2,511 research outputs found
A numerical study of the development of bulk scale-free structures upon growth of self-affine aggregates
During the last decade, self-affine geometrical properties of many growing
aggregates, originated in a wide variety of processes, have been well
characterized. However, little progress has been achieved in the search of a
unified description of the underlying dynamics. Extensive numerical evidence
has been given showing that the bulk of aggregates formed upon ballistic
aggregation and random deposition with surface relaxation processes can be
broken down into a set of infinite scale invariant structures called "trees".
These two types of aggregates have been selected because it has been
established that they belong to different universality classes: those of
Kardar-Parisi-Zhang and Edward-Wilkinson, respectively. Exponents describing
the spatial and temporal scale invariance of the trees can be related to the
classical exponents describing the self-affine nature of the growing interface.
Furthermore, those exponents allows us to distinguish either the compact or
non-compact nature of the growing trees. Therefore, the measurement of the
statistic of the process of growing trees may become a useful experimental
technique for the evaluation of the self-affine properties of some aggregates.Comment: 19 pages, 5 figures, accepted for publication in Phys.Rev.
Structural studies of cesium, lithium/cesium and sodium/cesium bis(trimethylsilyl)amide (HMDS) complexes
Reacting cesium fluoride with an equimolar n-hexane solution of lithium bis(trimethylsilyl)amide (LiHMDS), allows the isolation of CsHMDS (1) in 80% yield (after sublimation). This preparative route to 1 negates the need for pyrophoric Cs metal or organocesium reagents in its synthesis. If a 2:1 LiHMDS:CsF ratio is employed the heterobimetallic polymer [LiCs(HMDS)2]∞ 2 was isolated (57% yield). By combining equimolar quantities of NaHMDS and CsHMDS in hexane/toluene [NaCs(HMDS)2(toluene)]∞ 3 was isolated (62% yield). Attempts to prepare the corresponding potassium-cesium amide failed, and instead yielded the known monometallic polymer [Cs(HMDS)(toluene)]∞ 4. With the aim of expanding the structural diversity of Cs(HMDS) species, 1 was reacted with several different Lewis basic donor molecules of varying denticity; namely, (R,R)-N,N,N′,N′-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] and N,N,Nʹ,Nʹ-tetramethylethylenediamine (TMEDA), N,N,Nʹ,Nʹʹ,Nʹʹ-pentamethyldiethylenetriamine (PMDETA), tris[2-(dimethylamino)ethyl]amine (Me6-TREN) and tris[2-(2-methoxyethoxy)ethyl]amine (TMEEA). These reactions yielded dimeric [Cs(HMDS)·donor]2 5-7 [where donor is (R,R)-TMCDA, TMEDA and PMDETA respectively], the tetranuclear ‘open’-dimer [{Me6-TREN·Cs(HMDS)}2{Cs(HMDS)}2] 8 and the monomeric Cs(HMDS)·TMEEA 9. Complexes 2,3 and 5-9 were characterized by X-ray crystallography and in solution by multinuclear NMR spectroscopy
Observation of Very High Energy Gamma Rays from HESS J1804-216 with CANGAROO-III Telescopes
We observed the unidentified TeV gamma-ray source HESS J1804-216 with the
CANGAROO-III atmospheric Cerenkov telescopes from May to July in 2006. We
detected very high energy gamma rays above 600 GeV at the 10 sigma level in an
effective exposure of 76 hr. We obtained a differential flux of
(5.0+/-1.5_{stat}+/-1.6_{sys})\times 10^{-12}(E/1 TeV)^{-\alpha}
cm^{-2}s^{-1}TeV^{-1} with a photon index \alpha of 2.69 +/- 0.30_{stat} +/-
0.34_{sys}, which is consistent with that of the H.E.S.S. observation in 2004.
