6,480 research outputs found
Patterson Function from Low-Energy Electron Diffraction Measured Intensities and Structural Discrimination
Surface Patterson Functions have been derived by direct inversion of
experimental Low-Energy Electron Diffraction I-V spectra measured at multiple
incident angles. The direct inversion is computationally simple and can be used
to discriminate between different structural models. 1x1 YSi_2 epitaxial layers
grown on Si(111) have been used to illustrate the analysis. We introduce a
suitable R-factor for the Patterson Function to make the structural
discrimination as objective as possible. From six competing models needed to
complete the geometrical search, four could easily be discarded, achieving a
very significant and useful reduction in the parameter space to be explored by
standard dynamical LEED methods. The amount and quality of data needed for this
analysis is discussed.Comment: 5 pages, 4 figure
Superconductivity and charge carrier localization in ultrathin bilayers
/ (LSCO15/LCO) bilayers
with a precisely controlled thickness of N unit cells (UCs) of the former and M
UCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao}
(SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and
reciprocal space map (RSM) studies confirmed the epitaxial growth of the
bilayers and showed that a [LSCO15\_2/LCO\_2] bilayer is fully strained,
whereas a [LSCO15\_2/LCO\_7] bilayer is already partially relaxed. The
\textit{in situ} monitoring of the growth with reflection high energy electron
diffraction (RHEED) revealed that the gas environment during deposition has a
surprisingly strong effect on the growth mode and thus on the amount of
disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15
containing one plane each). For samples grown in pure
gas (growth type-B), the first LSCO15 UC next to the SLAO
substrate is strongly disordered. This disorder is strongly reduced if the
growth is performed in a mixture of and gas
(growth type-A). Electric transport measurements confirmed that the first UC of
LSCO15 next to the SLAO substrate is highly resistive and shows no sign of
superconductivity for growth type-B, whereas it is superconducting for growth
type-A. Furthermore, we found, rather surprisingly, that the conductivity of
the LSCO15 UC next to the LCO capping layer strongly depends on the thickness
of the latter. A LCO capping layer with 7~UCs leads to a strong localization of
the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity.
The magneto-transport data suggest a similarity with the case of weakly hole
doped LSCO single crystals that are in a so-called {"{cluster-spin-glass
state}"
Routing in the Space Internet: A contact graph routing tutorial
A Space Internet is possible, as long as the delay and disruption challenges imposed by the space environment are properly tackled. Because these conditions are not well addressed by terrestrial Internet, more capable Delay-Tolerant Networking (DTN) protocols and algorithms are being developed. In particular, the principles and techniques for routing among ground elements and spacecraft in near-Earth orbit and deep-space are enacted in the Contact Graph Routing (CGR) framework. CGR blends a set of non-trivial algorithm adaptations, space operations concepts, time-dynamic scheduling, and specific graph models. The complexity of that framework suggests a need for a focused discussion to facilitate its direct and correct apprehension. To this end, we present an in-depth tutorial that collects and organizes first-hand experience on researching, developing, implementing, and standardizing CGR. Content is laid out in a structure that considers the planning, route search and management, and forwarding phases bridging ground and space domains. We rely on intuitive graphical examples, supporting code material, and references to flight-grade CGR implementations details where pertinent. We hope this tutorial will serve as a valuable resource for engineers and that researchers can also apply the insights presented here to topics in DTN research.Fil: Fraire, Juan Andres. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. Universitat Saarland; AlemaniaFil: De Jonckère, Olivier. Technische Universität Dresden; AlemaniaFil: Burleigh, Scott C.. California Institute of Technology; Estados Unido
Ab-initio molecular dynamics simulation of hydrogen diffusion in -iron
First-principles atomistic molecular dynamics simulation in the
micro-canonical and canonical ensembles has been used to study the diffusion of
interstitial hydrogen in -iron. Hydrogen to Iron ratios between
