656 research outputs found
Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen
The ultrafast dynamics of clusters of trans-azobenzene anion (A–) solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A– was determined by monitoring in real time the transient of the A– rise, following an 800 nm excitation of A– (O2)n, where n=1–4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n>1 a quasi-O4 core is formed and that the dissociation occurs by a bond cleavage between A– and conglomerated (O2)n rather than a stepwise evaporation of O2. With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise (2- for A–O2 and A·O4-·(O2)n–2 for A–(O2)n, where n=2–4. Subsequent to an ultrafast electron recombination, A– rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A–-(O2)n nuclear coordinate and a slower component (1.6–36 ps, increasing with n) attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2–3 ps, with a change in the sign of the transient anisotropy between A–O2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters
Correlation in the transition metal based Heusler compounds CoMnSi and CoFeSi
Half-metallic ferromagnets like the full Heusler compounds with formula
XYZ are supposed to show an integer value of the spin magnetic moment.
Calculations reveal in certain cases of X = Co based compounds non-integer
values, in contrast to experiments. In order to explain deviations of the
magnetic moment calculated for such compounds, the dependency of the electronic
structure on the lattice parameter was studied theoretically. In local density
approximation (LDA), the minimum total energy of CoFeSi is found for the
experimental lattice parameter, but the calculated magnetic moment is about 12%
too low. Half-metallic ferromagnetism and a magnetic moment equal to the
experimental value of are found, however, only after increasing the
lattice parameter by more than 6%.
To overcome this discrepancy, the LDA scheme was used to respect on-site
electron correlation in the calculations. Those calculations revealed for
CoFeSi that an effective Coulomb-exchange interaction in the
range of about 2eV to 5eV leads to half-metallic ferromagnetism and the
measured, integer magnetic moment at the measured lattice parameter. Finally,
it is shown in the case of CoMnSi that correlation may also serve to
destroy the half-metallic behavior if it becomes too strong (for CoMnSi
above 2eV and for CoFeSi above 5eV). These findings indicate that on-site
correlation may play an important role in the description of Heusler compounds
with localized moments.Comment: submitted to Phys. Rev.
Geometric, electronic, and magnetic structure of CoFeSi: Curie temperature and magnetic moment measurements and calculations
In this work a simple concept was used for a systematic search for new
materials with high spin polarization. It is based on two semi-empirical
models. Firstly, the Slater-Pauling rule was used for estimation of the
magnetic moment. This model is well supported by electronic structure
calculations. The second model was found particularly for Co based Heusler
compounds when comparing their magnetic properties. It turned out that these
compounds exhibit seemingly a linear dependence of the Curie temperature as
function of the magnetic moment. Stimulated by these models, CoFeSi was
revisited. The compound was investigated in detail concerning its geometrical
and magnetic structure by means of X-ray diffraction, X-ray absorption and
M\"o\ss bauer spectroscopies as well as high and low temperature magnetometry.
The measurements revealed that it is, currently, the material with the highest
magnetic moment () and Curie-temperature (1100K) in the classes of
Heusler compounds as well as half-metallic ferromagnets. The experimental
findings are supported by detailed electronic structure calculations
Design of magnetic materials: CoCrFeAl
Doped Heusler compounds CoCrFeAl with varying Cr to Fe
ratio were investigated experimentally and theoretically. The electronic
structure of the ordered, doped Heusler compound CoCrFeAl
( was calculated using different types of band structure
calculations. The ordered compounds turned out to be ferromagnetic with small
Al magnetic moment being aligned anti-parallel to the 3d transition metal
moments. All compounds show a gap around the Fermi-energy in the minority
bands. The pure compounds exhibit an indirect minority gap, whereas the
ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism
(MCD) in X-ray absorption spectra was measured at the edges of Co,
Fe, and Cr of the pure compounds and the alloy in order to determine
element specific magnetic moments. Calculations and measurements show an
increase of the magnetic moments with increasing iron content. The
experimentally observed reduction of the magnetic moment of Cr can be explained
by Co-Cr site-disorder. The presence of the gap in the minority bands of
CoCrAl can be attributed to the occurrence of pure Co and mixed CrAl
(001)-planes in the structure. It is retained in structures with
different order of the CrAl planes but vanishes in the -structure with
alternating CoCr and CoAl planes.Comment: corrected author lis
Electronic structure, magnetism, and disorder in the Heusler compound CoTiSn
Polycrystalline samples of the half-metallic ferromagnet Heusler compound
CoTiSn have been prepared and studied using bulk techniques (X-ray
diffraction and magnetization) as well as local probes (Sn M\"ossbauer
spectroscopy and Co nuclear magnetic resonance spectroscopy) in order to
determine how disorder affects half-metallic behavior and also, to establish
the joint use of M\"ossbauer and NMR spectroscopies as a quantitative probe of
local ion ordering in these compounds. Additionally, density functional
electronic structure calculations on ordered and partially disordered
CoTiSn compounds have been carried out at a number of different levels of
theory in order to simultaneously understand how the particular choice of DFT
scheme as well as disorder affect the computed magnetization. Our studies
suggest that a sample which seems well-ordered by X-ray diffraction and
magnetization measurements can possess up to 10% of antisite (Co/Ti)
disordering. Computations similarly suggest that even 12.5% antisite Co/Ti
disorder does not destroy the half-metallic character of this material.