We also confirm the extended morphology of the source. By combining our result
with multi-wavelength observations, we discuss the possible counterparts of
HESS J1804-216 and the radiation mechanism based on leptonic and hadronic
processes for a supernova remnant and a pulsar wind nebula.Comment: 11 pages, 12 figures, Accepted in Ap
The K2-ESPRINT Project. I. Discovery of the Disintegrating Rocky Planet K2-22b with a Cometary Head and Leading Tail
We present the discovery of a transiting exoplanet candidate in the K2
Field-1 with an orbital period of 9.1457 hr: K2-22b. The highly variable
transit depths, ranging from 0\% to 1.3\%, are suggestive of a planet
that is disintegrating via the emission of dusty effluents. We characterize the
host star as an M-dwarf with K. We have obtained
ground-based transit measurements with several 1-m class telescopes and with
the GTC. These observations (1) improve the transit ephemeris; (2) confirm the
variable nature of the transit depths; (3) indicate variations in the transit
shapes; and (4) demonstrate clearly that at least on one occasion the transit
depths were significantly wavelength dependent. The latter three effects tend
to indicate extinction of starlight by dust rather than by any combination of
solid bodies. The K2 observations yield a folded light curve with lower time
resolution but with substantially better statistical precision compared with
the ground-based observations. We detect a significant "bump" just after the
transit egress, and a less significant bump just prior to transit ingress. We
interpret these bumps in the context of a planet that is not only likely
streaming a dust tail behind it, but also has a more prominent leading dust
trail that precedes it. This effect is modeled in terms of dust grains that can
escape to beyond the planet's Hill sphere and effectively undergo `Roche lobe
overflow,' even though the planet's surface is likely underfilling its Roche
lobe by a factor of 2.Comment: 22 pages, 16 figures. Final version accepted to Ap
Review of the effects of protection in marine protected areas: current knowledge and gaps
The effectiveness of marine protected areas (MPAs) and the conservation of marine environments must be based on reliable information on the quality of the marine environment that can be obtained in a reasonable timeframe. We reviewed studies that evaluated all aspects related to the effectiveness of MPAs in order to describe how the studies were conducted and to detect fields in which research is lacking. Existing parameters used to evaluate the effectiveness of MPAs are summarised. Two-hundred and twenty-two publications were reviewed. We identified the most commonly used study subjects and methodological approaches. Most of the studies concentrated on biological parameters. Peer reviewed studies were based on control vs. impact design. BACI and mBACI designs were used in very few studies. Through this review, we have identified gaps in the objectives assigned to MPAs and the way in which they have been evaluated. We suggest some guidelines aimedat improving the assessment of the effects of protection in MPAs
Kepler-47: A Transiting Circumbinary Multi-Planet System
We report the detection of Kepler-47, a system consisting of two planets
orbiting around an eclipsing pair of stars. The inner and outer planets have
radii 3.0 and 4.6 times that of the Earth, respectively. The binary star
consists of a Sun-like star and a companion roughly one-third its size,
orbiting each other every 7.45 days. With an orbital period of 49.5 days,
eighteen transits of the inner planet have been observed, allowing a detailed
characterization of its orbit and those of the stars. The outer planet's
orbital period is 303.2 days, and although the planet is not Earth-like, it
resides within the classical "habitable zone", where liquid water could exist
on an Earth-like planet. With its two known planets, Kepler-47 establishes that
close binary stars can host complete planetary systems.Comment: To appear on Science Express August 28, 11 pages, 3 figures, one
table (main text), 56 pages, 28 figures, 10 table
Identification and quantification of microplastics in wastewater using focal plane array-based reflectance micro-FT-IR imaging
Microplastics (<5 mm) have been documented in environmental samples on a global scale. While these pollutants may enter aquatic environments via wastewater treatment facilities, the abundance of microplastics in these matrices has not been investigated. Although efficient methods for the analysis of microplastics in sediment samples and marine organisms have been published, no methods have been developed for detecting these pollutants within organic-rich wastewater samples. In addition, there is no standardized method for analyzing microplastics isolated from environmental samples. In many cases, part of the identification protocol relies on visual selection before analysis, which is open to bias. In order to address this, a new method for the analysis of microplastics in wastewater was developed. A pretreatment step using 30% hydrogen peroxide (H2O2) was employed to remove biogenic material, and focal plane array (FPA)-based