Mechanisms of Manganese-Assisted Nonradiative Recombination in Cd(Mn)Se/Zn(Mn)Se Quantum Dots

Mechanisms of nonradiative recombination of electron-hole complexes in Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations of Mn$^{2+}$ ions are analyzed within the framework of single electron model of deep {\it 3d}-levels in semiconductors. In addition to the mechanisms caused by Coulomb and exchange interactions, which are related because of the Pauli principle, another mechanism due to {\it sp-d} mixing is considered. It is shown that the Coulomb mechanism reduces to long-range dipole-dipole energy transfer from photoexcited quantum dots to Mn$^{2+}$ ions. The recombination due to the Coulomb mechanism is allowed for any states of Mn$^{2+}$ ions and {\it e-h} complexes. In contrast, short-range exchange and ${\it sp-d}$ recombinations are subject to spin selection rules, which are the result of strong {\it lh-hh} splitting of hole states in quantum dots. Estimates show that efficiency of the {\it sp-d} mechanism can considerably exceed that of the Coulomb mechanism. The phonon-assisted recombination and processes involving upper excited states of Mn$^{2+}$ ions are studied. The increase in PL intensity of an ensemble of quantum dots in a magnetic field perpendicular to the sample growth plane observed earlier is analyzed as a possible manifestation of the spin-dependent recombination.Comment: 14 pages, 2 figure

Giant bivalves (Tridacna gigas) as recorders of ENSO variability

We compare monthly resolved oxygen isotope records derived from a giant bivalve shell, Tridacna gigas and massive Parkes corals collected along the northern coast of Papua New Guinea. This intercomparison study demonstrates that delta O-18 profiles obtained from these different aragonite-secreting organisms collected from within a 30 km range are correlated in great detail and record the timing and amplitude of seasonal and interannual (ENSO-related) variations in sea surface temperature (SST) and water isotopic composition which is closely related to rainfall. Furthermore, the T. gigas record is shown to be close to isotopic equilibrium with the local sea-water, in contrast to the corals which are approximately - 4 parts per thousand offset. These results reveal that living and fossil T. gigas clam shells have the potential to yield reliable records of past changes in seasonality and ENSO variability, as well as mean climate conditions. In particular, since the non-porous shells are generally more resistant to diagenesis than coral skeletons, they may provide robust estimates of past tropical climate for periods and locations where unaltered corals are absent. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved

Interannual climate variability in the Miocene: High resolution trace element and stable isotope ratios in giant clams

High resolution stable isotope and trace elemental ratios of a recent Tridacna squamosa from Vietnam and a Middle to Late Miocene (10–13 Ma) Tridacna gigas from Indonesia are presented. The seasonal pattern of modern sea surface temperature (SST) variability offshore Vietnam is faithfully recorded in the δ18O of the T. squamosa shell carbonate, confirming the potential of Tridacna shells as sub-annual resolution climate archives. Cultivation of the T. squamosa specimen in controlled conditions after recovery from the natural environment facilitated a quantitative calibration of the δ18O signal to ambient water temperatures. An age model for the Miocene T. gigas shell from Indonesia was therefore constructed on the basis of its δ18O profile, assuming a single-peak annual SST cycle. The magnitude of these oscillations was 5–7 °C. Mg/Ca and the growth-banding pattern in the Miocene T. gigas correlates well with shell δ18O during the later part of the organism's lifespan. Ba/Ca is negatively correlated to Mg/Ca, with a lag of several months, suggesting a different phasing of the annual primary productivity cycle from that of SST. Furthermore, δ18O and Mg/Ca show prominent deviations to warmer conditions with a periodicity of ~ 3 years. These shifts demonstrate the existence of substantial interannual sea surface temperature variability in the Miocene, a period with elevated global temperatures compared to the present day

Profiles of trace elements and stable isotopes derived from giant long-lived Tridacna gigas bivalves : Potential applications in paleoclimate studies

This study investigates the environmental and biological controls on trace element partitioning and stable isotope composition of modern giant long-lived bivalves (Tridacna gigas) with the aim to use these archives for paleoclimatic reconstructions. Firstly, the intra-shell variability is studied by measuring time equivalent profiles in the different shell layers characterised by different growth rates. Secondly, the inter-site variability is studied by comparing profiles derived from three modern specimens collected in sites across the Indo-Pacific region characterised by different ranges of temperature and productivity

Aerosol Concentrations in Relationship to Local Atmospheric Conditions on James Ross Island, Antarctica

International audienceSeveral important ice-free areas (e.g., Seymour Island, Cape Lamb on Vega Island, Terrapin Hill) are located in the Eastern Antarctic Peninsula region. The largest of these ice-free areas can be found on the Ulu Peninsula, James Ross Island, where this study was undertaken. The Ulu Peninsula covers an area of 312 km2, and has been found to be an important active High Latitude Dust source. In this study, aerosol concentrations and local wind properties are described together with their linkages and typical synoptic situations. The highest aerosol concentrations of 57 μg m-3 for PM10 were detected during high wind speed events that exceeded 10 m s-1, which is also a threshold level for activating local mineral material sources. Surface deposition of dust particles can have significant environmental impacts such as changes in properties of atmosphere or enhanced snow melting

Diffusion Tensor Imaging in Anxiety Disorders

Diffusion tensor imaging (DTI) can be used to examine the structural integrity of regional white matter and to map white matter tracts. DTI studies have been performed in several psychiatric disorders, especially in those for which a developmental or a neuropsychiatric component was postulated. Thus far, the use of DTI has been very limited in panic disorder, social anxiety disorder, and generalized anxiety disorder, and somewhat more extensive in post-traumatic stress disorder and obsessive-compulsive disorder. In most anxiety disorders, the results of DTI studies are in line with other structural and functional MRI findings and can be interpreted within the frameworks of existing models for the neurocircuitry of the various disorders. DTI findings could further enrich neurobiological models for anxiety disorders, although replication is often warranted, and studies in pediatric populations are lagging behind remarkably.Stress-related psychiatric disorders across the life spa