However, the use of an appropriate level of non-local DFT is crucial.Comment: 11 pages and 5 figure
Developing immortal cell lines from Xenopus embryos, four novel cell lines derived from Xenopus tropicalis
© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Gorbsky, G. J., Daum, J. R., Sapkota, H., Summala, K., Yoshida, H., Georgescu, C., Wren, J. D., Peshkin, L., & Horb, M. E. Developing immortal cell lines from Xenopus embryos, four novel cell lines derived from Xenopus tropicalis. Open Biology, 12(7), (2022): 220089, https://doi.org/10.1098/rsob.220089.The diploid anuran Xenopus tropicalis has emerged as a key research model in cell and developmental biology. To enhance the usefulness of this species, we developed methods for generating immortal cell lines from Nigerian strain (NXR_1018, RRID:SCR_013731) X. tropicalis embryos. We generated 14 cell lines that were propagated for several months. We selected four morphologically distinct lines, XTN-6, XTN-8, XTN-10 and XTN-12 for further characterization. Karyotype analysis revealed that three of the lines, XTN-8, XTN-10 and XTN-12 were primarily diploid. XTN-6 cultures showed a consistent mixed population of diploid cells, cells with chromosome 8 trisomy, and cells containing a tetraploid content of chromosomes. The lines were propagated using conventional culture methods as adherent cultures at 30°C in a simple, diluted L-15 medium containing fetal bovine serum without use of a high CO2 incubator. Transcriptome analysis indicated that the four lines were distinct lineages. These methods will be useful in the generation of cell lines from normal and mutant strains of X. tropicalis as well as other species of Xenopus.This work was supported by Whitman fellowships to G.J.G. from the Marine Biological Laboratory, by grant no. 1645105 to G.J.G. and MEH from the National Science Foundation and by grant no. P40OD010997 from the Office of the Director, National Institutes of Health. L.P. has been supported by grant no. R01HD073104 from the National Institute of Child Health and Development
A temporal dimension to the influence of pollen rewards on bee behaviour and fecundity in Aloe tenuior
The net effect of pollen production on fecundity in plants can range from negative – when self-pollen interferes with fecundity due to incompatibility mechanisms, to positive – when pollen availability is associated with increased pollinator visitation and fecundity due to its utilization as a reward. We investigated the responses of bees to pollen and nectar rewards, and the effects of these rewards on pollen deposition and fecundity in the hermaphroditic succulent shrub Aloe tenuior. Self-pollinated plants failed to set fruit, but their ovules were regularly penetrated by self-pollen tubes, which uniformly failed to develop into seeds as expected from ovarian self-incompatibility (or strong early inbreeding depression). Bees consistently foraged for pollen during the morning and early afternoon, but switched to nectar in the late afternoon. As a consequence of this differential foraging, we were able to test the relative contribution to fecundity of pollen- versus nectar-collecting flower visitors. We exposed emasculated and intact flowers in either the morning or late afternoon to foraging bees and showed that emasculation reduced pollen deposition by insects in the morning, but had little effect in the afternoon. Despite the potential for self-pollination to result in ovule discounting due to late-acting self-sterility, fecundity was severely reduced in artificially emasculated plants. Although there were temporal fluctuations in reward preference, most bee visits were for pollen rewards. Therefore the benefit of providing pollen that is accessible to bee foragers outweighs any potential costs to fitness in terms of gender interference in this species
Floral advertisement scent in a changing plant-pollinators market
Plant-pollinator systems may be considered as biological markets in which pollinators choose between different flowers that advertise their nectar/pollen rewards. Although expected to play a major role in structuring plant-pollinator interactions, community-wide patterns of flower scent signals remain largely unexplored. Here we show for the first time that scent advertisement is higher in plant species that bloom early in the flowering period when pollinators are scarce relative to flowers than in species blooming later in the season when there is a surplus of pollinators relative to flowers. We also show that less abundant flowering species that may compete with dominant species for pollinator visitation early in the flowering period emit much higher proportions of the generalist attractant β-ocimene. Overall, we provide a first community-wide description of the key role of seasonal dynamics of plant-specific flower scent emissions, and reveal the coexistence of contrasting plant signaling strategies in a plant-pollinator market
Lessons from the 2018 drought for management of local water supplies in upland areas : a tracer-based assessment
Funding Information: We would like to acknowledge financial support from the UK Natural Environment Research Council (project NE/P010334/1) via a CASE industrial studentship with Chivas Brothers. David Drummond, Katya Dimitrova-Petrova and Eva Loerke are thanked for assistance with fieldwork, while we acknowledge Dr Aaron Neill for his advice on young water fraction analyses. Trevor Buckley and staff at the Glenlivet Distillery are thanked for on-site assistance and supply of data and abstraction records. We thank Audrey Innes, Dr Bernhard Scheliga, and Dr Ilse Kamerling for their support with the laboratory isotope analysis. Publisher Copyright: © 2020 The Authors. Hydrological Processes published by John Wiley & Sons Ltd. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.Peer reviewedPublisher PD
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