450 research outputs found
On the Cosmic Evolution of Fe/Mg in QSO Absorption Line Systems
We investigate the variation of the ratio of the equivalent widths of the
FeII2600 line to the MgII2796,2803 doublet as a
function of redshift in a large sample of absorption lines drawn from the
JHU-SDSS Absorption Line Catalog. We find that despite large scatter, the
observed ratio shows a trend where the equivalent width ratio
decreases monotonically with
increasing redshift over the range . Selecting the
subset of absorbers where the signal-to-noise ratio of the MgII equivalent
width is 3 and modeling the equivalent width ratio
distribution as a gaussian, we find that the mean of the gaussian distribution
varies as . We discuss various possible
reasons for the trend. A monotonic trend in the Fe/Mg abundance ratio is
predicted by a simple model where the abundances of Mg and Fe in the absorbing
clouds are assumed to be the result of supernova ejecta and where the cosmic
evolution in the SNIa and core-collapse supernova rates is related to the
cosmic star-formation rate. If the trend in reflects the
evolution in the abundances, then it is consistent with the predictions of the
simple model.Comment: 10 pages, 4 figures, final version published in MNRA
Quasi-equilibrium optical nonlinearities in spin-polarized GaAs
Semiconductor Bloch equations, which microscopically describe the dynamics of
a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in
two limits: the coherent and the quasi-equilibrium regime. These equations have
been recently extended to include the spin degree of freedom, and used to
explain spin dynamics in the coherent regime. In the quasi-equilibrium limit,
one solves the Bethe-Salpeter equation in a two-band model to describe how
optical absorption is affected by Coulomb interactions within a
spin-unpolarized plasma of arbitrary density. In this work, we modified the
solution of the Bethe-Salpeter equation to include spin-polarization and light
holes in a three-band model, which allowed us to account for spin-polarized
versions of many-body effects in absorption. The calculated absorption
reproduced the spin-dependent, density-dependent and spectral trends observed
in bulk GaAs at room temperature, in a recent pump-probe experiment with
circularly polarized light. Hence our results may be useful in the microscopic
modelling of density-dependent optical nonlinearities in spin-polarized
semiconductors.Comment: 7 pages, 6 figure
Effect of well-width on the electro-optical properties of a quantum well
We record photoreflectance from Ge/GeSi modulation doped quantum wells
possessing V/cm perpendicular electric fields. Qualitatively very
different spectra are obtained from samples of well-width 100 \AA and 250 \AA.
Comparing the wavefunctions calculated from an \textbf{k.p}
theory, we find that while they remain confined in the narrower 100 \AA QW, the
electric field causes them to tunnel into the forbidden gap in the 250 \AA\
well. This implies that the samples should show a transition from the quantum
confined Franz-Keldysh effect to the bulk-like Franz-Keldysh effect. Close to
the band-edge where Franz-Keldysh effects are important, simulated
photoreflectance spectra reproduce the essential features of the experiment,
without any adjustable parameters.Comment: 8 pages, 8 figures. Submitted to Phys. Rev.
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Tbr1 instructs laminar patterning of retinal ganglion cell dendrites.
Visual information is delivered to the brain by >40 types of retinal ganglion cells (RGCs). Diversity in this representation arises within the inner plexiform layer (IPL), where dendrites of each RGC type are restricted to specific sublaminae, limiting the interneuronal types that can innervate them. How such dendritic restriction arises is unclear. We show that the transcription factor Tbr1 is expressed by four mouse RGC types with dendrites in the outer IPL and is required for their laminar specification. Loss of Tbr1 results in elaboration of dendrites within the inner IPL, while misexpression in other cells retargets their neurites to the outer IPL. Two transmembrane molecules, Sorcs3 and Cdh8, act as effectors of the Tbr1-controlled lamination program. However, they are expressed in just one Tbr1+ RGC type, supporting a model in which a single transcription factor implements similar laminar choices in distinct cell types by recruiting partially non-overlapping effectors
Rainfall intensification increases the contribution of rewetting pulses to soil heterotrophic respiration
Soil drying and wetting cycles promote carbon(C) release through large heterotrophic respiration pulses at rewetting, known as the “Birch” effect. Empirical evidence shows that drier conditions before rewetting and larger changes in soil moisture at rewetting cause larger respiration pulses. Because soil moisture varies in response to rainfall, these respiration pulses also depend on the random timing and intensity of precipitation. In addition to rewetting pulses, heterotrophic respiration continues during soil drying, eventually ceasing when soils are too dry to sustain microbial activity. The importance of respiration pulses in contributing to the overall soil heterotrophic respiration flux hasbeen demonstrated empirically, but no theoretical investigation has so far evaluated how the relative contribution ofthese pulses may change along climatic gradients or as precipitation regimes shift in a given location. To fill this gap,we start by assuming that heterotrophic respiration rates during soil drying and pulses at rewetting can be treated as random variables dependent on soil moisture fluctuations, and we develop a stochastic model for soil heterotrophic respi-ration rates that analytically links the statistical properties of respiration to those of precipitation. Model results show that both the mean rewetting pulse respiration and the mean respiration during drying increase with increasing mean pre-cipitation. However, the contribution of respiration pulses to the total heterotrophic respiration increases with decreasing precipitation frequency and to a lesser degree with decreas-ing precipitation depth, leading to an overall higher contribution of respiration pulses under future more intermittent and intense precipitation. Specifically, higher rainfall intermittency at constant total rainfall can increase the contribution of respiration pulses up to ∼10 % or 20 % of the total heterotrophic respiration in mineral and organic soils, respectively. Moreover, the variability of both components of soil heterotrophic respiration is also predicted to increase under these conditions. Therefore, with future more intermittent precipitation, respiration pulses and the associated nutrient release will intensify and become more variable, contributing more to soil biogeochemical cycling
Enduring strengths: analysing the UK’s current and potential economic strengths, and what they mean for its economic strategy, at the start of the decisive decade
Key to building a new economic strategy which can revitalise the UK economy after a decade of stagnation is understanding our current strengths, how these strengths evolve, and the trade-offs they present. The report uses global data on trade in goods and services and patenting to uncover where the UK’s relative strengths lie; we study the extent to which these strengths have changed over time and compare with international peers; and undertake several deep dives into the areas in which the UK has developed a specialism. Finally we consider what the UK’s mix of specialisms means for a wider economic strategy
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