spin to the strain and the influence of the quantum dot symmetry on the carrier-Cr spin coupling.

We studied the spin dynamics of a Cr atom incorporated in a II-VI semiconductor quantum dot using photon correlation techniques. We used recently developed singly Cr-doped CdTe/ZnTe quantum dots (A. Lafuente-Sampietro {\it et al.}, [1]) to access the spin of an individual magnetic atom. Auto-correlation of the photons emitted by the quantum dot under continuous wave optical excitation reveals fluctuations of the localized spin with a timescale in the 10 ns range. Cross-correlation gives quantitative transfer time between Cr spin states. A calculation of the time dependence of the spin levels population in Cr-doped quantum dots shows that the observed spin dynamics is controlled by the exciton-Cr interaction. These measurements also provide a lower bound in the 20 ns range for the intrinsic Cr spin relaxation time.

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)

An optical spin orientation is achieved for a Mn atom localized in a semiconductor quantum dot using quasiresonant excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. Relaxation times exceeding the micro-second range are measured (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetic state of a single magnetic atom (Mn) embedded in an individual semiconductor quantum dot is optically probed using micro-spectroscopy. A high degree of spin polarization can be achieved for an individual Mn atom localized in a quantum dot using quasi-resonant or fully-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. The Mn spin distribution prepared by optical pumping is fully conserved for a few microseconds. This opens the way to full optical control of the spin state of an individual magnetic atom in a solid state environment.

We have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence on the translational wavevector Kz. The behaviour of the Zee-man splittings corresponds to enhancement of the magnetic moments of the excitons. This enhancement is particularly marked when their translational kinetic energy becomes comparable with the exciton Rydberg and can be described by what appears to be a universal function of Kz. A model for the behaviour is outlined which involves motionally-induced mixing between the 1S hydrogenic exciton ground state and excited nP states. The observations imply that there are significant changes in the structure of the exciton as its translational kinetic energy increases. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of p-type modulation doped (Cd,Mn)Te quantum wells is studied with carrier density up to 5 X 10(11) cm(-2) at various spin splittings. This splitting can be made larger than the characteristic energies of the system thanks to the giant Zeeman effect. At small spin splitting and regardless of the carrier density, the photoluminescence exhibits a single line, which corresponds to the charged exciton in the singlet state. Above a certain spin splitting, the charged exciton is destabilized in favor of the exciton at vanishing hole density, and in favor of a double line at higher carrier density. It is found here that the charged exciton destabilization energy hardly depends on the carrier density. The double line is found to be band-to-band like, with the same initial state - where the holes have the same spin orientation - and final states that differ by some excitation of the 2D hole gas. In addition, the spin splitting needed to fully polarize the hole gas is twice smaller than expected from the single particle image and gives a unique insight into many-body effects in the hole gas.

We describe the magneto-photoluminescence of (Cd,Mn)Te/(Cd,Zn,Mg)Te quantum wells containing a 2D hole gas . The hole population can be tuned either optically in p-i-p structures, by above barrier illumination, or electrically by applying a bias voltage of a few volts on a p-i-n structure. We identify the photoluminescence at low field as due to the positively charged exciton X + , which is destabilized at a given value of the Zeeman splitting, due to its crossing with a state involving a triplet configuration of the trion hole pair. The X + looses its intensity in favor of a double line which exhibits features associated to band-to-band transitions at k=0 and k=k F (wave vector at Fermi energy).

New structures aiming at controlling ferromagnetic properties of diluted magnetic semiconductors quantum wells (QWs) are presented. The carrier density is monitored by applying voltage in p–i–n diode or adjusting distance between QW and surface.

Using photoluminescence and photoluminescence excitation, weinvestigate the magnetic properties of (Cd,Mn)Te quantum wells inserted in pin diodes. Such structures give us the opportunity to tune the carrier density by applying a bias of the order of 1 V. Results obtained in a small magnetic field show the presence of magnetic domains, which persist as long as the quantum well is populated.

