1,433 research outputs found
Spectroscopic imaging of single atoms within a bulk solid
The ability to localize, identify and measure the electronic environment of
individual atoms will provide fundamental insights into many issues in
materials science, physics and nanotechnology. We demonstrate, using an
aberration-corrected scanning transmission microscope, the spectroscopic
imaging of single La atoms inside CaTiO3. Dynamical simulations confirm that
the spectroscopic information is spatially confined around the scattering atom.
Furthermore we show how the depth of the atom within the crystal may be
estimated.Comment: 4 pages and 3 figures. Accepted in Phys.Rev.Let
Modeling the Dietary Pesticide Exposures of Young Children
A stepped approach was used to assess the exposures of 1 1/2 – 4 1/2-year-old children in the United Kingdom to residues of pesticides (dithiocarbamates; phosmet; carbendazim) found in apples and pears. The theoretical possibility that the acute reference dose (ARD) was being exceeded for a particular pesticide/fruit was tested by applying a combination of maximal variability and maximum measured residue relative to an average-body-weight consumer. The actual risk was then quantified by stochastically modeling consumption, from dietary survey data, with individual body weights, against published residue results for 2000–2002 and the variability of residue distribution within batches. The results, expressed as numbers of children per day likely to ingest more than the ARD, were in the range of 10–226.6 children per day, depending upon the pesticide and year of sampling. The implications for regulatory action are discussed
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
Surface Stress, Morphological Development, and Dislocation Nucleation During SixGe1-x Epitaxy
Utilizing Ge marker layer experiments combined with atomic number contrast (Z-contrast) imaging, we have studied the evolving surface morphology of SixGe1-x alloys during growth by molecular beam epitaxy. The marker layers map out the instability transition between planar two-dimensional (2D) growth and three-dimensional (3D) growth. The transition occurs via the gradual formation of a surface ripple as anticipated from instability theory. However, these undulations rapidly develop into crack-like surface instabilities which we simulate and explain by the mechanism of stress-driven surface diffusion. Finally, we model the large stresses associated with these features within a fracture mechanics formalism. This analysis demonstrates that crack-like instabilities provide ideal candidate sites for the nucleation of misfit dislocations
Sub-Angstrom Microscopy Through Incoherent Imaging and Image Reconstruction
Z-contrast scanning transmission electron microscopy (STEM) with a high-angle annular detector breaks the coherence of the imaging process, and provides an incoherent image of a crystal projection. Even in the presence of strong dynamical diffraction, the image can be accurately described as a convolution between an object function, sharply peaked at the projected atomic sites, and the probe intensity profile. Such an image can be inverted intuitively without the need for model structures, and therefore provides the important capability to reveal unanticipated interfacial arrangements. It represents a direct image of the crystal projection, revealing the location of the atomic columns and their relative high-angle scattering power. Since no phase is associated with a peak in the object function or the contrast transfer function, extension to higher resolution is also straightforward. Image restoration techniques such as maximum entropy, in conjunction with the 1.3 A probe anticipated for a 300 kV STEM, appear to provide a simple and robust route to the achievement of sub-Angstrom resolution electron microscopy
Thickness dependence of the exchange bias in epitaxial manganite bilayers
Exchange bias has been studied in a series of La2/3Ca1/3MnO3 / La1/3Ca2/3MnO3
bilayers grown on (001) SrTiO3 substrates by ozone-assisted molecular beam
epitaxy. The high crystalline quality of the samples and interfaces has been
verified using high-resolution X-ray diffractometry and Z-contrast scanning
transmission electron microscopy with electron energy loss spectroscopy. The
dependence of exchange bias on the thickness of the antiferromagnetic layer has
been investigated. A critical value for the onset of the hysteresis loop shift
has been determined. An antiferromagnetic anisotropy constant has been obtained
by fitting the results to the generalized Meiklejohn-Bean model.Comment: 7 pages. 8 figure
A Performance-Portable SYCL Implementation of CRK-HACC for Exascale
The first generation of exascale systems will include a variety of machine
architectures, featuring GPUs from multiple vendors. As a result, many
developers are interested in adopting portable programming models to avoid
maintaining multiple versions of their code. It is necessary to document
experiences with such programming models to assist developers in understanding
the advantages and disadvantages of different approaches.
To this end, this paper evaluates the performance portability of a SYCL
implementation of a large-scale cosmology application (CRK-HACC) running on
GPUs from three different vendors: AMD, Intel, and NVIDIA. We detail the
process of migrating the original code from CUDA to SYCL and show that
specializing kernels for specific targets can greatly improve performance
portability without significantly impacting programmer productivity. The SYCL
version of CRK-HACC achieves a performance portability of 0.96 with a code
divergence of almost 0, demonstrating that SYCL is a viable programming model
for performance-portable applications.Comment: 12 pages, 13 figures, 2023 International Workshop on Performance,
Portability & Productivity in HP
From SPEAKING to SEMIOSIS
This paper explores the quest for an account of the ‘total linguistic fact’. Speech act theory, sociolinguistics, linguistic anthropology and social semiotics have all attempted, in various ways and at various times, to find a way to describe as much as possible that is going on around any speech event. While this search for the total linguistic fact will always be a chimerical goal, this paper proposes a framework based on the acronym SEMIOSIS as one way of grasping the complexity of what is at play, comprising social relations, emotional and sensorial engagement, mobility, Iterative activity, objects and assemblages, socio- and translingual practices, interactivity, and spatial repertoires. Looking at data from a small Bangladeshi-run store in Tokyo, the paper shows how bringing in this wider set of concerns at least allows for a more comprehensive account of sociolinguistic moments.</jats:p
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