reflectance micro-Fourier-transform (FT-IR) imaging was shown to successfully image and identify different microplastic types (polyethylene, polypropylene, nylon-6, polyvinyl chloride, polystyrene). Microplastic-spiked wastewater samples were used to validate the methodology, resulting in a robust protocol which was nonselective and reproducible (the overall success identification rate was 98.33%). The use of FPA-based micro-FT-IR spectroscopy also provides a considerable reduction in analysis time compared with previous methods, since samples that could take several days to be mapped using a single-element detector can now be imaged in less than 9 h (circular filter with a diameter of 47 mm). This method for identifying and quantifying microplastics in wastewater is likely to provide an essential tool for further research into the pathways by which microplastics enter the environment.This work is funded by a NERC (Natural Environment Research Council) CASE studentship (NE/K007521/1) with contribution from industrial partner Fera Science Ltd., United Kingdom. The authors would like to thank Peter Vale, from Severn Trent Water Ltd, for providing access to and additionally Ashley Howkins (Brunel University London) for providing travel and assistance with the sampling of the Severn Trent wastewater treatment plant in Derbyshire, UK. We are grateful to Emma Bradley and Chris Sinclair for providing helpful suggestions for our research
Ballistic Josephson junctions in edge-contacted graphene
Hybrid graphene-superconductor devices have attracted much attention since
the early days of graphene research. So far, these studies have been limited to
the case of diffusive transport through graphene with poorly defined and modest
quality graphene-superconductor interfaces, usually combined with small
critical magnetic fields of the superconducting electrodes. Here we report
graphene based Josephson junctions with one-dimensional edge contacts of
Molybdenum Rhenium. The contacts exhibit a well defined, transparent interface
to the graphene, have a critical magnetic field of 8 Tesla at 4 Kelvin and the
graphene has a high quality due to its encapsulation in hexagonal boron
nitride. This allows us to study and exploit graphene Josephson junctions in a
new regime, characterized by ballistic transport. We find that the critical
current oscillates with the carrier density due to phase coherent interference
of the electrons and holes that carry the supercurrent caused by the formation
of a Fabry-P\'{e}rot cavity. Furthermore, relatively large supercurrents are
observed over unprecedented long distances of up to 1.5 m. Finally, in the
quantum Hall regime we observe broken symmetry states while the contacts remain
superconducting. These achievements open up new avenues to exploit the Dirac
nature of graphene in interaction with the superconducting state.Comment: Updated version after peer review. Includes supplementary material
and ancillary file with source code for tight binding simulation
A simple electron time-of-flight spectrometer for ultrafast vacuum ultraviolet photoelectron spectroscopy of liquid solutions
We present a simple electron time of flight spectrometer for time resolved photoelectron spectroscopy of liquid samples using a vacuum ultraviolet (VUV) source produced by high-harmonic generation. The field free spectrometer coupled with the time-preserving monochromator for the VUV at the Artemis facility of the Rutherford Appleton Laboratory achieves an energy resolution of 0.65 eV at 40 eV with a sub 100 fs temporal resolution. A key feature of the design is a differentially pumped drift tube allowing a microliquid jet to be aligned and started at ambient atmosphere while preserving a pressure of 10−1 mbar at the micro channel plate detector. The pumping requirements for photoelectron (PE) spectroscopy in vacuum are presented while the instrument performance is demonstrated with PE spectra of salt solutions in water. The capability of the instrument for time resolved measurements is demonstrated by observing the ultrafast (50 fs) vibrational excitation of water leading to temporary proton transfer
The Rossiter-McLaughlin effect in Exoplanet Research
The Rossiter-McLaughlin effect occurs during a planet's transit. It provides
the main means of measuring the sky-projected spin-orbit angle between a
planet's orbital plane, and its host star's equatorial plane. Observing the
Rossiter-McLaughlin effect is now a near routine procedure. It is an important
element in the orbital characterisation of transiting exoplanets. Measurements
of the spin-orbit angle have revealed a surprising diversity, far from the
placid, Kantian and Laplacian ideals, whereby planets form, and remain, on
orbital planes coincident with their star's equator. This chapter will review a
short history of the Rossiter-McLaughlin effect, how it is modelled, and will
summarise the current state of the field before describing other uses for a
spectroscopic transit, and alternative methods of measuring the spin-orbit
angle.Comment: Review to appear as a chapter in the "Handbook of Exoplanets", ed. H.
Deeg & J.A. Belmont
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