Recognition and analysis of local structure in polycrystalline configurations

A method is described for obtaining information about the local order existing in monoatomic model solids or real materials based on their atomistic configurations. An efficient algorithmic implementation is provided. The shape of the polyhedra formed by ‘relevant’ neighbors of each atom enter a pattern recognition method to resolve the type of the (usually non-ideal) crystal structure to which atoms surrounded by their relevant neighbors belong: hexagonal close-packed, face-centered cubic or body-centered cubic. Further, this approach allows for the analysis of icosahedral structure which preferably occurs in amorphous solids. Results of a molecular dynamics computer simulation illustrate how this method can be applied to contribute to an understanding of the mechanical and structural properties of solids (i) undergoing a steady shear stress and (ii) upon increasing temperature.     

Temporal detection in human vision: dependence on spatial frequency.

In the study of perception of temporal changes in luminance, it is customary to model perceptual performance as based on one or more linear filters. The task is then to estimate the temporal impulse responses or the representation of the impulse response in the frequency domain. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms (filters) in central vision for 1-cycle-per-degree (cpd) targets [Vision Res. 38, 1023 (1998)]. The same methods have been used to explore how properties of the estimated filters change with stimulus contrast energy [J. Opt. Soc. Am. A 14, 2557 (1997)]. We present estimated properties for temporal mechanisms that detect low spatial-frequency patterns. The results indicate that two filters provide the best model for performance when mask contrast is significant. There are also differences between properties for mechanisms that detect signal spatial frequencies of 1 cpd and 1/3 cpd. The sensitivity of the low-pass mechanism relative to the bandpass mechanism is reduced at 1/3 cpd, consistent with previous findings.     

A flexible algorithm for calculating pair interactions on SIMD architectures

Calculating interactions or correlations between pairs of particles is typically the most time-consuming task in particle simulation or correlation analysis. Straightforward implementations using a double loop over particle pairs have traditionally worked well, especially since compilers usually do a good job of unrolling the inner loop. In order to reach high performance on modern CPU and accelerator architectures, single-instruction multiple-data (SIMD) parallelization has become essential. Avoiding memory bottlenecks is also increasingly important and requires reducing the ratio of memory to arithmetic operations. Moreover, when pairs only interact within a certain cut-off distance, good SIMD utilization can only be achieved by reordering input and output data, which quickly becomes a limiting factor. Here we present an algorithm for SIMD parallelization based on grouping a fixed number of particles, e.g. 2, 4, or 8, into spatial clusters. Calculating all interactions between particles in a pair of such clusters improves data reuse compared to the traditional scheme and results in a more efficient SIMD parallelization. Adjusting the cluster size allows the algorithm to map to SIMD units of various widths. This flexibility not only enables fast and efficient implementation on current CPUs and accelerator architectures like GPUs or Intel MIC, but it also makes the algorithm future-proof. We present the algorithm with an application to molecular dynamics simulations, where we can also make use of the effective buffering the method introduces.     

Dislocation generation related to micro-cracks in Si wafers: High temperature in situ study with white beam X-ray topography

The generation and propagation of dislocations in Si at high temperature is observed in situ with white beam X-ray topography. For the heating experiments a double ellipsoidal mirror furnace was installed at the Topo–Tomo beamline of the ANKA synchrotron light source, Research Centre Karlsruhe, Germany. Details of the experimental set-up and the first results on the occurrence of dislocations are presented. Artificial damage was generated in commercial (1 0 0) Si wafers using a nanoindenter with various loads. The applied forces for each set of indents were varied from 100 to 500 mN, respectively. After heating to approx. 790 °C large area transmission topographs were taken every 30 min which were then compared to room temperature topographs before and after heating. At the outset straight 60°-dislocations with b = a/2〈1 1 0〉 originate from the 500 mN indents into the direction of the strongest temperature gradient. After 60 min at constant temperature an increase in the length and number of the dislocations in other directions is also observed. As a result of the continual thermal stressing dislocations develop from the 100 mN indents too.     

