123,969 research outputs found
Axisymmetric buckling of a spherical shell embedded in an elastic medium under uniaxial stress at infinity
The problem of a thin spherical linearly-elastic shell, perfectly bonded to an infinite linearly-elastic medium is considered. A constant axisymmetric stress field is applied at infinity in the matrix, and the displacement and stress fields in the shell and matrix are evaluated by means of harmonic potential functions. In order to examine the stability of this solution, the buckling problem of a shell which experiences this deformation is considered. Using Koiter's nonlinear shallow shell theory, restricting buckling patterns to those which are axisymmetric, and using the Rayleigh–Ritz method by expanding the buckling patterns in an infinite series of Legendre functions, an eigenvalue problem for the coefficients in the infinite series is determined. This system is truncated and solved numerically in order to analyse the behaviour of the shell as it undergoes buckling, and to identify the critical buckling stress in two cases — namely where the shell is hollow and the stress at infinity is either uniaxial or radial
Influence of Functional Groups on Charge Transport in Molecular Junctions
Using density functional theory (DFT), we analyze the influence of five
classes of functional groups, as exemplified by NO2, OCH3, CH3, CCl3, and I, on
the transport properties of a 1,4-benzenedithiolate (BDT) and
1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis
demonstrates how ideas from functional group chemistry may be used to engineer
a molecule's transport properties, as was shown experimentally and using a
semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show
that the qualitative change in conductance due to a given functional group can
be predicted from its known electronic effect (whether it is pi/sigma
donating/withdrawing). However, the influence of functional groups on a
molecule's conductance is very weak, as was also found in the BDA experiments.
The calculated DFT conductances for the BDA species are five times larger than
the experimental values, but good agreement is obtained after correcting for
self-interaction and image charge effects.Comment: 6 pages, 3 figures, J. Chem. Phys (in press
Algorithms for the workflow satisfiability problem engineered for counting constraints
The workflow satisfiability problem (WSP) asks whether there exists an
assignment of authorized users to the steps in a workflow specification that
satisfies the constraints in the specification. The problem is NP-hard in
general, but several subclasses of the problem are known to be fixed-parameter
tractable (FPT) when parameterized by the number of steps in the specification.
In this paper, we consider the WSP with user-independent counting constraints,
a large class of constraints for which the WSP is known to be FPT. We describe
an efficient implementation of an FPT algorithm for solving this subclass of
the WSP and an experimental evaluation of this algorithm. The algorithm
iteratively generates all equivalence classes of possible partial solutions
until, whenever possible, it finds a complete solution to the problem. We also
provide a reduction from a WSP instance to a pseudo-Boolean SAT instance. We
apply this reduction to the instances used in our experiments and solve the
resulting PB SAT problems using SAT4J, a PB SAT solver. We compare the
performance of our algorithm with that of SAT4J and discuss which of the two
approaches would be more effective in practice
Coarsening of Two Dimensional Foam on a Dome
In this paper we report on bubble growth rates and on the statistics of
bubble topology for the coarsening of a dry foam contained in the narrow gap
between two hemispheres. By contrast with coarsening in flat space, where
six-sided bubbles neither grow nor shrink, we observe that six sided bubbles
grow with time at a rate that depends on their size. This result agrees with
the modification to von Neumann's law predicted by J.E. Avron and D. Levine.
For bubbles with a different number of sides, except possibly seven, there is
too much noise in the growth rate data to demonstrate a difference with
coarsening in flat space. In terms of the statistics of bubble topology, we
find fewer 3, 4, and 5 sided bubbles, and more 6 and greater sided bubbles, in
comparison with the stationary distribution for coarsening in flat space. We
also find good general agreement with the Aboav-Weaire law for the average
number of sides of the neighbors of an n-sided bubble
The (2√3×3)rect. phase of alkylthiolate self-assembled monolayers on Au(111): a symmetry-constrained structural solution
Low-energy electron-diffraction (LEED) patterns of the Au(111)(2√3×3)rect.-butylthiolate surface phase (a structure also seen in longer alkane chain thiolate self-assembled monolayers) show missing diffracted beams characteristic of glide symmetry, but do not show the larger set of missing beams found in surface x-ray diffraction (SXRD). The difference can be attributed to the greatly enhanced role of multiple scattering in LEED, but the combination of symmetry constraints placed on possible structural models by the observed SXRD and LEED beam extinctions greatly reduces the number of possible structural models. Only three such models are identified, one of which is clearly incompatible with other published experimental data. The relative merits of the remaining models, both involving Au adatom-thiolate moieties, are discussed in the light of the results of previous experimental studies
The Propagation and Survival of Interstellar Grains
In this paper we discuss the propagation of dust through the interstellar
medium (ISM), and describe the destructive effects of stellar winds, jets, and
supernova shock waves on interstellar dust. We review the probability that
grains formed in stellar outflows or supernovae survive processing in and
propagation through the ISM, and incorporate themselves relatively unprocessed
into meteoritic bodies in the solar system. We show that very large (radii >= 5
micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the
pre-solar SiC and diamond grains extracted from meteorites, can survive the
passage through 100\kms shock waves relatively unscathed. High velocity (>= 250
km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the
stardust never encountered such fast shocks before incorporation into the solar
system. All grains should therefore retain traces of their passage through
interstellar shocks during their propagation through the ISM. The grain
surfaces should show evidence of processing due to sputtering and pitting due
to small grain cratering collisions on the micron-sized grains. This conclusion
seems to be in conflict with the evidence from the large grains recovered from
meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical
Implications of the Laboratory Study of Presolar Materials" AIP Conference
Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.
Experimental criteria for steering and the Einstein-Podolsky-Rosen paradox
We formally link the concept of steering (a concept created by Schrodinger
but only recently formalised by Wiseman, Jones and Doherty [Phys. Rev. Lett.
98, 140402 (2007)] and the criteria for demonstrations of
Einstein-Podolsky-Rosen (EPR) paradox introduced by Reid [Phys. Rev. A, 40, 913
(1989)]. We develop a general theory of experimental EPR-steering criteria,
derive a number of criteria applicable to discrete as well as
continuous-variables observables, and study their efficacy in detecting that
form of nonlocality in some classes of quantum states. We show that previous
versions of EPR-type criteria can be rederived within this formalism, thus
unifying these efforts from a modern quantum-information perspective and
clarifying their conceptual and formal origin. The theory follows in close
analogy with criteria for other forms of quantum nonlocality (Bell-nonlocality,
entanglement), and because it is a hybrid of those two, it may lead to insights
into the relationship between the different forms of nonlocality and the
criteria that are able to detect them.Comment: Changed title, updated references, minor corrections, added
journal-ref and DO
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