First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si nanowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets according to the Wulff shape. The Young's modulus, the equilibrium length and the constrained residual stress of a series of prismatic beams of differing sizes are found to have size dependences that scale like the surface area to volume ratio for all but the smallest beam. The results are compared with a continuum model and the results of classical atomistic calculations based on an empirical potential. We attribute the size dependence to specific physical structures and interactions. In particular, the hydrogen interactions on the surface and the charge density variations within the beam are quantified and used both to parameterize the continuum model and to account for the discrepancies between the two models and the first-principles results.Comment: 14 pages, 10 figure

Energy and Angular Distributions of Electrons from Ion Impact on Atomic and Molecular Hydrogen. I. 20- 114-keV H\u3csup\u3e+\u3c/sup\u3e + H\u3csub\u3e2\u3c/sub\u3e

Apparatus and procedures are described for the measurement of absolute cross sections, differential in ejected electron energy and angle, for ionization of atomic and molecular hydrogen by ion impact. A hemispherical electrostatic energy analyzer, rotatable from 15° to 165° with respect to the direction of the incident ion beam, was used to measure energy spectra of secondary electrons from 1.5 to 300 eV. Cross sections at ten angles (nine at some energies) and five incident-ion energies from 20 to 114 keV for H+ +H2 collisions are given. The doubly differential cross sections were integrated over angle and electron energy to obtain singly differential and total-ionization cross sections. The uncertainty in the doubly differential cross sections is 21% at a secondary energy of 1.5 eV decreasing to 18% at 10 eV and above. The total cross sections have a rms deviation of 12% from recommended values. A broad peak at 6 eV in the energy spectrum of electrons from low-energy H+ +H2 collisions is attributed to autoionization

Architecture of the parallel programming support environment

The Parallel Programming Support Environment (PPSE) is an experimental integrated set of tools for the design and construction of large software systems to run on parallel computers. The tools include a graphical de.sign editor, a graphical target machine description system, a task mapper/scheduler tool, parallel code generator, and graphical aids for performance analysis. The objective is, to the extent possible, to design and develop parallel software with little regard for the details of the architecture of the target ma.chine, programming language, or parallel computing paradigm that the program is to use

Combining opinions from several experts

This paper addresses the problem of aggregating a number of expert opinions which have been expressed in some numerical form. An important feature of sets of expert opinions is the possibility of stochastic dependence between members of the sets. We develop an approach for combining expert opinions which formally allows for such dependence. This approach is based on an extension of the Dempster-Shafer theory, a well-known calculus for reasoning with uncertainty in artificial intelligence

Weighted partial pattern matching

Pattern matching is the process of comparing two objects to see if they are similar to each other. Objects can be physical objects, situations, facts or events. The criteria of matching can be exact or partial. Traditional pattern matching systems rely on ex­act matches, in which a small set of predefined pattern matching rules are applied in all-or-none fashion. Partial matching is superior to exact matching because it can be used to identify the best from a set of options without insisting on a perfect match with what is required. Partial matching is important in retrieval from database and knowledge base sys­tems. We need to retrieve data from databases that are "like" data we are looking for. For example, a particular book can be selected from a collection of books in a library by partial matching a description of the subject we are interested in against a set of descriptions from a database that describes the library holdings

Wear of human teeth: a tribological perspective

The four main types of wear in teeth are attrition (enamel-on-enamel contact), abrasion (wear due to abrasive particles in food or toothpaste), abfraction (cracking in enamel and subsequent material loss), and erosion (chemical decomposition of the tooth). They occur as a result of a number of mechanisms including thegosis (sliding of teeth into their lateral position), bruxism (tooth grinding), mastication (chewing), toothbrushing, tooth flexure, and chemical effects. In this paper the current understanding of wear of enamel and dentine in teeth is reviewed in terms of these mechanisms and the major influencing factors are examined. In vitro tooth wear simulation and in vivo wear measurement and ranking are also discussed

A test case for the parallel programming support environment : parallelizing the analysis of satellite imagery data

Within a decade, the amount of raw data acquired each day by satellite -based instrumentation will exceed 1 terabyte (10¹² bytes). Virtually all quantitative data acquired about the Earth is useful [10]. Processing the raw data into useful information will be an enormous task . Much of the current data analysis software is FORTRAN dusty-deck code. To handle the data volume expected, not only will much of the currently used code need to be transformed or redesigned into parallel code, but many new parallel programs will need to be written from scratch. Although parallel computers are already in existence, the task of writing parallel software has proved to be exceedingly difficult [1]. The Parallel Programming Support Environment (PPSE) is intended to assist the parallel programer in the process of designing a parallel program. The primary focus of this work is to test the tools of PPSE on an actual mid-sized (4000 lines of code) FORTRAN program and report on the experience. The program (AVHTST) is part of a series of FORTRAN data analysis programs which automate the process of determining cloud properties from satellite imagery data. Although A VHTST has been in use for several years, it has been identified as being in need of parallelization in the literature [5]. The objective of this research is to concurrently examine viable methods of producing a useful parallel program while testing the utility of the PPSE tools. The product of this research is a prototype parallel version of AVHTST, a description of the usage of the PPSE tools, a list of possible improvements and extensions to the tools, and a plan for integrating the tools

Distributed computation system : distributed computing in a heterogeneous networked Unix environment

Large integer factorization exemplifies a class of hard computational problems requiring the power of a supercomputer but which have algorithms decomposable into many large indepen­dent computations. The availability of internetworking provides the opportunity to solve such problems in distributed fashion on ordinary machines. Such a distributed network might contain a heterogeneous collection of machines running under the administrative authority of different organizations in separate geographic locations. DCS uses standard UNIX BSD 4.2/4.3 features to implement a central scheduler daemon and remote grain server daemons providing support for an extended model of very large grain data flow computation. A system providing such a service needs to address the essential goals of reliability, fault tolerance, courtesy and security in addi­tion to dataflow functionality. In particular, checkpointing becomes essential for reliability and efficiency when grain lifetimes become comparable to system uptimes. A user control program makes grain requests and receives results from a central scheduler which launches and coordi­nates grain computations on remote grain servers. Grain servers supply checkpoint information at regular intervals to the central scheduler or a passive backup. Grain computations on busy or failed server hosts are automatically moved by the scheduler to quiescent hosts using checkpoint information. Additionally, the system transparently recovers from temporary failure of the cen­tral scheduler or the user control program, A simple paradigm for the organization and update protocol of a reliable backing store is introduced. Other design, implementation, and operational issues are discussed. DCS was used to run the computation finding the 34 digit penultimate prime factor of a 93 digit, "more wanted" number from the Cunningham Project [BRI88]

Soap Froths and Crystal Structures

We propose a physical mechanism to explain the crystal symmetries found in macromolecular and supramolecular micellar materials. We argue that the packing entropy of the hard micellar cores is frustrated by the entropic interaction of their brush-like coronas. The latter interaction is treated as a surface effect between neighboring Voronoi cells. The observed crystal structures correspond to the Kelvin and Weaire-Phelan minimal foams. We show that these structures are stable for reasonable areal entropy densities.Comment: 4 pages, RevTeX, 2 included eps figure