4,272 research outputs found
Traversable wormholes in a string cloud
We study spherically symmetric thin-shell wormholes in a string cloud
background in (3+1)-dimensional spacetime. The amount of exotic matter required
for the construction, the traversability and the stability under radial
perturbations, are analyzed as functions of the parameters of the model.
Besides, in the Appendices a non perturbative approach to the dynamics and a
possible extension of the analysis to a related model are briefly discussed.Comment: 21 pages, 10 figures; accepted for publication in Int. J. Mod. Phys.
Excitation properties of the divacancy in 4H-SiC
We investigate the quenching of the photoluminescence (PL) from the divacancy
defect in 4H-SiC consisting of a nearest-neighbour silicon and carbon
vacancies. The quenching occurs only when the PL is excited below certain
photon energies (thresholds), which differ for the four different inequivalent
divacancy configurations in 4H-SiC. Refined theoretical ab initio calculation
for the charge-transfer levels of the divacancy show very good agreement
between the position of the (0/-) level with respect to the conduction band for
each divacancy configurations and the corresponding experimentally observed
threshold, allowing us to associate the PL decay with conversion of the
divacancy from neutral to negative charge state due to capture of electrons
photoionized from other defects (traps) by the excitation. Electron
paramagnetic resonance measurements are conducted in dark and under excitation
similar to that used in the PL experiments and shed light on the possible
origin of traps in the different samples. A simple model built on this concept
agrees well with the experimentally-observed decay curves.Comment: 28 pages, 6 figure
Stability of singular jump-linear systems with a large state space : a two-time-scale approach
This paper considers singular systems that involve both continuous dynamics and discrete events with the coefficients being modulated by a continuous-time Markov chain. The underlying systems have two distinct characteristics. First, the systems are singular, that is, characterized by a singular coefficient matrix. Second, the Markov chain of the modulating force has a large state space. We focus on stability of such hybrid singular systems. To carry out the analysis, we use a two-time-scale formulation, which is based on the rationale that, in a large-scale system, not all components or subsystems change at the same speed. To highlight the different rates of variation, we introduce a small parameter ε>0. Under suitable conditions, the system has a limit. We then use a perturbed Lyapunov function argument to show that if the limit system is stable then so is the original system in a suitable sense for ε small enough. This result presents a perspective on reduction of complexity from a stability point of view
Initial correlations in nonequilibrium Falicov-Kimball model
The Keldysh boundary problem in a nonequilibrium Falicov-Kimball model in
infinite dimensions is studied within the truncated and self-consistent
perturbation theories, and the dynamical mean-field theory. Within the model
the system is started in equilibrium, and later a uniform electric field is
turned on. The Kadanoff-Baym-Wagner equations for the nonequilibrium Green
functions are derived, and numerically solved. The contributions of initial
correlations are studied by monitoring the system evolution. It is found that
the initial correlations are essential for establishing full electron
correlations of the system and independent on the starting time of preparing
the system in equilibrium. By examining the contributions of the initial
correlations to the electric current and the double occupation, we find that
the contributions are small in relation to the total value of those physical
quantities when the interaction is weak, and significantly increase when the
interaction is strong. The neglect of initial correlations may cause artifacts
in the nonequilibrium properties of the system, especially in the strong
interaction case
OPTIMIZING LIGHT-CURED COMPOSITE PROPERTIES WITH CAMPHROQUINONE AND BUTYLHYDROXYTOLUENE COMBINATIONS
