9,715 research outputs found
The random phase approximation applied to ice
Standard density functionals without van der Waals interactions yield an
unsatisfactory description of ice phases, specifically, high density phases
occurring under pressure are too unstable compared to the common low density
phase I observed at ambient conditions. Although the description is
improved by using functionals that include van der Waals interactions, the
errors in relative volumes remain sizable. Here we assess the random phase
approximation (RPA) for the correlation energy and compare our results to
experimental data as well as diffusion Monte Carlo data for ice. The RPA yields
a very balanced description for all considered phases, approaching the accuracy
of diffusion Monte Carlo in relative energies and volumes. This opens a route
towards a concise description of molecular water phases on surfaces and in
cavities
Pulsar timing analysis in the presence of correlated noise
Pulsar timing observations are usually analysed with least-square-fitting
procedures under the assumption that the timing residuals are uncorrelated
(statistically "white"). Pulsar observers are well aware that this assumption
often breaks down and causes severe errors in estimating the parameters of the
timing model and their uncertainties. Ad hoc methods for minimizing these
errors have been developed, but we show that they are far from optimal.
Compensation for temporal correlation can be done optimally if the covariance
matrix of the residuals is known using a linear transformation that whitens
both the residuals and the timing model. We adopt a transformation based on the
Cholesky decomposition of the covariance matrix, but the transformation is not
unique. We show how to estimate the covariance matrix with sufficient accuracy
to optimize the pulsar timing analysis. We also show how to apply this
procedure to estimate the spectrum of any time series with a steep red
power-law spectrum, including those with irregular sampling and variable error
bars, which are otherwise very difficult to analyse.Comment: Accepted by MNRA
Sport Mega-Events and Public Opposition: A Sociological Study of the London 2012 Olympics
This article examines the diverse forms of public opposition, protest, criticism, and complaint in the United Kingdom on the staging of the 2012 Olympic and Paralympic Games in London. Our discussion draws heavily on empirical research, primarily fieldwork and interviews in East London with local residents, opposition groups, business people, politicians, and other stakeholders. The article is separated into three main parts. First, we explore the setting and political?economic context for London 2012. The main Olympic setting?the London Borough of Newham?features very high levels of poverty and ethnic diversity. We argue that London 2012 represented a form of ?festival capitalism? that was part of a broader set of ?New Right two-step? policies in poor urban areas, involving initial Keynesian investment, followed by a deeper and far-reaching array of neo-liberal measures. Second, in the main part of the article, we identify and examine, in turn, six forms of public conflict, criticism, and complaint that centered on the Games, specifically national criticisms (e.g., on distribution of Olympic resources), local criticisms (e.g., on lack of jobs and business benefits), issue-specific campaigns (e.g., on the environment), ?glocal? protests against specific nations and sponsors (e.g., campaigns against BP, Dow, and Rio Tinto), neo-tribal transgressions and situationist spectacles (e.g., mass cycle rides near Olympic venues), and anti-Olympic forums and demonstrations (e.g., critical web sites, multi-group marches). Third, we set out briefly the importance of conducting research into critics and opponents of sport mega-events, and discuss different arguments on how the social impact of protest movements might have been intensified at London 2012. The findings in this article may be extended to examine critical public responses to the hosting of other mega-events in different settings
A Bayesian parameter estimation approach to pulsar time-of-arrival analysis
The increasing sensitivities of pulsar timing arrays to ultra-low frequency
(nHz) gravitational waves promises to achieve direct gravitational wave
detection within the next 5-10 years. While there are many parallel efforts
being made in the improvement of telescope sensitivity, the detection of stable
millisecond pulsars and the improvement of the timing software, there are
reasons to believe that the methods used to accurately determine the
time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More
specifically, the determination of the uncertainties on these TOAs, which
strongly affect the ability to detect GWs through pulsar timing, may be
unreliable. We propose two Bayesian methods for the generation of pulsar TOAs
starting from pulsar "search-mode" data and pre-folded data. These methods are
applied to simulated toy-model examples and in this initial work we focus on
the issue of uncertainties in the folding period. The final results of our
analysis are expressed in the form of posterior probability distributions on
the signal parameters (including the TOA) from a single observation.Comment: 16 pages, 4 figure
Ab-initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge
In order to search for the magnetic ground state of surface nanostructures we
extended first principles adiabatic spin dynamics to the case of fully
relativistic electron scattering. Our method relies on a constrained density
functional theory whereby the evolution of the orientations of the spin-moments
results from a semi-classical Landau-Lifshitz equation. This approach is
applied to a study of the ground state of a finite Co chain placed along a step
edge of a Pt(111) surface. As far as the ground state spin orientation is
concerned we obtain excellent agreement with the experiment. Furthermore we
observe noncollinearity of the atom-resolved spin and orbital moments. In terms
of magnetic force theorem calculations we also demonstrate how a reduction of
symmetry leads to the existence of canted magnetic states.Comment: 4 pages, ReVTeX + 3 figures (Encapsulated Postscript), submitted to
PR
Detecting Pulsars with Interstellar Scintillation in Variance Images
Pulsars are the only cosmic radio sources known to be sufficiently compact to
show diffractive interstellar scintillations. Images of the variance of radio
signals in both time and frequency can be used to detect pulsars in large-scale
continuum surveys using the next generation of synthesis radio telescopes. This
technique allows a search over the full field of view while avoiding the need
for expensive pixel-by-pixel high time resolution searches. We investigate the
sensitivity of detecting pulsars in variance images. We show that variance
images are most sensitive to pulsars whose scintillation time-scales and
bandwidths are close to the subintegration time and channel bandwidth.
Therefore, in order to maximise the detection of pulsars for a given radio
continuum survey, it is essential to retain a high time and frequency
resolution, allowing us to make variance images sensitive to pulsars with
different scintillation properties. We demonstrate the technique with
Murchision Widefield Array data and show that variance images can indeed lead
to the detection of pulsars by distinguishing them from other radio sources.Comment: 8 papes, 9 figures, accepted for publication in MNRA
Monellin (MNEI) at 1.15 Å resolution
The crystal structure of the sweet protein MNEI at 1.15 Å resolution reveals networks of alternate conformations and stably bound negative ions
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