12,434 research outputs found
Modified embedded-atom method interatomic potentials for the Mg-Al alloy system
We developed new modified embedded-atom method (MEAM) interatomic potentials
for the Mg-Al alloy system using a first-principles method based on density
functional theory (DFT). The materials parameters, such as the cohesive energy,
equilibrium atomic volume, and bulk modulus, were used to determine the MEAM
parameters. Face-centered cubic, hexagonal close packed, and cubic rock salt
structures were used as the reference structures for Al, Mg, and MgAl,
respectively. The applicability of the new MEAM potentials to atomistic
simulations for investigating Mg-Al alloys was demonstrated by performing
simulations on Mg and Al atoms in a variety of geometries. The new MEAM
potentials were used to calculate the adsorption energies of Al and Mg atoms on
Al (111) and Mg (0001) surfaces. The formation energies and geometries of
various point defects, such as vacancies, interstitial defects and
substitutional defects, were also calculated. We found that the new MEAM
potentials give a better overall agreement with DFT calculations and
experiments when compared against the previously published MEAM potentials.Comment: Fixed a referenc
Nanotube Piezoelectricity
We combine ab initio, tight-binding methods and analytical theory to study
piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity
of a heteropolar nanotube depends on its chirality and diameter and can be
understood starting from the piezoelectric response of an isolated planar
sheet, along with a structure specific mapping from the sheet onto the tube
surface. We demonstrate that coupling between the uniaxial and shear
deformation are only allowed in the nanotubes with lower chiral symmetry. Our
study shows that piezoelectricity of nanotubes is fundamentally different from
its counterpart in three dimensional (3D) bulk materials.Comment: 4 pages, with 3 postscript figures embedded. Uses REVTEX4 macros.
Also available at
http://www.physics.upenn.edu/~nsai/preprints/bn_piezo/index.htm
Impoundment of the Zipingpu reservoir and triggering of the 2008 Mw 7.9 Wenchuan earthquake, China
Impoundment of the Zipingpu reservoir (ZR), China, began in September 2005 and was followed 2.7 years later by the 2008 Mw 7.9 Wenchuan earthquake (WE) rupturing the Longmen Shan Fault (LSF), with its epicenter ~12 km away from the ZR. Based on the poroelastic theory, we employ three-dimensional finite element models to simulate the evolution of stress and pore pressure due to reservoir impoundment, and its effect on the Coulomb failure stress on the LSF. The results indicate that the reservoir impoundment formed a pore pressure front that slowly propagated through the crust with fluid diffusion. The reservoir loading induced either moderate or no increase of the Coulomb failure stress at the hypocenter prior to the WE. The Coulomb failure stress, however, grew ~9.3-69.1 kPa in the depth range of 1-8 km on the LSF, which may have advanced tectonic loading of the fault system by ~60-450 years. Due to uncertainties of fault geometry and hypocenter location of the WE, it is inconclusive whether impoundment of the ZR directly triggered the WE. However, a small event at the hypocenter could have triggered large rupture elsewhere on fault, where the asperities were weakened by the ZR. The microseismicity around the ZR also showed an expanding pattern from the ZR since its impoundment, likely associated with diffusion of a positive pore pressure pulse. These results suggest a poroelastic triggering effect (even if indirectly) of the WE due to the impoundment of the ZRThis project was supported in part by grants of NSFC 41090294 and 41374103, CEA LED2008A05, LED2013A04, and LED2009A02, NSF EAR 0911762 and EAR
0911466, and NASA ESI 1468758.
Academic licensing for the Abaqus software was provided by Dassault Systemes,
Simulia Inc. Chen Jiuhui of Institute of Geology, CEA provided the aftershock
data of the Wenchuan earthquake. Ma Wentao of Institute of Geology, CEA provided the microseismicity data in the vicinity of the Zipingpu reservoir. This work
was also partially supported by an NSF I/RD grant (EAR-1323052) for the corresponding author as a rotator program director at NSFPeer reviewe
A Population of Faint Non-Transient Low Mass Black Hole Binaries
We study the thermal and viscous stability of accretion flows in Low Mass
Black Hole Binaries (LMBHBs). We consider a model in which an inner
advection-dominated accretion flow (ADAF) is surrounded by a geometrically thin
accretion disk, the transition between the two zones occurring at a radius
R_tr. In all the known LMBHBs, R_tr appears to be such that the outer disks
could suffer from a global thermal-viscous instability. This instability is
likely to cause the transient behavior of these systems. However, in most
cases, if R_tr were slightly larger than the estimated values, the systems
would be globally stable. This suggests that a population of faint persistent
LMBHBs with globally stable outer disks could be present in the Galaxy. Such
LMBHBs would be hard to detect because they would lack large amplitude
outbursts, and because their ADAF zones would have very low radiative
efficiencies, making the systems very dim. We present model spectra of such
systems covering the optical and X-ray bands.Comment: LateX, 37 pages, 11 figures; Accepted for publication in The
Astrophysical Journa
A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721
We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines
in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging
Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721.
