396 research outputs found
Apollo guidance, navigation, and control: Candidate configuration trade study, Stellar-Inertial Measurement System (SIMS) for an Earth Observation Satellite (EOS)
The ten candidate SIMS configurations were reduced to three in preparation for the final trade comparison. The report emphasizes subsystem design trades, star availability studies, data processing (smoothing) methods, and the analytical and simulation studies at subsystem and system levels from which candidate accuracy estimates will be presented
Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory
Chalcogen vacancies are considered to be the most abundant point defects in
two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and
predicted to result in deep in-gap states (IGS). As a result, important
features in the optical response of 2D-TMDs have typically been attributed to
chalcogen vacancies, with indirect support from Transmission Electron
Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM
imaging measurements do not provide direct access to the electronic structure
of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a
direct probe of local electronic structure, the interpretation of the chemical
nature of atomically-resolved STM images of point defects in 2D-TMDs can be
ambiguous. As a result, the assignment of point defects as vacancies or
substitutional atoms of different kinds in 2D-TMDs, and their influence on
their electronic properties, has been inconsistent and lacks consensus. Here,
we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS,
and state-of-the-art ab initio density functional theory (DFT) and GW
calculations to determine both the structure and electronic properties of the
most abundant individual chalcogen-site defects common to 2D-TMDs.
Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our
results and analysis strongly suggest that the common chalcogen defects in our
2D-TMDs, prepared and measured in standard environments, are substitutional
oxygen rather than vacancies
Structural analysis of Pt(1 1 1)c(√3 × 5)rect.–CO using photoelectron diffraction
Core level shift scanned-energy mode photoelectron diffraction using the two distinct components of the C 1s emission has been used to determine the structure of the Pt(1 1 1)c(√3 × 5)rect.–CO phase formed by 0.6 ML of adsorbed CO. The results confirm earlier assignments of these components to CO in atop and bridging sites, further confirm that the best structural model involves a 2:1 occupation ratio of these two sites, and provides quantitative structural parameter values. In particular the Pt–C chemisorption bondlengths for the atop and bridging sites are, respectively, 1.86 ± 0.02 Å and 2.02 ± 0.04 Å. These values are closely similar to those found in the 0.5 ML coverage c(4 × 2) phase, involving an atop:bridge occupation ratio of 1:1, obtained in earlier quantitative low energy electron diffraction studies. The results also indicate a clear tilt of the molecular axis of atop CO species in this compression phase, consistent with the finding of an earlier electron-stimulated desorption ion angular distribution investigatio
Adsorption of CO on a Platinum (111) surface - a study within a four-component relativistic density functional approach
We report on results of a theoretical study of the adsorption process of a
single carbon oxide molecule on a Platinum (111) surface. A four-component
relativistic density functional method was applied to account for a proper
description of the strong relativistic effects. A limited number of atoms in
the framework of a cluster approach is used to describe the surface. Different
adsorption sites are investigated. We found that CO is preferably adsorbed at
the top position.Comment: 23 Pages with 4 figure
Fabrication of Nanostructured GaAs/AlGaAs Waveguide for Low-Density Polariton Condensation from a Bound State in the Continuum
Exciton-polaritons are hybrid light-matter states that arise from strong
coupling between an exciton resonance and a photonic cavity mode. As bosonic
excitations, they can undergo a phase transition to a condensed state that can
emit coherent light without a population inversion. This aspect makes them good
candidates for thresholdless lasers, yet short exciton-polariton lifetime has
made it difficult to achieve condensation at very low power densities. In this
sense, long-lived symmetry-protected states are excellent candidates to
overcome the limitations that arise from the finite mirror reflectivity of
monolithic microcavities. In this work we use a photonic symmetry protected
bound state in the continuum coupled to an excitonic resonance to achieve
state-of-the-art polariton condensation threshold in GaAs/AlGaAs waveguide.
Most important, we show the influence of fabrication control and how surface
passivation via atomic layer deposition provides a way to reduce exciton
quenching at the grating sidewalls
Cyclic Nucleotide-Gated Channels Contribute to Thromboxane A2-Induced Contraction of Rat Small Mesenteric Arteries
Background: Thromboxane A 2 (TxA 2)-induced smooth muscle contraction has been implicated in cardiovascular, renal and respiratory diseases. This contraction can be partly attributed to TxA2-induced Ca 2+ influx, which resulted in vascular contraction via Ca 2+-calmodulin-MLCK pathway. This study aims to identify the channels that mediate TxA2-induced Ca 2+ influx in vascular smooth muscle cells. Methodology/Principal Findings: Application of U-46619, a thromboxane A2 mimic, resulted in a constriction in endothelium-denuded small mesenteric artery segments. The constriction relies on the presence of extracellular Ca 2+, because removal of extracellular Ca 2+ abolished the constriction. This constriction was partially inhibited by an L-type Ca 2+ channel inhibitor nifedipine (0.5–1 mM). The remaining component was inhibited by L-cis-diltiazem, a selective inhibitor for CNG channels, in a dose-dependent manner. Another CNG channel blocker LY83583 [6-(phenylamino)-5,8-quinolinedione] had similar effect. In the primary cultured smooth muscle cells derived from rat aorta, application of U46619 (100 nM) induced a rise in cytosolic Ca 2+ ([Ca 2+]i), which was inhibited by L-cis-diltiazem. Immunoblot experiments confirmed the presence of CNGA2 protein in vascular smooth muscle cells. Conclusions/Significance: These data suggest a functional role of CNG channels in U-46619-induced Ca 2+ influx and contraction of smooth muscle cells
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