8,573 research outputs found
Surface Plasmon Dispersion Relations in Chains of Metallic Nanoparticles: Exact Quasistatic Calculation
We calculate the surface plasmon dispersion relations for a periodic chain of
spherical metallic nanoparticles in an isotropic host, including all multipole
modes in a generalized tight-binding approach. For sufficiently small particles
(, where is the wave vector and is the interparticle
separation), the calculation is exact. The lowest bands differ only slightly
from previous point-dipole calculations provided the particle radius , but differ substantially at smaller separation. We also
calculate the dispersion relations for many higher bands, and estimate the
group velocity and the exponential decay length for energy
propagation for the lowest two bands due to single-grain damping. For
, the result for is in qualitative agreement with experiments
on gold nanoparticle chains, while for larger , such as ,
and are expected to be strongly -dependent because of the multipole
corrections. When , we predict novel percolation effects in the
spectrum, and find surprising symmetry in the plasmon band structure. Finally,
we reformulate the band structure equations for a Drude metal in the time
domain, and suggest how to include localized driving electric fields in the
equations of motion.Comment: 19 pages 3 figures To be published in Phy. Rev.
Reasoning about Independence in Probabilistic Models of Relational Data
We extend the theory of d-separation to cases in which data instances are not
independent and identically distributed. We show that applying the rules of
d-separation directly to the structure of probabilistic models of relational
data inaccurately infers conditional independence. We introduce relational
d-separation, a theory for deriving conditional independence facts from
relational models. We provide a new representation, the abstract ground graph,
that enables a sound, complete, and computationally efficient method for
answering d-separation queries about relational models, and we present
empirical results that demonstrate effectiveness.Comment: 61 pages, substantial revisions to formalisms, theory, and related
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Dark matter substructure modelling and sensitivity of the Cherenkov Telescope Array to Galactic dark halos
Hierarchical structure formation leads to a clumpy distribution of dark
matter in the Milky Way. These clumps are possible targets to search for dark
matter annihilation with present and future -ray instruments. Many
uncertainties exist on the clump distribution, leading to disputed conclusions
about the expected number of detectable clumps and the ensuing limits that can
be obtained from non-detection. In this paper, we use the CLUMPY code to
simulate thousands of skymaps for several clump distributions. This allows us
to statistically assess the typical properties (mass, distance, angular size,
luminosity) of the detectable clumps. Varying parameters of the clump
distributions allows us to identify the key quantities to which the number of
detectable clumps is the most sensitive. Focusing our analysis on two extreme
clump configurations, yet consistent with results from numerical simulations,
we revisit and compare various calculations made for the Fermi-LAT instrument,
in terms of number of dark clumps expected and the angular power spectrum for
the Galactic signal. We then focus on the prospects of detecting dark clumps
with the future CTA instrument, for which we make a detailed sensitivity
analysis using open-source CTA software. Based on a realistic scenario for the
foreseen CTA extragalactic survey, and accounting for a post-trial sensitivity
in the survey, we show that we obtain competitive and complementary limits to
those based on long observation of a single bright dwarf spheroidal galaxy.Comment: 29 pages + appendix, 15 figures. V2: Sects. 3.3, 4, and 5.3 extended,
results unchanged (matching accepted JCAP version
Blazes: Coordination Analysis for Distributed Programs
Distributed consistency is perhaps the most discussed topic in distributed
systems today. Coordination protocols can ensure consistency, but in practice
they cause undesirable performance unless used judiciously. Scalable
distributed architectures avoid coordination whenever possible, but
under-coordinated systems can exhibit behavioral anomalies under fault, which
are often extremely difficult to debug. This raises significant challenges for
distributed system architects and developers. In this paper we present Blazes,
a cross-platform program analysis framework that (a) identifies program
locations that require coordination to ensure consistent executions, and (b)
automatically synthesizes application-specific coordination code that can
significantly outperform general-purpose techniques. We present two case
studies, one using annotated programs in the Twitter Storm system, and another
using the Bloom declarative language.Comment: Updated to include additional materials from the original technical
report: derivation rules, output stream label
Identifying Independence in Relational Models
The rules of d-separation provide a framework for deriving conditional
independence facts from model structure. However, this theory only applies to
simple directed graphical models. We introduce relational d-separation, a
theory for deriving conditional independence in relational models. We provide a
sound, complete, and computationally efficient method for relational
d-separation, and we present empirical results that demonstrate effectiveness.Comment: This paper has been revised and expanded. See "Reasoning about
Independence in Probabilistic Models of Relational Data"
http://arxiv.org/abs/1302.438
Count-Free Single-Photon 3D Imaging with Race Logic
Single-photon cameras (SPCs) have emerged as a promising technology for
high-resolution 3D imaging. A single-photon 3D camera determines the round-trip
time of a laser pulse by capturing the arrival of individual photons at each
camera pixel. Constructing photon-timestamp histograms is a fundamental
operation for a single-photon 3D camera. However, in-pixel histogram processing
is computationally expensive and requires large amount of memory per pixel.
Digitizing and transferring photon timestamps to an off-sensor histogramming
module is bandwidth and power hungry. Here we present an online approach for
distance estimation without explicitly storing photon counts. The two key
ingredients of our approach are (a) processing photon streams using race logic,
which maintains photon data in the time-delay domain, and (b) constructing
count-free equi-depth histograms. Equi-depth histograms are a succinct
representation for ``peaky'' distributions, such as those obtained by an SPC
pixel from a laser pulse reflected by a surface. Our approach uses a binner
element that converges on the median (or, more generally, to another quantile)
of a distribution. We cascade multiple binners to form an equi-depth
histogrammer that produces multi-bin histograms. Our evaluation shows that this
method can provide an order of magnitude reduction in bandwidth and power
consumption while maintaining similar distance reconstruction accuracy as
conventional processing methods.Comment: Accepted for presentation at the 2023 International Conference on
Computational Photograph
Data Near Here: Bringing Relevant Data Closer to Scientists
Large scientific repositories run the risk of losing value as their holdings expand, if it means increased effort for a scientist to locate particular datasets of interest. We discuss the challenges that scientists face in locating relevant data, and present our work in applying Information Retrieval techniques to dataset search, as embodied in the Data Near Here application
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