9,731 research outputs found
Accuracy of generalized gradient approximation functionals for density functional perturbation theory calculations
We assess the validity of various exchange-correlation functionals for
computing the structural, vibrational, dielectric, and thermodynamical
properties of materials in the framework of density-functional perturbation
theory (DFPT). We consider five generalized-gradient approximation (GGA)
functionals (PBE, PBEsol, WC, AM05, and HTBS) as well as the local density
approximation (LDA) functional. We investigate a wide variety of materials
including a semiconductor (silicon), a metal (copper), and various insulators
(SiO -quartz and stishovite, ZrSiO zircon, and MgO periclase).
For the structural properties, we find that PBEsol and WC are the closest to
the experiments and AM05 performs only slightly worse. All three functionals
actually improve over LDA and PBE in contrast with HTBS, which is shown to fail
dramatically for -quartz. For the vibrational and thermodynamical
properties, LDA performs surprisingly very good. In the majority of the test
cases, it outperforms PBE significantly and also the WC, PBEsol and AM05
functionals though by a smaller margin (and to the detriment of structural
parameters). On the other hand, HTBS performs also poorly for vibrational
quantities. For the dielectric properties, none of the functionals can be put
forward. They all (i) fail to reproduce the electronic dielectric constant due
to the well-known band gap problem and (ii) tend to overestimate the oscillator
strengths (and hence the static dielectric constant)
Quantifying the Drivers of Star Formation on Galactic Scales. I. The Small Magellanic Cloud
We use the star formation history of the Small Magellanic Cloud (SMC) to
place quantitative limits on the effect of tidal interactions and gas infall on
the star formation and chemical enrichment history of the SMC. The coincident
timing of two recent (< 4 Gyr) increases in the star formation rate and
SMC/Milky Way(MW) pericenter passages suggests that global star formation in
the SMC is driven at least in part by tidal forces due to the MW. The Large
Magellanic Cloud (LMC) is the other potential driver of star formation, but is
only near the SMC during the most recent burst. The poorly constrained LMC-SMC
orbit is our principal uncertainty. To explore the correspondence between
bursts and MW pericenter passages further, we model star formation in the SMC
using a combination of continuous and tidally-triggered star formation. The
behavior of the tidally-triggered mode is a strong inverse function of the
SMC-MW separation (preferred behavior ~ r^-5, resulting in a factor of ~100
difference in the rate of tidally-triggered star formation at pericenter and
apocenter). Despite the success of these closed-box evolutionary models in
reproducing the recent SMC star formation history and current chemical
abundance, they have some systematic shortcomings that are remedied by
postulating that a sizable infall event (~ 50% of the total gas mass) occured
about 4 Gyr ago. Regardless of whether this infall event is included, the
fraction of stars in the SMC that formed via a tidally triggered mode is > 10%
and could be as large as 70%.Comment: Accepted for publication in Ap
Gas Metallicity of Narrow-Line Regions in Narrow-Line Seyfert 1 Galaxies and Broad-Line Seyfert 1 Galaxies
We investigate gas metallicity of narrow-line regions in narrow-line Seyfert
1 galaxies (NLS1s) and broad-line ones (BLS1s) in order to examine whether or
not there is a difference in the gas metallicity between the two populations of
Seyfert 1 galaxies. We apply two methods to study this issue. One is to use the
emission-line flux ratio of [N II]6583/H_alpha in combination with some other
optical emission-line flux ratios. This method, which has been often applied to
Seyfert 2 galaxies, suggests that the gas metallicity of narrow-line regions is
indistinguishable or possibly higher in BLS1s than in NLS1s. On the contrary,
the other method in which only forbidden emission-line fluxes are used results
in that NLS1s tend to possess metal-richer gas in the narrow-line regions than
BLS1s. We point out that this inconsistency may be owing to the contamination
of the broad component of permitted lines into the narrow component of ones in
the first method. Since the results derived by using only forbidden
emission-line fluxes do not suffer from any uncertainty of the fitting function
for the broad component of Balmer lines, the results from this method are more
reliable than those derived by using permitted lines. We thus conclude that the
gas metallicity of narrow-line regions tends to be higher in NLS1s than in
BLS1s.Comment: 12 pages including 10 figures, to appear in The Astrophysical Journa
Dense Motion Estimation for Smoke
Motion estimation for highly dynamic phenomena such as smoke is an open
