Excitons and charged excitons in semiconductor quantum wells

A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width.Comment: 9 pages, 11 figure

Magnetic field dependence of the energy of negatively charged excitons in semiconductor quantum wells

A variational calculation of the spin-singlet and spin-triplet state of a negatively charged exciton (trion) confined to a single quantum well and in the presence of a perpendicular magnetic field is presented. We calculated the probability density and the pair correlation function of the singlet and triplet trion states. The dependence of the energy levels and of the binding energy on the well width and on the magnetic field strength was investigated. We compared our results with the available experimental data on GaAs/AlGaAs quantum wells and find that in the low magnetic field region (B<18 T) the observed transition are those of the singlet and the dark triplet trion (with angular momentum $L_z=-1$), while for high magnetic fields (B>25 T) the dark trion becomes optically inactive and possibly a transition to a bright triplet trion (angular momentum $L_z=0$) state is observed.Comment: 9 pages, 10 figures submitted to Phys. Rev.

La croûte océanique et les points chauds de la Polynésie française (Océan Pacifique central)

International audienceUne compilation de l’ensemble des données de géophysique marine disponibles sur l’ensemble de la Polynésie française et l’utilisation d’une nouvelle grille d’anomalie gravimétrique à l’air libre déduite de l’altimétrie satellitaire, permettent de proposer une nouvelle carte morphostructurale de la région comprise entre 0 et 30° de latitude sud et entre l20 et l65° de longitude ouest. A l’ouest de cette zone, la croûte océanique, d’âge crétacé, est générée à l’axe de la dorsale pacifique-antarctique ; elle est séparée de la croûte océanique créée à l’axe de la dorsale est-pacifique (direction Farallon) par une trace de direction grossièrement nord-sud. Cette dernière génération correspond à un taux d’expansion évoluant progressivement de 23 km/Ma à l’anomalie 34 (84 Ma) à 82 km/Ma à l’anomalie 7 (26 Ma). L’anomalie 7 marque le changement de direction entre le système Pacifique-Farallon et le nouveau système Pacifique-Nazca à un taux d’expansion moyen de l00 km/Ma. Cette période est suivie par une réorganisation majeure de la frontière de plaque, l’axe de la dorsale est-pacifique se décalant vers Tuamotu et de 250 à 400 km pour le segment des Australes. Les îles océaniques et les monts sous-marins sont particulièrement nombreux ; on en dénombre 421 dont l’élévation est supérieure à l000 m au-dessus des fonds océaniques avoisinants. Pour l’essentiel, ils correspondent à des constructions volcaniques hors-axe que l’on tente d’expliquer par la théorie des points chauds en les regroupant en alignements d’îles dont l’âge progresse régulièrement à un taux (Il cm/an) et dans une direction (N 29.5° E) qui correspondent au mouvement absolu de la plaque Pacifique. L’existence de six points chauds est généralement envisagée mais certains alignements posent problème quant à leur direction et leur progression en âge

Non-uniform meshes in gyrokinetic simulations

International audienc

Tissue-specific bioaccumulation of a wide range of legacy and emerging persistent organic contaminants in swordfish (Xiphias gladius) from Seychelles, Western Indian Ocean

Swordfish (Xiphias gladius) is a major marine resource of high economic value to industrial and artisanal fisheries. As a top predator with a long lifespan, it is prone to accumulate high levels of contaminants. The bioaccumulation of a wide range of both legacy and emerging persistent organic contaminants was investigated in the muscle, liver and gonads of swordfish collected from the Seychelles, western Indian Ocean. The detection of all target contaminants, some at frequencies above 80%, highlights their widespread occurrence, albeit at low levels. Mean concentrations in muscle were 5637, 491 and 331 pg g(-1) ww for organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs) and perfluoroalkyl substances (PFASs), respectively. XBFR mean concentrations were far below, i.e. 47 pg g(-1) ww. The data are among the first obtained for such a high diversity of contaminants in an oceanic top predator worldwide and constitute a benchmark of the contamination of Indian Ocean ecosystems

Legacy and emerging organic contaminants: Levels and profiles in top predator fish from the western Indian Ocean in relation to their trophic ecology

International audienceTuna and billfish are large pelagic fish of ecological importance in open oceans. As top predators with a long lifespan, they are prone to exposure to various contaminants such as persistent organic pollutants (POPs) and contaminants of emerging concern. In this study, three pollutant families were investigated, including polychlorinated biphenyls (PCBs), organochlorinated pesticides (OCPs) and perfluoroalkyl substances (PFASs), including perfluorooctane sulfonate (PFOS) and perfluorocarboxylic acids (PFCAs). Contamination was investigated in individuals from three tropical tuna species, namely bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis) and yellowfin (Thunnusalbacares) tunas and the billfish swordfish (Xiphias gladius), collected from various areas of the western Indian Ocean (WIO) in 2013-2014. Contamination levels and profiles were examined in fish muscle, together with biological parameters (fish length / age, sex, lipid content) and ecological tracers (carbon and nitrogen stable isotopes). POP levels were low in all species in comparison to other locations worldwide, revealing a low impact of anthropogenic organic contaminants in the WIO. A predominance of OCPs (especially DDTs) versus PCBs was highlighted in all species; PFASs were predominant over chlorinated POPs in tunas. Among the studied PFASs, long-chain PFCAs were found to prevail over PFOS in all species. Organic contaminant profiles differed across species according to their foraging habitat; swordfish and bigeye tuna, which both feed in deep oceanic layers, showed similarities in their contaminant profiles. Geographically, the distinct DDT profiles of fish from the Mozambique Channel suggested an exposure to different DDT sources, in line with regional use of this insecticide and coupled with an extended residence time of fish in the Channel. To our knowledge, the data presented here are among the first obtained for legacy and emerging organic contaminants in various species of large pelagic predators from the WIO

Legacy and emerging organic contaminants : levels and profiles in top predator fish from the western Indian Ocean in relation to their trophic ecology

Tuna and billfish are large pelagic fish of ecological importance in open oceans. As top predators with a long lifespan, they are prone to exposure to various contaminants such as persistent organic pollutants (POPs) and contaminants of emerging concern. In this study, three pollutant families were investigated, including polychlorinated biphenyls (PCBs), organochlorinated pesticides (OCPs) and perfluoroalkyl substances (PFASs), including perfluorooctane sulfonate (PFOS) and perfluorocarboxylic acids (PFCAs). Contamination was investigated in individuals from three tropical tuna species, namely bigeye (Thunnus obesus), skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tunas and the billfish swordfish (Xiphias gladius), collected from various areas of the western Indian Ocean (WIO) in 2013-2014. Contamination levels and profiles were examined in fish muscle, together with biological parameters (fish length / age, sex, lipid content) and ecological tracers (carbon and nitrogen stable isotopes). POP levels were low in all species in comparison to other locations worldwide, revealing a low impact of anthropogenic organic contaminants in the WIO. A predominance of OCPs (especially DDTs) versus PCBs was highlighted in all species; PFASs were predominant over chlorinated POPs in tunas. Among the studied PFASs, long-chain PFCAs were found to prevail over PFOS in all species. Organic contaminant profiles differed across species according to their foraging habitat; swordfish and bigeye tuna, which both feed in deep oceanic layers, showed similarities in their contaminant profiles. Geographically, the distinct DDT profiles of fish from the Mozambique Channel suggested an exposure to different DDT sources, in line with regional use of this insecticide and coupled with an extended residence time of fish in the Channel. To our knowledge, the data presented here are among the first obtained for legacy and emerging organic contaminants in various species of large pelagic predators from the WIO