2,626 research outputs found
Drift-diffusion model of the fragmentation of the external ring structure in the photoluminescence pattern of indirect excitons in coupled quantum wells
Under optical excitation, coupled quantum wells are known to reveal
fascinating features in the photoluminescence pattern originating from dipole
orientated indirect excitons. The appearance of an external ring has been
attributed to macroscopic charge separation in the quantum well plane. We
present a classical model of non-linear diffusion to account for the observed
fragmentation of the external ring into a periodic array of islands. The model
incorporates the Coulomb interactions between electrons, holes and indirect
excitons. At low temperatures, these interactions lead to pattern formation
similar to the experimentally observed ring fragmentation. The fragmentation is
found to persist to temperatures above the quantum degeneracy temperature of
indirect excitons.Comment: 5 pages, 3 figure
Kinetics of the inner ring in the exciton emission pattern in GaAs coupled quantum wells
We report on the kinetics of the inner ring in the exciton emission pattern.
The formation time of the inner ring following the onset of the laser
excitation is found to be about 30 ns. The inner ring was also found to
disappear within 4 ns after the laser termination. The latter process is
accompanied by a jump in the photoluminescence (PL) intensity. The spatial
dependence of the PL-jump indicates that the excitons outside of the region of
laser excitation, including the inner ring region, are efficiently cooled to
the lattice temperature even during the laser excitation. The ring formation
and disappearance are explained in terms of exciton transport and cooling.Comment: 19 pages, 6 figure
Excitation Energy Dependence of the Exciton Inner Ring
We report on the excitation energy dependence of the inner ring in the
exciton emission pattern. The contrast of the inner ring is found to decrease
with lowering excitation energy. Excitation by light tuned to the direct
exciton resonance is found to effectively suppress excitation-induced heating
of indirect excitons and facilitate the realization of a cold and dense exciton
gas. The excitation energy dependence of the inner ring is explained in terms
of exciton transport and cooling.Comment: 5 pages, 4 figure
Transport of Indirect Excitons in a Potential Energy Gradient
We realized a potential energy gradient - a ramp - for indirect excitons
using a shaped electrode at constant voltage. We studied transport of indirect
excitons along the ramp and observed that the exciton transport distance
increases with increasing density and temperature.Comment: 4 pages, 3 figure
X-ray absorption and re-emission from an ionised outflow in the Type 1 QSO 2MASS 234449+1221
We report on the analysis of a short XMM-Newton observation of the reddened
Type 1 QSO 2MASS 234449+1221 first identified in the Two Micron All-Sky Survey.
The underlying X-ray continuum is found to be typical of a broad-line active
galaxy, with photon index Gamma ~ 1.9. Low energy absorption can be modelled by
a column N_H ~ 10^22 cm^{-2} of moderately ionised gas or a smaller column of
cold gas. Addition of a soft X-ray emission component significantly improves
the fit in both cases. With the assumption that the soft X-ray flux represents
emission from gas photoionised by the incident X-ray continuum, a comparison of
the absorbed and emitted luminosities indicates a covering factor of ~ 8-17%.
The unusual opportunity to simultaneously observe and quantify ionised
absorption and emission in 2MASS 234449+1221 is due to the relatively large
opacity (for a Type 1 AGN) of the absorbing gas, which depresses the normally
strong continuum below ~ 1 keV. A comparison of the soft X-ray emission of
2MASS 234449+1221 with that of other Type 1 and Type 2 AGN suggests the
existence of an inner turbulent extension to ionised outflows, not detected in
current high resolution X-ray spectra.Comment: 5 pages, 6 figures, accepted for publication in MNRA
Electrostatic Conveyer for Excitons
We report on the study of indirect excitons in moving lattices - conveyers
created by a set of AC voltages applied to the electrodes on the sample
surface. The wavelength of this moving lattice is set by the electrode
periodicity, the amplitude is controlled by the applied voltage, and the
velocity is controlled by the AC frequency. We observed the dynamical
localization-delocalization transition for excitons in the conveyers and
measured its dependence on the exciton density and conveyer amplitude and
velocity. We considered a model for exciton transport via conveyers. The
theoretical simulations are in agreement with the experimental data.Comment: 4 pages, 4 figures + supplemental material including two video
Optical properties monitor: Experiment definition phase
The stability of materials used in the space environment will continue to be a limiting technology for space missions. The Optical Properties Monitor (OPM) Experiment provides a comprehensive space research program to study the effects of the space environment-both natural and induced-on optical, thermal and space power materials. The OPM Experiment was selected for definition under the NASA/OAST In-Space Technology Experiment Program. The results of the OPM Definition Phase are presented. The OPM Experiment will expose selected materials to the space environment and measure the effects with in-space optical measurements. In-space measurements include total hemispherical reflectance total integrated scatter and VUV reflectance/transmittance. The in-space measurements will be augmented with extensive pre- and post-flight sample measurements to determine other optical, mechanical, electrical, chemical or surface effects of space exposure. Environmental monitors will provide the amount and time history of the sample exposure to solar irradiation, atomic oxygen and molecular contamination
Black Hole Mass Estimates of Radio Selected Quasars
The black hole (BH) mass in the centre of AGN has been estimated for a sample
of radio-selected flat-spectrum quasars to investigate the relationship between
BH mass and radio properties of quasars. We have used the virial assumption
with measurements of the H FWHM and luminosity to estimate the central
BH mass. In contrast to previous studies we find no correlation between BH mass
and radio power in these AGN. We find a range in BH mass similar to that seen
in radio-quiet quasars from previous studies. We believe the reason that the
low BH mass radio-loud quasars have not been measured in previous studies is
due to optical selection effects which tend to miss the less optically luminous
radio-loud sources.Comment: 22 pages, 7 figures, accepted for publication in Ap
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