challenge for Computer Vision. Traditional dense motion estimation algorithms
have difficulties with non-rigid and large motions, both of which are
frequently observed in smoke motion. We propose an algorithm for dense motion
estimation of smoke. Our algorithm is robust, fast, and has better performance
over different types of smoke compared to other dense motion estimation
algorithms, including state of the art and neural network approaches. The key
to our contribution is to use skeletal flow, without explicit point matching,
to provide a sparse flow. This sparse flow is upgraded to a dense flow. In this
paper we describe our algorithm in greater detail, and provide experimental
evidence to support our claims.Comment: ACCV201
The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry
The recent observation of current-induced domain wall (DW) motion with large
velocity in ultrathin magnetic wires has opened new opportunities for
spintronic devices. However, there is still no consensus on the underlying
mechanisms of DW motion. Key to this debate is the DW structure, which can be
of Bloch or N\'eel type, and dramatically affects the efficiency of the
different proposed mechanisms. To date, most experiments aiming to address this
question have relied on deducing the DW structure and chirality from its motion
under additional in-plane applied fields, which is indirect and involves strong
assumptions on its dynamics. Here we introduce a general method enabling
direct, in situ, determination of the DW structure in ultrathin ferromagnets.
It relies on local measurements of the stray field distribution above the DW
using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in
diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire
and find clear signature of pure Bloch DWs. In contrast, we observe left-handed
N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the
presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co
interface. This method offers a new path for exploring interfacial DMI in
ultrathin ferromagnets and elucidating the physics of DW motion under current.Comment: Main text and Supplementary Information, 33 pages and 12 figure
Probabilistic analysis of the upwind scheme for transport
We provide a probabilistic analysis of the upwind scheme for
multi-dimensional transport equations. We associate a Markov chain with the
numerical scheme and then obtain a backward representation formula of
Kolmogorov type for the numerical solution. We then understand that the error
induced by the scheme is governed by the fluctuations of the Markov chain
around the characteristics of the flow. We show, in various situations, that
the fluctuations are of diffusive type. As a by-product, we prove that the
scheme is of order 1/2 for an initial datum in BV and of order 1/2-a, for all
a>0, for a Lipschitz continuous initial datum. Our analysis provides a new
interpretation of the numerical diffusion phenomenon
Pyrolysis-compound-specific hydrogen isotope analysis (δ2H Py-CSIA) of Mediterranean olive oils
The analysis of the hydrogen stable isotope composition (δ2H) of organic compounds provides information about its geographical origin. In this work, δ2H composition of specific compounds released by direct analytical pyrolysis (Py-CSIA) of extra virgin olive oils EVOOs was studied avoiding the use of any chemical and/or physical treatments, derivatization or previous separation steps. A collection of EVOOs from Mediterranean countries (Portugal, Spain, France, Tunisia and Turkey) was used for authentication of the olive oil samples. The δ2H value for 9 pyrolysis compounds present in all EVOOs, ranged between −112 and −267 mUr. These compounds were selected as possible surrogate descriptors linked to the olive oil geographic origin. Principal Component Analysis showed that δ2H was highly correlated with geographical longitude and annual temperature. Multiple Linear Regression analysis revealed that δ2H value of pyrolysis compounds can significantly (P < 0.05) predicts longitude, mean annual temperature and distance to the sea. The results suggest that the methodology used has a high potential to assess EVOOs geographic origin. This is the first report that evaluates δ2H directly from the pyrolysis products of olive oil using Py-CSIA. The approach used represents an innovative, fast, reproducible and reliable authentication technique
Muscle characteristics and maximal/explosive force production during isometric/dynamic actions of leg extensor muscles in untrained young men
Participaron en el estudio, de forma voluntaria cinco sujetos de sexo masculino, estudiantes de educación Física (22-26 años). El objetivo fue observar las relaciones entre el test de salto horizontal con los tests de salto vertical con contramovimiento y con flexión previa, tests de fuerza isométrica máxima y la distribución de los tipos de fibras musculares.
