Dynamics of a Mn spin coupled to a single hole confined in a quantum dot

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 $\mu s$ range or shorter. This hole-Mn spin relaxation is induced by the presence of valence band mixing in self-assembled quantum dots

Apports En Carbone Et Azote Dans Le Fleuve Niger À Tondibia (Niamey) : Résultats De Deux (2) Ans D’observations

The Niger River is the third more important river in Africa and drains a surface of about 2,120,000 km². It includes six hydrographic regions representing West African ecosystems. Despite the importance of this river at the regional and continental scale, little information has been collected on its biogeochemical characteristics and particularly on its role in the transportation and the transformation of matter (carbon, nitrogen and phosphorus). This study present the results of two years investigation, April 2011 to March 2013 in the middle Niger, upstream Niamey (Niger) city [2.01° E, 13.57° N], according to a bi-weekly observation frequency. The variables measured are temperature, conductivity, dissolved oxygen, pH, suspended matter, dissolved inorganic carbon (C) and nitrogen (N) and particulars organics C and N as well as isotopic composition of these elements. Daily discharges of the river come from hydrologic station measurement of the Niger authority Basin (NBA) at Niamey city. Hydrologic situation was strongly contrasted between the 2 years of survey. The mean discharge of the first year was only 673 m3 s-1 (one of the weakest discharge recorded on the river Niger at Niamey since 1940), while the mean discharge of the second year was 1,096 m3 s-1. Our results show that suspended matter, particular organic carbon, dissolved organic carbon and dissolved inorganic carbon are transported mainly during the local flood, induced by precipitations in July and August. The second flood (Guinean flood) which occurred, during November to January, is characterized by low temperatures and clean waters

Évaluation de l'état d'une eau de rivière par analyse multidimensionnelle utilisant certains paramètres caractéristiques de la matière organique dissoute

