5,957 research outputs found
Nominally forbidden transitions in the interband optical spectrum of quantum dots
We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs
self-assembled quantum dots by adopting an atomistic pseudopotential approach
to the single-particle problem followed by a configuration-interaction approach
to the many-body problem. We find three types of allowed transitions that would
be naively expected to be forbidden. (i) Transitions that are parity forbidden
in simple effective mass models with infinite confining wells (e.g. 1S-2S,
1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii)
light-hole--to--conduction transitions, enabled by the confinement of
light-hole states; and (iii) transitions that show and enhanced intensity due
to electron-hole configuration mixing with allowed transitions. We compare
these predictions with results of 8-band k.p calculations as well as recent
spectroscopic data. Transitions in (i) and (ii) explain recently observed
satellites of the allowed P-P transitions.Comment: Version published in Phys. Rev.
Factorization and Scaling in Hadronic Diffraction
In standard Regge theory with a pomeron intercept a(0)=1+\epsilon, the
contribution of the tripe-pomeron amplitude to the t=0 differential cross
section for single diffraction dissociation has the form d\sigma/dM^2(t=0) \sim
s^{2\epsilon}/(M^2)^{1+\epsilon}. For \epsilon>0, this form, which is based on
factorization, does not scale with energy. From an analysis of p-p and p-pbar
data from fixed target to collider energies, we find that such scaling actually
holds, signaling a breakdown of factorization. Phenomenologically, this result
can be obtained from a scaling law in diffraction, which is embedded in the
hypothesis of pomeron flux renormalization introduced to unitarize the triple
pomeron amplitude.Comment: 39 pages, Latex, 16 figure
Survey of charge symmetry breaking operators for dd -> alpha pi0
The charge-symmetry-breaking amplitudes for the recently observed d d ->
alpha pi0 reaction are investigated. Chiral perturbation theory is used to
classify and identify the leading-order terms. Specific forms of the related
one- and two-body tree level diagrams are derived. As a first step toward a
full calculation, a few tree-level two-body diagrams are evaluated at each
considered order, using a simplified set of d and alpha wave functions and a
plane-wave approximation for the initial dd state. The leading-order
pion-exchange term is shown to be suppressed in this model because of poor
overlap of the initial and final states. The higher-order one-body and
short-range (heavy-meson-exchange) amplitudes provide better matching between
the initial and final states and therefore contribute significantly and
coherently to the cross section. The consequences this might have for a full
calculation, with realistic wave functions and a more complete set of
amplitudes, are discussed.Comment: REVTeX 4, 35 pages, 8 eps figures, submitted to PR
Influence of chronic nitrate intoxication on the concentration of lipid hydroperoxides in the gastric mucosa
The excessive alimentary intake of nitrates into human and animal body can cause a cascade of changes in their organism. The sources of nitrates can be either products of vegetable origin (especially beetroot) or groundwater and drinking water (regions with developed agriculture). The first system of organs, which experiences the negative effect of nitrates, is the digestive system.
The purpose of this work was to establish the effect of chronic nitrate intoxication on the content of lipid hydroperoxides in rat stomach mucous membrane.
Materials and methods. We carried out experiment on 24 white rats. We divided animals into 2 groups: the control group (10) and the group of chronic nitrate intoxication (14). We modeled chronic nitrate intoxication by intragastric administration of sodium nitrate at a dose of 500 mg / kg for 30 days. The content of lipid hydroperoxides in 10% of the mucous membrane homogenate was determined by the method of VB Gavrilov. and MI Mishkorudnaya (1983). We analyzed the obtained results for normality by the Shapiro-Wilk method. Further, we compared data by the Student method. The difference was considered statistically significant if p <0.05.
Results and discussion. Chronic nitrate intoxication reduces the content of dien conjugates (DC) in the gastric mucosa by 8% relative to the control group. The concentration of triene conjugates (TC) increases by 20.51%. The content of octadiene conjugates (ODC) increases by 52.2%. Decrease in the concentration of DC under conditions of nitrate intoxication, we can explain by the increase in activity of superoxide dismutase [1]. The remaining hydroperoxides are products of the later stages of lipid peroxidation (LPO), which is controlled by the glutathione system. Thus, it can be argued that the excessive intake of nitrates in the body leads to a disruption in the functioning of the glutathione system.
Conclusion. Excess intake of nitrates in the body prevents the activation of LPO on early stages, but enhances lipid oxidation that has already begun
GEM operation in helium and neon at low temperatures
We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne
and Ne+H2 at temperatures in the range of 2.6-293 K. In He, at temperatures
between 62 and 293 K, the triple-GEM structures often operate at rather high
gains, exceeding 1000. There is an indication that this high gain is achieved
by Penning effect in the gas impurities released by outgassing. At lower
temperatures the gain-voltage characteristics are significantly modified
probably due to the freeze-out of impurities. In particular, the double-GEM and
single-GEM structures can operate down to 2.6 K at gains reaching only several
tens at a gas density of about 0.5 g/l; at higher densities the maximum gain
drops further. In Ne, the maximum gain also drops at cryogenic temperatures.
