Theory of nuclear induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots

We propose a model for spectral diffusion of localized spins in semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each nuclear spin flip-flop is assumed to be independent, the rate for this process being calculated by a method of moments. Our calculated spin decoherence time $T_{M}=0.64$ ms for donor electron spins in Si:P is a factor of two longer than spin echo decay measurements. For $^{31}$P nuclear spins we show that spectral diffusion is well into the motional narrowing regime. The calculation for GaAs quantum dots gives $T_{M}=10-50$ $\mu$s depending on the quantum dot size. Our theory indicates that nuclear induced spectral diffusion should not be a serious problem in developing spin-based semiconductor quantum computer architectures.Comment: 15 pages, 9 figures. Accepted for publication in Phys. Rev.

Analysing policy-induced effects on the performance of irrigated rice

Improving local rice production capacity is a key element on the agenda of most countries in the West African Economic and Monetary Union (WAEMU). There are several reasons for this drive: (1) the high levels of rice imports, which constitute a burden on the countries' financial resources; (2) the relatively high contribution of the commodity to national food security programmes; (3) income generation for smallholder farm communities; and (4) the contribution of rice to the improvement of nutritional status. The policy analysis matrix approach was used to evaluate the policy-induced effects of the WAEMU common external tariffs on the performance of irrigated rice production systems in Niger. The results show that the irrigated rice production system receives little protection, and that retail rice marketing channels are even less protected. The negative net policy effects indicate that greater incentives are needed for enhanced system performance

Economics in West African Irrigated-Rice Production

The main objective of this investigation is to contribute to the ongoing debate on the effects of the West African Economic and Monetary Union (WAEMU), common external tariff (CET) measures on the competitiveness of the irrigated-rice production system in Niger. The policy analysis matrix (PAM) approach was used to evaluate the various policy effects, based on farm-level, postharvest and rice-marketing data, as well as the financial and economic parity prices of different brands of imported-rice. Results from the PAM base-scenario model showed that under the evaluated CET policy irrigated rice production, activities were competitive and private operators had positive financial gains, meaning that private profitability was positive. Moreover, the irrigated-rice production enterprise revealed positive economic profitability for both retailers and wholesalers. Thus, it generated a net positive income for the national economy per unit of land devoted to this activity. One can argue that the irrigated rice system under the CET was generally competitive (positive private profitability) and had a potential for growth (positive economic profitability). Despite its competitiveness and efficiency, the irrigated rice production system still performs below potential because it lacks additional incentives. Some sensitivity analyses were performed with single-factor and simultaneous changes of several factors, which confirmed the necessity of providing further incentives to the system. The research results suggest that greater incentives should be given in terms of improving marketing channels, especially for retail rice marketing in which a great number of women rice traders are active

Asthme aigu grave de l’enfant : caractéristiques épidémiologiques, cliniques, thérapeutiques et évolutifs au Sénégal : Severe acute asthma of the child: epidemiological, clinical, therapeutic and evolutive characteristics in Senegal

Context and objective. The lethality of asthma is related to the occurrence of severe acute asthma which is a crisis that does not yield under initial bronchodilator therapy. The objective of this study was to describe the epidemiological, diagnostic, therapeutic and evolutionary characteristics of children hospitalized for a severe acute asthma attack. Methods. We conducted a retrospective study of 11 years (from January 1st, 2005 and December 31st, 2015) at the Pediatric Emergency and Respiratory Department of the Albert Royer Children's Hospital (CHNEAR). 89 Children aged 0 to 15 years hospitalized for severe asthma or severe acute asthma were included. Results. The hospital prevalence of severe asthma attacks was 0.18%. The average age of the children was 44.21 months and the sex ratio was 1.69. The hospitalizations peaks have been registered during the months of July and December. 6.6% of patients were undergoing treatment and 17.9% had already been hospitalized for severe asthma attacks. The main biological abnormalities were: hypoxemia (79.7%), anemia (66.29%) and leukocytosis (44.9%). Radiological abnormalities were dominated by pulmonary over distension (60.7%) and bronchial syndrome (36%). All patients were under oxygen, salbutamol and corticosteroids. One death was encountered. The average hospital length of stay was 3.75 days. The complications were pneumo-mediastinum / cervico-mediastinal emphysema in 4 cases, pneumothorax in 2 cases and atelectasis in 1 case. Conclusion. Severe acute asthma is a relatively rare condition, but it is always associated with a significant morbidity. Contexte et objectif. La létalité de l’asthme est liée à la survenue d’asthme aigu grave qui est une crise qui ne cède pas sous traitement bronchodilatateur initial. L’objectif de cette étude était de décrire les caractéristiques épidémiologiques, diagnostiques, thérapeutiques et évolutives des enfants hospitalisés pour une crise d’asthme aigu grave. Méthodes. Nous avons conduit une étude documentaire portant sur une période de 11 ans (1 janvier 2005-31 décembre 2015) et réalisée aux services des urgences pédiatriques et de pneumologie du centre hospitalier national d’enfants Albert Royer (CHNEAR). Etaient inclus, 89 enfants de 0 à 15ans hospitalisés pour crise d’asthme sévère ou asthme aigu grave. Résultats. La prévalence hospitalière des crises d’asthmes sévère était de 0,18%. Leur âge moyen était de 44,2 mois et le sexe ratio 1,6. Les pics d’hospitalisations ont été enregistrés durant les mois de Juillet et Décembre. 6,6% des patients étaient sous traitement de fond et 17,9% avaient déjà été hospitalisés pour crises d’asthmes sévères. L’hypoxémie (79,7%), l’anémie (66,2%) et l’hyperleucocytose (44,9%) étaient les principales anomalies biologiques. Les signes radiologiques étaient dominés par l’hyperinflation pulmonaire (60,7%) et le syndrome bronchique (36%). Tous les patients étaient ont bénéficié de l’oxygène, le salbutamol et les corticoïdes. Sous cette attitude thérapeutique, un décès avait été déploré. La durée moyenne de l’hospitalisation était de 3,75 jours. Les complications enregistrées étaient le pneumo-médiastin/emphysème cervico-médiastinal dans 4 cas, le pneumothorax dans 2 cas, la rupture trachéale dans 1 cas et l’atélectasie dans 1 cas. Conclusion. L’asthme aigu grave semble peu fréquent mais reste toujours associée à une morbidité non négligeable