Probing the light hole/heavy hole switching with correlated magneto-optical spectroscopy and chemical analysis on a single quantum dot

A whole series of complementary studies have been performed on the same single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive x-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si₃N₄ membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn, Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

A tomography diagnostic in the visible spectrum to investigate turbulence and coherent modes in the linear plasma column Mistral

In this paper, we describe a newly installed tomography diagnostic on the linear magnetized plasma device Mistral. The diagnostic is composed of 128 lines of sight covering a 20 cm diameter section and an acquisition rate up to 1 MHz. The measurements are done in the visible spectrum using silicon photomultiplier arrays that require a lower amplification voltage than photomultiplier tubes for similar gains and have a fast response. Tomographic reconstruction of a rotating plasma mode is shown to illustrate the capabilities of the diagnostic.

Quantitative Reconstructions of 3D Chemical Nanostructures in Nanowires

Energy dispersive X-ray spectrometry is used to extract a quantitative 3D composition profile of heterostructured nanowires. The analysis of hypermaps recorded along a limited number of projections, with a preliminary calibration of the signal associated with each element, is compared to the intensity profiles calculated for a model structure with successive shells of circular, elliptic, or faceted cross sections. This discrete tomographic technique is applied to II-VI nanowires grown by molecular beam epitaxy, incorporating ZnTe and CdTe and their alloys with Mn and Mg, with typical size down to a few nanometers and Mn or Mg content as low as 10%.

Structure and morphology in diffusion-driven growth of nanowires: the case of ZnTe

Gold-catalyzed ZnTe nanowires were grown at low temperature by molecular beam epitaxy on a ZnTe(111) B buffer layer, under different II/VI flux ratios, including with CdTe insertions. High-resolution electron microscopy and energy-dispersive X-ray spectroscopy (EDX) gave information about the crystal structure, polarity, and growth mechanisms. We observe, under stoichiometric conditions, the simultaneous presence of zinc-blende and wurtzite nanowires spread homogeneously on the same sample. Wurtzite nanowires are cylinder-shaped with a pyramidal-structured base. Zinc-blende nanowires are cone-shaped with a crater at their base. Both nanowires and substrate show a Te-ended polarity. Te-rich conditions favor zinc-blende nanowires, while Zn-rich suppress nanowire growth. Using a diffusion-driven growth model, we present a criterion for the existence of a crater or a pyramid at the base of the nanowires. The difference in nanowire morphology indicates lateral growth only for zinc-blende nanowires. The role of the direct impinging flux on the nanowire's sidewall is discussed.

Magnetization dynamics down to a zero field in dilute (Cd,Mn)Te quantum wells

The evolution of the magnetization in (Cd,Mn)Te quantum wells after a short pulse of magnetic field was determined from the giant Zeeman shift of spectroscopic lines. The dynamics in the absence of a static magnetic field was found to be up to 3 orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for the fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional relaxation channel.

Observation of strong-coupling effects in a diluted magnetic semiconductor Ga1-xFexN

The giant Zeeman splitting of free excitons is measured in Ga(1-x)Fe(x)N. Magneto-optical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe(3+) ions and holes in GaN, N_{0}beta(app)=+0.5+/-0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.

Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).

Probing the spin state of a single magnetic ion in an individual quantum dot

The magnetic state of a single magnetic ion (Mn2+) 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 Mn2+ ion (S=5/2) is analyzed in detail. The exciton-Mn2+ exchange interaction shifts the energy of the exciton depending on the Mn2+ spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements reveal that the emission intensities in both circular polarizations are controlled by the Mn2+ spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photocreated carriers. Under magnetic field, the electron-Mn interaction induces a mixing of the bright and dark exciton states.

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.

Large magnetoresistance effect due to spin injection into a nonmagnetic semiconductor

A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a nonmagnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the nonmagnetic semiconductor and can theoretically yield a positive magnetoresistance of 100%, when the spin flip length in the nonmagnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to 25% magnetoresistance.

[Non-complicated Horton's disease: initial treatment with methylprednisolone 500 mg/day bolus for three days followed by 20 mg/day prednisone-equivalent. Evaluation of 15 patients]

PURPOSE

To assess the efficacy and tolerance of three methylprednisolone boluses (500 mg/d) followed by a standard dose of prednisolone, 20 mg/d, as the initial treatment of non-complicated giant-cell arteritis.

METHOD

A retrospective study of 15 cases.

RESULTS

Six men and nine women with a mean age of 70.9 years were treated and followed for 41.5 months. Initial mean ESR was 83 mm; mean C-reactive protein level was 94.6 mg/L. The boluses were well tolerated, excepted in one patient who developed acute psychosis. After initiating the oral treatment, two patients presented signs of clinical relapse during the first month, and were given higher doses of corticosteroids. At 1 month, 12 patients were asymptomatic, nine of whom had normalized ESR and CRP. Mean ESR was 23; mean CRP was 13 mg/L. At 3 months, the mean prednisone dose delivered was 18.2 mg/d. Mean ESR was 12 mm. The cumulative prednisone dose given during the first year was 5,349 (+/- 2,512) mg. In the 13 patients who necessitated no more than 20 mg/d prednisone, no sequelae of giant-cell arteritis, no fractures nor major treatment intolerance occurred-during the first 2 years of treatment. Treatment was stopped in eight patients after a mean duration of 48.6 months.

CONCLUSION

Treatment with pulse methylprednisolone 500 mg/d for 3 days followed by 20 mg/d oral prednisone could be a valuable corticosteroid-sparing strategy in many patients with uncomplicated temporal arteritis.

Zener model description of ferromagnetism in zinc-blende magnetic semiconductors

Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature (T(C)) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T(C) of Ga(1-)(x)Mn(x)As and that of its II-VI counterpart Zn(1-)(x)Mn(x)Te and is used to predict materials with T(C) exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

DNA fragmentation induced in lymphocytes by gamma irradiation or dexamethasone: inhibition by diethyldithiocarbamate (DTC), potentiated by zinc

Apoptosis is a process of physiological cell death characterized by DNA fragmentation, chromatin condensation, loss of membrane asymmetry and cell lethality. In the present study, apoptosis induced in thymocytes by dexamethasone or gamma irradiation is evaluated by flow cytometry, gel electrophoresis and other techniques. Treatment of thymocytes with DTC or zinc shows that these products can inhibit radiation- or dexamethasone-induced apoptosis. Moreover, a synergistic effect is observed by using associations of both products (5 microM DTC + 50 microM ZnSO4): ZnSO4 potentiates the effect of DTC at concentrations for which the molecules used separately show a low efficacy. These results indicate that DNA fragmentation induced by dexamethasone or irradiation in thymocytes share some identical mechanisms.