The gain drop in Ne at low temperatures can be reestablished in Penning
mixtures of Ne+H2: very high gains, exceeding 10000, have been obtained in
these mixtures at 50-60 K, at a density of 9.2 g/l corresponding to that of
saturated Ne vapor near 27 K. The results obtained are relevant in the fields
of two-phase He and Ne detectors for solar neutrino detection and electron
avalanching at low temperatures.Comment: 13 pages, 14 figures. Accepted for publishing in Nucl. Instr. and
Meth.
Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field
The effect of an external electric field F on the excitonic photoluminescence
(PL) spectra of a symmetric coupled double quantum well (DQW) is investigated
both theoretically and experimentally. We show that the variational method in a
two-particle electron-hole wave function approximation gives a good agreement
with measurements of PL on a narrow DQW in a wide interval of F including
flat-band regime. The experimental data are presented for an MBE-grown DQW
consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide
pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers.
The bias voltage is applied along the growth direction. Spatially direct and
indirect excitonic transitions are identified, and the radius of the exciton
and squeezing of the exciton in the growth direction are evaluated
variationally. The excitonic binding energies, recombination energies,
oscillator strengths, and relative intensities of the transitions as functions
of the applied field are calculated. Our analysis demonstrates that this simple
model is applicable in case of narrow DQWs not just for a qualitative
description of the PL peak positions but also for the estimation of their
individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B
Single electron emission in two-phase xenon with application to the detection of coherent neutrino-nucleus scattering
We present an experimental study of single electron emission in ZEPLIN-III, a
two-phase xenon experiment built to search for dark matter WIMPs, and discuss
applications enabled by the excellent signal-to-noise ratio achieved in
detecting this signature. Firstly, we demonstrate a practical method for
precise measurement of the free electron lifetime in liquid xenon during normal
operation of these detectors. Then, using a realistic detector response model
and backgrounds, we assess the feasibility of deploying such an instrument for
measuring coherent neutrino-nucleus elastic scattering using the ionisation
channel in the few-electron regime. We conclude that it should be possible to
measure this elusive neutrino signature above an ionisation threshold of
3 electrons both at a stopped pion source and at a nuclear reactor.
Detectable signal rates are larger in the reactor case, but the triggered
measurement and harder recoil energy spectrum afforded by the accelerator
source enable lower overall background and fiducialisation of the active
volume
Improved detection of differentially represented DNA barcodes for high-throughput clonal phenomics
Cellular DNA barcoding has become a popular approach to study heterogeneity of cell populations and to identify clones with differential response to cellular stimuli. However, there is a lack of reliable methods for statistical inference of differentially responding clones. Here, we used mixtures of DNA-barcoded cell pools to generate a realistic benchmark read count dataset for modelling a range of outcomes of clone-tracing experiments. By accounting for the statistical properties intrinsic to the DNA barcode read count data, we implemented an improved algorithm that results in a significantly lower false-positive rate, compared to current RNA-seq data analysis algorithms, especially when detecting differentially responding clones in experiments with strong selection pressure. Building on the reliable statistical methodology, we illustrate how multidimensional phenotypic profiling enables one to deconvolute phenotypically distinct clonal subpopulations within a cancer cell line. The mixture control dataset and our analysis results provide a foundation for benchmarking and improving algorithms for clone-tracing experiments
Blue laser cooling transitions in Tm I
We have studied possible candidates for laser cooling transitions in
Tm in the spectral region 410 -- 420 nm. By means of saturation
absorption spectroscopy we have measured the hyperfine structure and rates of
two nearly closed cycling transitions from the ground state
to upper states
at
410.6 nm and
at
420.4 nm and evaluated the life times of the excited levels as 15.9(8) ns and
48(6) ns respectively. Decay rates from these levels to neighboring
opposite-parity levels are evaluated by means of Hartree-Fock calculations. We
conclude, that the strong transition at 410.6 nm has an optical leak rate of
less then and can be used for efficient laser cooling of
Tm from a thermal atomic beam. The hyperfine structure of two other
even-parity levels which can be excited from the ground state at 409.5 nm and
418.9 nm is also measured by the same technique. In addition we give a
calculated value of s for the rate of magnetic-dipole transition
at 1.14 m between the fine structure levels
of the ground state which can be
considered as a candidate for applications in atomic clocks.Comment: 8 pages, 5 figure
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