1,095 research outputs found
Isobaric-isothermal molecular dynamics computer simulations of the properties of water-1,2-dimethoxyethane model mixtures
Isothermal-isobaric molecular dynamics simulations have been performed to
examine a broad set of properties of the model water-1,2-dimethoxyethane (DME)
mixture as a function of composition. The SPC-E and TIP4P-Ew water models and
the modified TraPPE model for DME were applied. Our principal focus was to
explore the trends of behaviour of the structural properties in terms of the
radial distribution functions, coordination numbers and number of hydrogen
bonds between molecules of different species, and of conformations of DME
molecules. Thermodynamic properties, such as density, molar volume, enthalpy of
mixing and heat capacity at constant pressure have been examined. Finally, the
self-diffusion coefficients of species and the dielectric constant of the
system were calculated and analyzed.Comment: 14 pages, 9 figure
Effect of strontium and cooling rate upon eutectic temperatures of A319 aluminum alloy
DTA analysis was used to investigate the solidification reactions of alloy A319 with either 12 or 136 ppm of Sr added. Strontium does not affect primary solidification of (Al) dendrites but modifies the kinetics of the (Al)–Si eutectic. The effects of Sr level and of cooling rate on the characteristic temperatures for the (Al)–Si and other eutectic reactions are described
Effects of ion concentration and solvent composition on the properties of water-methanol solutions of NaCl. NPT molecular dynamics computer simulation results
Isothermal-isobaric molecular dynamics simulations are used to examine the
microscopic structure and other properties of a model solution consisting of
NaCl salt dissolved in water-methanol mixture. The SPC/E water model and the
united atom model for methanol are combined with the force field for ions by
Dang [J. Amer. Chem. Soc., 1995, 117, 6954] to describe the entire system. Our
principal focus is to study the effects of two variables, namely, the solvent
composition and ion concentrations on the solution's density, on the structural
properties, self-diffusion coefficients of the species and the dielectric
constant. Moreover, we performed a detailed analysis of the first coordination
numbers of the species. Trends of the behaviour of the average number of
hydrogen bonds between solvent molecules are evaluated.Comment: 18 pages, 13 figures, 1 tabl
Creation of the universe with a stealth scalar field
The stealth scalar field is a non-trivial configuration without any
back-reaction to geometry, which is characteristic for non-minimally coupled
scalar fields. Studying the creation probability of the de Sitter universe with
a stealth scalar field by the Hartle and Hawking's semi-classical method, we
show that the effect of the stealth field can be significant. For the class of
scalar fields we consider, creation with a stealth field is possible for a
discrete value of the coupling constant and its creation probability is always
less than that with a trivial scalar field. However, those creation rates can
be almost the same depending on the parameters of the theory.Comment: 7 pages; v2, references added; v3, creation of the open universe
adde
The conformal frame freedom in theories of gravitation
It has frequently been claimed in the literature that the classical physical
predictions of scalar tensor theories of gravity depend on the conformal frame
in which the theory is formulated. We argue that this claim is false, and that
all classical physical predictions are conformal-frame invariants. We also
respond to criticisms by Vollick [gr-qc/0312041], in which this issue arises,
of our recent analysis of the Palatini form of 1/R gravity.Comment: 9 pages, no figures, revtex; final published versio
MWA Tied-Array Processing IV: A Multi-Pixel Beamformer for Pulsar Surveys and Ionospheric Corrected Localisation
The Murchison Widefield Array (MWA) is a low-frequency aperture array capable
of high-time and frequency resolution astronomy applications such as pulsar
studies. The large field-of-view of the MWA (hundreds of square degrees) can
also be exploited to attain fast survey speeds for all-sky pulsar search
applications, but to maximise sensitivity requires forming thousands of
tied-array beams from each voltage-capture observation. The necessity of using
calibration solutions that are separated from the target observation both
temporally and spatially makes pulsar observations vulnerable to uncorrected,
frequency-dependent positional offsets due to the ionosphere. These offsets may
be large enough to move the source away from the centre of the tied-array beam,
incurring sensitivity drops of 30-50\% in Phase II extended array
configuration. We analyse these offsets in pulsar observations and develop a
method for mitigating them, improving both the source position accuracy and the
sensitivity. This analysis prompted the development of a multi-pixel
beamforming functionality that can generate dozens of tied-array beams
simultaneously, which runs a factor of ten times faster compared to the
original single-pixel version. This enhancement makes it feasible to observe
multiple pulsars within the vast field of view of the MWA and supports the
ongoing large-scale pulsar survey efforts with the MWA. We explore the extent
to which ionospheric offset correction will be necessary for the MWA Phase III
and the low-frequency Square Kilometre Array (SKA-Low).Comment: 10 pages, 5 figure
First-principle study of excitonic self-trapping in diamond
We present a first-principles study of excitonic self-trapping in diamond.
Our calculation provides evidence for self-trapping of the 1s core exciton and
gives a coherent interpretation of recent experimental X-ray absorption and
emission data. Self-trapping does not occur in the case of a single valence
exciton. We predict, however, that self-trapping should occur in the case of a
valence biexciton. This process is accompanied by a large local relaxation of
the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure
Pre-big bang model has Planck problem
The pre-big bang's kinetic driven inflationary mechanism is not an adequate
form of inflation: the Planck length grows more rapidly than the scale factor.
In order to explain our large universe, the resulting post-big bang universe
requires the same unnatural constants (Planck problem) as those of any other
non-inflationary big bang model.Comment: figures not include
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