181 research outputs found
Signature of the Overhauser field on the coherent spin dynamics of donor-bound electron in a single CdTe quantum well
We have studied the coherent spin dynamics in an oblique magnetic field of
electrons localized on donors and placed in the middle of a single CdTe quantum
well, by using a time-resolved optical technique: the photo-induced Faraday
rotation. We showed that this dynamics is affected by a weak Overhauser field
created via the hyperfine interaction of optically spin-polarized donor-bound
electrons with the surrounding nuclear isotopes carrying non-zero spins. We
have measured this nuclear field, which is on the order of a few mT and can
reach a maximum experimental value of 9.4 mT. This value represents 13 % of the
maximal nuclear polarization, and corresponds also to 13 % of maximal
electronic polarization.Comment: 15 pages, 4 figure
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
Hole spin dephasing time associated to hyperfine interaction in quantum dots
The spin interaction of a hole confined in a quantum dot with the surrounding
nuclei is described in terms of an effective magnetic field. We show that, in
contrast to the Fermi contact hyperfine interaction for conduction electrons,
the dipole-dipole hyperfine interaction is anisotropic for a hole, for both
pure or mixed hole states. We evaluate the coupling constants of the
hole-nuclear interaction and demonstrate that they are only one order of
magnitude smaller than the coupling constants of the electron-nuclear
interaction. We also study, theoretically, the hole spin dephasing of an
ensemble of quantum dots via the hyperfine interaction in the framework of
frozen fluctuations of the nuclear field, in absence or in presence of an
applied magnetic field. We also discuss experiments which could evidence the
dipole-dipole hyperfine interaction and give information on hole mixing.Comment: 35 pages, 7 figures and 2 table
Serum/plasma potassium monitoring using potentiometric point-of-care microanalyzers with improved ion selective electrodes
Different causes can trigger imbalances on homeostatic mechanisms between intracellular and extracellular compartments resulting in abnormal blood potassium concentrations (hypo or hyperkalemia). This can lead to serious consequences, even a life-threatening situation. Early diagnosis, treatment and follow-up are essential to minimize critical impacts in patients. Bedside determination of blood potassium is not accessible in all health care centers or in all emergency departments and far less common in this kind of centers in emerging countries. We have therefore proposed a portable, economic and long-lifetime potentiometric point-of-care (POC) analytical microsystem to deal with this question. It is a continuous flow microfluidic platform, made of cyclic olefin copolymer (COC), which combines microfluidics and a detection system based on the potentiometric technique containing a potassium selective electrode with a novel composition of polymeric membrane, which improves lifetime. Its size is smaller than a credit card and shows a linear range of Nernst calibration equation from 1 to 26 mM K+, a detection limit of 0.16 mM K+, a satisfactory repeatability and reproducibility, and an analysis frequency of 20 samples h−1, requiring only 25 μL as sample volume. Moreover, lifetime is as long as 9 months by intensive use. All these features comply with medical requirements. Human serum samples were analyzed with the developed device and the obtained results were compared with those provided by two methods: ICP-OES and another using ion selective electrodes. No significant differences were observed, demonstrating the suitability of the developed POC microanalyzer for bedside health applications
The 1600 CE Huaynaputina eruption as a possible trigger for persistent cooling in the North Atlantic region
Paleoclimate reconstructions have identified a period of exceptional summer and winter cooling in the North Atlantic region following the eruption of the tropical volcano Huaynaputina (Peru) in 1600 CE. A previous study based on numerical climate simulations has indicated a potential mechanism for the persistent cooling in a slowdown of the North Atlantic subpolar gyre (SPG) and consequent ocean–atmosphere feedbacks. To examine whether this mechanism could have been triggered by the Huaynaputina eruption, this study compares the simulations used in the previous study both with and without volcanic forcing and this SPG shift to reconstructions from annual proxies in natural archives and historical written records as well as contemporary historical observations of relevant climate and environmental conditions. These reconstructions and observations demonstrate patterns of cooling and sea-ice expansion consistent with, but not indicative of, an eruption trigger for the proposed SPG slowdown mechanism. The results point to possible improvements in future model–data comparison studies utilizing historical written records. Moreover, we consider historical societal impacts and adaptations associated with the reconstructed climatic and environmental anomalies.This research has been supported by the Swiss National Science Foundation (grant no. P2BEP1_175214), the Swiss National Science Foundation through the SNSF Sinergia CALDERA project (grant no. CRSII5_183571), the Spanish Sci-ence and Innovation Ministry (Ministerio de Ciencia e Innovación) through the STREAM project (grant no. PID2020-114746GBI00), Fonds de la Recherche Scientifique – FNRS and the FWO under the Excellence of Science (EOS) program through the PARAMOUR project (grant no. O0100718F, EOS ID no. 30454083), and the Georgetown Environment Initiative.Peer ReviewedPostprint (published version
