National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Elastic Scattering Corrections in AES and XPS. II. Estimating Attenuation Lengths and Conditions Required for their Valid Use in Overlayer/Substrate Experiments

Abstract: We examine substrate/overlayer experiments and the equations commonly used to quantify overlayer thicknesses. Comparisons with accurate Monte-Carlo simulations show that using attenuation lengths (rather than inelastic mean free paths) eliminates most of the error due to elastic scattering without increasing the complexity of the quantification. We give attenutation lengths for 27 elements, calculated by the criterion that systematic errors in such quantifications should be minimized. These are therefore the best attenuation length values to use in layerwise quantification. We show that, provided these attenuation length values are used, the error in estimation of the thickness of an overlayer due to elastic scattering can be limited to +(5% +1 A) for an emission angle ≤58° from the surface normal, and +(10%a + 1 A) for an emission angle ≤63° from the surface normal. This accuracy is acceptable for most analytical work. Other methods (such as analytical transport theory) are much more complicated, and achieve a high precision that is often unnecessary in view of other uncertainties typically present in these experiments (such as errors due to surface morphology and diffraction effects). The results presented here, using the full theory, show that the analyst's simple straight-line approximation is in fact of adequate accuracy, provided that the correct values of attenuation length are used. Simple semi-empirical equations are presented, which allow the analyst to estimate the attenuation length for electrons of kinetic energy between 50 and 2000 eV, to a standard uncertainty of 6%.

A Facile and Novel Synthesis of Ag–Graphene-Based Nanocomposites

Abstract: Nanospacers for graphene: A facile and scalable process for the synthesis of aqueous solutions of isolated silver-decorated graphene sheets (see image) is presented. The silver–graphene nanocomposite film shows a 109-fold increase in electrical conductivity as compared to the graphite oxide film.

An Updated Edlén Equation for the Refractive Index of Air

Abstract: The Edlen equation for the refractive index of air, published in 1966, is still widely used. Subsequent to its formulation, however, improved data have become available on the density of air and the refractivity of water vapour. The practical temperature scale has also been revised and the carbon dioxide content of normal laboratory air has increased. These effects result in a discrepancy of typically 1 × 10-7. The consequent revision of the equation brings the agreement between calculation and experiment within an experimental uncertainty of ± 3 × 10-8.

Atomic clock performance enabling geodesy below the centimetre level

Abstract: The passage of time is tracked by counting oscillations of a frequency reference, such as Earth’s revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10−17 level1–5. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer’s reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented values of three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4 × 10−18, measurement instability of 3.2 × 10−19 and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of [−7 ± (5)stat ± (8)sys] × 10−19, where ‘stat’ and ‘sys’ indicate statistical and systematic uncertainty, respectively. Although sensitivity to differences in gravity potential could degrade the performance of the clocks as terrestrial standards of time, this same sensitivity can be used as a very sensitive probe of geopotential5–9. Near the surface of Earth, clock comparisons at the 1 × 10−18 level provide a resolution of one centimetre along the direction of gravity, so the performance of these clocks should enable geodesy beyond the state-of-the-art level. These optical clocks could further be used to explore geophysical phenomena10, detect gravitational waves11, test general relativity12 and search for dark matter13–17. Improved techniques allow the measurement of a frequency difference with an uncertainty of the order of 10–19 between two independent atomic optical lattice clocks, suggesting that they may be able to improve state-of-the-art geodetic techniques.

Bat algorithm for constrained optimization tasks

Abstract: In this study, we use a new metaheuristic optimization algorithm, called bat algorithm (BA), to solve constraint optimization tasks. BA is verified using several classical benchmark constraint problems. For further validation, BA is applied to three benchmark constraint engineering problems reported in the specialized literature. The performance of the bat algorithm is compared with various existing algorithms. The optimal solutions obtained by BA are found to be better than the best solutions provided by the existing methods. Finally, the unique search features used in BA are analyzed, and their implications for future research are discussed in detail.