Experimental results obtained on Zn 1− x Mn x Te layers and Cd 1− x Mn x Te quantum wells are surprisingly well understood in the framework of a mean field model of the carrier-induced ferromagnetic transition, provided the real structure of the valence band is properly taken into account. Here we put the emphasis on effects leading to deviations from the standard model: RKKY oscillations in Zn 1− x Mn x Te when the Mn concentration is not high enough with respect to the carrier density, electronic disorder and localization at low carrier density in Zn 1− x Mn x Te epilayers and Cd 1− x Mn x Te quantum wells. Importantly, the modulation of magnetic properties through modulation of the carrier density, either optically or using the electric potential in a pin diode, is demonstrated.

International audience; Using the emission of the positively charged exciton as a probe, we analyze the dynamics of the optical pumping and the dynamics of the relaxation of a Mn spin exchange coupled with a confined hole spin in a II-VI semiconductor quantum dot. The hole-Mn spin can be efficiently initialized in a few tens of ns under optical injection of spin-polarized carriers. We show that this optical pumping process and its dynamics are controlled by electron-Mn flip-flops within the positively charged exciton-Mn complex. The pumping mechanism and its magnetic field dependence are theoretically described by a model including the dynamics of the electron-Mn complex in the excited state and the dynamics of the hole-Mn complex in the ground state of the positively charged quantum dot. We measure at zero magnetic field a spin-relaxation time of the hole-Mn spin in the μs range or shorter. This hole-Mn spin relaxation is induced by the presence of valence-band mixing in self-assembled quantum dots.

We have studied the growth conditions to obtain the chemically well ordered L1 0 phase of the FePd alloy deposited on ZnSe and CdZnMgTe II–VI semiconductors. We have observed that the segregation of Se or Te atoms at the surface involved a dramatic effect on the formation of the L1 0 phase. The strong anisotropy exhibited along the growth direction allows to observe, by magnetic force microscopy, the perpendicular magnetic configuration with up and down domains. Comparably, we have grown, on the same materials, Cu/Ni multilayers showing a clear perpendicular magnetic anisotropy. Optical measurements on a semimagnetic heterostructure covered by a ferromagnetic metal layer suggest fluctuations of the Fermi level at the interface between the semiconductor and the metal.

Magneto optical micro-spectroscopy allowed us to access the magnetic field dependency of the photoluminescence of Cd(Mn)Te/ZnTe single quantum dots. We report a strongly non-linear behavior of the exciton and biexciton energies as a function of magnetic field which clearly differs from the expected and usually observed giant Zeeman effect. We demonstrate that the existence of a positive feedback loop between photocarriers and Mn spins results in heating the Mn system and can explain the observed phenomenon. This shows that controlling the coupling of the dot with its environment is a corner stone to achieve spin manipulation in quantum dots.

International audience; We analyze the spin dynamics of an individual magnetic atom (Mn) inserted in a II-VI semiconductor quantum dot under resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that for particular conditions of laser detuning and excitation intensity, the spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. This spin population trapping mechanism is controlled by the combined effect of the coupling with the laser field and the coherent interaction between the different Mn spin states induced by an anisotropy of the strain in the plane of the quantum dot.

International audience; We report on the dynamics of optically induced nuclear spin polarization in individual CdTe/ZnTe quantum dots loaded with one electron by modulation doping. The fine structure of the hot trion (charged exciton X− with an electron in the P shell) is identified in photoluminescence excitation spectra. A negative polarization rate of the photoluminescence, optical pumping of the resident electron, and the built up of dynamic nuclear spin polarization (DNSP) are observed in time-resolved optical pumping experiments when the quantum dot is excited at higher energy than the hot trion triplet state. The time and magnetic field dependence of the polarization rate of the X− emission allows us to probe the dynamics of formation of the DNSP in the optical pumping regime. We demonstrate using time-resolved measurements that the creation of a DNSP at B=0 T efficiently prevents longitudinal spin relaxation of the electron caused by fluctuations of the nuclear spin bath. The DNSP is built in the microsecond range at high excitation intensity. A relaxation time of the DNSP in about 10 μm is observed at B=0 T and significantly increases under a magnetic field of a few milli-Tesla. We discuss mechanisms responsible for the fast initialization and relaxation of the diluted nuclear spins in this system.

