3,299 research outputs found
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
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
The speaking vocabulary of 454 third grade children.
Thesis (Ed.M.)--Boston Universit
Spin-dependent electron dynamics and recombination in GaAs(1-x)N(x) alloys at room temperature
We report on both experimental and theoretical study of conduction-electron
spin polarization dynamics achieved by pulsed optical pumping at room
temperature in GaAs(1-x)N(x) alloys with a small nitrogen content (x = 2.1,
2.7, 3.4%). It is found that the photoluminescence circular polarization
determined by the mean spin of free electrons reaches 40-45% and this giant
value persists within 2 ns. Simultaneously, the total free-electron spin decays
rapidly with the characteristic time ~150 ps. The results are explained by
spin-dependent capture of free conduction electrons on deep paramagnetic
centers resulting in dynamical polarization of bound electrons. We have
developed a nonlinear theory of spin dynamics in the coupled system of
spin-polarized free and localized carriers which describes the experimental
dependencies, in particular, electron spin quantum beats observed in a
transverse magnetic field.Comment: 5 pages, 4 figures, Submitted to JETP Letter
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints
Context. The availability of asteroseismic constraints for a large sample of
red giant stars from the CoRoT and Kepler missions paves the way for various
statistical studies of the seismic properties of stellar populations.
Aims. We use the first detailed spectroscopic study of 19 CoRoT red-giant
stars (Morel et al 2014) to compare theoretical stellar evolution models to
observations of the open cluster NGC 6633 and field stars.
Methods. In order to explore the effects of rotation-induced mixing and
thermohaline instability, we compare surface abundances of carbon isotopic
ratio and lithium with stellar evolution predictions. These chemicals are
sensitive to extra-mixing on the red-giant branch.
Results. We estimate mass, radius, and distance for each star using the
seismic constraints. We note that the Hipparcos and seismic distances are
different. However, the uncertainties are such that this may not be
significant. Although the seismic distances for the cluster members are self
consistent they are somewhat larger than the Hipparcos distance. This is an
issue that should be considered elsewhere. Models including thermohaline
instability and rotation-induced mixing, together with the seismically
determined masses can explain the chemical properties of red-giants targets.
However, with this sample of stars we cannot perform stringent tests of the
current stellar models. Tighter constraints on the physics of the models would
require the measurement of the core and surface rotation rates, and of the
period spacing of gravity-dominated mixed modes. A larger number of stars with
longer times series, as provided by Kepler or expected with Plato, would help
for ensemble asteroseismology.Comment: Accepted 03/05/201
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
Which aspects of health care are most valued by people living with HIV in high-income countries? A systematic review.
BACKGROUND: Increasing numbers of people with HIV are living into older age and experiencing comorbidities. The development of new models of care to meet the needs of this population is now a priority. It is important that the views and preferences of patients inform the development of services in order to maintain high levels of patient satisfaction and engagement. The aim of this systematic review was to determine which aspects of healthcare are particularly valued by people living with HIV. METHODS: We searched electronic databases and reference lists of relevant articles. The search strategy was developed to identify articles reporting on HIV positive patients' perceptions, evaluations or experiences of healthcare services and factors associated with satisfaction with care. Peer-reviewed papers and conference abstracts were included if the study reported on aspects of health care that were valued by people living with HIV, data were collected during the era of combination therapy (from 1996 onwards), and the paper was published in English. A thematic approach to data synthesis was used. RESULTS: Twenty-three studies met the inclusion criteria. Studies used both qualitative and quantitative methods. Six studies specifically reported on relative importance to patients of different aspects of care. The valued aspects of care identified were grouped into seven themes. These highlighted the importance to patients of: a good health care professional-patient relationship, HIV specialist knowledge, continuity of care, ease of access to services, access to high quality information and support, effective co-ordination between HIV specialists and other healthcare professionals, and involvement in decisions about treatment and care. We were unable to determine the relative importance to patients of different aspects of care because of methodological differences between the studies. CONCLUSIONS: This review identified several attributes of healthcare that are valued by people living with HIV, many of which would be relevant to any future reconfiguration of services to meet the needs of an ageing population. Further research is required to determine the relative importance to patients of different aspects of care
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