259 research outputs found
Strain induced coherent dynamics of coupled carriers and Mn spins in a quantum dot
We report on the coherent dynamics of the spin of an individual magnetic atom
coupled to carriers in a semiconductor quantum dot which has been investigated
by resonant photoluminescence of the positively charged exciton (X+). We
demonstrate that a positively charged CdTe/ZnTe quantum dot doped with a single
Mn atom forms an ensemble of optical Lambda systems which can be addressed
independently. We show that the spin dynamics of the X+Mn complex is dominated
by the electron-Mn exchange interaction and report on the coherent dynamics of
the electron-Mn spin system that is directly observed in the time domain.
Quantum beats reflecting the coherent transfer of population between
electron-Mn spin states, which are mixed by an anisotropic strain in the plane
of the quantum dot, are clearly observed. We finally highlight that this strain
induced coherent coupling is tunable with an external magnetic field
Resonant photoluminescence and dynamics of a hybrid Mn-hole spin in a positively charged magnetic quantum dot
We analyze, through resonant photoluminescence, the spin dynamics of an
individual magnetic atom (Mn) coupled to a hole in a semiconductor quantum dot.
The hybrid Mn-hole spin and the positively charged exciton in a CdTe/ZnTe
quantum dot forms an ensemble of systems which can be addressed
optically. Auto-correlation of the resonant photoluminescence and resonant
optical pumping experiments are used to study the spin relaxation channels in
this multilevel spin system. We identified for the hybrid Mn-hole spin an
efficient relaxation channel driven by the interplay of the Mn-hole exchange
interaction and the coupling to acoustic phonons. We also show that the optical
systems are connected through inefficient spin-flips than can be
enhanced under weak transverse magnetic field. The dynamics of the resonant
photoluminescence in a p-doped magnetic quantum dot is well described by a
complete rate equation model. Our results suggest that long lived hybrid
Mn-hole spin could be obtained in quantum dot systems with large
heavy-hole/light-hole splitting
Optical Stark Effect and Dressed Excitonic States in a Mn-doped Quantum Dot
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 exciton-to-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
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
p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic CdMnTe quantum wells
We present a study of p-type doping of CdTe and CdMnTe 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 cm. Surface doping was successfully applied
to obtain carrier-induced ferromagnetism in a CdMnTe quantum well.
The observed temperature dependence of photoluminescence spectra, and the
critical temperature, correspond well to those previously reported for
ferromagnetic quantum wells doped with nitrogen.Comment: 4 figure
Electron-nuclei spin dynamics in II-VI semiconductor quantum dots
We report on the dynamics of optically induced nuclear spin polarization in
individual CdTe/ZnTe quantum dots loaded with one electron by modulation
doping. The fine structure of the hot trion (charged exciton with an
electron in the -shell) is identified in photoluminescence excitation
spectra. A negative polarisation rate of the photoluminescence, optical pumping
of the resident electron and the built-up of dynamic nuclear spin polarisation
(DNSP) are observed in time-resolved optical pumping experiments when the
quantum dot is excited at higher energy than the hot trion triplet state. The
time and magnetic field dependence of the polarisation rate of the
emission allows to probe the dynamics of formation of the DNSP in the optical
pumping regime. We demonstrate using time-resolved measurements that the
creation of a DNSP at B=0T efficiently prevents longitudinal spin relaxation of
the electron caused by fluctuations of the nuclear spin bath. The DNSP is built
in the microsecond range at high excitation intensity. A relaxation time of the
DNSP in about 10 microseconds is observed at and significantly increases
under a magnetic field of a few milli-Tesla. We discuss mechanisms responsible
for the fast initialisation and relaxation of the diluted nuclear spins in this
system
Pollution physico-chimique et bactériologique d’un écosystème aquatique et ses risques écotoxicologiques: Cas du lac Nokoue au Sud Benin
La protection des écosystèmes aquatiques est indispensable à l’équilibre écologique des espèces halieutiques et à une saine alimentation des populations locales. Le lac Nokoué situé au Sud du Bénin est sous l’influence de plusieurs formes de pollution qui résultent pour la plupart des activités anthropiques. Ce travail se propose de faire une évaluation des diverses formes de pollution que subi aujourd’hui ce lac, afin d’alerter sur les risques écotoxicologiques. Pour atteindre cet objectif, des visites du terrain d’étude, des analyses physico-chimiques et bactériologiques ont été effectuées, des entretiens avec des personnes ressources ont été organisés et des enquêtes de terrain ont été menées auprès des populations locales. Les résultats issus des analyses physico-chimiques et bactériologiques de l’eau du lac Nokoué ont permis d’identifier les sources des divers types de polluants du lac Nokoué. La teneur en oxygène dissous varie de 3,62 mg /L à 2,76 mg/L ; la DBO5 de 29 mg/L à 10 mg/L ; la DCO est de 253 mg/L ; la teneur en ammonium est supérieure à 8 mg/L ; la teneur en nitrite avoisine 0,7 mg /L par endroit ; la teneur en coliformes totaux fluctue de 4000 /100 mL à 6000/100 mL ; la teneur en streptocoques fécaux varie de 500/100mL à 4000/100 mL. Ces résultats montrent que le lac Nokoué est pollué. Des stratégies pour limiter la pollution du lac doivent être mises en place par les décideurs à tous les niveaux afin d’éviter des problèmes d’écosanté aux populations du lac Nokoué, ce qui pourrait constituer aussi un frein pour le développement du pays.Keywords: Pollution, écosystème, lac Nokoué, risques écotoxicologique
Coagulation of some humic acid solutions by moringa oleifera lam seeds: effect on chlorine requirement
No Abstract. Bulletin of the Chemical Society of Ethiopia Vol. 15 (2) 2001: pp. 119-12
Institutional Repositories and Academic Social Networks: Competition or Complement? A Study of Open Access Policy Compliance vs. ResearchGate Participation
INTRODUCTION The popularity of academic social networks like ResearchGate and Academia.edu indicates that scholars want to share their work, yet for universities with open access (OA) policies, these sites may be competing with institutional repositories (IRs) for content. This article seeks to reveal researcher practices, attitudes, and motivations around uploading their work to ResearchGate and complying with an institutional OA Policy through a study of faculty at the University of Rhode Island (URI). METHODS We conducted a population study to examine the participation by 558 full-time URI faculty members in the OA Policy and ResearchGate followed by a survey of 728 full-time URI faculty members about their participation in the two services. DISCUSSION The majority of URI faculty do not participate in the OA Policy or use ResearchGate. Authors’ primary motivations for participation are sharing their work more broadly and increasing its visibility and impact. Faculty who participate in ResearchGate are more likely to participate in the OA Policy, and vice versa. The fact that the OA Policy targets the author manuscript and not the final published article constitutes a significant barrier to participation. CONCLUSION Librarians should not view academic social networks as a threat to open access. Authors’ strong preference for sharing the final, published version of their articles provides support for calls to hasten the transition to a Gold OA publishing system. Misunderstandings about the OA Policy and copyright indicate a need for librarians to conduct greater education and outreach to authors about options for legally sharing articles
- …