156,790 research outputs found
First-Principles Studies of Local Order in Relaxor Ferroelectrics
A key to optimizing the growth of the new single-crystal relaxor
ferroelectrics is resolving basic questions concerning their structural
properties and energetics. We report on initial first-principles total energy
and force calculations, examining the energetics of local order in PZN type
relaxors.Comment: 9 pages, LaTeX (aipproc), three eps figure
Type 2 diabetes, but not obesity, prevalence is positively associated with ambient temperature
Acknowledgements This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB13030000), the National Science foundation of China (microevolution program NSFC91431102) and the 1000 talents program. We are grateful to members of the Molecular Energetics group in Beijing and the Energetics group in Aberdeen, UK for valuable discussion of the data and analysis. All data analysed in this paper are already available in the public domain.Peer reviewedPublisher PD
Variations on a theme of AGN-driven outflows: luminosity evolution and ambient density distribution
Galactic outflows are now commonly observed in starburst and active galactic
nuclei (AGN) host galaxies. Yet, there is no clear consensus on their physical
driving mechanism(s). We have previously shown that AGN radiative feedback,
driven by radiation pressure on dust, can account for the observed dynamics and
energetics of galactic outflows, provided that radiation trapping is taken into
account. Here we generalise our model results by explicitly considering the
temporal evolution of the central AGN luminosity, and the shell mass evolution
in different ambient density distributions. In the case of fixed-mass shells,
the high observed values of the momentum ratio () and
energy ratio () may be attributed to either
radiation trapping or AGN luminosity decay. In contrast, for expanding shells
sweeping up mass from the surrounding environment, a decay in AGN luminosity
cannot account for the observed high energetics, and radiation trapping is
necessarily required. Indeed, strong radiation trapping, e.g. due to high
dust-to-gas ratios, can considerably boost the outflow energetics. We obtain a
distinct radial dependence for the outflow energetics (,
) in the case of radiation trapping and luminosity decay, which
may help discriminate between the two scenarios. In this framework, the
recently discovered `fossil' outflows, with anomalously high values of the
energetics, may be interpreted as relics of past AGN activity. The observed
outflow properties may therefore provide useful constraints on the past history
of AGN activity and/or the physical conditions of the outflow launch region.Comment: accepted for publication in MNRA
A magnetically driven origin for the low luminosity GRB 170817A associated with GW170817
The gamma-ray burst GRB 170817A associated with GW170817 is subluminous and
subenergetic compared with other typical short GRBs. It may be due to a
relativistic jet viewed off-axis, or a structured jet, or cocoon emission.
Giant flares from magnetars may possibly be ruled out. However, the luminosity
and energetics of GRB 170817A is coincident with that of magnetar giant flares.
After the coalescence of the binary neutron star, a hypermassive neutron star
may be formed. The hypermassive neutron star may have magnetar-strength
magnetic field. During the collapse of the hypermassive neutron star, the
magnetic field energy will also be released. This giant-flare-like event may
explain the the luminosity and energetics of GRB 170817A. Bursts with similar
luminosity and energetics are expected in future neutron star-neutron star or
neutron star-black hole mergers.Comment: 6 pages, 1 figure, accepted in Research in Astronomy and Astrophysic
Breakdown of the static picture of defect energetics in halide perovskites: the case of the Br vacancy in CsPbBr3
We consider the Br vacancy in CsPbBr3 as a prototype for the impact of
structural dynamics on defect energetics in halide perovskites (HaPs). Using
first-principles molecular dynamics based on density functional theory, we find
that the static picture of defect energetics breaks down; the energy of the Br
vacancy level is found to be intrinsically dynamic, oscillating by as much as 1
eV on the ps time scale at room temperature. These significant energy
fluctuations are correlated with the distance between the neighboring Pb atoms
across the vacancy and with the electrostatic potential at these Pb atomic
sites. We expect this unusually strong coupling of structural dynamics and
defect energetics to bear important implications for both experimental and
theoretical analysis of defect characteristics in HaPs. It may also hold
significant ramifications for carrier transport and defect tolerance in this
class of photovoltaic materials.Comment: 5 figures, 1 tabl
Energetics of the AK13 Semi-Local Kohn-Sham Exchange Energy Functional
The recent non-empirical semi-local exchange functional of Armiento and
K\"ummel, the AK13 [PRL 111, 036402 (2013)] incorporates a number of features
reproduced by higher-order theory. The AK13 potential behaves analogously with
the discontinuous jump associated with the derivative discontinuity at integer
particle numbers. Recent works have established that AK13 gives a qualitatively
improved orbital description compared to other semi-local methods, and
reproduces a band structure closer to higher-order theory. However, its
energies and energetics are inaccurate. The present work further investigates
the deficiency in energetics. In addition to AK13 results, we find that
applying the local-density approximation (LDA) non-self-consistently on the
converged AK13 density gives very reasonable energetics with equilibrium
lattice constants and bulk moduli well described across 14 systems. We also
confirm that the attractive orbital features of AK13 are retained even after
full structural relaxation. Hence, the deficient energetics cannot be a result
of the AK13 orbitals having adversely affected the quality of the electron
density compared to that of usual semi-local functionals; an improved orbital
description and good energetics are not in opposition. We also prove that the
non-self-consistent scheme is equivalent to using a single external-potential
dependent functional in an otherwise consistent KS-DFT scheme. Furthermore, our
results also demonstrate that, while an internally consistent KS functional is
presently missing, non-self-consistent LDA on AK13 orbitals works as a
practical non-empirical computational scheme to predict geometries, bulk
moduli, while retaining the band structure features of AK13 at the
computational cost of semi-local DFT.Comment: 7 pages, 4 figure
Numerical studies of baroclinic instability at small Richardson number
Baroclinic instability at small Richardson number consists of competition between symmetric (Solberg) modes, zonal (Eady) modes, and possibly other modes whose orientation is neither symmetric nor zonal. In FY-84, a study of the fully nonlinear development and energetics of the symmetric modes was completed. The nonlinear effects were quite strong and, in some senses, unexpected. Secondly, a study of the dependency of the energetics of the symmetric modes upon the physical parameters was completed
Flare energetics
In this investigation of flare energetics, researchers sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energics of these flares. The role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model is discussed. Finally, a critique of the current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested
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