3,659 research outputs found
Exciton dynamics in WSe2 bilayers
We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved
photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for
both the direct exciton with ~ 3 ps and the indirect optical
transition with ~ 25 ps. For temperatures between 4 to 150 K
remains constant. Following polarized laser excitation, we observe
for the direct exciton transition at the K point of the Brillouin zone
efficient optical orientation and alignment during the short emission time
. The evolution of the direct exciton polarization and intensity as a
function of excitation laser energy is monitored in PL excitation (PLE)
experiments.Comment: 4 pages, 3 figure
The inner circumstellar disk of the UX Ori star V1026 Sco
The UX Ori type variables (named after the prototype of their class) are
intermediate-mass pre-main sequence objects. One of the most likely causes of
their variability is the obscuration of the central star by orbiting dust
clouds. We investigate the structure of the circumstellar environment of the
UX~Ori star V1026 Sco (HD 142666) and test whether the disk inclination is
large enough to explain the UX Ori variability. We observed the object in the
low-resolution mode of the near-infrared interferometric VLTI/AMBER instrument
and derived H- and K-band visibilities and closure phases. We modeled our AMBER
observations, published Keck Interferometer observations, archival MIDI/VLTI
visibilities, and the spectral energy distribution using geometric and
temperature-gradient models. Employing a geometric inclined-ring disk model, we
find a ring radius of 0.15 +- 0.06 AU in the H band and 0.18 +- 0.06 AU in the
K band. The best-fit temperature-gradient model consists of a star and two
concentric, ring-shaped disks. The inner disk has a temperature of
1257^{+133}_{-53} K at the inner rim and extends from 0.19 +- 0.01 AU to 0.23
+- 0.02 AU. The outer disk begins at 1.35^{+0.19}_{-0.20} AU and has an inner
temperature of 334^{+35}_{-17} K. The derived inclination of
48.6^{+2.9}_{-3.6}deg approximately agrees with the inclination derived with
the geometric model (49 +- 5deg in the K band and 50 +- 11deg in the H band).
The position angle of the fitted geometric and temperature-gradient models are
163 +- 9deg (K band; 179 +- 17deg in the H band) and 169.3^{+4.2}_{-6.7}deg,
respectively. The narrow width of the inner ring-shaped model disk and the disk
gap might be an indication for a puffed-up inner rim shadowing outer parts of
the disk. The intermediate inclination of ~50deg is consistent with models of
UX Ori objects where dust clouds in the inclined disk obscure the central star
Lunar laser ranging in infrfared at hte Grasse laser station
For many years, lunar laser ranging (LLR) observations using a green
wavelength have suffered an inhomogeneity problem both temporally and
spatially. This paper reports on the implementation of a new infrared detection
at the Grasse LLR station and describes how infrared telemetry improves this
situation. Our first results show that infrared detection permits us to densify
the observations and allows measurements during the new and the full Moon
periods. The link budget improvement leads to homogeneous telemetric
measurements on each lunar retro-reflector. Finally, a surprising result is
obtained on the Lunokhod 2 array which attains the same efficiency as Lunokhod
1 with an infrared laser link, although those two targets exhibit a
differential efficiency of six with a green laser link
Exciton states in monolayer MoSe2: impact on interband transitions
We combine linear and non-linear optical spectroscopy at 4K with ab initio
calculations to study the electronic bandstructure of MoSe2 monolayers. In
1-photon photoluminescence excitation (PLE) and reflectivity we measure a
separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we
detect for the A- and B-exciton the 2p state 180meV above the respective 1s
state. In second harmonic generation (SHG) spectroscopy we record an
enhancement by more than 2 orders of magnitude of the SHG signal at resonances
of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our
post-Density Functional Theory calculations show in the conduction band along
the direction a local minimum that is energetically and in k-space
close to the global minimum at the K-point. This has a potentially strong
impact on the polarization and energy of the excitonic states that govern the
interband transitions and marks an important difference to MoS2 and WSe2
monolayers.Comment: 8 pages, 3 figure
Abolishing user fees for children and pregnant women trebled uptake of malaria-related interventions in Kangaba, Mali.
Malaria is the most common cause of morbidity and mortality in children under 5 in Mali. Health centres provide primary care, including malaria treatment, under a system of cost recovery. In 2005, Médecins sans Frontieres (MSF) started supporting health centres in Kangaba with the provision of rapid malaria diagnostic tests and artemisinin-based combination therapy. Initially MSF subsidized malaria tests and drugs to reduce the overall cost for patients. In a second phase, MSF abolished fees for all children under 5 irrespective of their illness and for pregnant women with fever. This second phase was associated with a trebling of both primary health care utilization and malaria treatment coverage for these groups. MSF's experience in Mali suggests that removing user fees for vulnerable groups significantly improves utilization and coverage of essential health services, including for malaria interventions. This effect is far more marked than simply subsidizing or providing malaria drugs and diagnostic tests free of charge. Following the free care strategy, utilization of services increased significantly and under-5 mortality was reduced. Fee removal also allowed for more efficient use of existing resources, reducing average cost per patient treated. These results are particularly relevant for the context of Mali and other countries with ambitious malaria treatment coverage objectives, in accordance with the United Nations Millennium Development Goals. This article questions the effectiveness of the current national policy, and the effectiveness of reducing the cost of drugs only (i.e. partial subsidies) or providing malaria tests and drugs free for under-5s, without abolishing other related fees. National and international budgets, in particular those that target health systems strengthening, could be used to complement existing subsidies and be directed towards effective abolition of user fees. This would contribute to increasing the impact of interventions on population health and, in turn, the effectiveness of aid
Nitrogen depletion in field red giants: mixing during the He flash?
