18,485 research outputs found
Photon induced secondary electron emission
Numerical models for predicting photon-induced secondary electron emission are presented. The results are compared with experimental measurements made using a Co-60 gamma ray source
Wetting layer thickness and early evolution of epitaxially strained thin films
We propose a physical model which explains the existence of finite thickness
wetting layers in epitaxially strained films. The finite wetting layer is shown
to be stable due to the variation of the non-linear elastic free energy with
film thickness. We show that anisotropic surface tension gives rise to a
metastable enlarged wetting layer. The perturbation amplitude needed to
destabilize this wetting layer decreases with increasing lattice mismatch. We
observe the development of faceted islands in unstable films.Comment: 4 pages, 3 eps figure
Astronomy using basic Mark 2 very long baseline interferometry
Two experiments were performed in April and September 1976 to determine precise positions of radio sources using conventional Mark 2 VLBI techniques. Four stations in the continental United States observed at a wavelength of 18 cm. The recording bandwidth was 2 MHz. The preliminary results using analyses of fringe rate and delay are discussed and the source positions compared with the results of other measurements
Model of surface instabilities induced by stress
We propose a model based on a Ginzburg-Landau approach to study a strain
relief mechanism at a free interface of a non-hydrostatically stressed solid,
commonly observed in thin-film growth. The evolving instability, known as the
Grinfeld instability, is studied numerically in two and three dimensions.
Inherent in the description is the proper treatment of nonlinearities. We find
these nonlinearities can lead to competitive coarsening of interfacial
structures, corresponding to different wavenumbers, as strain is relieved. We
suggest ways to experimentally measure this coarsening.Comment: 4 pages (3 figures included
Women as moral pioneers? Experiences of first trimester antenatal screening
Copyright @ 2005 Elsevier Ltd.The implementation of innovative medical technologies can raise unprecedented ethical, legal and social dilemmas. This is particularly so in the area of antenatal screening, which is dominated by the language of risk and probabilities. Second trimester serum screening for Down's syndrome and neural tube defects has a well-established place in antenatal care. Increasingly, first trimester screening with biochemical and ultrasound markers is being proposed as advance on this, yielding higher detection rates of Down's syndrome at an earlier gestational age. This article explores the experiences of 14 women offered innovative first trimester screening, which takes place within the context of a detailed ultrasound scan. The study is set within the UK, where recent policy changes mean that the offer of screening for fetal anomalies, particularly Down's syndrome, will become a routine part of antenatal care and offered to all pregnant women. This paper focuses on the significance of the scan in first trimester screening, and some of the potential dilemmas for women that can result from this. It then discusses the ways in which women made their decisions about screening, in particular, their work as ‘moral pioneers’. We found that the part played by the ultrasound scan in first trimester screening, particularly in relation to the higher-quality images now being obtained, has the potential to introduce new and novel ethical dilemmas for pregnant women. Although concerns have been raised about pregnant women viewing ultrasound scans as benign, many of the women reported having thought carefully through their own moral beliefs and values prior to screening. It seems that whatever other implications they may have, first trimester screening technologies will continue the tradition of pregnant women acting as ‘moral pioneers’ in increasingly complex settings.ESRC/MRC Innovative Health Technologies Programme for funding the project (grant no: L218252042). CW acknowledges the support of The Wellcome Trust Biomedical Ethics Programme in funding her postdoctoral fellowship, which enabled her to work on this project
The effect of quantization on the FCIQMC sign problem
The sign problem in Full Configuration Interaction Quantum Monte Carlo
(FCIQMC) without annihilation can be understood as an instability of the
psi-particle population to the ground state of the matrix obtained by making
all off-diagonal elements of the Hamiltonian negative. Such a matrix, and hence
the sign problem, is basis dependent. In this paper we discuss the properties
of a physically important basis choice: first versus second quantization. For a
given choice of single-particle orbitals, we identify the conditions under
which the fermion sign problem in the second quantized basis of antisymmetric
Slater determinants is identical to the sign problem in the first quantized
basis of unsymmetrized Hartree products. We also show that, when the two
differ, the fermion sign problem is always less severe in the second quantized
basis. This supports the idea that FCIQMC, even in the absence of annihilation,
improves the sign problem relative to first quantized methods. Finally, we
point out some theoretically interesting classes of Hamiltonians where first
and second quantized sign problems differ, and others where they do not.Comment: 4 pages w/ 2 page appendix, 2 figures, 1 tabl
Stress-driven instability in growing multilayer films
We investigate the stress-driven morphological instability of epitaxially
growing multilayer films, which are coherent and dislocation-free. We construct
a direct elastic analysis, from which we determine the elastic state of the
system recursively in terms of that of the old states of the buried layers. In
turn, we use the result for the elastic state to derive the morphological
evolution equation of surface profile to first order of perturbations, with the
solution explicitly expressed by the growth conditions and material parameters
of all the deposited layers. We apply these results to two kinds of multilayer
structures. One is the alternating tensile/compressive multilayer structure,
for which we determine the effective stability properties, including the effect
of varying surface mobility in different layers, its interplay with the global
misfit of the multilayer film, and the influence of asymmetric structure of
compressive and tensile layers on the system stability. The nature of the
asymmetry properties found in stability diagrams is in agreement with
experimental observations. The other multilayer structure that we study is one
composed of stacked strained/spacer layers. We also calculate the kinetic
critical thickness for the onset of morphological instability and obtain its
reduction and saturation as number of deposited layers increases, which is
consistent with recent experimental results. Compared to the single-layer film
growth, the behavior of kinetic critical thickness shows deviations for upper
strained layers.Comment: 27 pages, 11 figures; Phys. Rev. B, in pres
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