156 research outputs found
Towards a Linear-Scaling DFT Technique: The Density Matrix Approach
A recently proposed linear-scaling scheme for density-functional
pseudopotential calculations is described in detail. The method is based on a
formulation of density functional theory in which the ground state energy is
determined by minimization with respect to the density matrix, subject to the
condition that the eigenvalues of the latter lie in the range [0,1].
Linear-scaling behavior is achieved by requiring that the density matrix should
vanish when the separation of its arguments exceeds a chosen cutoff. The
limitation on the eigenvalue range is imposed by the method of Li, Nunes and
Vanderbilt. The scheme is implemented by calculating all terms in the energy on
a uniform real-space grid, and minimization is performed using the
conjugate-gradient method. Tests on a 512-atom Si system show that the total
energy converges rapidly as the range of the density matrix is increased. A
discussion of the relation between the present method and other linear-scaling
methods is given, and some problems that still require solution are indicated.Comment: REVTeX file, 27 pages with 4 uuencoded postscript figure
Seismic Reflection Imaging of the Low-Angle Panamint Normal Fault System, Eastern California
Shallowly dipping (<30°) low-angle normal faults (LANFs) have been documented globally; however, examples of active LANFs in continental settings are limited. The western margin of the Panamint Range in eastern California is defined by a LANF that dips west beneath Panamint Valley and has evidence of Quaternary motion. In addition, high-angle dextral-oblique normal faults displace middle to late Quaternary alluvial fans near the range front. To image shallow (<1 km depth), crosscutting relationships between the low- and high-angle faults along the range front, we acquired two high-resolution P wave seismic reflection profiles. The northern, 4.6-km-long profile crosses the 2-km-wide Wildrose graben and the southern, 0.8-km-long profile extends onto the Panamint Valley playa, ~7.5 km S of Ballarat, CA. The profile across the Wildrose graben reveals a robust, low-angle reflector interpreted to represent the LANF separating Plio-Pleistocene alluvial fanglomerate and Proterozoic metasedimentary deposits. High-angle faults interpreted in the seismic profile correspond to fault scarps on Quaternary alluvial fan surfaces. Interpretation of the reflection data suggests that the high-angle faults vertically displace the LANF up to 80 m within the Wildrose graben. Similarly, the profile south of Ballarat reveals a low-angle reflector, which appears both rotated and displaced up to 260 m by high-angle faults. These results suggest that near the Panamint range front, the high-angle faults are the dominant active structures. We conclude that at least at shallow (<1 km) depths, the LANF we imaged is not active today
Basis Functions for Linear-Scaling First-Principles Calculations
In the framework of a recently reported linear-scaling method for
density-functional-pseudopotential calculations, we investigate the use of
localized basis functions for such work. We propose a basis set in which each
local orbital is represented in terms of an array of `blip functions'' on the
points of a grid. We analyze the relation between blip-function basis sets and
the plane-wave basis used in standard pseudopotential methods, derive criteria
for the approximate equivalence of the two, and describe practical tests of
these criteria. Techniques are presented for using blip-function basis sets in
linear-scaling calculations, and numerical tests of these techniques are
reported for Si crystal using both local and non-local pseudopotentials. We
find rapid convergence of the total energy to the values given by standard
plane-wave calculations as the radius of the linear-scaling localized orbitals
is increased.Comment: revtex file, with two encapsulated postscript figures, uses epsf.sty,
submitted to Phys. Rev.
United classification of cosmic gamma-ray bursts and their counterparts
United classification of gamma-ray bursts and their counterparts is
established on the basis of measured characteristics: photon energy E and
emission duration T. The founded interrelation between the mentioned
characteristics of events consists in that, as the energy increases, the
duration decreases (and vice versa). The given interrelation reflects the
nature of the phenomenon and forms the E-T diagram, which represents a natural
classification of all observed events in the energy range from 10E9 to 10E-6 eV
and in the corresponding interval of durations from about 10E-2 up to 10E8 s.
