H. Boukari

Light and Electric Field Control of Ferromagnetism in Magnetic Quantum Structures

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 Cd(0.96)Mn(0.04)Te quantum well placed in various built-in electric fields. It is shown that both light beam and bias voltage generate an isothermal and reversible crossover 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.

Optical spin orientation of a single manganese atom in a semiconductor quantum dot using quasiresonant photoexcitation.

A hight degree of spin polarization is achieved for a Mn atom localized in a semiconductor quantum dot using quasi-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.

p-type doping of II–VI heterostructures from surface states: Application to ferromagnetic Cd1−xMnxTe quantum wells

We present a study of p-type doping of CdTe and Cd1−xMnxTe quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding 2×1011 cm−2. Surface doping was applied to obtain samples with Cd1−xMnxTe quantum well with up to x=9.3% containing hole gas. We could also increase the growth temperature up to 280 °C, which results in sharper photoluminescence lines, when compared to the similar nitrogen doped samples. Carrier-induced ferromagnetism was observed in surface doped samples.

Optical probing of spin fluctuations of a single paramagnetic Mn atom in a semiconductor quantum dot

L. Besombes, ∗ Y. Leger, J. Bernos, H. Boukari, H. Mariette, J.P. Poizat, J. Fernández-Rossier, and R. Aguado CEA-CNRS group ”Nanophysique et Semiconducteurs”, Institut Néel, CNRS & Université Joseph Fourier, 25 avenue des Martyrs, 38042 Grenoble, France Departamento de F́ısica Aplicada, Universidad de Alicante, San Vicente del Raspeig, 03690 Spain Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain Abstract 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.

Optical initialization, readout, and dynamics of a Mn spin in a quantum dot

This work is supported by the French ANR contract QuAMOS and Fondation Nanoscience (RTRAGrenoble). J.F.R. acknowledges funding from MEC-Spain (Grants No. MAT07-67845 and CONSOLIDER No. CSD2007-0010).

Ferromagnetism in II–VI-based semiconductor structures

Abstract 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.

Optical control of the spin of a magnetic atom in a semiconductor quantum dot

Abstract: The control of single spins in solids is a key but challenging step for any spin-based solid-state quantumcomputing device. Thanks to their expected long coherence time, localized spins on magnetic atoms in a semiconductor host could be an interesting media to store quantum information in the solid state. Optical probing and control of the spin of individual or pairs of Manganese (Mn) atoms (S = 5/2) have been obtained in II-VI and IIIV semiconductor quantum dots during the last years. In this paper, we review recently developed optical control experiments of the spin of an individual Mn atoms in II-VI semiconductor self-assembled or strain-free quantum dots (QDs).We first show that the fine structure of the Mn atom and especially a strained induced magnetic anisotropy is the main parameter controlling the spin memory of the magnetic atom at zero magnetic field. We then demonstrate that the energy of any spin state of a Mn atom or pairs of Mn atom can be independently tuned by using the optical Stark effect induced by a resonant laser field. The strong coupling with the resonant laser field modifies the Mn fine structure and consequently its dynamics.We then describe the spin dynamics of a Mn atom under this strong resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that the Mn 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. Finally, we discuss the spin dynamics of a Mn atom in strain-free QDs and show that these structures should permit a fast optical coherent control of an individual Mn spin.

Light and electric-field control of ferromagnetism in Cd1-xMnxTe based quantum wells

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.

Optical control of a Mn spin embedded in a quantum dot

We demonstrate a high degree of an optical spin preparation of a single Mn atom embedded in a CdTe/ZnTe quantum dot (QD). Due to the strong exchange interaction of the manganese atom with an exciton injected into the QD the spin orientation can be achieved by quasi-resonant or fully-resonant optical creation of the polarized electron-hole pairs. A measured spin memory of the isolated Mn atom, in most of the cases, is in the microsecond range, and depends on the built-in strain in the quantum dot. During the resonant optical pumping process exciton spin-flip can occur without a change of the Mn spin providing a way to directly read the dynamics of the pumped spin state. The manganese spin orientation is achieved in a few tens of ns.

Resonant Optical Pumping, Read‐out and Dynamics of a Mn Spin in a Quantum Dot

We have investigated the spin preparation efficiency by optical pumping of single Mn atoms embedded in CdTe/ZnTe quantum dots. Monitoring the time dependence of the intensity of the resonant fluorescence during the optical pumping process allows direct probing of the efficiency of the Mn spin initialization, providing preparation and read‐out in the same step. The efficiency measured can reach 75% at zero magnetic field and occurs in the tens of nanosecond range when a laser resonantly drives at saturation one of the quantum dot transition.