Los resultados en la distancia y altura de vuelo conseguidas en los tests de salto horizontal, salto desde flexión previa sin contramovimiento y salto con contramovimiento fueron respectivamente 2,53 m ± 0,17, 0,32 m ± 0,05 y 0,40 m ± 0,05. No se encontraron correlaciones significativas entre las fibras tipo I, tipo IIA y tipo IIB y los tests realizados. Tampoco se encontraron correlaciones significativas entre la máxima fuerza, la pendiente de fuerza isométrica y entre la distancia y alturas de salto. Por otro lado, tampoco se encontraron correlaciones entre la fuerza y la potencia en el salto horizontal (2,53 B W ± 0,17 y 63,4 W/kg ± 5,4, respectivamente) y la distancia lograda.
El test de fuerza isométrica máxima se realizó con dos diferentes ángulos de flexión de rodillas: 90" y 120". Tanto el pico de máxima fuerza isométrica como la máxima pendiente de desarrollo de la fuerza fueron superiores al realizar el test con una flexión de rodillas de 120" (2268 N ± 576 y 8962 N/s ± 1945 frente a 1538 N ± 300 y 7398 N/s ± 1557 con 90" de flexión). Por último, no se encontraron relaciones significativas entre la fuerza isométrica máxima y la fuerza explosiva en acciones dinámicas
The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution
Despite compelling evidence that stellar bars drive gas into the inner 1--2
kpc or circumnuclear (CN) region of galaxies, there are few large, high
resolution studies of the CN molecular gas and star formation (SF). We study a
sample of local barred non-starbursts and starbursts with high-resolution CO,
optical, Ha, RC, Br-gamma, and HST data, and find the following. (1) The inner
kpc of bars differs markedly the outer disk and hosts molecular gas surface
densities Sigma-gas-m of 500-3500 Msun pc-2, gas mass fractions of 10--30 %,
and epicyclic frequencies of several 100--1000 km s-1 kpc-1.Consequently,
gravitational instabilities can only set in at high gas densities and grow on a
short timescale (few Myr). This high density, short timescale, `burst' mode may
explain why powerful starbursts tend to be in the CN region of galaxies. (2) We
suggest that the variety in CO morphologies is due to different stages of
bar-driven inflow. At late stages, most of the CN gas is inside the outer inner
Lindblad resonance (OILR), and has predominantly circular motions. Across the
sample, we find bar pattern speeds with upper limits of 43 to 115 km s-1 kpc-1
and OILR radii of > 500 pc. (3) Barred starbursts and non-starbursts have CN
SFRs of 3--11 and 0.1--2 Msun yr-1, despite similar CN gas mass. Sigma-gas-m in
the starbursts is larger (1000--3500 Msun pc-2) and close to the Toomre
critical density over a large region. (4) Molecular gas makes up 10%--30% of
the CN dynamical mass (6--30 x 10^9 Msun).In the starbursts, it fuels CN SFRs
of 3--11 Msun yr-1, building young, massive, high V/sigma components. We
present evidence for such a pseudo-bulge in NGC 3351. Implications for secular
evolution along the Hubble sequence are discussed.Comment: Accepted by the Astrophysical Journal. Paper length reduced to fit
within APJ page limits. Version of paper with high resolution figures is at
http://www.as.utexas.edu/~sj/papers/ms-hires-sj05a.ps.g
Modeling the radial abundance distribution of the transition galaxy ngc 1313
NGC 1313 is the most massive disk galaxy showing a flat radial abundance
distribution in its interstellar gas, a behavior generally observed in
magellanic and irregular galaxies. We have attempted to reproduce this flat
abundance distribution using a multiphase chemical evolution model, which has
been previously used sucessfully to depict other spiral galaxies along the
Hubble morphological sequence. We found that it is not possible to reproduce
the flat radial abundance distribution in NGC 1313, and at the same time, be
consistent with observed radial distributions of other key parameters such the
surface gas density and star formation profiles. We conclude that a more
complicated galactic evolution model including radial flows, and possibly mass
loss due to supernova explosions and winds, is necessary to explain the
apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri
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