The proposed classification results in the consequences, which are principal
for the theory and practical study of the phenomenon.Comment: Keywords Gamma rays: burst
Control de cambios / Rastrea los cambios / El camino cambia: Reflexiones sobre un mundo en transformación
“Track changes: Reflecting on a transforming world” was the theme chosen to invite panels, papers, posters and alternative presentations to be part of the 2019 international congress of SIEF that was held in Santiago de Compostela, Galicia (Spain). This introduction includes a description of the content of the congress, the rationale of the choice of plenaries and some reflections about the outcomes of the congress.El lema elegido para presentar paneles, ponencias, posters y presentaciones en formatos alternativos para el congreso internacional 2019 de SIEF -que tuvo lugar en Santiago de Compostela, Galicia (España)- fue “Track changes: Reflecting on a transforming world”. Esta introducción incluye una descripción del contenido del congreso, la idea para la elección de las plenarias y algunas reflexiones sobre los resultados del congreso.
 
On the sensitivity of the HAWC observatory to gamma-ray bursts
We present the sensitivity of HAWC to Gamma Ray Bursts (GRBs). HAWC is a very
high-energy gamma-ray observatory currently under construction in Mexico at an
altitude of 4100 m. It will observe atmospheric air showers via the water
Cherenkov method. HAWC will consist of 300 large water tanks instrumented with
4 photomultipliers each. HAWC has two data acquisition (DAQ) systems. The main
DAQ system reads out coincident signals in the tanks and reconstructs the
direction and energy of individual atmospheric showers. The scaler DAQ counts
the hits in each photomultiplier tube (PMT) in the detector and searches for a
statistical excess over the noise of all PMTs. We show that HAWC has a
realistic opportunity to observe the high-energy power law components of GRBs
that extend at least up to 30 GeV, as it has been observed by Fermi LAT. The
two DAQ systems have an energy threshold that is low enough to observe events
similar to GRB 090510 and GRB 090902b with the characteristics observed by
Fermi LAT. HAWC will provide information about the high-energy spectra of GRBs
which in turn could help to understanding about e-pair attenuation in GRB jets,
extragalactic background light absorption, as well as establishing the highest
energy to which GRBs accelerate particles
Orbital Observations of Dust Lofted by Daytime Convective Turbulence
Over the past several decades, orbital observations of lofted dust have revealed the importance of mineral aerosols as a climate forcing mechanism on both Earth and Mars. Increasingly detailed and diverse data sets have provided an ever-improving understanding of dust sources, transport pathways, and sinks on both planets, but the role of dust in modulating atmospheric processes is complex and not always well understood. We present a review of orbital observations of entrained dust on Earth and Mars, particularly that produced by the dust-laden structures produced by daytime convective turbulence called “dust devils”. On Earth, dust devils are thought to contribute only a small fraction of the atmospheric dust budget; accordingly, there are not yet any published accounts of their occurrence from orbit. In contrast, dust devils on Mars are thought to account for several tens of percent of the planet’s atmospheric dust budget; the literature regarding martian dust devils is quite rich. Because terrestrial dust devils may temporarily contribute significantly to local dust loading and lowered air quality, we suggest that martian dust devil studies may inform future studies of convectively-lofted dust on Earth
Tides in colliding galaxies
Long tails and streams of stars are the most noticeable upshots of galaxy
collisions. Their origin as gravitational, tidal, disturbances has however been
recognized only less than fifty years ago and more than ten years after their
first observations. This Review describes how the idea of galactic tides
emerged, in particular thanks to the advances in numerical simulations, from
the first ones that included tens of particles to the most sophisticated ones
with tens of millions of them and state-of-the-art hydrodynamical
prescriptions. Theoretical aspects pertaining to the formation of tidal tails
are then presented. The third part of the review turns to observations and
underlines the need for collecting deep multi-wavelength data to tackle the
variety of physical processes exhibited by collisional debris. Tidal tails are
not just stellar structures, but turn out to contain all the components usually
found in galactic disks, in particular atomic / molecular gas and dust. They
host star-forming complexes and are able to form star-clusters or even
second-generation dwarf galaxies. The final part of the review discusses what
tidal tails can tell us (or not) about the structure and content of present-day
galaxies, including their dark components, and explains how tidal tails may be
used to probe the past evolution of galaxies and their mass assembly history.
On-going deep wide-field surveys disclose many new low-surface brightness
structures in the nearby Universe, offering great opportunities for attempting
galactic archeology with tidal tails.Comment: 46 pages, 13 figures, Review to be published in "Tidal effects in
Astronomy and Astrophysics", Lecture Notes in Physics. Comments are most
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