63 research outputs found
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 ), 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 ) 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 lâensemble des donnĂ©es de gĂ©ophysique marine disponibles sur lâensemble de la PolynĂ©sie française et lâutilisation dâune nouvelle grille dâanomalie gravimĂ©trique Ă lâair libre dĂ©duite de lâaltimĂ©trie satellitaire, permettent de proposer une nouvelle carte morphostructurale de la rĂ©gion comprise entre 0 et 30° de latitude sud et entre l20 et l65° de longitude ouest. A lâouest de cette zone, la croĂ»te ocĂ©anique, dâĂąge crĂ©tacĂ©, est gĂ©nĂ©rĂ©e Ă lâaxe de la dorsale pacifique-antarctique ; elle est sĂ©parĂ©e de la croĂ»te ocĂ©anique crĂ©Ă©e Ă lâaxe de la dorsale est-pacifique (direction Farallon) par une trace de direction grossiĂšrement nord-sud. Cette derniĂšre gĂ©nĂ©ration correspond Ă un taux dâexpansion Ă©voluant progressivement de 23 km/Ma Ă lâanomalie 34 (84 Ma) Ă 82 km/Ma Ă lâanomalie 7 (26 Ma). Lâanomalie 7 marque le changement de direction entre le systĂšme Pacifique-Farallon et le nouveau systĂšme Pacifique-Nazca Ă un taux dâexpansion moyen de l00 km/Ma. Cette pĂ©riode est suivie par une rĂ©organisation majeure de la frontiĂšre de plaque, lâaxe de la dorsale est-pacifique se dĂ©calant vers Tuamotu et de 250 Ă 400 km pour le segment des Australes. Les Ăźles ocĂ©aniques et les monts sous-marins sont particuliĂšrement nombreux ; on en dĂ©nombre 421 dont lâĂ©lĂ©vation est supĂ©rieure Ă l000 m au-dessus des fonds ocĂ©aniques avoisinants. Pour lâessentiel, ils correspondent Ă des constructions volcaniques hors-axe que lâon tente dâexpliquer par la thĂ©orie des points chauds en les regroupant en alignements dâĂźles dont lâĂąge progresse rĂ©guliĂšrement Ă un taux (Il cm/an) et dans une direction (N 29.5° E) qui correspondent au mouvement absolu de la plaque Pacifique. Lâexistence de six points chauds est gĂ©nĂ©ralement envisagĂ©e mais certains alignements posent problĂšme quant Ă leur direction et leur progression en Ăąge
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
- âŠ