The 1600 CE Huaynaputina eruption as a possible trigger for persistent cooling in the North Atlantic region
Paleoclimate reconstructions have identified a period of exceptional summer and winter cooling in the North Atlantic region following the eruption of the tropical volcano Huaynaputina (Peru) in 1600 CE. A previous study based on numerical climate simulations has indicated a potential mechanism for the persistent cooling in a slowdown of the North Atlantic subpolar gyre (SPG) and consequent ocean-atmosphere feedbacks. To examine whether this mechanism could have been triggered by the Huaynaputina eruption, this study compares the simulations used in the previous study both with and without volcanic forcing and this SPG shift to reconstructions from annual proxies in natural archives and historical written records as well as contemporary historical observations of relevant climate and environmental conditions. These reconstructions and observations demonstrate patterns of cooling and sea-ice expansion consistent with, but not indicative of, an eruption trigger for the proposed SPG slowdown mechanism. The results point to possible improvements in future model-data comparison studies utilizing historical written records. Moreover, we consider historical societal impacts and adaptations associated with the reconstructed climatic and environmental anomalies
Determinação de selenometionina por quimiolumiescência em castanha do Brasil.
Selênio é reconhecido como um micronutriente essencial. Porém, a bioutilidade deste elemento está intrísecamente dependente da sua forma química. Espécies orgânicas de selênio são mais facilmente absorvidas por organismos humanos quando comparado a suas espécies inorgânicas. Castanhas do Brasil podem ser consideradas uma boa fonte de selênio, pois apresenta concentrações significativas deste elemento, principalmente na forma de selenometionina.1 Neste trabalho, sistema para análise em fluxo para a determinação de selenometionina em amostras de castanha do Brasil empregando quimioluminescência foi desenvolvido, utilizando uma célula de reação com estrutura em vórtex construída com tecnologia LTCC2
Cinética da adsorção de Cd, Co, Cr, Cu, K, Ni e Zn em soluções aquosas usando zeólita natural integrada à tecnologia LTCC.
Materiais naturais, disponíveis em grandes quantidades e que possam ser empregados como adsorventes de baixo custo para o tratamento de efluentes, vem sendo o alvo de inúmeras pesquisas. Quitosana, zeólitas, esponjas naturais e carvão ativado são empregados com sucesso para este propósito1. Dentro deste contexto, o objetivo do trabalho foi investigar a capacidade de remoção de Cd, Co, Cr, Cu, K, Ni e Zn através de uma amostra de zeólita natural integrada a dispositivos cerâmicos mediante a tecnologia LTCC
Live synthesis of selective carbon dots as fluorescent probes for cobalt determination in water with an automatic microanalyzer
Altres ajuts: acords transformatius de la UABA new strategy integrating the straight synthesis of carbon dots (CDs) and their direct use for the determination of heavy metals by means of fuorescence quenching is presented. The proposal consists of a modular analyzer, which includes a low temperature co-fred ceramics (LTCC) microreactor for the synthesis of CDs and a cyclic olefn copolymer (COC) microfuidic platform, which automatically performs a reverse fow injection analysis (rFIA) protocol for the determination of heavy metal ions in water by CD fuorescence quenching. As a proof of concept, nitrogen-doped CDs were synthesized from acrylic acid and ethylenediamine (ED) with quantum yields (QYs) of up to 44%, which are selective to cobalt. With the described system, we synthesized homogeneous CDs without the need for further purifcation and with the minimum consumption of reagents, and optimized fuorescence measurements can be performed with freshly obtained luminescent nanomaterials that have not undergone decomposition processes. They have an averagehydrodynamic diameter of 4.2±0.9 nm and maximum excitation and emission wavelengths at 358 nm and 452 nm, respectively. The system allows the automatic dilution and bufering of the synthesized CDs and the sample prior to the determination of cobalt. The concentration of cobalt was determined with good sensitivity and a limit of detection of 7 μg·L−1 with a linear range of 0.02-1 mg·L−1 of Co2+. Spiked tap water and river water samples were analyzed, obtaining recovery from 98 to 104%. This demonstrates the potential of the equipment as an efcient on-site control system for heavy metal monitoring in water
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