670 research outputs found
Solid state concentration quenching of organic fluorophores in PMMA
In this study the concentration quenching phenomenon is explored for seven organic singlet emitters (Rhodamine 6G, Pyridine 2, Lumogen F Red 305, Perylene, Coumarin 102, DCM and DCJTB) in an inert host of poly(methyl methacrylate) (PMMA). Combining fluorescence lifetime and quantum yield measurements on samples of different molecular separation allows a deep decay rate analysis to be performed yielding, for each fluorophore, a monomial power law that indicates the strength and type of interaction. The fluorophores studied exhibit interactions in between that of FRET-like dipole–dipole (R−6) and surface–surface (R−2) with many lying close to that expected for surface–dipole (R−3) interactions. With no observed dependence on molecular structure it is concluded that the concentration quenching rate in singlet emitters follows a power law as kCQ = aR−3.1±0.7 with aggregation expected to increase the magnitude of the observed power
Evolution of String-Wall Networks and Axionic Domain Wall Problem
We study the cosmological evolution of domain walls bounded by strings which
arise naturally in axion models. If we introduce a bias in the potential, walls
become metastable and finally disappear. We perform two dimensional lattice
simulations of domain wall networks and estimate the decay rate of domain
walls. By using the numerical results, we give a constraint for the bias
parameter and the Peccei-Quinn scale. We also discuss the possibility to probe
axion models by direct detection of gravitational waves produced by domain
walls.Comment: 19 pages, 7 figures; revised version of the manuscript, accepted for
publication in JCA
Retinotopic remapping of the visual system in deaf adults
Sound is a vital cue in helping hearing people orient their gaze and attention towards events outside their central line of sight, especially in the far periphery, where vision is poor. Without sound cues, deaf individuals must rely on vision as an ‘early warning system’ for peripheral events, and in fact numerous behavioural studies demonstrate that deaf adults have superior visual sensitivity, particularly to far peripheral stimuli. We asked whether an increased demand on peripheral vision throughout development might be reflected in early visual brain structures. Using functional magnetic resonance imaging (fMRI), we mapped visual field representations in 16 early, profoundly deaf adults and 16 hearing age-matched controls. To target the far periphery, we used wide-field retinotopic mapping stimuli to map visual field eccentricity out to 72°, well beyond conventional mapping studies. Deaf individuals exhibited a larger representation of the far peripheral visual field in both the primary visual cortex and the lateral geniculate nucleus of the thalamus. Importantly, this was not due to a total expansion of the visual map, as there was no difference between groups in overall size of either structure, but a smaller representation of the central visual field in the deaf group, suggesting a redistribution of neural resources. Here, we demonstrate for the first time that the demands placed on vision due to lifelong hearing loss can sculpt visual maps at the first level of inputs from the retina, increasing neural resources for processing stimuli in the far peripheral visual field
New evidence for strong nonthermal effects in Tycho's supernova remnant
For the case of Tycho's supernova remnant (SNR) we present the relation
between the blast wave and contact discontinuity radii calculated within the
nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is
demonstrated that these radii are confirmed by recently published Chandra
measurements which show that the observed contact discontinuity radius is so
close to the shock radius that it can only be explained by efficient CR
acceleration which in turn makes the medium more compressible. Together with
the recently determined new value erg of the SN
explosion energy this also confirms our previous conclusion that a TeV
gamma-ray flux of erg/(cms) is to be expected from
Tycho's SNR. Chandra measurements and the HEGRA upper limit of the TeV
gamma-ray flux together limit the source distance to kpc.Comment: 5 pages, 4 figures. Accepted for publication in Astrophysics and
Space Science, Proc. of "The Multi-Messenger Approach to High-Energy
Gamma-ray Sources (Third Workshop on the Nature of Unidentified High-Energy
Sources)", Barcelona, July 4-7, 200
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Spring-mass behavioural adaptations to acute changes in prosthetic blade stiffness during submaximal running in unilateral transtibial prosthesis users
YesBackground: Individuals with lower-limb amputation can use running specific prostheses (RSP) that store and
then return elastic energy during stance. However, it is unclear whether varying the stiffness category of the
same RSP affects spring-mass behaviour during self-selected, submaximal speed running in individuals with
unilateral transtibial amputation.
Research question: The current study investigates how varying RSP stiffness affects limb stiffness, running performance,
and associated joint kinetics in individuals with a unilateral transtibial amputation.
Methods: Kinematic and ground reaction force data were collected from eight males with unilateral transtibial
amputation who ran at self-selected submaximal speeds along a 15 m runway in three RSP stiffness conditions;
recommended habitual stiffness (HAB) and, following 10-minutes of familiarisation, stiffness categories above
(+1) and below (-1) the HAB. Stance-phase centre of mass velocity, contact time, limb stiffness’ and joint/RSP
work were computed for each limb across RSP stiffness conditions.