L'évaluation de la qualité de l'eau de rivières pose un certain nombre de problèmes non encore résolus. Parmi ceux-là, nous en avons retenu un qui porte sur la méthodologie généralement adoptée pour réaliser cette évaluation et qui est à la base de nombreux autres problèmes. Actuellement en France, ainsi que dans de nombreux autres pays européens, la qualité de l'eau est évaluée à partir de grilles de valeurs limites de paramètres physico-chimiques ou biologiques. L'usage de telles grilles sous-entend que les paramètres qui la composent sont indépendants les uns des autres alors qu'il n'en est rien. En effet, il est indispensable de considérer les milieux aquatiques comme étant des systèmes chimiques, physiques et biologiques plus ou moins éloignés de leurs états d'équilibre et pour lesquels les paramètres qui les décrivent, considérés à un instant donné, sont liés les uns des autres par des relations qui évoluent avec leurs états.En plus du problème précédent, il faut rappeler que toute évaluation de l'état d'une masse d'eau (au sens de la Directive Cadre européenne sur l'Eau (DCE)) nécessite que soit précisé au préalable un état de référence. La diversité, hors anthropisation, des milieux aquatiques pose alors le problème du choix de telles références.Ce travail qui a pour cadre un cours d'eau du centre de la France (la Vienne) s'inscrit donc en partie dans cette double problématique.S'appuyant sur un suivi analytique mensuel en huit points, réalisé au cours d'une période de 27 mois entre 1999 et 2002, nous avons tenté de montrer que la méthode bien connue de l'analyse en composante principale (ACP) avec étude des corrélations, pouvait, moyennant quelques préalables, être un outil permettant de préciser avec une bonne objectivité l'état, ainsi que l'évolution spatio-temporelle de ces eaux.Nous avons également tenté de montrer que l'un de ces préalables était la prise en compte dans l'ACP de variables caractéristiques de la matière organique dissoute (MOD). Les paramètres classiques actuellement utilisés, en particulier par le système français d'évaluation de la qualité de l'eau (SEQ-Eau), qui sont essentiellement minéraux, ne suffisent pas à eux seuls pour expliquer et mettre en évidence les effets liés aux apports anthropiques.Dans cette étude, nous nous sommes donc attachés de différentes manière à montrer sur ce milieu, que les représentations ACP prenaient en compte les principales observations connues comme liées aux rejets polluants.Satisfactory systems for evaluating river water quality have not yet been established. In France, as in many other European countries, water quality is evaluated using physico-chemical or biological parameter grids. This method implicitly assumes that these parameters are independent from each other and that a universal state of reference exists. In fact, when taking into account the great diversity of rivers it is clear that any evaluation of water quality requires that reference conditions be specified for each water system. Although it is one of the objectives of the new European Water Framework Directive, this problem has not yet been resolved. In addition, it is important to note that the state of water is characterised by dependent analytical parameters, and that the relationships linking these parameters evolve with the state of the water; this variable dependence must be taken into account. Lastly, it is important to note that organic matter is generally underestimated by current water quality evaluation systems.In the present study an attempt was made to evaluate the water quality of the Vienne River situated in western France. This study provided us with the opportunity to verify the accuracy of the current system. A monthly analytical monitoring program at eight study sites was carried out during the period 1999-2001. It allowed us to demonstrate that the well-known method of the principal component analysis (PCA), together with correlation analysis, was an appropriate tool for this purpose. More specifically, we showed that when using parameters characteristic of organic matter, this tool is well adapted for monitoring spatial and temporal changes in water quality. As this method is graphical, it made the choice of a reference point possible.This study showed that traditional parameters currently used, which are primarily inorganic, were not sufficient to identify anthropogenic contributions. For this reason we used unconventional organic variables, formerly used to study the structure of the dissolved organic matter (DOM) in surface waters, such as:- the distribution of the various organic fractions, such as humic (or hydrophobic) substances and non humic (or hydrophilic) substances, after filtration through macroporous XAD8 and XAD4 resins, under well-defined conditions;- the distribution of the apparent molecular weights, by ultrafiltration through membranes with molecular weight cut-offs of 30, 10 and 3 kDaltons;- fluorescence spectrometry;- the ratio UV-absorbance at 254 nm/dissolved organic carbon (SUVA).It was noted in this study that these organic variables, coupled with those classically analysed (Chemical Oxygen Demand, COD; Biological Oxygen Demand, BOD5), were often mainly responsible for the variations in the water quality. Principal Component Analysis was used to extract the main factors associated with water quality variability. The graphical representation obtained underlined that the greatest variability of the points characteristic of the stations could be observed on the first principal component. This component was linked to the organic variables. The second principal component was linked to inorganic variables indicative of the geological substrate. In addition, it was shown that the temporal dispersion of stations in the PCA graphical representation can be used to evaluate (either from the variance of the PCA factors, or from the box plots of these same factors) the disturbances related to the contributions of anthropogenic origin. The proportion of variability in water quality due to hydrological conditions could be also highlighted by PCA

La mise à l'équilibre des eaux tropicales : conséquences sur la déferrisation

L'étude de l'équilibre calco-carbonique d'une eau souterraine de l'Afrique de l'Ouest par la méthode de LEGRAND et POIRIER, a conduit à la détermination des conditions de traitement pour neutraliser l'agressivité naturelle de l'eau, due essentiellement à une forte teneur en acide carbonique et réduire ainsi son caractère corrosif. le problème de l'évolution du fer présent en relation avec la mise à l'équilibre a été également étudiée.L'application réalisée sur un pilote de laboratoire a permis d'optimiser les principales étapes du traitement, c'est-à-dire l'addition d'une base forte, la précipitation et la filtration des hydroxydes ferriques qui se forment à la suite de la neutralisation de l'acide carbonique.Studies on calcium carbonate equilibrium of West African ground water, a sample of potable water distributed in Lomé (Togo), were carried out using the method described by LEGRAND and POIRIER. It is a standard graphic method in which calculations include all basic (Ca++, HCO3-, CO3--, CO2, H2O+, OH-), and characteristic (Mg++, Na+, K+, SO4--, Cl- ...) chemical parameters of natural water. Processing of a computer program for calculations enabled the equilibrium and saturation curve [CO2] = f[Ca++], to be obtained. The aggressivity to the lime deposit character, different conditions of treatment, aeration, and addition of an appropriate base to neutralise the nature aggressivity of water due to its high dissolved carbon dioxide content could also be defined.The problem of the presence of iron was studied; high concentration of iron in natural ground water lead to various problems : developing of tastes, staining and discoloration of clothes, and growth of iron bacteria in the distribution system. Its removal from water is accomplished by means of the oxidation of iron (II) to iron (III) followed by the precipitation of relatively insoluble ferric hydroxide. The oxidation rate is highly dependent on pH. Thus, the process of removal of carbon dioxide by stripping or neutralisation raises the pH of the water. If the pH is raised high enough, an insoluble form of iron will precipitate. Another important aspect of the chemical behaviour of iron is the formation of complex ions with inorganic as well as organic ligands; silica in water forms a relatively stable complex with iron (III). The difficulty in removing iron from water is often associated with this phenomenon.Continuous flow studies were carried out to improve important steps of treatment such as addition of a strong base, and precipitation and filtration of ferric hydroxide produced during carbonic acid neutralization. They show the weak stability of silicato-iron (III) complex at high pH value and prove that practically complete iron removal may be obtained

