4,713 research outputs found
Dynamics from diffraction
A model-independent approach for the extraction of detailed
lattice dynamical information from neutron powder diffraction data is described. The technique is based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement.
Phonon dispersion curves extracted in this way are shown to
reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. The extent to which diffraction data are sensitive to lattice dynamics is explored in a
range of materials. The prospect that such detailed dynamical information might be accessible using comparatively facile experiments such as neutron
powder diffraction is incredibly valuable when studying systems for which established spectroscopic methods are prohibitive or
inappropriate
Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions
By using a symmetry motivated basis to evaluate local distortions against
pair distribution function data (PDF), we show without prior bias, that the
off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone
centred and rhombohedral-like in nature across its known ferroelectric and
paraelectric phases. With our newly-gained insight we construct a simple Monte
Carlo (MC) model which captures our main experimental findings and demonstrate
how the rich crystallographic phase diagram of BaTiO3 emerges from correlations
of local symmetry-breaking distortions alone. Our results strongly support the
order-disorder picture for these phase transitions, but can also be reconciled
with the soft-mode theory of BaTiO3 that is supported by some spectroscopic
techniques.Comment: 5 pages, 3 figure
A zinc transporter gene required for development of the nervous system.
The essentiality of zinc for normal brain development is well established. It has been suggested that primary and secondary zinc deficiencies can contribute to the occurrence of numerous human birth defects, including many involving the central nervous system. In a recent study, we searched for zinc transporter genes that were critical for neurodevelopment. We confirmed that ZIP12 is a zinc transporter encoded by the gene slc39a12 that is highly expressed in the central nervous systems of human, mouse, and frog (Xenopus tropicalis).Using loss-of-function methods, we determined that ZIP12 is required for neuronal differentiation and neurite outgrowth and necessary for neurulation and embryonic viability. These results highlight an essential need for zinc regulation during embryogenesis and nervous system development. We suggest that slc39a12 is a candidate gene for inherited neurodevelopmental defects in humans
Parabolic groups acting on one-dimensional compact spaces
Given a class of compact spaces, we ask which groups can be maximal parabolic
subgroups of a relatively hyperbolic group whose boundary is in the class. We
investigate the class of 1-dimensional connected boundaries. We get that any
non-torsion infinite f.g. group is a maximal parabolic subgroup of some
relatively hyperbolic group with connected one-dimensional boundary without
global cut point. For boundaries homeomorphic to a Sierpinski carpet or a
2-sphere, the only maximal parabolic subgroups allowed are virtual surface
groups (hyperbolic, or virtually ).Comment: 10 pages. Added a precision on local connectedness for Lemma 2.3,
thanks to B. Bowditc
Orbital Dimer Model for Spin-Glass State in YMoO
The formation of a spin glass usually requires both structural disorder and
frustrated magnetic interactions. Consequently, the origin of spin-glass
behaviour in YMoO in which magnetic Mo ions occupy a
frustrated pyrochlore lattice with minimal compositional disorder has been
a longstanding question. Here, we use neutron and X-ray pair-distribution
function (PDF) analysis to develop a disorder model that resolves apparent
incompatibilities between previously-reported PDF, EXAFS and NMR studies and
provides a new and physical mechanism for spin-glass formation. We show that
Mo ions displace according to a local "2-in/2-out" rule on each Mo
tetrahedron, driven by orbital dimerisation of Jahn-Teller active Mo
ions. Long-range orbital order is prevented by the macroscopic degeneracy of
dimer coverings permitted by the pyrochlore lattice. Cooperative O
displacements yield a distribution of MoOMo angles, which in turn
introduces disorder into magnetic interactions. Our study demonstrates
experimentally how frustration of atomic displacements can assume the role of
compositional disorder in driving a spin-glass transition.Comment: 6 pages, 3 figure
Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision:Examples from the South African Interior
Resistant bedrock outcrops can exert control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. To examine how variation in lithologic resistance impacts landscape development in the post-orogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and Schoonspruit are widely variable, with channel bed denudation rates ranging from 14-255 m/Myr and valley side and top denudation rates ranging from 11-50 m/Myr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (~20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 kyr). These results suggest that in the post-orogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics
Virus Infections in Children in a Respiratory Intensive Care Unit during an Influenza Epidemic
The incidence and severity of virus infections in a children's respiratory intensive care unit during an influenza A epidemic were studied. Infections caused by the Port Chalmers strain of influenza A or by an adenovirus were associated with severe, often fatal, pneumonia, whereas infections caused by respiratory syncytial virus or cytomegalovirus carried a good prognosis
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