1,322 research outputs found
Optical characteristics of single wavelength-tunable InAs/InGaAsP/InP(100) quantum dots emitting at 1.55 um
We have studied the emission properties of individual InAs quantum dots (QDs)
grown in an InGaAsP matrix on InP(100) by metal-organic vapor-phase epitaxy.
Low-temperature microphotoluminescence spectroscopy shows emission from single
QDs around 1550 nm with characteristic exciton-biexciton behavior, and a
biexciton antibinding energy of more than 2 meV. Temperature-dependent
measurements reveal negligible optical-phonon induced broadening of the exciton
line up to 50 K, and emission from the exciton state clearly persists above 70
K. Furthermore, we find no measurable polarized fine structure splitting of the
exciton state within the experimental precision. These results are encouraging
for the development of a controllable photon source for fiber-based quantum
information and cryptography systems.Comment: 3 pages, 4 figures, submitted AP
Charged exciton emission at 1.3 m from single InAs quantum dots grown by metalorganic chemical vapor deposition
We have studied the emission properties of self-organized InAs quantum dots
(QDs) grown in an InGaAs quantum well by metalorganic chemical vapor
deposition. Low-temperature photoluminescence spectroscopy shows emission from
single QDs around 1300 nm; we clearly observe the formation of neutral and
charged exciton and biexciton states, and we obtain a biexciton binding energy
of 3.1 meV. The dots exhibit an s-p shell splitting of approximately 100 meV,
indicating strong confinement.Comment: 3 pages, 3 figures, submitted AP
Fine structure and magneto-optics of exciton, trion, and charged biexciton states in single InAs quantum dots emitting at 1.3 um
We present a detailed investigation into the optical characteristics of
individual InAs quantum dots (QDs) grown by metalorganic chemical vapor
deposition, with low temperature emission in the telecoms window around 1300
nm. Using micro-photoluminescence (PL) spectroscopy we have identified neutral,
positively charged, and negatively charged exciton and biexciton states.
Temperature-dependent measurements reveal dot-charging effects due to
differences in carrier diffusivity. We observe a pronounced linearly polarized
splitting of the neutral exciton and biexciton lines (~250 ueV) resulting from
asymmetry in the QD structure. This asymmetry also causes a mixing of the
excited trion states which is manifested in the fine structure and polarization
of the charged biexciton emission; from this data we obtain values for the
ratio between the anisotropic and isotropic electron-hole exchange energies of
(Delta1)/(Delta0)= 0.2--0.5. Magneto-PL spectroscopy has been used to
investigate the diamagnetic response and Zeeman splitting of the various
exciton complexes. We find a significant variation in g-factor between the
exciton, the positive biexciton, and the negative biexciton; this is also
attributed to anisotropy effects and the difference in lateral extent of the
electron and hole wavefunctions.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Non-Breaking Undular Hydraulic Jump
The discusser performed a large number of experiments on undular hydraulic jumps (CHANSON 1993, 1995a). Most results were reported in CHANSON and MONTES (1995) and CHANSON (1995b, 1995c). The discusser wishes to stress several aspects of undular jump flows and he will show that the work of REINAUER and HAGER did not bring really new information
Oral pathobiont induces systemic inflammation and metabolic changes associated with alteration of gut microbiota.
Periodontitis has been implicated as a risk factor for metabolic disorders such as type 2 diabetes, atherosclerotic vascular diseases, and non-alcoholic fatty liver disease. Although bacteremias from dental plaque and/or elevated circulating inflammatory cytokines emanating from the inflamed gingiva are suspected mechanisms linking periodontitis and these diseases, direct evidence is lacking. We hypothesize that disturbances of the gut microbiota by swallowed bacteria induce a metabolic endotoxemia leading metabolic disorders. To investigate this hypothesis, changes in the gut microbiota, insulin and glucose intolerance, and levels of tissue inflammation were analysed in mice after oral administration of Porphyromonas gingivalis, a representative periodontopathogens. Pyrosequencing revealed that the population belonging to Bacteroidales was significantly elevated in P. gingivalis-administered mice which coincided with increases in insulin resistance and systemic inflammation. In P. gingivalis-administered mice blood endotoxin levels tended to be higher, whereas gene expression of tight junction proteins in the ileum was significantly decreased. These results provide a new paradigm for the interrelationship between periodontitis and systemic diseases
Viscous Instanton for Burgers' Turbulence
We consider the tails of probability density functions (PDF) for different
characteristics of velocity that satisfies Burgers equation driven by a
large-scale force. The saddle-point approximation is employed in the path
integral so that the calculation of the PDF tails boils down to finding the
special field-force configuration (instanton) that realizes the extremum of
probability. We calculate high moments of the velocity gradient
and find out that they correspond to the PDF with where is the
Reynolds number. That stretched exponential form is valid for negative
with the modulus much larger than its root-mean-square (rms)
value. The respective tail of PDF for negative velocity differences is
steeper than Gaussian, , as well as
single-point velocity PDF . For high
velocity derivatives , the general formula is found:
.Comment: 15 pages, RevTeX 3.