Light and electric field control of ferromagnetism in Cd 0.96 Mn 0.04 Te quantum wells is demonstrated. Ferromagnetic transition is observed at a constant temperature as a function of sheet concentration of 2D free carrier gas in the quantum well. The carrier concentration is controlled either by light with photon energy above the band gap of the barriers or by applying a voltage in a p-i-n diode structure.

International audience; We present a systematic study of the CdTe/ZnTe quantum dots (QDs) formation induced by tellurium (Te) deposition and desorption. The investigation of the QDs formation was performed by reflection high electron energy diffraction analysis as a function of the Te deposition time and temperature. We show that the Te can be deposited at a temperature as high as 210 degrees C which leads to a significant shortening of the time needed to fabricate one layer of CdTe QDs. Our investigation also reveals that a critical amount of Te is required to drive the QDs formation. This fast Te induced process to fabricate CdTe QDs was then applied to the fabrication of samples with multiple CdTe/ZnTe QDs. Transmission electron microscopy images suggest that vertical stacking of CdTe dots can be performed. This opens a way towards vertical coupling between CdTe QDs

International audience; We report on the observation of spin-dependent optically dressed states and the optical Stark effect on an individual Mn spin in a semiconductor quantum dot. The vacuum-to-exciton or the exciton-to-biexciton transitions in a Mn-doped quantum dot are optically dressed by a strong laser field, and the resulting spectral signature is measured in photoluminescence. We demonstrate that the energy of any spin state of a Mn atom can be independently tuned by using the optical Stark effect induced by a control laser. High resolution spectroscopy reveals a power-, polarization-, and detuning-dependent Autler-Townes splitting of each optical transition of the Mn-doped quantum dot. This experiment demonstrates an optical resonant control of the exciton-Mn system.

Time-resolved Kerr rotation experiments in CdMnTe quantum wells provide the evidence of mixed spin excitations of the two-dimensional electron gas and magnetic ions. The onset of strong coupling between electron and Mn spin modes reveals the collective (spin-wave) nature of electronic spin excitations probed by this method. We show that resonant exchange coupling between electron-spin waves and magnetic ions spin-flip excitations provide insights in the many-body physics of the two-dimensional electron gas.

We have investigated the spin preparation efficiency by optical pumping of individual Mn atoms embedded in CdTe/ZnTe quantum dots. Monitoring the time dependence of the intensity of the fluorescence during the resonant optical pumping process in individual quantum dots allows to directly probe the dynamics of the initialization of the Mn spin. This technique presents the convenience of including preparation and read-out of the Mn spin in the same step. Our measurements demonstrate that Mn spin initialization, at zero magnetic field, can reach an efficiency of 75% and occurs in the tens of \emph{ns} range when a laser resonantly drives at saturation one of the quantum dot transition. We observe that the efficiency of optical pumping changes from dot to dot and is affected by a magnetic field of a few tens of mT applied in Voigt or Faraday configuration. This is attributed to the local strain distribution at the Mn location which predominantly determines the dynamics of the Mn spin under weak magnetic field. The spectral distribution of the spin-flip scattered photons from quantum dots presenting a weak optical pumping efficiency reveals a significant spin relaxation for the exciton split in the exchange field of the Mn spin., 10 pages, 8 figures

International audience; We have shown recently that the magnetic properties of excitions change significantly as the excitions acquire kinetic energy. In particular, the excition magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectrascopic studies of excitions confined in quantum wells of large width (greater than five times the excition Bohr radius) and these motion-induced changes in the magnetic properties have now been observed for CdTe, Znse, ZnTe and GaAs. The present paper summarises these phenomena. with particular focus on CdTe and ZnSe, and shows that the changes can be accounted or by motion induced mixing between the excition ground and higher lying states. The mixing is caused by the gamma term in the Luttinger Hamiltonian which describes the dispersion curves for the valence hand and, as a result the form of the excition wavefunction becomes motion dependent. For both materials, excellent agreement is obtained between experiment and the results predicted by this mechanism. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

