140 research outputs found
High In-content InGaN layers synthesized by plasma-assisted molecular-beam epitaxy: growth conditions, strain relaxation and In incorporation kinetics
We report the interplay between In incorporation and strain relaxation
kinetics in high-In-content InxGa1-xN (x = 0.3) layers grown by plasma-assisted
molecular-beam epitaxy. For In mole fractions x = 0.13-0.48, best structural
and morphological quality is obtained under In excess conditions, at In
accumulation limit, and at a growth temperature where InGaN decomposition is
active. Under such conditions, in situ and ex situ analysis of the evolution of
the crystalline structure with the growth thickness points to an onset of
misfit relaxation after the growth of 40 nm, and a gradual relaxation during
more than 200 nm which results in an inhomogeneous strain distribution along
the growth axis. This process is associated with a compositional pulling
effect, i.e. indium incorporation is partially inhibited in presence of
compressive strain, resulting in a compositional gradient with increasing In
mole fraction towards the surface
Raman signatures of classical and quantum phases in coupled dots: A theoretical prediction
We study electron molecules in realistic vertically coupled quantum dots in a
strong magnetic field. Computing the energy spectrum, pair correlation
functions, and dynamical form factor as a function of inter-dot coupling via
diagonalization of the many-body Hamiltonian, we identify structural
transitions between different phases, some of which do not have a classical
counterpart. The calculated Raman cross section shows how such phases can be
experimentally singled out.Comment: 9 pages, 2 postscript figures, 1 colour postscript figure, Latex 2e,
Europhysics Letters style and epsfig macros. Submitted to Europhysics Letter
Phosphoribulokinase abundance is not limiting the Calvin-Benson-Bassham cycle in Chlamydomonas reinhardtii
Improving photosynthetic efficiency in plants and microalgae is of utmost importance to support the growing world population and to enable the bioproduction of energy and chemicals. Limitations in photosynthetic light conversion efficiency can be directly attributed to kinetic bottlenecks within the Calvin-Benson-Bassham cycle (CBBC) responsible for carbon fixation. A better understanding of these bottlenecks in vivo is crucial to overcome these limiting factors through bio-engineering. The present study is focused on the analysis of phosphoribulokinase ( PRK) in the unicellular green alga Chlamydomonas reinhardtii. We have characterized a PRK knock-out mutant strain and showed that in the absence of PRK, Chlamydomonas cannot grow photoautotrophically while functional complementation with a synthetic construct allowed restoration of photoautotrophy. Nevertheless, using standard genetic elements, the expression of PRK was limited to 40% of the reference level in complemented strains and could not restore normal growth in photoautotrophic conditions suggesting that the CBBC is limited. We were subsequently able to overcome this initial limitation by improving the design of the transcriptional unit expressing PRK using diverse combinations of DNA parts including PRK endogenous promoter and introns. This enabled us to obtain strains with PRK levels comparable to the reference strain and even overexpressing strains. A collection of strains with PRK levels between 16% and 250% of WT PRK levels was generated and characterized. Immunoblot and growth assays revealed that a PRK content of approximate to 86% is sufficient to fully restore photoautotrophic growth. This result suggests that PRK is present in moderate excess in Chlamydomonas. Consistently, the overexpression of PRK did not increase photosynthetic growth indicating that that the endogenous level of PRK in Chlamydomonas is not limiting the Calvin-Benson-Bassham cycle under optimal conditions
Combination of electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy to determine indium concentration in InGaN thin film structures
We demonstrate a method to determine the indium concentration, x, of In x Ga1-x N thin films by combining plasmon excitation studies in electron energy-loss spectroscopy (EELS) with a novel way of quantification of the intensity of x-ray lines in energy-dispersive x-ray spectroscopy (EDXS). The plasmon peak in EELS of InGaN is relatively broad. We fitted a Lorentz function to the main plasmon peak to suppress noise and the influence from the neighboring Ga 3d transition in the spectrum, which improves the precision in the evaluation of the plasmon peak position. As the indium concentration of InGaN is difficult to control during high temperature growth due to partial In desorption, the nominal indium concentrations provided by the growers were not considered reliable. The indium concentration obtained from EDXS quantification using Oxford Instrument ISIS 300 x-ray standard quantification software often did not agree with the nominal indium concentration, and quantification using K and L lines was inconsistent. We therefore developed a self-consistent iterative procedure to determine the In content from thickness-dependent k-factors, as described in recent work submitted to Journal of Microscopy. When the plasmon peak position is plotted versus the indium concentration from EDXS we obtain a linear relationship over the whole compositional range, and the standard error from linear least-squares fitting shows that the indium concentration can be determined from the plasmon peak position to within Δx = ± 0.037 standard deviation
