1,688,464 research outputs found
Low temperature scale for a 1 to 20 degree Kelvin region
New temperature scale, accurate to better than plus or minus 0.001 Kelvin over low temperature region, is based on National Bureau of Standards 1955 platinum resistance thermometer scale and utilizes precise susceptibility measurements on two paramagnetic salts
Reciprocity in Social Networks with Capacity Constraints
Directed links -- representing asymmetric social ties or interactions (e.g.,
"follower-followee") -- arise naturally in many social networks and other
complex networks, giving rise to directed graphs (or digraphs) as basic
topological models for these networks. Reciprocity, defined for a digraph as
the percentage of edges with a reciprocal edge, is a key metric that has been
used in the literature to compare different directed networks and provide
"hints" about their structural properties: for example, are reciprocal edges
generated randomly by chance or are there other processes driving their
generation? In this paper we study the problem of maximizing achievable
reciprocity for an ensemble of digraphs with the same prescribed in- and
out-degree sequences. We show that the maximum reciprocity hinges crucially on
the in- and out-degree sequences, which may be intuitively interpreted as
constraints on some "social capacities" of nodes and impose fundamental limits
on achievable reciprocity. We show that it is NP-complete to decide the
achievability of a simple upper bound on maximum reciprocity, and provide
conditions for achieving it. We demonstrate that many real networks exhibit
reciprocities surprisingly close to the upper bound, which implies that users
in these social networks are in a sense more "social" than suggested by the
empirical reciprocity alone in that they are more willing to reciprocate,
subject to their "social capacity" constraints. We find some surprising linear
relationships between empirical reciprocity and the bound. We also show that a
particular type of small network motifs that we call 3-paths are the major
source of loss in reciprocity for real networks
Momentum isotropisation in random potentials
When particles are multiply scattered by a random potential, their momentum
distribution becomes isotropic on average. We study this quantum dynamics
numerically and with a master equation. We show how to measure the elastic
scattering time as well as characteristic isotropisation times, which permit to
reconstruct the scattering phase function, even in rather strong disorder.Comment: 5 pages, paper contributed to Lyon BEC 2012; v2 minor changes,
version published in prin
Characterization of Si/Si_(1-y)C_y superlattices grown by surfactant assisted molecular beam epitaxy
Si/Si_(0.97)C_(0.03) superlattices grown on Si(001) substrates by Sb surfactant assisted molecular beam epitaxy are characterized by in situ reflection high energy electron diffraction (RHEED), atomic force microscopy, transmission electron microscopy (TEM), and high resolution x‐ray diffraction. The RHEED shows that, in the absence of Sb, the growth front roughens during Si_(0.97)C_(0.03) growth and smooths during subsequent Si growth. In contrast, when Sb is present, the growth front remains smooth throughout the growth. This observation is confirmed by cross‐sectional TEM, which reveals that for samples grown without the use of Sb, the Si/Si_(0.97)C_(0.03) interfaces (Si_(0.97)C_(0.03) on Si) are much more abrupt than the Si_(0.97)C_(0.03)/Si interfaces. In the case of Sb assisted growth, there is no observable difference in abruptness between the two types of interfaces. Atomic force microscopy micrographs of the Si_(0.97)C_(0.03) surface reveal features that could be the source of the roughness observed by RHEED and TEM
Compatible finite element methods for numerical weather prediction
This article takes the form of a tutorial on the use of a particular class of
mixed finite element methods, which can be thought of as the finite element
extension of the C-grid staggered finite difference method. The class is often
referred to as compatible finite elements, mimetic finite elements, discrete
differential forms or finite element exterior calculus. We provide an
elementary introduction in the case of the one-dimensional wave equation,
before summarising recent results in applications to the rotating shallow water
equations on the sphere, before taking an outlook towards applications in
three-dimensional compressible dynamical cores.Comment: To appear in ECMWF Seminar proceedings 201
Duality and zero-point length of spacetime
The action for a relativistic free particle of mass receives a
contribution from a path segment of infinitesimal length . Using
this action in a path integral, one can obtain the Feynman propagator for a
spinless particle of mass . If one of the effects of quantizing gravity is
to introduce a minimum length scale in the spacetime, then one would
expect the segments of paths with lengths less than to be suppressed in
the path integral. Assuming that the path integral amplitude is invariant under
the `duality' transformation , one can calculate the modified
Feynman propagator. I show that this propagator is the same as the one obtained
by assuming that: quantum effects of gravity leads to modification of the
spacetime interval to . This equivalence suggests a
deep relationship between introducing a `zero-point-length' to the spacetime
and postulating invariance of path integral amplitudes under duality
transformations.Comment: Revtex document; 4 page
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