357 research outputs found
Experimental Determination of Nucleation Scaling Law for Small Charged Particles
We investigated the nucleation process at the molecular level. Controlled
sticking of individual atoms onto mass selected clusters over a wide mass range
has been carried out for the first time. We measured the absolute unimolecular
nucleation cross sections of cationic sodium clusters Na_{n}^{+} in the range
n=25-200 at several collision energies. The widely used hard sphere
approximation clearly fails for small sizes: not only should vapor-to-liquid
nucleation theories be modified, but also, through the microreversibility
principle, cluster decay rate statistical models
Impaired Competence for Pretense in Children with Autism: Exploring Potential Cognitive Predictors.
Lack of pretense in children with autism has been explained by a number of theoretical explanations, including impaired mentalising, impaired response inhibition, and weak central coherence. This study aimed to empirically test each of these theories. Children with autism (n=60) were significantly impaired relative to controls (n=65) when interpreting pretense, thereby supporting a competence deficit hypothesis. They also showed impaired mentalising and response inhibition, but superior local processing indicating weak central coherence. Regression analyses revealed that mentalising significantly and independently predicted pretense. The results are interpreted as supporting the impaired mentalising theory and evidence against competing theories invoking impaired response inhibition or a local processing bias. The results of this study have important implications for treatment and intervention
Surface reconstruction induced geometries of Si clusters
We discuss a generalization of the surface reconstruction arguments for the
structure of intermediate size Si clusters, which leads to model geometries for
the sizes 33, 39 (two isomers), 45 (two isomers), 49 (two isomers), 57 and 61
(two isomers). The common feature in all these models is a structure that
closely resembles the most stable reconstruction of Si surfaces, surrounding a
core of bulk-like tetrahedrally bonded atoms. We investigate the energetics and
the electronic structure of these models through first-principles density
functional theory calculations. These models may be useful in understanding
experimental results on the reactivity of Si clusters and their shape as
inferred from mobility measurements.Comment: 9 figures (available from the author upon request) Submitted to Phys.
Rev.
Assessing pragmatic communication in children with Down syndrome
Purpose: Successful communication depends on language content, language form, and language use (pragmatics).Down syndrome (DS) experience communication difficulties, however little is known about their pragmatic profile, particularly during early school years. The purpose of the present study was to explore the nature of pragmatic communication in children with DS. Method: Twenty-nine six-year-old children with DS were assessed, in the areas of 1) initiation, 2) scripted language, 3) understanding context and 4) nonverbal communication, as reported by children’s parents via the Children’s Communication Checklist-2 (Bishop, 2003). Additionally, the relationships between pragmatics and measures of vocabulary, nonverbal mental ability and social functioning were explored. Results: Children with DS were impaired relative to norms from typically developing children in all areas of pragmatics. A profile of relative strengths and weaknesses was found in the children with DS; the area of nonverbal communication was significantly stronger, while the area of understanding context was significantly poorer, relative to the other areas of pragmatics assessed in these children. Relationships between areas of pragmatics and other linguistic areas, as well as aspects of vocabulary and social functioning were observed. Conclusions: By the age of six children with DS experience significantly impaired pragmatic communication, with a clear profile of relative strengths and weaknesses. The study highlights the need to teach children with DS pragmatic skills as a component of communication, alongside language content and form
Excitation and relaxation in atom-cluster collisions
Electronic and vibrational degrees of freedom in atom-cluster collisions are
treated simultaneously and self-consistently by combining time-dependent
density functional theory with classical molecular dynamics. The gradual change
of the excitation mechanisms (electronic and vibrational) as well as the
related relaxation phenomena (phase transitions and fragmentation) are studied
in a common framework as a function of the impact energy (eV...MeV). Cluster
"transparency" characterized by practically undisturbed atom-cluster
penetration is predicted to be an important reaction mechanism within a
particular window of impact energies.Comment: RevTeX (4 pages, 4 figures included with epsf
Magic Numbers of Silicon Clusters
A structural model for intermediate sized silicon clusters is proposed that
is able to generate unique structures without any dangling bonds. This
structural model consists of bulk-like core of five atoms surrounded by
fullerene-like surface. Reconstruction of the ideal fullerene geometry results
in the formation of crown atoms surrounded by -bonded dimer pairs. This
model yields unique structures for \Si{33}, \Si{39}, and \Si{45} clusters
without any dangling bonds and hence explains why these clusters are least
reactive towards chemisorption of ammonia, methanol, ethylene, and water. This
model is also consistent with the experimental finding that silicon clusters
undergo a transition from prolate to spherical shapes at \Si{27}. Finally,
reagent specific chemisorption reactivities observed experimentally is
explained based on the electronic structures of the reagents.Comment: 4 pages + 3 figures (postscript files after \end{document}
Verbal short-term memory deficits in Down syndrome: phonological, semantic, or both?