International audience; For low Mn-doped Cd(Mn)Te/ZnTe single quantum dots, we report a nonlinear behavior of exciton energy as a function of magnetic field, which differs from the usual giant Zeeman effect. This effect is modeled by a positive feedback loop in the heating process of Mn spins by carriers photocreated in the vicinity of the dot. It results in a highly nonlinear and magnetic field dependent Mn spin heating process. This effect prevails when dealing with magnetic dot deposited on a thin two-dimensional layer and containing about 1% Mn. This shows that controlling the coupling between the magnetic dot and its local environment is a corner stone to achieve nanoscale spin manipulation.

International audience; An optical spin orientation is achieved for a Mn atom localized in a semiconductor quantum dot using quasiresonant excitation at zero magnetic field. Optically created spin-polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism show that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots.

The magnetic state of a single magnetic ion (Mn 2+ ) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn 2+ ion ( S = 5 / 2 ) is analyzed in detail. The exciton–Mn 2+ exchange interaction shifts the energy of the exciton depending on the Mn 2+ spin component and six emission lines are observed at zero magnetic field. The QD exciton emission is then a direct probe of the Mn spin state. The limiting factors of such detection are then presented: they are mainly due to the influence of geometrical effects such as the anisotropic strain distribution into the QD and its shape anisotropy. In the last part, we show how we can manipulate and understand quantitatively the interaction between the single Mn spin and a confined carrier by controlling the charge state of a quantum dot ( 0 , ± 1 electron) with an electrostatic (and optical) gate. This opens the way to an electrical or optical control of the magnetic properties of a single atom. To cite this article: L. Besombes et al., C. R. Physique 9 (2008).

During the growth of ZnTe by molecular beam epitaxy, we observed that, depending on the substrate temperature and flux ratio, the RHEED intensity oscillations can exhibit strong beats. Those beats are interpreted as a consequence of the coexistence, during growth, of c(2 × 2) and (2 × 1) surface reconstruction domains at the surface of (0 0 1)-ZnTe. The intensity beats superimposed on the regular oscillations are responsible for strong phase shift in the RHEED oscillations, which can be seriously misleading for an accurate determination of the growth rate.

We analyzed the photoluminescence intermittency generated by a single paramagnetic spin localized in an individual semiconductor quantum dot. The statistics of the photons emitted by the quantum dot reflect the quantum fluctuations of the localized spin interacting with the injected carriers. Photon correlation measurements which are reported here reveal unique signatures of these fluctuations. A phenomenological model is proposed to quantitatively describe these observations, allowing a measurement of the spin dynamics of an individual magnetic atom at zero magnetic field. These results demonstrate the existence of an efficient spin relaxation channel arising from a spin-exchange with individual carriers surrounding the quantum dot. A theoretical description of a spin-flip mechanism involving spin exchange with surrounding carriers gives relaxation times in good agreement with the measured dynamics.

Cross-sectional scanning tunneling microscopy is used to study in the atomic scale the structural properties of ZnSeTe∕ZnTe multiple quantum wells and N:ZnTe delta-doped structures. Some peculiar effects are found on the cleaved (110) ZnTe surface plane, which have not been observed in III–V semiconductors. In particular, cleavage induced monatomic wide vacancy chains are always present on the Te sublattice. Furthermore, the semiconductor surface is manipulated when certain positive voltages are applied to the sample. Regarding the heterostructures, the ZnSeTe∕ZnTe quantum wells are found to have abrupt interfaces and the Se concentration is determined to be significantly larger than the nominal value.