Results: With increased RSP stiffness, prosthetic limb stiffness increased, whilst intact limb stiffness decreased
slightly (
The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables
The temperatures of electrons and ions in the post-shock accretion region of
a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass
flow rates or for sufficiently weak magnetic fields. At lower mass flow rates
or in stronger magnetic fields, efficient cyclotron cooling will cool the
electrons faster than the electrons can cool the ions and a two-temperature
flow will result. Here we investigate the differences in polarized radiation
expected from mCV post-shock accretion columns modeled with one- and
two-temperature hydrodynamics. In an mCV model with one accretion region, a
magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along
with a relatively generic geometric orientation of the system, we find that in
the ultraviolet either a single linear polarization pulse per binary orbit or
two pulses per binary orbit can be expected, depending on the accretion column
hydrodynamic structure (one- or two-temperature) modeled. Under conditions
where the physical flow is two-temperature, one pulse per orbit is predicted
from a single accretion region where a one-temperature model predicts two
pulses. The intensity light curves show similar pulse behavior but there is
very little difference between the circular polarization predictions of one-
and two-temperature models. Such discrepancies indicate that it is important to
model some aspect of two-temperature flow in indirect imaging procedures, like
Stokes imaging, especially at the edges of extended accretion regions, were the
specific mass flow is low, and especially for ultraviolet data.Comment: Accepted for publication in Astrophysics & Space Scienc
Thermal inflation, baryogenesis and axions
In a previous paper, we proposed a simple extension of the Minimal
Supersymmetric Standard Model which gives rise to thermal inflation and
baryogenesis in a natural and remarkably consistent way. In this paper, we
consider the special case of our model, which is the minimal
way to incorporate a Peccei-Quinn symmetry. The axino/flatino becomes the
lightest supersymmetric particle with m_\axino \sim 1 to 10 \GeV and is
typically over-produced during the flaton decay. Interestingly though, the dark
matter abundance is minimized for m_\axino \sim 1 \GeV, to
10^{12} \GeV and |\mu| \sim 400 \GeV to 2 \TeV at an abundance coincident
with the observed abundance and with significant amounts of both axions and
axinos. Futhermore, for these values the baryon abundance naturally matches the
observed abundance.Comment: 53 pages, 20 figures. Figures reformatted, minor changes in Section
3, references adde
Transfer learning for galaxy morphology from one survey to another
© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.Deep Learning (DL) algorithms for morphological classification of galaxies have proven very successful, mimicking (or even improving) visual classifications. However, these algorithms rely on large training samples of labelled galaxies (typically thousands of them). A key question for using DL classifications in future Big Data surveys is how much of the knowledge acquired from an existing survey can be exported to a new dataset, i.e. if the features learned by the machines are meaningful for different data. We test the performance of DL models, trained with Sloan Digital Sky Survey (SDSS) data, on Dark Energy survey (DES) using images for a sample of 5000 galaxies with a similar redshift distribution to SDSS. Applying the models directly to DES data provides a reasonable global accuracy ( 90%), but small completeness and purity values. A fast domain adaptation step, consisting in a further training with a small DES sample of galaxies (500-300), is enough for obtaining an accuracy > 95% and a significant improvement in the completeness and purity values. This demonstrates that, once trained with a particular dataset, machines can quickly adapt to new instrument characteristics (e.g., PSF, seeing, depth), reducing by almost one order of magnitude the necessary training sample for morphological classification. Redshift evolution effects or significant depth differences are not taken into account in this study.Peer reviewedFinal Accepted Versio
Size-fractionated labile trace elements in the Northwest Pacific and Southern Oceans
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 126 (2011): 108-113, doi:10.1016/j.marchem.2011.04.004.Photosynthesis by marine phytoplankton requires bioavailable forms of several trace elements that are found in extremely low concentrations in the open ocean. We have compared the concentration, lability and size distribution (< 1 nm and < 10 nm) of a suite
of trace elements that are thought to be limiting to primary productivity as well as a toxic element (Pb) in two High Nutrient Low Chlorophyll (HNLC) regions using a new dynamic speciation technique, Diffusive Gradients in Thin-film (DGT). The labile species trapped within the DGT probes have a size that is smaller or similar than the pore size of algal cell walls and thus present a proxy for bioavailable species.
Total Dissolvable trace element concentrations (TD concentration) varied between 0.05 nM (Co) and 4.0 nM (Ni) at K2 (Northwest Pacific Ocean) and between 0.026 nM (Co) and 4.7 nM (Ni) in the Southern Ocean. The smallest size fractionated labile concentrations (< 1 nm) observed at Southern Ocean sampling stations ranged between 0.002 nM (Co) and 2.1 nM (Ni). Moreover, large differences in bioavailable fractions (ratio of labile to TD concentration) were observed between the trace elements. In the Northwest Pacific Ocean Fe, Cu and Mn had lower labile fractions (between 10 and 44%) than Co, Cd, Ni and Pb (between 80 and 100%). In the Southern Ocean a similar trend was observed, and in addition: (1) Co, Cd, Ni and Pb have lower labile fractions in the Southern Ocean than in the Northwest Pacific and (2) the ratios of <1nm to dissolvable element concentrations at some Southern Ocean stations were very low and varied between 4 and16 %.This research was supported by Federal Science Policy Office, Brussels, through contracts EV/03/7A, SD/CA/03A, the Research Foundation Flanders through grant G.0021.04 and Vrije Universiteit Brussel via grant GOA 22, as well as for K2, the VERTIGO program funding primarily by the US National Science Foundation programs in Chemical and Biological Oceanograph
Organic photovoltaic devices with enhanced efficiency processed from non-halogenated binary solvent blends
The development of processing routes to fabricate organic photovoltaic devices (OPVs) using non-halogenated solvents is a necessary step towards their eventual commercialisation. To address this issue, we have used Hansen solubility parameter analysis to identify a non-halogenated solvent blend based on a mixture of carbon disulphide and acetone. This solvent blend was then used to deposit a donor–acceptor polymer–fullerene thin-film that was then used as the active layer of bulk-heterojunction OPV. For the benchmark polymer:fullerene system PCDTBT:PC70BM, a power conversion efficiency of 6.75% was achieved; a 20% relative improvement over reference cells processed using the chlorinated-solvent chlorobenzene. Improvements in device efficiency are attributed to an increase in electron and hole conductivity resulting from enhanced fullerene crystallisation; a property that leads to enhanced device efficiency through improved charge extraction
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