Search for Direct CP Violation in Non-Leptonic Decays of Charged and Hyperons
A search for direct CP violation in the non-leptonic decays of hyperons has
been performed. In comparing the product of the decay parameters,
, in terms of an asymmetry parameter,
, between hyperons and anti-hyperons in the charged and decay sequence, we found no evidence of
direct CP violations. The parameter was measured to be .Comment: Submitted for publication; RevTex, 13 pages, 4 figure
Measurement of Decay Parameters for Decay
Based on 1.35 million polarized events, we measure the parameter
to be for
decay. New results for the parameters
and are also presented. Assuming that the
CP-violating phase-shift difference is negligible, we deduce the strong
phase-shift difference between the P-wave and S-wave amplitudes of the
final state to be . This strong phase-shift difference reduces the theoretical
uncertainty in estimating the level of CP violation in -hyperon decay.Comment: To be published in Physical Review Letter
Neutral weak currents in pion electroproduction on the nucleon
Parity violating asymmetry in inclusive scattering of longitudinally
polarized electrons by unpolarized protons with or meson
production, is calculated as a function of the momentum transfer squared
and the total energy of the -system. This asymmetry, which is
induced by the interference of the one-photon exchange amplitude with the
parity-odd part of the -exchange amplitude, is calculated for the
processes ( is a virtual photon and
a virtual Z-boson) considering the -contribution in the channel,
the standard Born contributions and vector meson ( and )
exchanges in the channel. Taking into account the known isotopic properties
of the hadron electromagnetic and neutral currents, we show that the P-odd term
is the sum of two contributions. The main term is model independent and it can
be calculated exactly in terms of fundamental constants. It is found to be
linear in . The second term is a relatively small correction which is
determined by the isoscalar component of the electromagnetic current. Near
threshold and in the -region, this isoscalar part is much smaller (in
absolute value) than the isovector one: its contribution to the asymmetry
depend on the polarization state (longitudinal or transverse) of the virtual
photon.Comment: 30 pages 9 figure
Evidence of relationship between strain and In-incorporation:Growth of N-polar In-rich InAlN buffer layer by OMCVD
International audienc
Low-Energy Parity-Violation and New Physics
The new physics sensitivity of a variety of low-energy parity-violating (PV)
observables is analyzed. A comparison is made between atomic PV for a single
isotope, atomic PV using isotope ratios, and PV electron-hadron and
electron-electron scattering. The complementarity among these observables, as
well as with high-energy processes, is emphasized. Theoretical uncertainties
entering the interpretation of low-energy measurements are discussed.Comment: 37 pages, 2 figures. Omissions to Tables I and V corrected along with
some minor typographical errors. To appear in Phys. Rev.
Spatially structured environmental filtering of collembolan traits in late successional salt marsh vegetation
Both the environment and the spatial configuration of habitat patches are important factors that shape community composition and affect species diversity patterns. Species have traits that allow them to respond to their environment. Our current knowledge on environment to species traits relationships is limited in spite of its potential importance for understanding community assembly and ecosystem function. The aim of our study was to examine the relative roles of environmental and spatial variables for the small-scale variation in Collembola (springtail) communities in a Dutch salt marsh. We used a trait-based approach in combination with spatial statistics and variance partitioning, between environmental and spatial variables, to examine the important ecological factors that drive community composition. Turnover of trait diversity across space was lower than for species diversity. Most of the variation in community composition was explained by small-scale spatial variation in topography, on a scale of 4-6 m, most likely because it determines the effect of inundation, which restricts where habitat generalists can persist. There were only small pure spatial effects on species and trait diversity, indicating that biotic interactions or dispersal limitation probably were less important for structuring the community at this scale. Our results suggest that for springtails, life form (i.e. whether they live in the soil or litter or on the surface/in vegetation) is an important and useful trait to understand community assembly. Hence, using traits in addition to species identity when analysing environment-organism relationships results in a better understanding of the factors affecting community composition
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