International audience; The critical spin splitting needed to fully polarize a two-dimensional heavy-hole gas in a (Cd,Mn)Te quantum well thanks to the giant Zeeman effect is measured through photoluminescence and transmission spectroscopy. While the splitting between two circularly polarized photoluminescence remains equal to the bare spin splitting, the critical spin splitting which governs the polarization is enhanced, in good agreement with the prediction of a model of hole-hole interactions previously applied to conduction electrons. Consequences for carrier induced ferromagnetism are discussed.

The spectroscopic properties of a spin-polarized two-dimensional hole gas are studied in modulation doped ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ quantum wells with variable carrier density up to $5\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The giant Zeeman effect, which is characteristic of diluted magnetic semiconductors, induces a significant spin splitting even at very small values of the applied field. Several methods of measuring the carrier density (Hall effect, filling factors of the Landau levels at high field, various manifestations of Moss-Burstein shifts) are described and calibrated. The value of the spin splitting needed to fully polarize the hole gas shows a strong enhancement of the spin susceptibility of the hole gas due to the carrier-carrier interaction. At small values of the spin splitting, whatever the carrier density (nonzero) is, photoluminescence lines are due to the formation of charged excitons in the singlet state. Spectral shifts in photoluminescence and in transmission (including an ``excitonic Moss-Bustein shift'') are observed and discussed in terms of excitations of the partially or fully polarized hole gas. At large spin splitting, and without changing the carrier density, the singlet state of the charged exciton is destabilized in favor of a triplet state configuration of holes. The binding energy of the singlet state is thus measured and found to be independent of the carrier density (in contrast to the splitting between the charged exciton and the neutral exciton lines). The state stable at large spin splitting is close to the neutral exciton at low carrier density, and close to an uncorrelated electron-hole pair at the largest values of the carrier density achieved. The triplet state gives rise to a characteristic double-line structure with an indirect transition to the ground state (with a strong phonon replica) and a direct transition to an excited state of the hole gas.

New structures aiming at controlling ferromagnetic properties of Diluted Magnetic Semiconductors quantum wells are presented. The carrier density is monitored by applying voltage in p-i-n diode or adjusting a distance between quantum well and surface. Surface doping was successfully applied to obtain samples with CdMnTe quantum well with up to 9.3% Mn concentration.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Experimental results obtained on Zn 1− x Mn x Te layers and Cd 1− x Mn x Te quantum wells are surprisingly well understood in the framework of a mean field model of the carrier-induced ferromagnetic transition, provided the real structure of the valence band is properly taken into account. Here we put the emphasis on effects leading to deviations from the standard model: RKKY oscillations in Zn 1− x Mn x Te when the Mn concentration is not high enough with respect to the carrier density, electronic disorder and localization at low carrier density in Zn 1− x Mn x Te epilayers and Cd 1− x Mn x Te quantum wells. Importantly, the modulation of magnetic properties through modulation of the carrier density, either optically or using the electric potential in a pin diode, is demonstrated.

A strong influence of illumination and electric bias on the Curie temperature and saturation value of the magnetization is demonstrated for semiconductor structures containing a modulation-doped p-type Cd0.96Mn0.04Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible cross-over between the paramagnetic and ferromagnetic phases, in the way that is predetermined by the structure design. The observed behavior is in quantitative agreement with the expectations for systems, in which ferromagnetic interactions are mediated by the weakly disordered two-dimensional hole liquid., Comment: 4 pages and 3 figures

We report remarkable changes in the magnetic properties of excitons as they acquire kinetic energy in wide (001) quantum wells of CdTe, ZnSe, ZnTe and GaAs. In particular, the magnetic moments (g-values) when magnetic fields are applied in the growth direction are shown to. have a strong dependence on the translational wavevector K-z. The dependence is particularly marked when Kz is of the order of the inverse of the exciton Bohr radius or, equivalently, when the translational kinetic energy becomes comparable with the exciton Rydberg. The behaviour is ascribed to motionally-induced mixing between the 1S hydrogenic exciton ground state and excited nP states and is describable by what appears to be a universal formula.