220 research outputs found
Towards bose-einstein condensation of semiconductor excitons: The biexciton polarization effect
We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells. © 2009 The American Physical Society
Temperature-dependent electron Landé g factor and the interband matrix element of GaAs
Very high precision measurements of the electron Landé g factor in GaAs are presented using spin-quantum beat spectroscopy at low excitation densities and temperatures ranging from 2.6 to 300 K. In colligation with available data for the temperature-dependent effective mass temperature dependence of the interband matrix element within a common five-level k⋅p theory can model both parameters consistently. A strong decrease in the interband matrix element with increasing temperature consistently closes a long lasting gap between experiment and theory and substantially improves the modeling of both parameters. © 2009 The American Physical Society
Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases
We report a comprehensive experimental study and detailed model analysis of
the terahertz dielectric response and density kinetics of excitons and unbound
electron-hole pairs in GaAs quantum wells. A compact expression is given, in
absolute units, for the complex-valued terahertz dielectric function of
intra-excitonic transitions between the 1s and higher-energy exciton and
continuum levels. It closely describes the terahertz spectra of resonantly
generated excitons. Exciton ionization and formation are further explored,
where the terahertz response exhibits both intra-excitonic and Drude features.
Utilizing a two-component dielectric function, we derive the underlying exciton
and unbound pair densities. In the ionized state, excellent agreement is found
with the Saha thermodynamic equilibrium, which provides experimental
verification of the two-component analysis and density scaling. During exciton
formation, in turn, the pair kinetics is quantitatively described by a Saha
equilibrium that follows the carrier cooling dynamics. The terahertz-derived
kinetics is, moreover, consistent with time-resolved luminescence measured for
comparison. Our study establishes a basis for tracking pair densities via
transient terahertz spectroscopy of photoexcited quasi-two-dimensional
electron-hole gases.Comment: 14 pages, 8 figures, final versio
Electron spin orientation under in-plane optical excitation in GaAs quantum wells
We study the optical orientation of electron spins in GaAs/AlGaAs quantum
wells for excitation in the growth direction and for in-plane excitation. Time-
and polarization-resolved photoluminescence excitation measurements show, for
resonant excitation of the heavy-hole conduction band transition, a negligible
degree of electron spin polarization for in-plane excitation and nearly 100%
for excitation in the growth direction. For resonant excitation of the
light-hole conduction band transition, the excited electron spin polarization
has the same (opposite) direction for in-plane excitation (in the growth
direction) as for excitation into the continuum. The experimental results are
well explained by an accurate multiband theory of excitonic absorption taking
fully into account electron-hole Coulomb correlations and heavy-hole light-hole
coupling.Comment: 10 pages, 4 figures, final versio
Traces of stimulated bosonic exciton-scattering in semiconductor luminescence
We observe signatures of stimulated bosonic scattering of excitons, a
precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of
semiconductor quantum wells. The optical decay of a spinless molecule of two
excitons (biexciton) into an exciton and a photon with opposite angular momenta
is subject to bosonic enhancement in the presence of other excitons. In a spin
polarized gas of excitons the bosonic enhancement breaks the symmetry of two
equivalent decay channels leading to circularly polarized luminescence of the
biexciton with the sign opposite to the excitonic luminescence. Comparison of
experiment and many body theory proves stimulated scattering of excitons, but
excludes the presence of a fully condensed BEC-like state.Comment: 5 page
Zebrafish Stem/Progenitor Factor msi2b Exhibits Two Phases of Activity Mediated by Different Splice Variants
The Musashi (Msi) family of RNA-binding proteins is important in stem and differentiating cells in many species. Here, we present a zebrafish gene/protein trap line gt(msi2b-citrine)(ct) (57) (a) that expresses a Citrine fusion protein with endogenous Msi2b. Our results reveal two phases of Msi2b expression: ubiquitous expression in progenitor cells in the early embryo and later, tissue-specific expression in differentiating cells in the olfactory organ, pineal gland, and subpopulations of neurons in the central nervous system (CNS). Interestingly, this division between early and late phases is paralleled by differential expression of msi2b alternative splicing products. Whereas the full-length and long variant v3 Msi2b predominate at early stages, the later expression of variants