124,357 research outputs found
Band Symmetries and Singularities in Twisted Multilayer Graphene
The electronic spectra of rotationally faulted graphene bilayers are
calculated using a continuum formulation for small fault angles that identifies
two distinct electronic states of the coupled system. The low energy spectra of
one state features a Fermi velocity reduction which ultimately leads to
pairwise annihilation and regeneration of its low energy Dirac nodes. The
physics in the complementary state is controlled by pseudospin selection rules
that prevent a Fermi velocity renormalization and produce second generation
symmetry-protected Dirac singularities in the spectrum. These results are
compared with previous theoretical analyses and with experimental data.Comment: 5 pages, 3 figure
Functional PCA for Remotely Sensed Lake Surface Water Temperature Data
Functional principal component analysis is used to investigate a high-dimensional surface water temperature data set of Lake Victoria, which has been produced in the ARC-Lake project. Two different perspectives are adopted in the analysis: modelling temperature curves (univariate functions) and temperature surfaces (bivariate functions). The latter proves to be a better approach in the sense of both dimension reduction and pattern detection. Computational details and some results from an application to Lake Victoria data are presented
Analytical investigation of rotor wake formation and geometry
A number of refinements in the computer code were worked out and tested. Three codes have been written to date. One program is for an isolated wing and is being used to compare with data for the vortex wake (Weston). The second code is for an isolated wing with a streamwise vortex passing above it. This program is being used to validate the computational procedure for incorporating the vortex into the Euler equation calculations. The third program is the hovering rotor code which is the overall objective of the research. The optimization calculations for a hovering helicopter rotor have been completed
Legibility of electroluminescent instrument panels investigated
Legibility studies of several EL /electroluminescent/ displays correlate reading time and accuracy with number size, stroke/width ratio, indicia size, pointer width, contrast, ambient illumination, and color background and and contrast. Human factor criteria established on non-EL displays may not apply to EL displays
Simulating Astro-H Observations of Sloshing Gas Motions in the Cores of Galaxy Clusters
Astro-H will be the first X-ray observatory to employ a high-resolution
microcalorimeter, capable of measuring the shift and width of individual
spectral lines to the precision necessary for estimating the velocity of the
diffuse plasma in galaxy clusters. This new capability is expected to bring
significant progress in understanding the dynamics, and therefore the physics,
of the intracluster medium. However, because this plasma is optically thin,
projection effects will be an important complicating factor in interpreting
future Astro-H measurements. To study these effects in detail, we performed an
analysis of the velocity field from simulations of a galaxy cluster
experiencing gas sloshing, and generated synthetic X-ray spectra, convolved
with model Astro-H Soft X-ray Spectrometer (SXS) responses. We find that the
sloshing motions produce velocity signatures that will be observable by Astro-H
in nearby clusters: the shifting of the line centroid produced by the
fast-moving cold gas underneath the front surface, and line broadening produced
by the smooth variation of this motion along the line of sight. The line shapes
arising from inviscid or strongly viscous simulations are very similar,
indicating that placing constraints on the gas viscosity from these
measurements will be difficult. Our spectroscopic analysis demonstrates that,
for adequate exposures, Astro-H will be able to recover the first two moments
of the velocity distribution of these motions accurately, and in some cases
multiple velocity components may be discerned. The simulations also confirm the
importance of accurate treatment of PSF scattering in the interpretation of
Astro-H/SXS spectra of cluster plasmas.Comment: 27 pages, 20 figures, submitted to the Astrophysical Journa
Application of remote sensing data to surveys of the Alaskan environment
Coupling of satellite data to resource management problems in Alaska is implemented through feasibility studies of applicability of Landsat data to specific environmental surveys in ecology, agriculture, hydrology, wildlife management, oceanography, geology, etc.; and using the results of these studies to extend the benefits of satellite data applications to the operational needs of mission-oriented agencies of federal, state, and regional governments, as well as private industry. Activities designed to encourage the participation of users in the Landsat program at levels most appropriate to the users' interests are described and include: observation, coordination, and information exchange; training courses and workshops; data exchange; consulting services; data processing services; user participation in University research projects; and university participation in the operational projects of user agencies. Progress in these areas is reported. The effectiveness of this broad-based approach in overcoming the initial apprehensiveness of users is demonstrated
An X-ray-UV correlation in Cen X-4 during quiescence
Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is
seen to be variable on timescales from hundreds of seconds to years, suggesting
that at least in this object, low-level accretion is important during
quiescence. Here we present results from recent XMM-Newton and Swift
observations of Cen X-4, where the X-ray flux (0.5 - 10 keV) varies by a factor
of 6.5 between the brightest and faintest states. We find a positive
correlation between the X-ray flux and the simultaneous near-UV flux, where as
there is no significant correlation between the X-ray and simultaneous optical
(V, B) fluxes. This suggests that while the X-ray and UV emitting regions are
somehow linked, the optical region originates elsewhere. Comparing the
luminosities, it is plausible that the UV emission originates due to
reprocessing of the X-ray flux by the accretion disk, with the hot inner region
of the disk being a possible location for the UV emitting region. The optical
emission, however, could be dominated by the donor star. The X-ray/UV
correlation does not favour the accretion stream-impact point as the source of
the UV emission.Comment: 8 pages, 3 figures, accepted for publication in MNRA
Application of remote sensing data to surveys of the Alaskan environment
There are no author-identified significant results in this report
Finite-Temperature Quasicontinuum: Molecular Dynamics without All the Atoms
Using a combination of statistical mechanics and finite-element interpolation, we develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasicontinuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nanoindentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter
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