Three-dimensional coordinates of individual atoms in materials revealed by electron tomography.

Crystallography, the primary method for determining the 3D atomic positions in crystals, has been fundamental to the development of many fields of science. However, the atomic positions obtained from crystallography represent a global average of many unit cells in a crystal. Here, we report, for the first time, the determination of the 3D coordinates of thousands of individual atoms and a point defect in a material by electron tomography with a precision of ∼19 pm, where the crystallinity of the material is not assumed. From the coordinates of these individual atoms, we measure the atomic displacement field and the full strain tensor with a 3D resolution of ∼1 nm(3) and a precision of ∼10(-3), which are further verified by density functional theory calculations and molecular dynamics simulations. The ability to precisely localize the 3D coordinates of individual atoms in materials without assuming crystallinity is expected to find important applications in materials science, nanoscience, physics, chemistry and biology.

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning.

Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide ∼4.5 nm thick covers air-exposed samples which increases the film's coercivity. HCl etching efficiently removes this oxide and in situ surface treatment of etched samples enables surface magnetic contrast to be observed in SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L(3,2) edge XMCD data is discussed.

The effect of a 1-year multiple micronutrient or n-3 fatty acid fortified food intervention on morbidity in Indian school children.

BACKGROUND/OBJECTIVES: Few studies have shown that supplementation with micronutrients (MNs) or n-3 fatty acids may have health benefits such as reduced morbidity in schoolchildren. The effect of a combination of these nutrients has never been investigated. This study aimed to determine the effect of a combination of two different doses of MN and n-3 fatty acids on morbidity in schoolchildren in Bangalore, India. SUBJECTS/METHODS: In all 598 children (6-10 years) received foods fortified with either high (100% recommended dietary allowance) or low (15% recommended dietary allowance) MN, combined with either high (900 mg α-linolenic acid (ALA) plus 100 mg docosahexaenoic acid) or low (140 mg ALA) n-3 fatty acids for 1 year. Morbidity was measured by weekly self-reports using a structured questionnaire. Poisson regression analyses of episodes/child/year and duration/episode adjusted for age and sex were performed on clusters of symptoms, including upper and lower respiratory tract infections (URTI and LRTI), gastrointestinal complaints (GI) and general symptoms of illness to observe MN and n-3 fatty acid treatment effects. RESULTS: Children consuming high n-3 fatty acids had significantly fewer episodes of URTI/child/year (relative risk (RR)=0.88, 95% confidence interval (CI): 0.79, 0.97) and significantly shorter duration/episode of URTI (RR=0.81, 95% CI: 0.78, 0.85), LRTI (RR=0.91, 95% CI: 0.85, 0.97), GI complaints (RR=0.79, 95% CI: 0.74, 0.85) and general symptoms (RR=0.90, 95% CI: 0.82, 0.98) compared with children who received low n-3 fatty acid intervention. The high MN intervention reduced the duration of general symptoms (RR=0.89, 95% CI: 0.82, 0.98). CONCLUSION: Although n-3 fatty acids may be beneficial for reducing illness in Indian schoolchildren, more research is needed to confirm presence of combined effect with MN.

Calibrating thermal and scanning tunnelling microscope induced desorption and diffusion for the chemisorbed chlorobenzene/Si(111)7 × 7 system.

The precise calibration of thermally driven processes in scanning tunnelling microscope (STM) manipulation experiments, especially at room temperature and above, is necessary to uncover an accurate picture of non-thermal dynamical processes such as desorption induced by electronic transitions, driven by the STM current. Here we probe the displacement (the sum of desorption and diffusion) of chlorobenzene molecules chemisorbed on the Si(111)-7 × 7 surface, induced both by the STM electrical current and by heat. We also establish truly passive imaging inspection parameters. The activation energy for pure thermal displacement is 580 ± 20 meV, possibly associated with excitation to a physisorbed precursor state. STM induced displacement shows a marked decrease with increasing temperature, once the thermal effects are removed.

The surface science of quasicrystals.

The surfaces of quasicrystals have been extensively studied since about 1990. In this paper we review work on the structure and morphology of clean surfaces, and their electronic and phonon structure. We also describe progress in adsorption and epitaxy studies. The paper is illustrated throughout with examples from the literature. We offer some reflections on the wider impact of this body of work and anticipate areas for future development.

Achieving epitaxy between incommensurate materials by quasicrystalline interlayers.

Epitaxial interfaces of commensurate periodic materials can be characterized by a locking into registry of their atomic structure. This characteristic is identified as a natural framework to capture the essence of epitaxy also for systems including quasicrystalline materials. The resulting general definition for epitaxy requires a matching of reciprocal lattice points. The consequences for the real space structure of an epitaxial interface between quasiperiodic and periodic materials are explored and an experimental realization of such an interface is presented. It is demonstrated that due to their higher number of reciprocal lattice basis vectors (exceeding three), quasicrystals can provide interlayers epitaxially linking incommensurate materials.

Surface phase transition in H/W(110) induced by tuning the fermi surface nesting vector by hydrogen loading.

At a hydrogen coverage of one monolayer, W(110) is known to exhibit a Fermi nesting in its electronic surface states with a nesting vector q{N} of 0.9 A{-1} along [001]. Here we show that additional H adsorption allows a controlled tuning of q{N}. As q{N} approaches the commensurate value of 1.0 A{-1}, its signature in inelastic He-atom scattering becomes more pronounced, finally disappearing as a surface charge density wave (CDW) develops and the surface symmetry changes from c(2 x 2) to a p(8 x 2) superstructure. The gradual change in q{N} is attributed to an energetic shift of the spin-polarized electronic surface states that eventually form the surface CDW.

Quasicrystalline electronic states of a one-dimensionally modulated Ag film.

Ag films on GaAs(110) exhibit a one-dimensional quasiperiodic modulation, resulting in a Fibonacci sequence of parallel stripes with two different widths. Valence level photoemission shows that the Ag electronic states acquire a unique character along the quasiperiodic direction, distinctively manifested by a hierarchy of energy level replicas and avoided crossings at characteristic intervals in reciprocal space. These observations demonstrate the strong influence of the one-dimensional quasiperiodic potential on the Ag film states.

Quasicrystalline epitaxial single element monolayers on icosahedral Al-pd-mn and decagonal Al-ni-co quasicrystal surfaces.

Single element quasicrystalline monolayers were prepared by deposition of antimony and bismuth on the fivefold surface of icosahedral Al71.5Pd21Mn8.5 and the tenfold surface of decagonal Al71.8Ni14.8Co13.4. Elastic helium atom scattering and low energy electron diffraction of the monolayers show Bragg peaks at the bulk derived positions of the clean surfaces, revealing highly ordered quasicrystalline epitaxial films. Their adatom densities of (0.9+/-0.2)x10(15) cm(-2) and (0.8+/-0.2)x10(15) cm(-2) on Al-Pd-Mn and Al-Ni-Co, respectively, correspond to roughly one adatom per Al atom of the quasicrystalline substrate surfaces.

Quasicrystalline valence bands in decagonal AlNiCo

Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials are accompanied by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model have provided theoretical insights. But the basic properties of the valence electronic states--whether they are extended as in periodic crystals or localized as in amorphous materials--are still largely unresolved. Here we investigate the electronic bandstructure of quasicrystals through angle-resolved photoemission experiments on decagonal Al71.8Ni14.8Co13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are not dominated by localization.