2 \times 2 \times 2$ supercell. We find that
the average optimum absorption site and the barrier for diffusion depend on the
concentration of interestitials. Iron Debye temperature decreases monotonically
for increasing concentration of interstitial hydrogen, proving that iron-iron
interatomic potential is significantly weakened in the presence of a large
number of diffusing hydrogen atoms
Hydrogen in -iron: stress and diffusion
First-principles density-functional theory has been used to investigate
equilibrium geometries, total energies, and diffusion barriers for H as an
interstitial impurity absorbed in -Fe. Internal strains/stresses upon
hydrogen absorption are a crucial factor to understand preferred absorption
sites and diffusion. For high concentrations, H absorbs near the octahedral
site favoring a large tetragonal distortion of the BCC lattice. For low
concentration, H absorbs near the tetrahedral site minimizing the elastic
energy stored on nearby cells. Diffusion paths depend on the concentration
regime too; hydrogen diffuses about ten times faster in the distorted BCT
lattice. External stresses of several GPa modify barriers by 10%, and
diffusion rates by 30%
The complex nature of the nuclear star cluster in FCC 277
Recent observations have shown that compact nuclear star clusters (NSCs) are
present in up to 80% of galaxies. However, detailed studies of their dynamical
and chemical properties are confined mainly to spiral galaxy hosts, where they
are more easily observed. In this paper we present our study of the NSC in FCC
277, a nucleated elliptical galaxy in the Fornax cluster. We use a combination
of adaptive optics assisted near-infrared integral field spectroscopy, Hubble
Space Telescope imaging, and literature long slit data. We show that while the
NSC does not appear to rotate within our detection limit of ~6 km/s, rotation
is detected at larger radii, where the isophotes appear to be disky, suggesting
the presence of a nuclear disk. We also observe a distinct central velocity
dispersion drop that is indicative of a dynamically cold rotating sub-system.
Following the results of orbit-based dynamical modelling, co-rotating as well
as counter-rotating stellar orbits are simultaneously needed to reproduce the
observed kinematics. We find evidence for varying stellar populations, with the
NSC and nuclear disk hosting younger and more metal rich stars than the main
body of the galaxy. We argue that gas dissipation and some level of merging
have likely played an important role in the formation of the nucleus of this
intermediate-mass galaxy. This is in contrast to NSCs in low-mass early- type
galaxies, which may have been formed primarily through the infall of star
clusters.Comment: 10 pages, 11 figures, MNRAS in press, changes to this version:
co-author adde
Polymer brush collapse under shear flow
Shear responsive surfaces offer potential advances in a number of
applications. Surface functionalisation using polymer brushes is one route to
such properties, particularly in the case of entangled polymers. We report on
neutron reflectometry measurements of polymer brushes in entangled polymer
solutions performed under controlled shear, as well as coarse-grained computer
simulations corresponding to these interfaces. Here we show a reversible and
reproducible collapse of the brushes, increasing with the shear rate. Using two
brushes of greatly different chain lengths and grafting densities, we
demonstrate that the dynamics responsible for the structural change of the
brush are governed by the free chains in solution rather than the brush itself,
within the range of parameters examined. The phenomenon of the brush collapse
could find applications in the tailoring of nanosensors, and as a way to
dynamically control surface friction and adhesion
Cavography with hydrogen peroxide. Accidental injection of hydrogen peroxide into the inferior vena cava
Contrast cavography during removal of a vena cava filter in pa- tients treated via thrombectomy for pulmonary embolism is a widely performed procedure, which may involve potentially serious com- plications if the human factor fails. Accidental injection of hydrogen peroxide (H2O2) after mistaking it for contrast material can have fatal consequences. We present the case of a patient who was acciden- tally injected with a high quantity of hydrogen peroxide. The prepa- ration of a preoperative checklist, the amount of H202 injected and the operator’s skill in aspirating the substance are decisive factors in avoiding such complications
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