228 research outputs found
-algebras and quantum dynamics: some existence results
We discuss the possibility of defining an algebraic dynamics within the
settings of -algebras. Compared with our previous results on this
subject, the main improvement here is that we are not assuming the existence of
some hamiltonian for the {\em full} physical system. We will show that, under
suitable conditions, the dynamics can still be defined via some limiting
procedure starting from a given {\em regularized sequence}
Representable states on quasi-local quasi *-algebras
Continuing a previous analysis originally motivated by physics, we consider
representable states on quasi-local quasi *-algebras, starting with examining
the possibility for a {\em compatible} family of {\em local} states to give
rise to a {\em global} state. Some properties of {\em local modifications} of
representable states and some aspects of their asymptotic behavior are also
considered.Comment: In press in Jpurnal of Mathematical Physic
Energy Distribution of a Stringy Charged Black Hole
The energy distribution associated with a stringy charged black hole is
studied using M{\o}ller's energy-momentum complex. Our result is reasonable and
it differs from that known in literature using Einstein's energy-momentum
complex.Comment: Latex, no figure
Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W
Screw dislocations in bcc metals display non-planar cores at zero temperature
which result in high lattice friction and thermally activated strain rate
behavior. In bcc W, electronic structure molecular statics calculations reveal
a compact, non-degenerate core with an associated Peierls stress between 1.7
and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations
can only be gained by using more efficient atomistic simulations based on
semiempirical interatomic potentials. In this paper we assess the suitability
of five different potentials in terms of static properties relevant to screw
dislocations in pure W. As well, we perform molecular dynamics simulations of
stress-assisted glide using all five potentials to study the dynamic behavior
of screw dislocations under shear stress. Dislocations are seen to display
thermally-activated motion in most of the applied stress range, with a gradual
transition to a viscous damping regime at high stresses. We find that one
potential predicts a core transformation from compact to dissociated at finite
temperature that affects the energetics of kink-pair production and impacts the
mechanism of motion. We conclude that a modified embedded-atom potential
achieves the best compromise in terms of static and dynamic screw dislocation
properties, although at an expense of about ten-fold compared to central
potentials
The influence of transition metal solutes on dislocation core structure and values of Peierls stress and barrier in tungsten
Several transition metals were examined to evaluate their potential for
improving the ductility of tungsten. The dislocation core structure and Peierls
stress and barrier of screw dislocations in binary
tungsten-transition metal alloys (WTM) were investigated using
first principles electronic structure calculations. The periodic quadrupole
approach was applied to model the structure of dislocation. Alloying
with transition metals was modeled using the virtual crystal approximation and
the applicability of this approach was assessed by calculating the equilibrium
lattice parameter and elastic constants of the tungsten alloys. Reasonable
agreement was obtained with experimental data and with results obtained from
the conventional supercell approach. Increasing the concentration of a
transition metal from the VIIIA group, i.e. the elements in columns headed by
Fe, Co and Ni, leads to reduction of the elastic constant and
increase of elastic anisotropy A=. Alloying W with a group
VIIIA transition metal changes the structure of the dislocation core from
symmetric to asymmetric, similar to results obtained for WRe
alloys in the earlier work of Romaner {\it et al} (Phys. Rev. Lett. 104, 195503
(2010))\comments{\cite{WRECORE}}. In addition to a change in the core symmetry,
the values of the Peierls stress and barrier are reduced. The latter effect
could lead to increased ductility in a tungsten-based
alloy\comments{\cite{WRECORE}}. Our results demonstrate that alloying with any
of the transition metals from the VIIIA group should have similar effect as
alloying with Re.Comment: 12 pages, 8 figures, 3 table
Temporally stable coherent states for infinite well and P\"oschl-Teller potentials
This paper is a direct illustration of a construction of coherent states
which has been recently proposed by two of us (JPG and JK). We have chosen the
example of a particle trapped in an infinite square-well and also in
P\"oschl-Teller potentials of the trigonometric type. In the construction of
the corresponding coherent states, we take advantage of the simplicity of the
solutions, which ultimately stems from the fact they share a common SU(1,1)
symmetry \`a la Barut--Girardello. Many properties of these states are then
studied, both from mathematical and from physical points of view.Comment: 48 pages, 21 figure
Evolutionary history of endogenous Human Herpesvirus 6 reflects human migration out of Africa
Human herpesvirus 6A and 6B (HHV-6) can integrate into the germline, and as a result, âŒ70 million people harbor the genome of one of these viruses in every cell of their body. Until now, it has been largely unknown if 1) these integrations are ancient, 2) if they still occur, and 3) whether circulating virus strains differ from integrated ones. Here, we used next-generation sequencing and mining of public human genome data sets to generate the largest and most diverse collection of circulating and integrated HHV-6 genomes studied to date. In genomes of geographically dispersed, only distantly related people, we identified clades of integrated viruses that originated from a single ancestral event, confirming this with fluorescent in situ hybridization to directly observe the integration locus. In contrast to HHV-6B, circulating and integrated HHV-6A sequences form distinct clades, arguing against ongoing integration of circulating HHV-6A or âreactivationâ of integrated HHV-6A. Taken together, our study provides the first comprehensive picture of the evolution of HHV-6, and reveals that integration of heritable HHV-6 has occurred since the time of, if not before, human migrations out of Africa
Study and suppression of the microstructural anisotropy generated during the consolidation of a carbonyl iron powder by field-assisted hot pressing
Published OnlineA spherical carbonyl iron powder was consolidated by the field-assisted hot pressing technique using graphite tools at two different temperatures, both above the austenitizing temperature. The microstructures obtained exhibited a compositional gradient in carbon along the consolidated material. Thus, the outer rim of the cylindrical samples was composed of cementite and pearlite that gradually turned to pearlite, leading to a fully ferritic microstructure at the core of the sample. The increase in the temperature has led to a higher introduction of carbon within the sample. The interposition of a thin tungsten foil between the graphite die/punches and the powders has significantly reduced the diffusion of the carbon through the iron matrix and has suppressed the microstructural anisotropy.Publicad
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