184 research outputs found
Charge transfer and coherence dynamics of tunnelling system coupled to a harmonic oscillator
We study the transition probability and coherence of a two-site system,
interacting with an oscillator. Both properties depend on the initial
preparation. The oscillator is prepared in a thermal state and, even though it
cannot be considered as an extended bath, it produces decoherence because of
the large number of states involved in the dynamics. In the case in which the
oscillator is intially displaced a coherent dynamics of change entangled with
oscillator modes takes place. Coherency is however degraded as far as the
oscillator mass increases producing a increasingly large recoherence time.
Calculations are carried on by exact diagonalization and compared with two
semiclassical approximations. The role of the quantum effects are highlighted
in the long-time dynamics, where semiclassical approaches give rise to a
dissipative behaviour. Moreover, we find that the oscillator dynamics has to be
taken into account, even in a semiclassical approximation, in order to
reproduce a thermally activated enhancement of the transition probability
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Lattice matched VolmerâWeber growth of Fe3Si on GaAs(001) - the influence of the growth rate
We investigate the formation of lattice matched single-crystalline Fe3Si/GaAs(001) ferromagnet/semiconductor hybrid structures by Volmer-Weber island growth, starting from the epitaxial growth of isolated Fe3Si islands up to the formation of continuous films as a result of island coalescence. We find coherent defect-free layers exhibiting compositional disorder near the Fe3Si/GaAs - interface for higher growth rates, whereas they are fully ordered for lower growth rates. © 2019 IOP Publishing Ltd
In situ x-ray diffraction study of epitaxial growth of ordered Fe3Si films
Molecular beam epitaxy of Fe3Si on GaAs(001) is studied in situ by grazing
incidence x-ray diffraction. Layer-by-layer growth of Fe3Si films is observed
at a low growth rate and substrate temperatures near 200 degrees Celsius. A
damping of x-ray intensity oscillations due to a gradual surface roughening
during growth is found. The corresponding sequence of coverages of the
different terrace levels is obtained. The after-deposition surface recovery is
very slow. Annealing at 310 degrees Celsius combined with the deposition of one
monolayer of Fe3Si restores the surface to high perfection and minimal
roughness. Our stoichiometric films possess long-range order and a high quality
heteroepitaxial interface.Comment: 8 pages, 3 figure
PHOTOSYNTHESIS AND CALCIFICATION IN THE COCCOLITHOPHORE EMILIANIA HUXLEYI AND TWO HERMATYPIC CORALS, PORITES PORITES AND ACROPORA SP.
PhDMost global calcification is carried out by organisms which are also
photosynthetic. In this study, the coccolithophore Emiliania huxleyi (Lohmann)
Hay and Mohler and two species of hermatypic coral were used to:
examine the effect of dissolved inorganic carbon (DIC) and light on
photosynthesis and calcification; and
determine the extent to which these two processes interact.
A novel method of producing coccolith-less (non-calcifying) cells from
calcifying cells of the same strain of E huxley! was developed thus allowing
photosynthesis and calcification to be studied separately. The kinetics of
photosynthesis in both types of cell, and of calcification in coccolith-bearing cells,
were shown to be biphasic with respect to DIC concentration. The hiatus in all
three cases was located at 1 mM DIC. This unusual pattern was shown to be
the product of two carbon uptake mechanisms: an anion exchanger working at all
DIC concentrations and an external carbonic anhydrase active only at low DIC
concentrations.
In contrast to the commonly-held view, this study demonstrated that
calcification did not promote photosynthesis in E. huxleyi. Nevertheless, there
was clearly strong biological control of calcification in this alga since DIC uptake
was mediated by an anion transporter and a dehydroxylating enzyme.
This work also showed that in E huxleyi, DIC addition enhanced
photosynthesis at both limiting and saturating photon flux densities and that
bicarbonate affected photochemical processes directly. Photosystem II activity
was stimulated and non-photochemical quenching was reduced, possibly
protecting the photosynthetic apparatus from damage by light.
In the two corals; Porites porites and Acropora sp., strong biological
control of calcium carbonate precipitation was also evident. Again, calcification
did not stimulate photosynthesis. Calcification rates of Acropora sp. were
monitored in the dark and although these were lower than in the light, they still
increased dramatically with bicarbonate addition. This showed that high
concentrations of the bicarbonate ion can compensate for the lack of light.
Hence, it seems that in hermatypic corals, light-dependence of calcification may
be facultative and not obligate. It is therefore clear from the results of this study
that calcification and photosynthesis are not as closely coupled as has been
previously thought.
In neither E. huxleyi, nor in the hermatypic corals, were photosynthetic and
calcification rates saturated at the present ambient DIC concentration of
seawater
Microbial players and processes involved in phytoplankton bloom utilization in the water column of a fast-flowing, river-dominated estuary
© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in MicrobiologyOpen 6 (2017): e467, doi:10.1002/mbo3.467.Fueled by seasonal phytoplankton blooms, the Columbia River estuary is a natural bioreactor for organic matter transformations. Prior metagenome analyses indicated high abundances of diverse Bacteroidetes taxa in estuarine samples containing phytoplankton. To examine the hypothesis that Bacteroidetes taxa have important roles in phytoplankton turnover, we further analyzed metagenomes from water collected along a salinity gradient at 0, 5, 15, 25, and 33 PSU during bloom events. Size fractions were obtained by using a 3-ÎŒm prefilter and 0.2-ÎŒm collection filter. Although this approach targeted bacteria by removing comparatively large eukaryotic cells, the metagenome from the ES-5 sample (5 PSU) nevertheless contained an abundance of diatom DNA. Biogeochemical measurements and prior studies indicated that this finding resulted from the leakage of cellular material due to freshwater diatom lysis at low salinity. Relative to the other metagenomes, the bacterial fraction of ES-5 was dramatically depleted of genes annotated as Bacteroidetes and lysogenic bacteriophages, but was overrepresented in DNA of protists and Myxococcales bacterivores. We suggest the following equally plausible scenarios for the microbial response to phytoplankton lysis: (1) Bacteroidetes depletion in the free-living fraction may at least in part be caused by their attachment to fluvial diatoms as the latter are lysed upon contact with low-salinity estuarine waters; (2) diatom particle colonization is likely followed by rapid bacterial growth and lytic phage infection, resulting in depletion of lysogenic bacteriophages and host bacteria; and (3) the subsequent availability of labile organic matter attracted both grazers and predators to feed in this estuarine biogeochemical âhotspot,â which may have additionally depleted Bacteroidetes populations. These results represent the first detailed molecular analysis of the microbial response to phytoplankton lysis at the freshwaterâbrackish water interface in the fast-flowing Columbia River estuary.National Science Foundation Grant Numbers: OCE 0424602, MCB 064446
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
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Structural properties of Co2TiSi films on GaAs(001)
Co2TiSi films were grown by molecular beam epitaxy on GaAs(001) and analyzed using reflection high-energy electron diffraction, and electron microscopy. In addition, X-ray diffraction was combined with lattice parameter calculations by density functional theory comparing the L21 and B2 structures and considering the influence of non-stoichiometry. Columnar growth is found and attributed to inhomogeneous epitaxial strain from non-random alloying. In films with thicknesses up to 13 nm, these columns may be the origin of perpendicular magnetization with the easy axis perpendicular to the sample surface. We found L21 and B2 ordered regions, however the [Co]/[Ti]-ratio is changing in dependence of the position in the film. The resulting columnar structure is leading to anisotropic B2-ordering with the best order parallel to the axes of the columns
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