The current study examined the phonological and semantic contributions to the verbal short-term memory (VSTM) deficit in Down syndrome (DS) by experimentally manipulating the phonological and semantic demands of VSTM tasks. The performance of 18 individuals with DS (ages 11–25) and 18 typically developing children (ages 3–10) matched pairwise on receptive vocabulary and gender was compared on four VSTM tasks, two tapping phonological VSTM (phonological similarity, nonword discrimination) and two tapping semantic VSTM (semantic category, semantic proactive interference). Group by condition interactions were found on the two phonological VSTM tasks (suggesting less sensitivity to the phonological qualities of words in DS), but not on the two semantic VSTM tasks. These findings suggest that a phonological weakness contributes to the VSTM deficit in DS. These results are discussed in relation to the DS neuropsychological and neuroanatomical phenotype
Effect of hydrogen on ground state structures of small silicon clusters
We present results for ground state structures of small SiH (2 \leq
\emph{n} \leq 10) clusters using the Car-Parrinello molecular dynamics. In
particular, we focus on how the addition of a hydrogen atom affects the ground
state geometry, total energy and the first excited electronic level gap of an
Si cluster. We discuss the nature of bonding of hydrogen in these
clusters. We find that hydrogen bonds with two silicon atoms only in SiH,
SiH and SiH clusters, while in other clusters (i.e. SiH,
SiH, SiH, SiH, SiH and SiH) hydrogen is bonded
to only one silicon atom. Also in the case of a compact and closed silicon
cluster hydrogen bonds to the cluster from outside. We find that the first
excited electronic level gap of Si and SiH fluctuates as a function
of size and this may provide a first principles basis for the short-range
potential fluctuations in hydrogenated amorphous silicon. Our results show that
the addition of a single hydrogen can cause large changes in the electronic
structure of a silicon cluster, though the geometry is not much affected. Our
calculation of the lowest energy fragmentation products of SiH clusters
shows that hydrogen is easily removed from SiH clusters.Comment: one latex file named script.tex including table and figure caption.
Six postscript figure files. figure_1a.ps and figure_1b.ps are files
representing Fig. 1 in the main tex
Thermodynamics of tin clusters
We report the results of detailed thermodynamic investigations of the
Sn cluster using density-functional molecular dynamics. These
simulations have been performed over a temperature range of 150 to 3000 K, with
a total simulation time of order 1 ns. The prolate ground state and low-lying
isomers consist of two tricapped trigonal prism (TTP) units stacked end to end.
The ionic specific heat, calculated via a multihistogram fit, shows a small
peak around 500 K and a shoulder around 850 K. The main peak occurs around 1200
K, about 700 K higher than the bulk melting temperature, but significantly
lower than that for Sn. The main peak is accompanied by a sharp change
in the prolate shape of the cluster due to the fusion of the two TTP units to
form a compact, near spherical structure with a diffusive liquidlike ionic
motion. The small peak at 500 K is associated with rearrangement processes
within the TTP units, while the shoulder at 850 K corresponds to distortion of
at least one TTP unit, preserving the overall prolate shape of the cluster. At
all temperatures observed, the bonding remains covalent.Comment: Latex File and EPS Figures. 18 pages,11 Figures. Submitted to Phys.
Rev.
Spatial contrast sensitivity in adolescents with autism spectrum disorders
Adolescents with autism spectrum disorders (ASD) and typically developing (TD) controls underwent a rigorous psychophysical assessment that measured contrast sensitivity to seven spatial frequencies (0.5-20 cycles/degree). A contrast sensitivity function (CSF) was then fitted for each participant, from which four measures were obtained: visual acuity, peak spatial frequency, peak contrast sensitivity, and contrast sensitivity at a low spatial frequency. There were no group differences on any of the four CSF measures, indicating no differential spatial frequency processing in ASD. Although it has been suggested that detail-oriented visual perception in individuals with ASD may be a result of differential sensitivities to low versus high spatial frequencies, the current study finds no evidence to support this hypothesis
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