1,180 research outputs found
Dark energy records in lensed cosmic microwave background
We consider the weak lensing effect induced by linear cosmological
perturbations on the cosmic microwave background (CMB) polarization
anisotropies. We find that the amplitude of the lensing peak in the BB mode
power spectrum is a faithful tracer of the dark energy dynamics at the onset of
cosmic acceleration. This is due to two reasons. First, the lensing power is
non-zero only at intermediate redshifts between the observer and the source,
keeping record of the linear perturbation growth rate at the corresponding
epoch. Second, the BB lensing signal is expected to dominate over the other
sources. The lensing distortion on the TT and EE spectra do exhibit a similar
dependence on the dark energy dynamics, although those are dominated by primary
anisotropies. We investigate and quantify the effect by means of exact tracking
quintessence models, as well as parameterizing the dark energy equation of
state in terms of the present value () and its asymptotic value in the
past (); in the interval allowed by the present constraints on dark
energy, the variation of induces a significant change in the BB
mode lensing amplitude. A Fisher matrix analysis, under conservative
assumptions concerning the increase of the sample variance due to the lensing
non-Gaussian statistics, shows that a precision of order 10% on both
and is achievable by the future experiments probing a large sky
area with angular resolution and sensitivity appropriate to detect the lensing
effect on the CMB angular power spectrum. These results show that the CMB can
probe the differential redshift behavior of the dark energy equation of state,
beyond its average.Comment: New version including substantial text change, three more figures and
two more table
Atomic-scale compensation phenomena at polar interfaces
The interfacial screening charge that arises to compensate electric fields of
dielectric or ferroelectric thin films is now recognized as the most important
factor in determining the capacitance or polarization of ultrathin
ferroelectrics. Here we investigate using aberration-corrected electron
microscopy and density functional theory how interfaces cope with the need to
terminate ferroelectric polarization. In one case, we show evidence for ionic
screening, which has been predicted by theory but never observed. For a
ferroelectric film on an insulating substrate, we found that compensation can
be mediated by interfacial charge generated, for example, by oxygen vacancies.Comment: 3 figure
Receiver Operating Characteristic Analysis for Detecting Explosives-related Threats
The Department of Homeland Security (DHS) and the Transportation Security Administration (TSA) are interested in developing a standardized testing procedure for determining the performance of candidate detection systems. This document outlines a potential method for judging detection system performance as well as determining if combining the information from a legacy system with a new system can signiÃÂÃÂÃÂÃÂcantly improve performance. In this document, performance corresponds to the Neyman-Pearson criterion applied to the Receiver Operating Characteristic (ROC) curves of the detection systems in question. A simulation was developed to investigate how the amount of data provided by the vendor in the form of the ROC curve eÃÂÃÂÃÂäects the performance of the combined detection system. Furthermore, the simulation also takes into account the potential eÃÂÃÂÃÂäects of correlation and how this information can also impact the performance of the combined system
Suppression of Octahedral Tilts and Associated Changes of Electronic Properties at Epitaxial Oxide Heterostructure Interfaces
Epitaxial oxide interfaces with broken translational symmetry have emerged as
a central paradigm behind the novel behaviors of oxide superlattices. Here, we
use scanning transmission electron microscopy to demonstrate a direct,
quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen
octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate
how the change of crystal symmetry is accommodated. Combined with low-loss
electron energy loss spectroscopy imaging, we demonstrate a mesoscopic
antiferrodistortive phase transition and elucidate associated changes in
electronic properties in a thin layer directly adjacent to the interface
A Delphi Study exploring the barriers to Dyslexia diagnosis and support: A parent’s perspective
The Rose Report (Rose, Independent review of the primary curriculum (England); 2009) outlined a set of recommendations for the management of dyslexia in the United Kingdom after a range of issues were found. Despite these recommendations, recent reports indicate that issues are still prevalent in the diagnosis process and support offered for dyslexic children. The Delphi method was employed to gain parental consensus as to the most significant barriers to diagnosis and delivery of support for children with dyslexia, as well as solutions to overcoming these barriers. Parents of primary school children with dyslexia were recruited for the study and were presented with a three round iterative questionnaire surrounding their experience of their child's dyslexia management. Parents' experiences of their child's diagnosis were explored to provide a first-hand account of the diagnosis procedure. Two overarching issues were identified: parents perceive that teachers have a lack of training around dyslexia, both initially and from continued professional development, and parents believe there is insufficient funding for dyslexia in schools and local authorities. Overall, the study indicated that better guidance is needed to ensure that reform and spending leads to tangible change in the identification of dyslexia and provision of support for children with dyslexia in primary education in the United Kingdom
Self-avoiding walks crossing a square
We study a restricted class of self-avoiding walks (SAW) which start at the
origin (0, 0), end at , and are entirely contained in the square on the square lattice . The number of distinct
walks is known to grow as . We estimate as well as obtaining strict upper and lower bounds,
We give exact results for the number of SAW of
length for and asymptotic results for .
We also consider the model in which a weight or {\em fugacity} is
associated with each step of the walk. This gives rise to a canonical model of
a phase transition. For the average length of a SAW grows as ,
while for it grows as
. Here is the growth constant of unconstrained SAW in . For we provide numerical evidence, but no proof, that the
average walk length grows as .
We also consider Hamiltonian walks under the same restriction. They are known
to grow as on the same lattice. We give
precise estimates for as well as upper and lower bounds, and prove that
Comment: 27 pages, 9 figures. Paper updated and reorganised following
refereein
General duality for abelian-group-valued statistical-mechanics models
We introduce a general class of statistical-mechanics models, taking values
in an abelian group, which includes examples of both spin and gauge models,
both ordered and disordered. The model is described by a set of ``variables''
and a set of ``interactions''. A Gibbs factor is associated to each variable
and to each interaction. We introduce a duality transformation for systems in
this class. The duality exchanges the abelian group with its dual, the Gibbs
factors with their Fourier transforms, and the interactions with the variables.
High (low) couplings in the interaction terms are mapped into low (high)
couplings in the one-body terms. The idea is that our class of systems extends
the one for which the classical procedure 'a la Kramers and Wannier holds, up
to include randomness into the pattern of interaction. We introduce and study
some physical examples: a random Gaussian Model, a random Potts-like model, and
a random variant of discrete scalar QED. We shortly describe the consequence of
duality for each example.Comment: 26 pages, 2 Postscript figure
Measurement of the Electric and Magnetic Polarizabilities of the Proton
The Compton scattering cross section on the proton has been measured at
laboratory angles of 90 and 135 using tagged photons in the
energy range 70--100 MeV and simultaneously using untagged photons in the range
100--148~MeV. With the aid of dispersion relations, these cross sections were
used to extract the electric and magnetic polarizabilities, and
respectively, of the proton. We find
in agreement with a model-independent dispersion sum rule, and
where the errors shown are statistical, systematic, and model-dependent,
respectively. A comparison with previous experiments is given and global values
for the polarizabilities are extracted.Comment: 35 pages, 11 PostScript figures, uses RevTex 3.
Molecular dynamics simulations of oxide memory resistors (memristors)
Reversible bipolar nano-switches that can be set and read electronically in a
solid-state two-terminal device are very promising for applications. We have
performed molecular-dynamics simulations that mimic systems with oxygen
vacancies interacting via realistic potentials and driven by an external bias
voltage. The competing short- and long-range interactions among charged mobile
vacancies lead to density fluctuations and short-range ordering, while
illustrating some aspects of observed experimental behavior, such as memristor
polarity inversion.Comment: 15 pages, 5 figure
- …