1,399 research outputs found
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
Legal determinants of external finance revisited : the inverse relationship between investor protection and societal well-being
This paper investigates relationships between corporate governance traditions and quality of life as measured by a number of widely reported indicators. It provides an empirical analysis of indicators of societal health in developed economies using a classification based on legal traditions. Arguably the most widely cited work in the corporate governance literature has been the collection of papers by La Porta et al. which has shown, inter alia, statistically significant relationships between legal traditions and various proxies for investor protection. We show statistically significant relationships between legal traditions and various proxies for societal health. Our comparative evidence suggests that the interests of investors may not be congruent with the interests of wider society, and that the criteria for judging the effectiveness of approaches to corporate governance should not be restricted to financial metrics
Dynamical phenomena in Fibonacci Semiconductor Superlattices
We present a detailed study of the dynamics of electronic wavepackets in
Fibonacci semiconductor superlattices, both in flat band conditions and subject
to homogeneous electric fields perpendicular to the layers. Coherent
propagation of electrons is described by means of a scalar Hamiltonian using
the effective-mass approximation. We have found that an initial Gaussian
wavepacket is filtered selectively when passing through the superlattice. This
means that only those components of the wavepacket whose wavenumber belong to
allowed subminibands of the fractal-like energy spectrum can propagate over the
entire superlattice. The Fourier pattern of the transmitted part of the
wavepacket presents clear evidences of fractality reproducing those of the
underlying energy spectrum. This phenomenon persists even in the presence of
unintentional disorder due to growth imperfections. Finally, we have
demonstrated that periodic coherent-field induced oscillations (Bloch
oscillations), which we are able to observe in our simulations of periodic
superlattices, are replaced in Fibonacci superlattices by more complex
oscillations displaying quasiperiodic signatures, thus sheding more light onto
the very peculiar nature of the electronic states in these systems.Comment: 7 pagex, RevTex, 5 Postscript figures. Physical Review B (in press
Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice
The similarity between matter waves in periodic potential and solid-state
physics processes has triggered the interest in quantum simulation using
Bose-Fermi ultracold gases in optical lattices. The present work evidences the
similarity between electrons moving under the application of oscillating
electromagnetic fields and matter waves experiencing an optical lattice
modulated by a frequency difference, equivalent to a spatially shaken periodic
potential. We demonstrate that the tunneling properties of a Bose-Einstein
condensate in shaken periodic potentials can be precisely controlled. We take
additional crucial steps towards future applications of this method by proving
that the strong shaking of the optical lattice preserves the coherence of the
matter wavefunction and that the shaking parameters can be changed
adiabatically, even in the presence of interactions. We induce reversibly the
quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press
Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field
We study a resistively shunted semiconductor superlattice subject to a
high-frequency electric field. Using a balance equation approach that
incorporates the influence of the electric circuit, we determine numerically a
range of amplitude and frequency of the ac field for which a dc bias and
current are generated spontaneously and show that this region is likely
accessible to current experiments. Our simulations reveal that the Bloch
frequency corresponding to the spontaneous dc bias is approximately an integer
multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure
Magnetotransport Properties of Quasi-Free Standing Epitaxial Graphene Bilayer on SiC: Evidence for Bernal Stacking
We investigate the magnetotransport properties of quasi-free standing
epitaxial graphene bilayer on SiC, grown by atmospheric pressure graphitization
in Ar, followed by H intercalation. At the charge neutrality point the
longitudinal resistance shows an insulating behavior, which follows a
temperature dependence consistent with variable range hopping transport in a
gapped state. In a perpendicular magnetic field, we observe quantum Hall states
(QHSs) both at filling factors () multiple of four (), as
well as broken valley symmetry QHSs at and . These results
unambiguously show that the quasi-free standing graphene bilayer grown on the
Si-face of SiC exhibits Bernal stacking.Comment: 12 pages, 5 figure
A direct physical interaction between Nanog and Sox2 regulates embryonic stem cell self-renewal
Embryonic stem (ES) cell self-renewal efficiency is determined by the Nanog protein level. However, the protein partners of Nanog that function to direct self-renewal are unclear. Here, we identify a Nanog interactome of over 130 proteins including transcription factors, chromatin modifying complexes, phosphorylation and ubiquitination enzymes, basal transcriptional machinery members, and RNA processing factors. Sox2 was identified as a robust interacting partner of Nanog. The purified NanogâSox2 complex identified a DNA recognition sequence present in multiple overlapping Nanog/Sox2 ChIP-Seq data sets. The Nanog tryptophan repeat region is necessary and sufficient for interaction with Sox2, with tryptophan residues required. In Sox2, tyrosine to alanine mutations within a triple-repeat motif (S X T/S Y) abrogates the NanogâSox2 interaction, alters expression of genes associated with the Nanog-Sox2 cognate sequence, and reduces the ability of Sox2 to rescue ES cell differentiation induced by endogenous Sox2 deletion. Substitution of the tyrosines with phenylalanine rescues both the Sox2âNanog interaction and efficient self-renewal. These results suggest that aromatic stacking of Nanog tryptophans and Sox2 tyrosines mediates an interaction central to ES cell self-renewal
Addressing inequities in maternal health among women living in communities of social disadvantage and ethnic diversity.
The response to the coronavirus outbreak and how the disease and its societal consequences pose risks to already vulnerable groups such those who are socioeconomically disadvantaged and ethnic minority groups. Researchers and community groups analysed how the COVID-19 crisis has exacerbated persisting vulnerabilities, socio-economic and structural disadvantage and discrimination faced by many communities of social disadvantage and ethnic diversity, and discussed future strategies on how best to engage and involve local groups in research to improve outcomes for childbearing women experiencing mental illness and those living in areas of social disadvantage and ethnic diversity. Discussions centred around: access, engagement and quality of care; racism, discrimination and trust; the need for engagement with community stakeholders; and the impact of wider social and economic inequalities. Addressing biomedical factors alone is not sufficient, and integrative and holistic long-term public health strategies that address societal and structural racism and overall disadvantage in society are urgently needed to improve health disparities and can only be implemented in partnership with local communities
Reverse Transcriptase-Coupled Quantitative Real Time PCR Analysis of Cell-Free Transcription on the Chromatin-Assembled p21 Promoter
Background: Cell-free eukaryotic transcription assays have contributed tremendously to the current understanding of the molecular mechanisms that govern transcription at eukaryotic promoters. Currently, the conventional G-less cassette transcription assay is one of the simplest and fastest methods for measuring transcription in vitro. This method requires several components, including the radioisotope labelling of RNA product during the transcription reaction followed by visualization of transcripts using autoradiography. Methodology/Principal Findings: To further simplify and expedite the conventional G-less cassette transcription assay, we have developed a method to incorporate a reverse transcriptase-coupled quantitative real time PCR (RT-qPCR). By using DNA template depletion steps that include DNA template immobilization, Trizol extraction and DNase I treatment, we have successfully enriched p21 promoter-driven transcripts over DNA templates. The quantification results of RNA transcripts using the RT-qPCR assay were comparable to the results of the conventional G-less cassette transcription assay both in naked DNA and chromatin-assembled templates. Conclusions: We first report a proof-of-concept demonstration that incorporating RT-qPCR in cell-free transcription assays can be a simpler and faster alternative method to the conventional radioisotope-mediated transcription assays. This method will be useful for developing high throughput in vitro transcription assays and provide quantitative data for RNA transcript
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