Spin relaxation due to the Bir-Aronov-Pikus mechanism in intrinsic and -type GaAs quantum wells from a fully microscopic approach
We study the electron spin relaxation in intrinsic and -type (001) GaAs
quantum wells by constructing and numerically solving the kinetic spin Bloch
equations. All the relevant scatterings are explicitly included, especially the
spin-flip electron-heavy hole exchange scattering which leads to the
Bir-Aronov-Pikus spin relaxation. We show that, due to the neglection of the
nonlinear terms in the electron-heavy hole exchange scattering in the
Fermi-golden-rule approach, the spin relaxation due to the Bir-Aronov-Pikus
mechanism is greatly exaggerated at moderately high electron density and low
temperature in the literature. We compare the spin relaxation time due to the
Bir-Aronov-Pikus mechanism with that due to the D'yakonov-Perel' mechanism
which is also calculated from the kinetic spin Bloch equations with all the
scatterings, especially the spin-conserving electron-electron and
electron-heavy hole scatterings, included. We find that, in intrinsic quantum
wells, the effect from the Bir-Aronov-Pikus mechanism is much smaller than that
from the D'yakonov-Perel' mechanism at low temperature, and it is smaller by no
more than one order of magnitude at high temperature. In -type quantum
wells, the spin relaxation due to the Bir-Aronov-Pikus mechanism is also much
smaller than the one due to the D'yakonov-Perel' mechanism at low temperature
and becomes comparable to each other at higher temperature when the hole
density and the width of the quantum well are large enough. We claim that
unlike in the bulk samples, the Bir-Aronov-Pikus mechanism hardly dominates the
spin relaxation in two-dimensional samples.Comment: 10 pages, 6 figures, Phys. Rev. B 77, 2008, in pres
Cell cycle arrest determines adult neural stem cell ontogeny by an embryonic Notch-nonoscillatory Hey1 module
Quiescent neural stem cells (NSCs) in the adult mouse brain are the source of neurogenesis that regulates innate and adaptive behaviors. Adult NSCs in the subventricular zone are derived from a subpopulation of embryonic neural stem-progenitor cells (NPCs) that is characterized by a slower cell cycle relative to the more abundant rapid cycling NPCs that build the brain. Yet, how slow cell cycle can cause the establishment of adult NSCs remains largely unknown. Here, we demonstrate that Notch and an effector Hey1 form a module that is upregulated by cell cycle arrest in slowly dividing NPCs. In contrast to the oscillatory expression of the Notch effectors Hes1 and Hes5 in fast cycling progenitors, Hey1 displays a non-oscillatory stationary expression pattern and contributes to the long-term maintenance of NSCs. These findings reveal a novel division of labor in Notch effectors where cell cycle rate biases effector selection and cell fate
Superdiffusivity of the 1D lattice Kardar-Parisi-Zhang equation
The continuum Kardar-Parisi-Zhang equation in one dimension is lattice
discretized in such a way that the drift part is divergence free. This allows
to determine explicitly the stationary measures. We map the lattice KPZ
equation to a bosonic field theory which has a cubic anti-hermitian
nonlinearity. Thereby it is established that the stationary two-point function
spreads superdiffusively.Comment: 21 page
Dynamical equations for high-order structure functions, and a comparison of a mean field theory with experiments in three-dimensional turbulence
Two recent publications [V. Yakhot, Phys. Rev. E {\bf 63}, 026307, (2001) and
R.J. Hill, J. Fluid Mech. {\bf 434}, 379, (2001)] derive, through two different
approaches that have the Navier-Stokes equations as the common starting point,
a set of steady-state dynamic equations for structure functions of arbitrary
order in hydrodynamic turbulence. These equations are not closed. Yakhot
proposed a "mean field theory" to close the equations for locally isotropic
turbulence, and obtained scaling exponents of structure functions and an
expression for the tails of the probability density function of transverse
velocity increments. At high Reynolds numbers, we present some relevant
experimental data on pressure and dissipation terms that are needed to provide
closure, as well as on aspects predicted by the theory. Comparison between the
theory and the data shows varying levels of agreement, and reveals gaps
inherent to the implementation of the theory.Comment: 16 pages, 23 figure
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