348 research outputs found
Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations
Using density-functional-theory calculations, we have identified new stable
configurations for tri-, tetra-, and penta-vacancies in silicon. These new
configurations consist of combinations of a ring-hexavacancy with three, two,
or one interstitial atoms, respectively, such that all atoms remain fourfold.
As a result, their formation energies are lower by 0.6, 1.0, and 0.6 eV,
respectively, than the ``part of a hexagonal ring'' configurations, believed up
to now to be the lowest-energy states
Particle dynamics of a cartoon dune
The spatio-temporal evolution of a downsized model for a desert dune is
observed experimentally in a narrow water flow channel. A particle tracking
method reveals that the migration speed of the model dune is one order of
magnitude smaller than that of individual grains. In particular, the erosion
rate consists of comparable contributions from creeping (low energy) and
saltating (high energy) particles. The saltation flow rate is slightly larger,
whereas the number of saltating particles is one order of magnitude lower than
that of the creeping ones. The velocity field of the saltating particles is
comparable to the velocity field of the driving fluid. It can be observed that
the spatial profile of the shear stress reaches its maximum value upstream of
the crest, while its minimum lies at the downstream foot of the dune. The
particle tracking method reveals that the deposition of entrained particles
occurs primarily in the region between these two extrema of the shear stress.
Moreover, it is demonstrated that the initial triangular heap evolves to a
steady state with constant mass, shape, velocity, and packing fraction after
one turnover time has elapsed. Within that time the mean distance between
particles initially in contact reaches a value of approximately one quarter of
the dune basis length
Temporal Modulation of the Control Parameter in Electroconvection in the Nematic Liquid Crystal I52
I report on the effects of a periodic modulation of the control parameter on
electroconvection in the nematic liquid crystal I52. Without modulation, the
primary bifurcation from the uniform state is a direct transition to a state of
spatiotemporal chaos. This state is the result of the interaction of four,
degenerate traveling modes: right and left zig and zag rolls. Periodic
modulations of the driving voltage at approximately twice the traveling
frequency are used. For a large enough modulation amplitude, standing waves
that consist of only zig or zag rolls are stabilized. The standing waves
exhibit regular behavior in space and time. Therefore, modulation of the
control parameter represents a method of eliminating spatiotemporal chaos. As
the modulation frequency is varied away from twice the traveling frequency,
standing waves that are a superposition of zig and zag rolls, i.e. standing
rectangles, are observed. These results are compared with existing predictions
based on coupled complex Ginzburg-Landau equations
Changes in Prevalence and Severity of Domestic Violence During the COVID-19 Pandemic:A Systematic Review
Background: To contain the spread of COVID-19, governmental measures were implemented in many countries. Initial evidence suggests that women and men experience increased anger and aggression during COVID-19 lockdowns. Not surprisingly, media reports and initial empirical evidence highlight an increased risk for domestic violence (DV) during the pandemic. Nonetheless, a systematic review of studies utilizing participants' reports of potential changes in DV prevalence and severity during the pandemic as compared to pre-pandemic times is needed.Objective: To examine empirical, peer-reviewed studies, pertaining to the potential change in prevalence and severity of different types of DV during the COVID-19 pandemic, as reported by study participants.Data Sources: Electronic EMBASE, MEDLINE, PsycINFO, and CINAHL searches were conducted for the period between 2020 and January 5, 2022. References of eligible studies were integrated by using a snowballing technique.Study Selection: A total of 22 primary, empirical, peer-reviewed studies published in English or German were included.Results: Of the 22 studies, 19 were cross-sectional whereas 3 included both pre-pandemic and during pandemic assessments. Data synthesis indicates that severity of all types of DV as well as the prevalence of psychological/emotional and sexual DV increased for a significant number of victims in the general population during the pandemic. Evidence for changes in prevalence regarding economic/financial, physical, and overall DV remains inconclusive. There was considerable between-study variation in reported prevalence depending on region, sample size, assessment time, and measure.Conclusions: Data synthesis partly supports the previously documented increase in DV. Governmental measures should consider the availability of easily accessible, anonymous resources. Awareness and knowledge regarding DV need to be distributed to improve resources and clinical interventions
Clustering of vacancy defects in high-purity semi-insulating SiC
Positron lifetime spectroscopy was used to study native vacancy defects in
semi-insulating silicon carbide. The material is shown to contain (i) vacancy
clusters consisting of 4--5 missing atoms and (ii) Si vacancy related
negatively charged defects. The total open volume bound to the clusters
anticorrelates with the electrical resistivity both in as-grown and annealed
material. Our results suggest that Si vacancy related complexes compensate
electrically the as-grown material, but migrate to increase the size of the
clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure
Fundamental scaling laws of on-off intermittency in a stochastically driven dissipative pattern forming system
Noise driven electroconvection in sandwich cells of nematic liquid crystals
exhibits on-off intermittent behaviour at the onset of the instability. We
study laser scattering of convection rolls to characterize the wavelengths and
the trajectories of the stochastic amplitudes of the intermittent structures.
The pattern wavelengths and the statistics of these trajectories are in
quantitative agreement with simulations of the linearized electrohydrodynamic
equations. The fundamental distribution law for the durations
of laminar phases as well as the power law of the amplitude distribution
of intermittent bursts are confirmed in the experiments. Power spectral
densities of the experimental and numerically simulated trajectories are
discussed.Comment: 20 pages and 17 figure
Scalar correlations in a quark plasma and low mass dilepton production
We investigate possible consequences of resonant scalar interactions for
dilepton production from a quark plasma at the chiral phase transition. It is
found that this production mechanism is strongly suppressed compared to the
Born process and has no significance for present experiments.Comment: 7 pages revtex, 2 ps figure
Modulation of Localized States in Electroconvection
We report on the effects of temporal modulation of the driving force on a
particular class of localized states, known as worms, that have been observed
in electroconvection in nematic liquid crystals. The worms consist of the
superposition of traveling waves and have been observed to have unique, small
widths, but to vary in length. The transition from the pure conduction state to
worms occurs via a backward bifurcation. A possible explanation of the
formation of the worms has been given in terms of coupled amplitude equations.
Because the worms consist of the superposition of traveling waves, temporal
modulation of the control parameter is a useful probe of the dynamics of the
system. We observe that temporal modulation increases the average length of the
worms and stabilizes worms below the transition point in the absence of
modulation.Comment: 4 pages, 4 figure
Hadronization of a Quark-Gluon Plasma in the Chromodielectric Model
We have carried out simulations of the hadronization of a hot, ideal but
effectively massive quark-gluon gas into color neutral clusters in the
framework of the semi-classical SU(3) chromodielectric model. We have studied
the possible quark-gluon compositions of clusters as well as the final mass
distribution and spectra, aiming to obtain an insight into relations between
hadronic spectral properties and the confinement mechanism in this model.Comment: 34 pages, 37 figure
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