3,758 research outputs found
Theoretical X-Ray Absorption Debye-Waller Factors
An approach is presented for theoretical calculations of the Debye-Waller
factors in x-ray absorption spectra. These factors are represented in terms of
the cumulant expansion up to third order. They account respectively for the net
thermal expansion , the mean-square relative displacements
, and the asymmetry of the pair distribution function
. Similarly, we obtain Debye-Waller factors for x-ray and
neutron scattering in terms of the mean-square vibrational amplitudes .
Our method is based on density functional theory calculations of the dynamical
matrix, together with an efficient Lanczos algorithm for projected phonon
spectra within the quasi-harmonic approximation. Due to anharmonicity in the
interatomic forces, the results are highly sensitive to variations in the
equilibrium lattice constants, and hence to the choice of exchange-correlation
potential. In order to treat this sensitivity, we introduce two prescriptions:
one based on the local density approximation, and a second based on a modified
generalized gradient approximation. Illustrative results for the leading
cumulants are presented for several materials and compared with experiment and
with correlated Einstein and Debye models. We also obtain Born-von Karman
parameters and corrections due to perpendicular vibrations.Comment: 11 pages, 8 figure
Improving measurements of SF6 for the study of atmospheric transport and emissions
Sulfur hexafluoride (SF6) is a potent greenhouse gas and useful atmospheric tracer. Measurements of SF6 on global and regional scales are necessary to estimate emissions and to verify or examine the performance of atmospheric transport models. Typical precision for common gas chromatographic methods with electron capture detection (GC-ECD) is 1–2%. We have modified a common GC-ECD method to achieve measurement precision of 0.5% or better. Global mean SF6 measurements were used to examine changes in the growth rate of SF6 and corresponding SF6 emissions. Global emissions and mixing ratios from 2000–2008 are consistent with recently published work. More recent observations show a 10% decline in SF6 emissions in 2008–2009, which seems to coincide with a decrease in world economic output. This decline was short-lived, as the global SF6 growth rate has recently increased to near its 2007–2008 maximum value of 0.30±0.03 pmol mol−1 (ppt) yr−1 (95% C.L.)
Electromagnetically induced transparency in superconducting quantum circuits : Effects of decoherence, tunneling and multi-level cross-talk
We explore theoretically electromagnetically-induced transparency (EIT) in a
superconducting quantum circuit (SQC). The system is a persistent-current flux
qubit biased in a configuration. Previously [Phys. Rev. Lett. 93,
087003 (2004)], we showed that an ideally-prepared EIT system provides a
sensitive means to probe decoherence. Here, we extend this work by exploring
the effects of imperfect dark-state preparation and specific decoherence
mechanisms (population loss via tunneling, pure dephasing, and incoherent
population exchange). We find an initial, rapid population loss from the
system for an imperfectly prepared dark state. This is followed by a
slower population loss due to both the detuning of the microwave fields from
the EIT resonance and the existing decoherence mechanisms. We find analytic
expressions for the slow loss rate, with coefficients that depend on the
particular decoherence mechanisms, thereby providing a means to probe,
identify, and quantify various sources of decoherence with EIT. We go beyond
the rotating wave approximation to consider how strong microwave fields can
induce additional off-resonant transitions in the SQC, and we show how these
effects can be mitigated by compensation of the resulting AC Stark shifts
Formation of shock waves in a Bose-Einstein condensate
We consider propagation of density wave packets in a Bose-Einstein
condensate. We show that the shape of initially broad, laser-induced, density
perturbation changes in the course of free time evolution so that a shock wave
front finally forms. Our results are well beyond predictions of commonly used
zero-amplitude approach, so they can be useful in extraction of a speed of
sound from experimental data. We discuss a simple experimental setup for shock
propagation and point out possible limitations of the mean-field approach for
description of shock phenomena in a BEC.Comment: 8 pages & 6 figures, minor changes, more references, to appear in
Phys. Rev.
Towards a first principles description of phonons in NiPt disordered alloys: the role of relaxation
Using a combination of density-functional perturbation theory and the
itinerant coherent potential approximation, we study the effects of atomic
relaxation on the inelastic incoherent neutron scattering cross sections of
disordered NiPt alloys. We build on previous work, where
empirical force constants were adjusted {\it ad hoc} to agree with experiment.
After first relaxing all structural parameters within the local-density
approximation for ordered NiPt compounds, density-functional perturbation
theory is then used to compute phonon spectra, densities of states, and the
force constants. The resulting nearest-neighbor force constants are first
compared to those of other ordered structures of different stoichiometry, and
then used to generate the inelastic scattering cross sections within the
itinerant coherent potential approximation. We find that structural relaxation
substantially affects the computed force constants and resulting inelastic
cross sections, and that the effect is much more pronounced in random alloys
than in ordered alloys.Comment: 8 pages, 3 eps figures, uses revtex
Thoughts and acts of self-harm, and suicidal ideation, in online gamblers
Gambling problems have been linked to suicidal ideation and enhanced risk of suicide attempts. However, we know very little about the factors associated with either thoughts or acts of self-harm amongst people who gamble. A web-based study of 4125 online gamblers (79% males; mean age 35.5 years), analysed using hierarchical multiple regression, revealed that self-reported non-gambling-related self-harm was negatively related to age and marital status, and positively related to problematic alcohol use. Self-reported acts of self-harm both related and unrelated to gambling were associated with drug misuse. Thoughts and acts of gambling-related self-harm were associated with problem gambling, gambling involvement and parental problem gambling. All types of self-harm were associated with mood disorder symptoms, unemployment and certain gambling motivations. When tailoring assessment and interventions for individuals at risk for gambling-related deliberate self-harm, it is important to recognize that contributory factors may include some that differ from those for deliberate self-harm in general, and that there is potential value in evaluating gambling involvement and motivations, and history of parental gambling.supported by the Responsibility in Gambling Trust and the Economic and Social Research Council under grant number 164-25-000
Statistical Basis for Predicting Technological Progress
Forecasting technological progress is of great interest to engineers, policy
makers, and private investors. Several models have been proposed for predicting
technological improvement, but how well do these models perform? An early
hypothesis made by Theodore Wright in 1936 is that cost decreases as a power
law of cumulative production. An alternative hypothesis is Moore's law, which
can be generalized to say that technologies improve exponentially with time.
