74 research outputs found
Tagging of Biomedical Articles on CiteULike: A Comparison of User, Author and Professional Indexing
No full textThis paper examines the context of online indexing from the viewpoint of three different groups: users, authors, and professional indexers. User tags, author keywords and descriptors were collected from academic journal articles, which were both indexed in Pubmed and tagged on CiteULike, and analysed. Descriptive statistics, informetric measures, and thesaural term comparison shows that there are important differences in the use of keywords between the three groups in addition to similarities which can be used to enhance support for search and browse. While tags and author keywords were found that matched descriptors exactly, other terms which did not match but provided important expansion to the indexing lexicon were found. These additional terms could be used to enhance support for searching and browsing in article databases as well as to provide invaluable data for entry vocabulary and emergent terminology for regular updates to indexing systems. Additionally, the study suggests that tags support organisation by association to task, projects and subject while making important connections to traditional systems which classify into subject categories
Neutron scattering studies of the zinc-paratacamite mineral family
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 201-214).The magnetic properties of the geometrically frustrated quantum magnets clinoatacamite, Cu2(OH)3Cl, and herbertsmithite, ZnCu3(OH)6Cl2, are studied by means of neutron scattering measurements as well as specific heat, susceptibility, and mag-netization measurements. These materials are studied to investigate the nature of the ground state of the spin-1 2 kagomé lattice antiferromagnet, as such a system is considered ideal for the emergence of spin liquid physics. Clinoatacamite, a distorted kagomé lattice antiferromagnet with weak inter-plane coupling, undergoes a Néel or- dering transition at TN ~/~ 6.2 K and shows evidence of a static local moment in the disordered phase below 18 K. Our experiments suggest two-dimensional Ising fluctuations at the Néel transition. A proposed spin ordering model is developed that suggests an order structure below TN and two-dimensional short range order of the kagomé plane spins up to 18 K. The inelastic spectrum is analyzed in terms of spin waves in an ordered kagomé lattice antiferromagnet with a Dzyaloshinskii-Moriya interaction. Herbertsmithite is the first structurally perfect spin- 1 2 kagomé lattice antiferromagnet. Susceptibility, specific heat, and neutron scattering measurements show no sign of any spin freezing or transition to a long range ordered state down to 50 mK. The data shows magnetic excitations extending adjacent to the ground state, suggesting the lack of any measurable spin gap. Several hypotheses are explored as possible explanations for the apparent lack of a spin gap.(cont.) Dynamic susceptibility data display an unusual scaling relation, suggesting proximity to a quantum critical point. In sum, a wide range of data suggest that herbertsmithite displays a disordered gapless spin liquid ground state.by Joel Strader Helton.Ph.D
Mercer 5: A probable new globular cluster in the Galactic bulge
No full textWe present a detailed study of a dust-obscured Galactic star cluster Mercer 5 ([MCM2005b] 5) in an extremely crowded field in the Milky Way. Near-infrared (near-IR) photometry from United Kingdom Infrared Digital Sky Surveys (UKIDSS) and the Son of ISAAC on the New Technology Telescope (SofI/NTT), combined with near-IR spectroscopy also from SofI, indicates that it is almost certainly a Galactic globular cluster, located at the edge of the Galactic bulge. The cluster suffers ~9 mag of visual extinction, with strong evidence for an extinction gradient across the cluster. A simulation of the differential reddening in the cluster using empirical data from NGC 6539 (chosen because it had high signal-to-noise ratio data and low field star contamination) as a template mimics the observations extremely well. This simulation and other arguments are used to indicate that the most prominent clump of stars in the colour-magnitude diagrams is a horizontal branch clump. On this basis we conclude that the cluster is at a distance of ~5.5kpc and suffers from visual extinction ranging from ~8.5 to ~12.5 mag. Alternative explanations for its nature, such as a young cluster or an old open cluster, are much less likely, on the grounds of no visible main sequence or stars with IR excesses for the former and location versus lifetime arguments for the latter. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Topographic studies of Torpedo acetylcholine receptor subunits as a transmembrane complex
No full textThe exposure of the four subunits of the acetylcholine receptor from Torpedo californica on both the extracellular and cytoplasmic faces of the postsynaptic membranes of the electroplaque cells has been investigated. Sealed membrane vesicles containing no protein components other than the receptor were isolated and were shown to have 95% of their synaptic surfaces facing the medium. The susceptibility of the four receptor subunits in these preparations to hydrolysis by trypsin both from the external and from the internal medium was used to investigate the exposure of the subunits on the synaptic and cytoplasmic surfaces of the membrane. It was shown by sodium dodecyl sulfate gel electrophoresis of the tryptic products that all four subunits are exposed on the extracellular surface to a similar degree. All four subunits are also exposed on the internal surface of the membrane, but the apparent degree of exposure varies with the subunit size, the larger subunits being more exposed. The results are discussed in terms of a possible topographic model of the receptor as a transmembrane protein complex
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