We combine simultaneous constraints on stellar evolutionary status from asteroseismology, and on nitrogen abundances derived from large spectroscopic surveys, to follow nitrogen surface abundances all along the evolution of a low-mass star, comparing model expectations with data. After testing and calibrating the observed yields from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, we first show that nitrogen surface abundances follow the expected trend after the first dredge-up occurred, i.e. that the more massive is the star, the more nitrogen is enhanced. Moreover, the behaviour of nitrogen data along the evolution confirms the existence of non-canonical extramixing on the red giant branch (RGB) for all low-mass stars in the field. But more surprisingly, the data indicate that nitrogen has been depleted between the RGB tip and the red clump. This may suggest that some nitrogen has been burnt near or at the He flash episode.This work was partly supported by the European Union FP7 programme through ERC grant number 320360. NL acknowledges financial support from the Marie Curie Intra-European fellowship (FP7-PEOPLE-2012-IEF) and the CNES postdoctoral fellowship 2016. AM and YE acknowledge the support of the UK Science and Technology Facilities Council (STFC). Funding for the Stellar Astrophysics Centre (SAC) is provided by The Danish National Research Foundation (Grant agreement no. DNRF106)
AMBER/VLTI high spectral resolution observations of the Br emitting region in HD 98922. A compact disc wind launched from the inner disc region
We analyse the main physical parameters and the circumstellar environment of
the young Herbig Be star HD 98922. We present AMBER/VLTI high spectral
resolution (R =12000) interferometric observations across the Br line,
accompanied by UVES high-resolution spectroscopy and SINFONI-AO assisted
near-infrared integral field spectroscopic data. To interpret our observations,
we develop a magneto-centrifugally driven disc-wind model. Our analysis of the
UVES spectrum shows that HD 98922 is a young (~5x10^5 yr) Herbig Be star
(SpT=B9V), located at a distance of 440(+60-50) pc, with a mass accretion rate
of ~9+/-3x10^(-7) M_sun yr^(-1). SINFONI K-band AO-assisted imaging shows a
spatially resolved circumstellar disc-like region (~140 AU in diameter) with
asymmetric brightness distribution. Our AMBER/VLTI UT observations indicate
that the Br emitting region (radius ~0.31+/-0.04 AU) is smaller than
the continuum emitting region (inner dust radius ~0.7+/-0.2 AU), showing
significant non-zero V-shaped differential phases (i.e. non S-shaped, as
expected for a rotating disc). The value of the continuum-corrected pure
Br line visibility at the longest baseline (89 m) is ~0.8+/-0.1, i.e.
the Br emitting region is partially resolved. Our modelling suggests
that the observed Br line-emitting region mainly originates from a disc
wind with a half opening angle of 30deg, and with a mass-loss rate of ~2x10(-7)
M_sun yr^(-1). The observed V-shaped differential phases are reliably
reproduced by combining a simple asymmetric continuum disc model with our
Br disc-wind model. The Br emission of HD 98922 can be modelled
with a disc wind that is able to approximately reproduce all interferometric
observations if we assume that the intensity distribution of the dust continuum
disc is asymmetric.Comment: Accepted for publication on Astronomy \& Astrophysics. High
resolution figures published on the main journal (see Astronomy &
Astrophysics: Forthcoming) or at
www.researchgate.net/profile/Alessio_Caratti_o_Garatti/publication
Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots
In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum
dots in longitudinal magnetic fields applied along the growth axis we observe
in addition to the expected bright states also nominally dark transitions for
both charged and neutral excitons. We uncover a strongly non-monotonous, sign
changing field dependence of the bright neutral exciton splitting resulting
from the interplay between exchange and Zeeman effects. Our theory shows
quantitatively that these surprising experimental results are due to
magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of
systems with C_3v point-group symmetry.Comment: 5 pages, 3 figure
12C/13C isotopic ratios in red-giant stars of the open cluster NGC 6791
Carbon isotope ratios, along with carbon and nitrogen abundances, are derived
in a sample of 11 red-giant members of one of the most metal-rich clusters in
the Milky Way, NGC 6791. The selected red-giants have a mean metallicity and
standard deviation of [Fe/H]=+0.39+-0.06 (Cunha et al. 2015). We used high
resolution H-band spectra obtained by the SDSS-IV Apache Point Observatory
Galactic Evolution Experiment (APOGEE). The advantage of using high-resolution
spectra in the H-band is that lines of CO are well represented and their line
profiles are sensitive to the variation of 12C/13C. Values of the 12C/13C ratio
were obtained from a spectrum synthesis analysis. The derived 12C/13C ratios
varied between 6.3 and 10.6 in NGC 6791, in agreement with the final isotopic
ratios from thermohaline-induced mixing models. The ratios derived here are
combined with those obtained for more metal poor red-giants from the literature
to examine the correlation between 12C/13C, mass, metallicity and evolutionary
status.Comment: Accepted for publication in MNRAS, 9 pages, 4 figures, 2 table
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