Fitting the STIS data with magnetic white dwarf model spectra, we derive a
field strength of B~144MG and an effective temperature of 17000K<Teff<23000K.
This measurement makes RX J1554.2+2721 only the third cataclysmic variable
containing a white dwarf with a field exceeding 100MG. Similar to the other
high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble
mass transfer, which suggests that a considerable number of high-field polars
may still remain undiscovered.Comment: 4 pages, accepted for ApJ Letter
Interplay of Spin-Orbit Interactions, Dimensionality, and Octahedral Rotations in Semimetallic SrIrO
We employ reactive molecular-beam epitaxy to synthesize the metastable
perovskite SrIrO and utilize {\it in situ} angle-resolved photoemission
to reveal its electronic structure as an exotic narrow-band semimetal. We
discover remarkably narrow bands which originate from a confluence of strong
spin-orbit interactions, dimensionality, and both in- and out-of-plane IrO
octahedral rotations. The partial occupation of numerous bands with strongly
mixed orbital characters signals the breakdown of the single-band Mott picture
that characterizes its insulating two-dimensional counterpart,
SrIrO, illustrating the power of structure-property relations for
manipulating the subtle balance between spin-orbit interactions and
electron-electron interactions
Pressure induced high-spin to low-spin transition in FeS evidenced by x-ray emission spectroscopy
We report the observation of the pressure-induced high-spin to low-spin
transition in FeS using new high-pressure synchrotron x-ray emission
spectroscopy techniques. The transition is evidenced by the disappearance of
the low-energy satellite in the Fe K emission spectrum of FeS. Moreover,
the phase transition is reversible and closely related to the structural phase
transition from a manganese phosphide-like phase to a monoclinic phase. The
study opens new opportunities for investigating the electronic properties of
materials under pressure.Comment: ReVTeX, 4 pages, 3 figures inserted with epsfig. minor modifications
before submission to PR
Theoretical and numerical studies of chemisorption on a line with precursor layer diffusion
We consider a model for random deposition of monomers on a line with
extrinsic precursor states. As the adsorbate coverage increases, the system
develops non-trivial correlations due to the diffusion mediated deposition
mechanism. In a numeric simulation, we study various quantities describing the
evolution of the island structure. We propose a simple, self-consistent theory
which incorporates pair correlations. The results for the correlations, island
density number, average island size and probabilities of island nucleation,
growth and coagulation show good agreement with the simulation data.Comment: 17 pages(LaTeX), 11 figures(1 PS file, uuencoded), submmited to Phys.
Rev.
Trimaximal neutrino mixing from vacuum alignment in A4 and S4 models
Recent T2K results indicate a sizeable reactor angle theta_13 which would
rule out exact tri-bimaximal lepton mixing. We study the vacuum alignment of
the Altarelli-Feruglio A4 family symmetry model including additional flavons in
the 1' and 1" representations and show that it leads to trimaximal mixing in
which the second column of the lepton mixing matrix consists of the column
vector (1,1,1)^T/sqrt{3}, with a potentially large reactor angle. In order to
limit the reactor angle and control the higher order corrections, we propose a
renormalisable S4 model in which the 1' and 1" flavons of A4 are unified into a
doublet of S4 which is spontaneously broken to A4 by a flavon which enters the
neutrino sector at higher order. We study the vacuum alignment in the S4 model
and show that it predicts accurate trimaximal mixing with approximate
tri-bimaximal mixing, leading to a new mixing sum rule testable in future
neutrino experiments. Both A4 and S4 models preserve form dominance and hence
predict zero leptogenesis, up to renormalisation group corrections.Comment: 24 pages, 2 figures, version to be published in JHE
A dairy-based protein, calcium and vitamin D supplement reduces falls and femoral neck bone loss in aged care residents : a cluster randomised trial
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