53,494 research outputs found
Numerical study of the effect of structure and geometry on van der Waals forces
We use multipolar expansions to find the force on a gold coated sphere above
a gold substrate; we study both an empty gold shell and a gold coated
polystyrene sphere. We find four characteristic separation ranges. In the first
region, which for the empty gold shell occurs for distances, d, smaller than
the thickness of the coating, the result agrees with that on a solid gold
sphere and varies as d^(-2); for larger separations there is a region where the
force behaves as if the coating is strictly two dimensional and varies as
d^(-5/2); in the third region the dependence is more unspecific; in the forth
region when d is larger than the radius, the force varies as d^(-4). For
homogeneous objects of more general shapes we introduce a numerical method
based on the solution of an integral equation for the electric field over a
system of objects with arbitrary shapes. We study the effect of shape and
orientation on the van der Waals interaction between an object and a substrate
and between two objects.Comment: 8 pages, presented in the QFEXT07 conference, submitted to Journal of
Physics
On compact hyperbolic Coxeter d-polytopes with d+4 facets
We show that there is no compact hyperbolic Coxeter d-polytope with d+4
facets for d>7. This bound is sharp: examples of such polytopes up to dimension
7 were found by Bugaenko (1984). We also show that in dimension d=7 the
polytope with 11 facets is unique.Comment: v2: the paper is rewritten. A new section added in which
7-dimensional polytopes are classified. 43 pages, a lot of figure
On prisms, M\"obius ladders and the cycle space of dense graphs
For a graph X, let f_0(X) denote its number of vertices, d(X) its minimum
degree and Z_1(X;Z/2) its cycle space in the standard graph-theoretical sense
(i.e. 1-dimensional cycle group in the sense of simplicial homology theory with
Z/2-coefficients). Call a graph Hamilton-generated if and only if the set of
all Hamilton circuits is a Z/2-generating system for Z_1(X;Z/2). The main
purpose of this paper is to prove the following: for every s > 0 there exists
n_0 such that for every graph X with f_0(X) >= n_0 vertices, (1) if d(X) >=
(1/2 + s) f_0(X) and f_0(X) is odd, then X is Hamilton-generated, (2) if d(X)
>= (1/2 + s) f_0(X) and f_0(X) is even, then the set of all Hamilton circuits
of X generates a codimension-one subspace of Z_1(X;Z/2), and the set of all
circuits of X having length either f_0(X)-1 or f_0(X) generates all of
Z_1(X;Z/2), (3) if d(X) >= (1/4 + s) f_0(X) and X is square bipartite, then X
is Hamilton-generated. All these degree-conditions are essentially
best-possible. The implications in (1) and (2) give an asymptotic affirmative
answer to a special case of an open conjecture which according to [European J.
Combin. 4 (1983), no. 3, p. 246] originates with A. Bondy.Comment: 33 pages; 5 figure
Extremal properties for dissections of convex 3-polytopes
A dissection of a convex d-polytope is a partition of the polytope into
d-simplices whose vertices are among the vertices of the polytope.
Triangulations are dissections that have the additional property that the set
of all its simplices forms a simplicial complex. The size of a dissection is
the number of d-simplices it contains. This paper compares triangulations of
maximal size with dissections of maximal size. We also exhibit lower and upper
bounds for the size of dissections of a 3-polytope and analyze extremal size
triangulations for specific non-simplicial polytopes: prisms, antiprisms,
Archimedean solids, and combinatorial d-cubes.Comment: 19 page
Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor
In most applications of ultrashort pulse lasers, temporal compressors are
used to achieve a desired pulse duration in a target or sample, and precise
temporal characterization is important. The dispersion-scan (d-scan) pulse
characterization technique usually involves using glass wedges to impart
variable, well-defined amounts of dispersion to the pulses, while measuring the
spectrum of a nonlinear signal produced by those pulses. This works very well
for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much
more difficult to measure this way. Here we demonstrate the concept of
self-calibrating d-scan, which extends the applicability of the d-scan
technique to pulses of arbitrary duration, enabling their complete measurement
without prior knowledge of the introduced dispersion. In particular, we show
that the pulse compressors already employed in chirped pulse amplification
(CPA) systems can be used to simultaneously compress and measure the temporal
profile of the output pulses on-target in a simple way, without the need of
additional diagnostics or calibrations, while at the same time calibrating the
often-unknown differential dispersion of the compressor itself. We demonstrate
the technique through simulations and experiments under known conditions.
Finally, we apply it to the measurement and compression of 27.5 fs pulses from
a CPA laser.Comment: 11 pages, 5 figures, Scientific Reports, in pres
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