Study of molecular spin-crossover complex Fe(phen)2(NCS)2 thin films

We report on the growth by evaporation under high vacuum of high-quality thin films of Fe(phen)2(NCS)2 (phen=1,10-phenanthroline) that maintain the expected electronic structure down to a thickness of 10 nm and that exhibit a temperature-driven spin transition. We have investigated the current-voltage characteristics of a device based on such films. From the space charge-limited current regime, we deduce a mobility of 6.5x10-6 cm2/V?s that is similar to the low-range mobility measured on the widely studied tris(8-hydroxyquinoline)aluminium organic semiconductor. This work paves the way for multifunctional molecular devices based on spin-crossover complexes

Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction

We report a detailed investigation of the first stages of the growth of self-organized Fe clusters on the reconstructed Au(111) surface by grazing incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are in "local epitaxy" whereas the subsequent layers adopt first a strained fcc lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial relationship. The structural evolution is discussed in relation with the magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200

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 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 figure

COVID-19 vaccination coverage for half a million non-EU migrants and refugees in England

Despite evidence suggesting that some migrants are at risk of under-immunization and have experienced severe health inequities during the pandemic, data are limited on migrants’ COVID-19 vaccine coverage globally. Here we linked data from non-European Union migrants and resettled refugees to the national COVID-19 vaccination dataset in England. We estimated patterns in second and third dose delays and overdue doses between 12 December 2020 and 20 April 2022 by age, visa type and ethnicity. Of the 465,470 linked records, 91.8% (427,073/465,470) of migrants received a second dose and 51.3% (238,721/465,470) received a third. Refugees had the highest risk of delayed second (adjusted odds ratio 1.66; 95% confidence interval 1.55–1.79) and third dose (1.55; 1.43–1.69). Black migrants were twice as likely to have a second dose delayed (2.37; 2.23–2.54) than white migrants, but this trend reversed for the third dose. Older migrants (>65 years) were four times less likely to have received their second or third dose compared with the general population in England aged >65 or older. Policymakers, researchers and practitioners should work to understand and address personal and structural barriers to vaccination for diverse migrant populations

ETIOLOGIES DES PLEURESIES DU SUJET AGE AU TOGO

Objective: The aim of this study was to review the current aetiologic profile of pleurisies in the elderly people. Materials and method: We carried out a retrospective study based on the data of 126 patients, old of at least 55 years, hospitalised from January 2007 to December 2011 for pleurisy in the pneumology department of Sylvanus Olympio teaching hospital of Lome. Results: Pleurisies represent 23,11 % of elderly people’s hospitalisation motives. The Sex-ratio was 1 and the average age of the patients 65 ± 09 years old. The clinical aspect was dominated by thoracic pain (88,88 %) and cough (69,84 %). The chest X-ray revealed pleural effusion at the left side in 48,41 %, at right side in 46,83% and of medium abundance in 57,93 %. The liquid was citrine in 53,97 %, haematic in 30,95% and purulent in 15,08% . Pleurisies were of cancerous origin in 32,54 %, tubercular in 17,46 % and bacterial not tubercular in 14,58%. No aetiology had been found in 33,33 %. The mortality was 27,78 % and due to cancers and idiopathic pleurisies in 86 % during three months’ follow up. Conclusion: Cancers are the first aetiology of pleurisies followed by tuberculosis in elderly people. The acquisition of the new means of pleural exploration is important to reduce the proportion of idiopathic pleurisies