1,298 research outputs found
Lessons on Eternal Traversable Wormholes in AdS
We attempt to construct eternal traversable wormholes connecting two
asymptotically AdS regions by introducing a static coupling between their dual
CFTs. We prove that there are no semiclassical traversable wormholes with
Poincar\'e invariance in the boundary directions in higher than two spacetime
dimensions. We critically examine the possibility of evading our result by
coupling a large number of bulk fields. Static, traversable wormholes with less
symmetry may be possible, and could be constructed using the ingredients we
develop here.Comment: 22 pages, 4 figures. v2: minor additions, matches published versio
Piezoelectric Phononic Plates: Retrieving the Frequency Band Structure via All-electric Experiments
We propose an experimental technique based on all-electric measurements to
retrieve the frequency response of a one-dimensional piezoelectric phononic
crystal plate, structured periodically with millimeter-scaled metallic strips
on its two surfaces. The metallic electrodes, used for the excitation of
Lamb-like guided modes in the plate, ensure at the same time control of their
dispersion by means of externally loaded electric circuits that offer
non-destructive tunability in the frequency response of these structures. Our
results, in very good agreement with finite-element numerical predictions,
reveal interesting symmetry aspects that are employed to analyze the frequency
band structure of such crystals. More importantly, Lamb-like guided modes
interact with electric-resonant bands induced by inductance loads on the plate,
whose form and symmetry are discussed and analyzed in depth, showing
unprecedented dispersion characteristics.Comment: This is the version of the article before peer review or editing, as
submitted by an author to Smart Materials and Structures. IOP Publishing Ltd
is not responsible for any errors or omissions in this version of the
manuscript or any version derived from it. The Version of Record is available
online at https://doi.org/10.1088/1361-665X/ab4aa
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
Skyrmion-skyrmion and skyrmion-edge repulsions in skyrmion-based racetrack memory
Magnetic skyrmions are promising for building next-generation magnetic
memories and spintronic devices due to their stability, small size and the
extremely low currents needed to move them. In particular, skyrmion-based
racetrack memory is attractive for information technology, where skyrmions are
used to store information as data bits instead of traditional domain walls.
Here we numerically demonstrate the impacts of skyrmion-skyrmion and
skyrmion-edge repulsions on the feasibility of skyrmion-based racetrack memory.
The reliable and practicable spacing between consecutive skyrmionic bits on the
racetrack as well as the ability to adjust it are investigated. Clogging of
skyrmionic bits is found at the end of the racetrack, leading to the reduction
of skyrmion size. Further, we demonstrate an effective and simple method to
avoid the clogging of skyrmionic bits, which ensures the elimination of
skyrmionic bits beyond the reading element. Our results give guidance for the
design and development of future skyrmion-based racetrack memory.Comment: 15 pages, 6 figure
A Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence for a strong rotating jet
Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet
C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure
interferometer on 11 March, 1997. The observations were conducted in both
single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral
resolution. Images of CO emission with 1.7 to 3" angular resolution were
obtained. The ON-OFF and interferometric spectra show a velocity shift with
sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35
h. The peak position of the CO images moves perpendicularly to the spin axis
direction in the plane of the sky. This suggests the presence of a CO jet,
which is active night and day at about the same extent, and is spiralling with
nucleus rotation. The high quality of the data allows us to constrain the
characteristics of this CO jet. We have developed a 3-D model to interpret the
temporal evolution of CO spectra and maps. The CO coma is represented as the
combination of an isotropic distribution and a spiralling gas jet, both of
nucleus origin. Spectra and visibilities (the direct output of interferometric
data) analysis shows that the CO jet comprises ~40% the total CO production and
is located at a latitude ~20 degrees North on the nucleus surface. Our
inability to reproduce all observational characteristics shows that the real
structure of the CO coma is more complex than assumed, especially in the first
thousand kilometres from the nucleus. The presence of another moving CO
structure, faint but compact and possibly created by an outburst, is
identified.Comment: 20 pages, 26 figures. Accepted for publication in Astronomy &
Astrophysic
Organic Matter in Space - an Overview
Organic compounds are ubiquitous in space: they are found in diffuse clouds,
in the envelopes of evolved stars, in dense star-forming regions, in
protoplanetary disks, in comets, on the surfaces of minor planets, and in
meteorites and interplanetary dust particles. This brief overview summarizes
the observational evidence for the types of organics found in these regions,
with emphasis on recent developments. The Stardust sample-return mission
provides the first opportunity to study primitive cometary material with
sophisticated equipment on Earth. Similarities and differences between the
types of compounds in different regions are discussed in the context of the
processes that can modify them. The importance of laboratory astrophysics is
emphasized.Comment: Introductory overview lecture presented at IAU Symposium 251,
"Organic matter in space", held at Hong Kong, February 2008; to appear in IAU
Symposium 251 proceedings, Cambridge University Press, ed. S. Kwok et a
Precipitation-Redispersion of Cerium Oxide Nanoparticles with Poly(Acrylic Acid) : Towards Stable Dispersions
We exploit a precipitation-redispersion mechanism for complexation of short
chain polyelectrolytes with cerium oxide nanoparticles to extend their
stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of
monodisperse cationic nanocrystalline particles having a hydrodynamic diameter
of 10 nm and a molecular weight 400000 gmol-1. We show that short chain
uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads
to macroscopic precipitation. As the pH is increased, the solution
spontaneously redisperses into a clear solution of single particles with an
anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide
nanosols and their hybrid polymer-inorganic complexes in solution are
investigated by static and dynamic light scattering, X-ray scattering, and by
chemical analysis. Quantitative analysis of the redispersed sol gives rise to
an estimate of 40 - 50 polymer chains per particle for stable suspension. This
amount represents 20 % of the mass of the polymer-nanoparticle complexes. This
complexation adds utility to the otherwise unstable cerium oxide dispersions by
extending the range of stability of the sols in terms of pH, ionic strength and
concentration.Comment: 6 Figures, 23 pages, submitte
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