4,274 research outputs found
Comparison of Global and Local Adaptive Coordinates for Density Functional Calculations
A globally-adaptive curvilinear coordinate formalism is shown to be easily
convertible to a class of curvilinear transformations locally optimized around
atom sites by a few parameters. Parameter transferability is established for a
demanding test case, and the results of the two methods are shown to be
comparable. Computational efficiencies realized in the local method are
discussed.Comment: 21 pages, 4 figure
fMRI Evidence for Modality-Specific Processing of Conceptual Knowledge on Six Modalities
Traditional theories assume that amodal representations, such as feature lists and semantic
networks, represent conceptual knowledge about the world. According to this view, the
sensory, motor, and introspective states that arise during perception and action are irrelevant
to representing knowledge. Instead the conceptual system lies outside modality-specific
systems and operates according to different principles. Increasingly, however, researchers
report that modality-specific systems become active during purely conceptual tasks,
suggesting that these systems play central roles in representing knowledge (for a review, see
Martin, 2001, Handbook of Functional Neuroimaging of Cognition). In particular,
researchers report that the visual system becomes active while processing visual properties,
and that the motor system becomes active while processing action properties. The present
study corroborates and extends these findings. During fMRI, subjects verified whether or not
properties could potentially be true of concepts (e.g., BLENDER-loud). Subjects received
only linguistic stimuli, and nothing was said about using imagery. Highly related false
properties were used on false trials to block word association strategies (e.g., BUFFALOwinged).
To assess the full extent of the modality-specific hypothesis, properties were
verified on each of six modalities. Examples include GEMSTONE-glittering (vision),
BLENDER-loud (audition), FAUCET-turned (motor), MARBLE-cool (touch),
CUCUMBER-bland (taste), and SOAP-perfumed (smell). Neural activity during property
verification was compared to a lexical decision baseline. For all six sets of the modalityspecific
properties, significant activation was observed in the respective neural system.
Finding modality-specific processing across six modalities contributes to the growing
conclusion that knowledge is grounded in modality-specific systems of the brain
Towards a Model for the Progenitors of Gamma-Ray Bursts
We consider models for gamma-ray bursts in which a collimated jet expands
either into a homogeneous medium or into a stellar wind environment, and
calculate the expected afterglow temporal behavior. We show that (i) following
a break and a faster decay, afterglows should exhibit a flattening, which may
be detectable in both the radio and optical bands; (ii) Only observations at
times much shorter than a day can clearly distinguish between a fireball
interacting with a homogeneous medium and one interacting with a stellar wind.
Using our results we demonstrate that constraints can be placed on progenitor
models. In particular, existing data imply that while some long duration bursts
may be produced by collapses of massive stars, it is almost certain that not
all long duration bursts are produced by such progenitors.Comment: 13 pages; Submitted to Ap
Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
Wolf-Rayet (WR) stars have a severe impact on their environments owing to
their strong ionizing radiation fields and powerful stellar winds. Since these
winds are considered to be driven by radiation pressure, it is theoretically
expected that the degree of the wind mass-loss depends on the initial
metallicity of WR stars. Following our comprehensive studies of WR stars in the
Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates
for all seven putatively single WN stars known in the SMC. Based on these data,
we discuss the impact of a low-metallicity environment on the mass loss and
evolution of WR stars. The quantitative analysis of the WN stars is performed
with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical
properties of our program stars are obtained from fitting synthetic spectra to
multi-band observations. In all SMC WN stars, a considerable surface hydrogen
abundance is detectable. The majority of these objects have stellar
temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to
10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates
and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By
comparing the mass-loss rates derived for WN stars in different Local Group
galaxies, we conclude that a clear dependence of the wind mass-loss on the
initial metallicity is evident, supporting the current paradigm that WR winds
are driven by radiation. A metallicity effect on the evolution of massive stars
is obvious from the HRD positions of the SMC WN stars at high temperatures and
high luminosities. Standard evolution tracks are not able to reproduce these
parameters and the observed surface hydrogen abundances. Homogeneous evolution
might provide a better explanation for their evolutionary past.Comment: 18+12 pages; 22+8 figures; accepted for publication in A&
Systematic treatment of displacements, strains and electric fields in density-functional perturbation theory
The methods of density-functional perturbation theory may be used to
calculate various physical response properties of insulating crystals including
elastic, dielectric, Born charge, and piezoelectric tensors. These and other
important tensors may be defined as second derivatives of the total energy with
respect to atomic-displacement, electric-field, or strain perturbations, or as
mixed derivatives with respect to two of these perturbations. The resulting
tensor quantities tend to be coupled in complex ways in polar crystals, giving
rise to a variety of variant definitions. For example, it is generally
necessary to distinguish between elastic tensors defined under different
electrostatic boundary conditions, and between dielectric tensors defined under
different elastic boundary conditions. Here, we describe an approach for
computing all of these various response tensors in a unified and systematic
fashion. Applications are presented for two materials, wurtzite ZnO and
rhombohedral BaTiO3, at zero temperature.Comment: 14 pages. Uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/xfw_sys/index.htm
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