Other alternatives were proposed by Goddard, Sinclair et al., and Nordhaus.
These hypotheses have not previously been rigorously tested. Using a new
database on the cost and production of 62 different technologies, which is the
most expansive of its kind, we test the ability of six different postulated
laws to predict future costs. Our approach involves hindcasting and developing
a statistical model to rank the performance of the postulated laws. Wright's
law produces the best forecasts, but Moore's law is not far behind. We discover
a previously unobserved regularity that production tends to increase
exponentially. A combination of an exponential decrease in cost and an
exponential increase in production would make Moore's law and Wright's law
indistinguishable, as originally pointed out by Sahal. We show for the first
time that these regularities are observed in data to such a degree that the
performance of these two laws is nearly tied. Our results show that
technological progress is forecastable, with the square root of the logarithmic
error growing linearly with the forecasting horizon at a typical rate of 2.5%
per year. These results have implications for theories of technological change,
and assessments of candidate technologies and policies for climate change
mitigation
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Long-term trends in the foraging ecology and habitat use of an endangered species: an isotopic perspective.
Evaluating long-term drivers of foraging ecology and population productivity is crucial for providing ecological baselines and forecasting species responses to future environmental conditions. Here, we examine the trophic ecology and habitat use of North Atlantic leatherback turtles (St. Croix nesting population) and investigate the effects of large-scale oceanographic conditions on leatherback foraging dynamics. We used bulk and compound-specific nitrogen isotope analysis of amino acids (CSIA-AA) to estimate leatherback trophic position (TP) over an 18-year period, compare these estimates with TP estimates from a Pacific leatherback population, and elucidate the pre-nesting habitat use patterns of leatherbacks. Our secondary objective was to use oceanographic indices and nesting information from St. Croix leatherbacks to evaluate relationships between trophic ecology, nesting parameters, and regional environmental conditions measured by the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation. We found no change in leatherback TP over time and no difference in TP between Atlantic and Pacific leatherbacks, indicating that differences in trophic ecology between populations are an unlikely driver of the population dichotomy between Pacific and Atlantic leatherbacks. Isotope data suggested that St. Croix leatherbacks inhabit multiple oceanic regions prior to nesting, although, like their conspecifics in the Pacific, individuals exhibit fidelity to specific foraging regions. Leatherback nesting parameters were weakly related to the NAO, which may suggest that positive NAO phases benefit St. Croix leatherbacks, potentially through increases in resource availability in their foraging areas. Our data contribute to the understanding of leatherback turtle ecology and potential mechanistic drivers of the dichotomy between populations of this protected species
Vitamin D deficiency in Malawian adults with pulmonary tuberculosis : risk factors and treatment outcomes
The study was supported by a Wellcome Trust (London, UK) Clinical PhD Fellowship awarded to DS (086757/Z/08/A) and the Malawi Liverpool Wellcome Trust (MLW) Core grant from the Wellcome Trust.SETTING: Vitamin D deficiency is common in African adults with tuberculosis (TB), and may be exacerbated by the metabolic effects of anti-tuberculosis drugs and antiretroviral therapy (ART). It is unclear whether vitamin D deficiency influences response to antituberculosis treatment. OBJECTIVES : To describe risk factors for baseline vitamin D deficiency in Malawian adults with pulmonary TB, assess the relationship between serum 25-hydroxy vitamin D (25[OH]D) concentration and treatment response, and evaluate whether the administration of anti-tuberculosis drugs and ART is deleterious to vitamin D status during treatment. DESIGN: A prospective longitudinal cohort study. RESULTS : The median baseline 25(OH)D concentration of the 169 patients (58% human immunodeficiency virus [HIV] infected) recruited was 57 nmol/l; 47 (28%) had vitamin D deficiency (<50 nmol/l). Baseline 25(OH)D concentrations were lower during the cold season (P < 0.001), with food insecurity (P = 0.034) or in patients who consumed alcohol (P = 0.019). No relationship between vitamin D status and anti-tuberculosis treatment response was found. 25(OH)D concentrations increased during anti-tuberculosis treatment, irrespective of HIV status or use of ART. CONCLUSIONS : Vitamin D deficiency is common among TB patients in Malawi, but this does not influence treatment response. Adverse metabolic effects of drug treatment may be compensated by the positive impact of clinical recovery preventing exacerbation of vitamin D deficiency during anti-tuberculosis treatment.Publisher PDFPeer reviewe
Editorial: Advances in the Biology and Conservation of Marine Turtles
(First Paragraph) Marine turtles have been the subject of research over many decades, inspired by their unique life history and necessitated by their declining populations from a suite of human impacts including direct harvest, bycatch in marine fisheries, pollution, and climate change. Despite this, much about marine turtle biology has remained a mystery (Godley et al., 2008; Rees et al., 2016; Wildermann et al., 2018), but the rate of scientific discovery is increasing rapidly. As research techniques and conservation practices expand, the marine turtle research community has kept abreast of these developments and their application to marine turtles. In this special Research Topic, researchers submitted articles related to cutting-edge work in biology and conservation. The resulting 10 articles bring new insights across marine turtle movement, conservation, and methodological and analytical techniques, as well as other understudied areas and issues
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