poster abstractPolymerization shrinkage is an inherent property in dental composite that has major effects on its clinical performance. Many strategies on minimizing the shrinkage have been explored in the past. Here we propose that, by op-timizing the dose combinations of photoinitiator and polymerization inhibitor, we can effectively reduce the polymerization shrinkage stress without sacri-ficing the mechanical properties of dental composite. The objective of this study therefore was to investigate the effects of a common photoinitiator, camphroquinone (CQ), and inhibitor, butylhydroxytoluene (BHT), at clinical-ly-relevant concentration combinations on the shrinkage properties and me-chanical properties of light-cured composite. Samples were prepared by mix-ing bisphenol-A-glycidyl methacrylate, urethane dimethacrylate, and tetraethyleneglycol dimethacrylate at a 1:1:1 ratio. Borosilicate glass fillers constituted 70% of the resin weight. Sixteen groups of resin composite were prepared from the combination of four CQ (0.1%, 0.5%, 1.0%, and 1.5%) and four BHT (0.0%, 0.5%, 1.0%, and 1.5%) levels. Six properties were tested, including Flexural strength (FS) flexural modulus (FM), degree of conversion (DC), contraction stress (CS), stress rate (SR), and gel point (GP). The effects of CQ and BHT combinations on each of these properties were evaluated using two-way analysis of variance (ANOVA). Groups with low CQ and BHT showed moderate values for FS, FM, SR and CS with DC around 70%. Increasing the BHT concentration caused a decrease in SR, CS, DC and an increase in GP values. Increasing the CQ content gave a steady increase in values for FS and FM. Notable, in CQ=1.5% group, increasing BHT from 0 to 1.5% result in a statistically significant decrease in polymeri-zation shrinkage stress (p<0.05) while maintain the same mechanical prop-erties. In this project, we successfully demonstrated that the polymerization shrinkage of resin composite can be tailored through CQ and BHT combina-tions with high CQ and high BHT showing the most promising results
Elastic properties of grafted microtubules
We use single-particle tracking to study the elastic properties of single
microtubules grafted to a substrate. Thermal fluctuations of the free
microtubule's end are recorded, in order to measure position distribution
functions from which we calculate the persistence length of microtubules with
contour lengths between 2.6 and 48 micrometers. We find the persistence length
to vary by more than a factor of 20 over the total range of contour lengths.
Our results support the hypothesis that shearing between protofilaments
contributes significantly to the mechanics of microtubules.Comment: 9 pages, 3 figure
A new root-knot nematode, Meloidogyne moensi n. sp. (Nematoda : Meloidogynidae), parasitizing Robusta coffee from Western Highlands, Vietnam
A new root-knot nematode, parasitizing Robusta coffee in Dak Lak Province, Western Highlands of Vietnam, is described as Meloidogyne moensi n. sp. Morphological and molecular analyses demonstrated that this species differs clearly from other previously described root-knot nematodes. Morphologically, the new species is characterized by a swollen body of females with a small posterior protuberance that elongated from ovoid to saccate; perineal patterns with smooth striae, continuous and low dorsal arch; lateral lines marked as a faint space or linear depression at junction of the dorsal and ventral striate; distinct phasmids; perivulval region free of striae; visible and wide tail terminus surrounding by concentric circles of striae; medial lips of females in dumbbell-shaped and slightly raised above lateral lips; female stylet is normally straight with posteriorly sloping stylet knobs; lip region of second stage juvenile (J2) is not annulated; medial lips and labial disc of J2 formed dumbbell shape; lateral lips are large and triangular; tail of J2 is conoid with rounded unstriated tail tip; distinct phasmids and hyaline; dilated rectum. Meloidogyne moensi n. sp. is most similar to M. africana, M. ottersoni by prominent posterior protuberance. Results of molecular analysis of rDNA sequences including the D2-D3 expansion regions of 28S rDNA, COI, and partial COII/16S rRNA of mitochondrial DNA support for the new species status
Global action-angle coordinates for completely integrable systems with noncompact invariant submanifolds
The obstruction to the existence of global action-angle coordinates of
Abelian and noncommutative (non-Abelian) completely integrable systems with
compact invariant submanifolds has been studied. We extend this analysis to the
case of noncompact invariant submanifolds.Comment: 13 pages, to be published in J. Math. Phys. (2007
Dilaton thin-shell wormholes supported by a generalized Chaplygin gas
In this article, we construct spherical thin-shell wormholes with charge in
dilaton gravity. The exotic matter required for the construction is provided by
a generalized Chaplygin gas. We study the stability under perturbations
preserving the symmetry. We find that the increase of the coupling between the
dilaton and the electromagnetic fields reduces the range of the parameters for
which stable configurations are possible.Comment: 14 pages, 6 figures. v3: typos correcte
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