369 research outputs found
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.
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
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
Correlation between the latent heats and cohesive energies of metal clusters
ProducciĂłn CientĂficaDissociation energies have been determined for Al_n^+ clusters (n = 25â83) using a new experimental approach that takes into account the latent heat of melting. According to the arguments presented here, the cohesive energies of the solidlike clusters are made up of contributions from the dissociation energies of the liquidlike clusters and the latent heats for melting. The size-dependent variations in the measured dissociation energies of the liquidlike clusters are small and the variations in the cohesive energies of solidlike clusters result almost entirely from variations in the latent heats for melting. To compare with the measured cohesive energies, density-functional theory has been used to search for the global minimum energy structures. Four groups of low energy structures were found: Distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and âdisordered.â For most cluster sizes, the measured and calculated cohesive energies are strongly correlated. The calculations show that the variations in the cohesive energies (and the latent heats) result from a combination of geometric and electronic shell effects. For some clusters an electronic shell closing is responsible for the enhanced cohesive energy and latent heat (e.g., n = 37), while for others (e.g., n = 44) a structural shell closing is the cause
Size--sensitive melting characteristics of gallium clusters: Comparison of Experiment and Theory for Ga and Ga
Experiments and simulations have been performed to examine the
finite-temperature behavior of Ga and Ga clusters.
Specific heats and average collision cross sections have been measured as a
function of temperature, and the results compared to simulations performed
using first principles Density--Functional Molecular--Dynamics. The
experimental results show that while Ga apparently undergoes a
solid--liquid transition without a significant peak in the specific--heat,
Ga melts with a relatively sharp peak. Our analysis of the
computational results indicate a strong correlation between the ground--state
geometry and the finite--temperature behavior of the cluster. If the
ground--state geometry is symmetric and "ordered" the cluster is found to have
a distinct peak in the specific--heat. However, if the ground--state geometry
is amorphous or "disordered" the cluster melts without a peak in the
specific--heat.Comment: 6 figure
Electronic effects on melting: Comparison of aluminum cluster anions and cations
ProducciĂłn CientĂficaHeat capacities have been measured as a function of temperature for aluminum cluster anions with 35â70 atoms. Melting temperatures and latent heats are determined from peaks in the heat capacities; cohesive energies are obtained for solid clusters from the latent heats and dissociation energies determined for liquid clusters. The melting temperatures, latent heats, and cohesive energies for the aluminum cluster anions are compared to previous measurements for the corresponding cations. Density functional theory calculations have been performed to identify the global minimum energy geometries for the cluster anions. The lowest energy geometries fall into four main families: distorted decahedral fragments, fcc fragments, fcc fragments with stacking faults, and âdisorderedâ roughly spherical structures. The comparison of the cohesive energies for the lowest energy geometries with the measured values allows us to interpret the size variation in the latent heats. Both geometric and electronic shell closings contribute to the variations in the cohesive energies (and latent heats), but structural changes appear to be mainly responsible for the large variations in the melting temperatures with cluster size. The significant charge dependence of the latent heats found for some cluster sizes indicates that the electronic structure can change substantially when the cluster melts
Substituting a copper atom modifies the melting of aluminum clusters
ProducciĂłn CientĂficaHeat capacities have been measured for Al(nâ1)Cuâ clusters (n = 49â62) and compared with results for pure Aln+ clusters. Al(nâ1)Cuâ and Aln+ have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n = 55 and 56 for pure aluminum clusters does not occur for the Al(nâ1)Cuâ analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Aln+ isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the AlâAl bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution
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}
- âŠ