in differentiating tissues appears to be tissue specific. Using the gt(msi2b-citrine)(ct) (57) (a), we characterized tissue-specific expression of Msi2b with cellular resolution in subsets of differentiating cells in the olfactory organ, pineal gland, CNS, and ventral neural tube. By performing transcription activator-like effectors nuclease-mediated biallelic genome editing or morpholino knockdown of Msi2b in zebrafish, our results show that early inactivation of Msi2b results in severe embryonic defects including hypertrophy of the ventricles and shortening of the body, consistent with an important role in cell proliferation and survival. Moreover, specific inactivation of Msi2b full-length indicates that this species is essential for the early role of Msi2b. This line provides a valuable tool both for live imaging of the endogenous Msi2b at subcellular resolution and manipulation of Msi2b-expressing cells
Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells
Pump-probe experiments are used as a controllable way to investigate the
properties of photoexcited semiconductors, in particular, the absorption
saturation. We present an experiment-theory comparison for ZnSe quantum wells,
investigating the energy renormalization and bleaching of the excitonic
resonances. Experiments were performed with spin-selective excitation and
above-bandgap pumping. The model, based on the semiconductor Bloch equations in
the screened Hartree-Fock approximation, takes various scattering processes
into account phenomenologically. Comparing numerical results with available
experimental data, we explain the experimental results and find that the
electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics,
modeling of femtosecond pump-probe experiments, electron spin-flip tim
Laminin β1a controls distinct steps during the establishment of digestive organ laterality
Visceral organs, including the liver and pancreas, adopt asymmetric positions to ensure proper function. Yet the molecular and cellular mechanisms controlling organ laterality are not well understood. We identified a mutation affecting zebrafish laminin β1a (lamb1a) that disrupts left-right asymmetry of the liver and pancreas. In these mutants, the liver spans the midline and the ventral pancreatic bud remains split into bilateral structures. We show that lamb1a regulates asymmetric left-right gene expression in the lateral plate mesoderm (LPM). In particular, lamb1a functions in Kupffer’s vesicle (KV), a ciliated organ analogous to the mouse node, to control the length and function of the KV cilia. Later during gut-looping stages, dynamic expression of Lamb1a is required for the bilayered organization and asymmetric migration of the LPM. Loss of Lamb1a function also results in aberrant protrusion of LPM cells into the gut. Collectively, our results provide cellular and molecular mechanisms by which extracellular matrix proteins regulate left-right organ morphogenesis
The electron temperatures of SDSS high-metallicity giant extragalactic HII regions
Spectra of high-metallicity (12+log(O/H) > 8.2) HII regions where oxygen
auroral lines are measurable in both the O+ and O++ zones, have been extracted
from the Data Release 6 of the Sloan Digital Sky Survey (SDSS). Our final
sample consists of 181 SDSS spectra of HII regions in galaxies in the redshift
range from ~0.025 to ~0.17. The t_2,O-t_3,O diagram is examined. In the SDSS
HII regions, the electron temperature t_2,O is found to have a large scatter at
a given value of the electron temperature t_3,O. The majority of the SDSS HII
regions lie below the t_2,O-t_3,O relation derived for HII regions in nearby
galaxies, i.e. the positions of the SDSS HII regions show a systematic shift
towards lower t_2,O temperatures or/and towards higher t_3,O temperatures. The
scatter and shift of the SDSS HII regions in the t_2,O-t_3,O diagram can be
understood if they are composite nebulae excited by two or more ionizing
sources of different temperatures.Comment: 14 pages, 11 figures accepted for publication in the MNRA
Enantioseparation of Dansylated Amino Acids by Ligand-exchange Capillary Electrophoresis Using L-phenylalaninamide, L-lysine or L-threonine as Chiral Selector
In recent years enantioseparation of both active pharmaceutical ingredients and bio molecules
such as amino acids became more and more necessary because in most cases the two stereo forms exhibit
different pharmacological effects. This article deals with the chiral separation of dansylated amino acids
by ligand-exchange capillary electrophoresis using L-phenylalaninamide, L-lysine and L-threonine as chiral
selectors. Experiments with different central metal ions such as Cu(II), Co(II), Cd(II), Ni(II) and Zn(II)
were carried out. Optimal conditions were found out by studying the effect of the pH and the selector molarity
on the chiral resolution. Best separation was obtained for the Cu(II)/L-lysine complex, showing a
chiral resolution up to 17 for Dns-DL-Met. (doi: 10.5562/cca1762
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