Superfluidity in thin4He films: Comparison of onset on particular crystal facets and on disordered substrates

Superfluid flow in unsaturated⁴He films on highly ordered pyrolytic graphite, (001) cleaved MgO, epitaxial gold on mica, polycrystalline gold, and glass has been studied by the heat transport technique. Using a criterion for onset suggested by the Kosterlitz-Thouless theory, we confirm that the locus of onset in the variablesT andp/p ₀ is substrate independent within experimental error, whereas the film thickness at onset varies widely with substrate. The reason for this is not understood. Several aspects of the Kosterlitz-Thouless theory are verified: On polycrystalline gold the film flow impedance decreases approaching the transition from the normal side, consistent with the dependence calculated by Ambegaokar et al. On the superfluid side of onset, power law relations are found between temperature gradient and current, in agreement with the prediction based on thermal dissociation of vortex pairs in the velocity field. The exponents are consistent with the expected variation of the renormalized superfluid density with coverage, including a Nelson-Kosterlitz jump smeared by finite-velocity effects. On MgO we obtain only an upper limit for the film coverage at onset; its consistency with the Nelson-Kosterlitz jump depends on the interpretation of neutron scattering data.Supported in part by NSF grant DMR 78-22697.     

Specific exemplar retention and prototype abstraction in young and old adults.

Examined the effects of age differences in retention of information about specific concept members on 40 older adults' (mean age 72.3 yrs) and 40 younger adults' (mean age 20.6 yrs) ability to abstract central tendency information. The mean Wechsler Adult Intelligence Scale—Revised (WAIS—R) Vocabulary scaled score for the young adults was significantly lower than that of the older adults. Ss were presented with a series of visual patterns that were organized around a prototype and were then presented with these same patterns plus a set of new patterns varying in prototype similarity in a recognition test. It was found that young Ss retained more information about specific acquisition set exemplars, which resulted in slightly different recognition responses for new patterns. However, the recognition behavior of both young and older Ss appeared to be governed by the same rules. It is suggested that the organization of conceptual information does not change with age but that the poorer retention of specific item information in older adults may result in a less complete representation. (14 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Endothelin receptor blockade prevents the rise in pulmonary vascular resistance after cardiopulmonary bypass in lambs with increased pulmonary blood flow

BACKGROUND: Children with increased pulmonary blood flow may experience morbidity as the result of increased pulmonary vascular resistance after operations in which cardiopulmonary bypass is used. Plasma levels of endothelin-1, a potent vasoactive substance implicated in pulmonary hypertension, are increased after cardiopulmonary bypass. OBJECTIVES: In a lamb model of increased pulmonary blood flow after in utero placement of an aortopulmonary shunt, we characterized the changes in pulmonary vascular resistance induced by hypothermic cardiopulmonary bypass and investigated the role of endothelin-1 and endothelin-A receptor activation in postbypass pulmonary hypertension. METHODS: In eleven 1-month-old lambs, the shunt was closed, and vascular pressures and blood flows were monitored. An infusion of a selective endothelin-A receptor blocker (PD 156707; 1.0 mg/kg/h) or drug vehicle (saline solution) was then begun 30 minutes before cardiopulmonary bypass and continued for 4 hours after bypass. The hemodynamic variables were monitored, and plasma endothelin-1 concentrations were determined before, during, and for 6 hours after cardiopulmonary bypass. RESULTS: After 90 minutes of hypothermic cardiopulmonary bypass, both pulmonary arterial pressure and pulmonary vascular resistance increased significantly in saline-treated lambs during the 6-hour study period (P <.05). In lambs pretreated with PD 156707, pulmonary arterial pressure and pulmonary vascular resistance decreased (P <. 05). After bypass, plasma endothelin-1 concentrations increased in all lambs; there was a positive correlation between postbypass pulmonary vascular resistance and plasma endothelin-1 concentrations (P <.05). CONCLUSIONS: This study suggests that endothelin-A receptor-induced pulmonary vasoconstriction mediates, in part, the rise in pulmonary vascular resistance after cardiopulmonary bypass. Endothelin-A receptor antagonists may decrease morbidity in children at risk for postbypass pulmonary hypertension. This potential therapy warrants further investigation.