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.

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 modulationdoped 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.Soon after the discovery of carrier-controlled ferromagnetism in Mn-doped III-V [1] and II-VI [2] semiconductor compounds, it has become clear that these systems offer unprecedented opportunities to exploit the powerful methods developed for tuning carrier densities in semiconductor quantum structures, in order to control the magnetic characteristics in these systems [2,3,4,5,6]. Such a control opens new prospects for information storage and processing, as well as it makes it possible to examine the behavior of strongly correlated systems as a function of externally controllable parameters. In the case of III-V magnetic semiconductors, Koshihara et al.[3] detected an enhancement of ferromagnetism by illumination of an (In,Mn)As/GaSb heterostructure, an effect assigned to the presence of an interfacial electric field that drives the photo-holes to the magnetically active (In,Mn)As layer. More recently, Ohno et al.[6] demonstrated that a gate voltage of ±125 V changes the Curie temperature T C by about 1 K in a field-effect transistor structure containing an (In,Mn)As quantum well (QW).In the case of II-VI diluted magnetic semiconductors (DMS), Mn does not introduce any carriers. Hence, holemediated ferromagnetic interactions can be induced by modulation-doping of heterostructures [7]. Due to the valence band structure, T C is typically lower in II-VI than in III-V DMS. At the same time, however, it may be expected [2,5] that, owing to the small background hole density, the strength of the carrier mediated ferromagnetic interactions can be tuned over a wider range in II-VI than in III-V DMS.In this paper, we present photoluminescence (PL) studies of modulation-doped p-type (Cd,Mn)Te QW. The (Cd,Zn,Mg)Te barriers are doped either p-or n-type, so that p-i-p or p-i-n structures are formed. The QW in these systems are ferromagnetic below about 3 K. We show that, depending of the sample layout, the ferromagnetism is either destroyed or enhanced during illumination by photons with energy greater than the band gap of the barrier material. In both cases, the switching process is isothermal and reversible. Moreover, we demonstrate that the reverse biasing of the p-i-n diode by a voltage smaller than 1 V turns the ferromagnet into a paramagnetic material. Importantly, this strong effect of light and electric field can be readily explained by considering the distributi...

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 readout 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 nanoseconds 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 millitesla 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.

We report on the observation of spin dependent optically dressed states and optical Stark effect on an individual Mn spin in a semiconductor quantum dot. The vacuum-to-exciton or the excitonto-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 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 a complete optical resonant control of the exciton-Mn system.Semiconductor quantum dots (QDs) exhibit discrete excitonic energy levels with an atom-like light-matter interaction. Resonant optical excitation of QDs has allowed to observe the absorption of a single QD [1] and the efficient preparation of the quantum state of a single confined carrier [2,3] It has also been demonstrated that the optical Stark effect can be used to compensate the exchange splitting in anisotropic QDs to produce entangled photon pairs [10].We show in this letter that the energy of any spin state of an individual Mn atom embedded in a II-VI semiconductor QD, can be tuned using the optical Stark effect induced by a strong laser field. Under resonant excitation, hybrid light-matter states are created independently for all Mn spin states. The corresponding Rabi energy measured through the Autler-Townes splitting can reach 250 µeV . At low temperature, the energies that control the dynamics of an isolated Mn spin in a CdTe QD, like the strain induced magnetic anisotropy or hyperfine coupling to the nuclei, are weaker than the observed optical Stark shifts. This opens a way to control the dynamics of a single Mn spin in its solid state environment. We also report optical Stark shifts and optically dressed states of the Mn exchanged coupled with the exciton or biexciton. Finally, we discuss a particular situation where two Mn spin states are mixed by the coupling between bright and dark excitons. In spite of the Mn spin mixing, we show that an optical manipulation of individual spin states can be performed.The sample used in this study is grown on a ZnTe substrate and contains CdTe QDs. A 6.5 monolayer thick CdTe layer is deposited at 280 • C by atomic layer epitaxy on a ZnTe barrier grown by molecular beam epitaxy at 360 • C. The dots are formed by a Tellurium deposition/desorption process [11] and protected by a 100 nm thick ZnTe top barrier. The QDs are 10-20 nm wide and few nm high. Mn atoms are introduced during the growth of the CdTe layer. The Mn concentration is adjusted to optimize the probability to detect one Mn per dot.Optical addressing of QDs containing a single magnetic atom is achieved using micro-spectroscopy techniques. A high refractive index hemispherical solid immersion lens is mounted on the surface of the sample to enhance the spatia...

International audienceWe 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 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.

Molecular beam epitaxy technique has been used to deposit a single layer and a bilayer of MoSe2 on sapphire. Extensive characterizations including in-situ and ex-situ measurements show that the layered MoSe2 grows in a scalable manner on the substrate and reveals characteristics of a stoichiometric 2H-phase. The layered MoSe2 exhibits polycrystalline features with domains separated by defects and boundaries. Temperature and magnetic field dependent resistivity measurements unveil a carrier hopping character described within two-dimensional variable range hopping mechanism. Moreover, a negative magnetoresistance was observed, stressing a fascinating feature of the charge transport under the application of a magnetic field in the layered MoSe2 system. This negative magnetoresistance observed at millimeter-scale is similar to that observed recently at room temperature inWS2 flakes at a micrometer scale [Zhang et al., Appl. Phys. Lett. 108, 153114 (2016)]. This scalability highlights the fact that the underlying physical mechanism is intrinsic to these two-dimensional materials and occurs at very short scale.

The spectroscopic properties of a spin polarized two-dimensional hole gas are studied in modulation doped Cd1−xMnxTe quantum wells with variable carrier density up to 5 10 11 cm −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 MossBurstein shifts) are described and calibrated. The value of the spin splitting needed to fully polarize the hole gas, evidences a strong enhancement of the spin susceptibility of the hole gas due to carriercarrier interaction. At small values of the spin splitting, whatever the carrier density (non zero) 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 favour 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 with 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.