473 research outputs found
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the -network
Semiconductor glasses exhibit many unique optical and electronic anomalies.
We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508
(2010)) in which several of these anomalies arise from deep midgap electronic
states residing on high-strain regions intrinsic to the activated transport
above the glass transition. Here we demonstrate at the molecular level how this
scenario is realized in an important class of semiconductor glasses, namely
chalcogen and pnictogen containing alloys. Both the glass itself and the
intrinsic electronic midgap states emerge as a result of the formation of a
network composed of -bonded atomic -orbitals that are only weakly
hybridized. Despite a large number of weak bonds, these -networks are
stable with respect to competing types of bonding, while exhibiting a high
degree of structural degeneracy. The stability is rationalized with the help of
a hereby proposed structural model, by which -networks are
symmetry-broken and distorted versions of a high symmetry structure. The latter
structure exhibits exact octahedral coordination and is fully
covalently-bonded. The present approach provides a microscopic route to a fully
consistent description of the electronic and structural excitations in vitreous
semiconductors.Comment: 22 pages, 17 figures, revised version, final version to appear in J.
Chem. Phy
Job Mobility and Sorting: Theory and Evidence
I derive a measure of job mobility that reflects individuals ability to sort into the preferred jobs. Relying on the Survey of Income and Program Participation, I find that educational attainment tends to have a strong positive effect on internal (i.e., within firms) and external (i.e., between firms) job mobility. General experience and occupation-specific human capital have only a limited effect on both internal and external mobility. The impact of being versatile on an individual s external job mobility is substantial and similar in magnitude as the effect of a college degree on a high school dropout s external mobility
The Structure, Dynamics and Electronic Structure of Liquid Ag-Se Alloys Investigated by Ab Initio Simulation
Ab initio molecular-dynamics simulations have been used to investigate the
structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x)
at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations
are based on density-functional theory in the local density approximation and
on the pseudopotential plane-wave method. The reliability of the simulations is
confirmed by detailed comparisons with very recent neutron diffraction results
for the partial structure factors and radial distribution functions (RDF) of
the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the
Se-Se RDF with increasing Se content. This change is due to the formation of Se
clusters bound by covalent bonds, the Se-Se bond length being almost the same
as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for
higher x a large fraction of 3-fold coordinated Se atoms is also found. It is
shown that the equilibrium fractions of Se present as isolated atoms and in
clusters can be understood on a simple charge-balance model based on an ionic
interpretation. The Ag and Se diffusion coefficients both increase with Se
content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics
reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the
density of states with composition arise directly from the formation of Se-Se
covalent bonds. Results for the electronic conductivity obtained using the
Kubo-Greenwood approximation are in adequate agreement with experiment for
l-Ag2Se, but not for the high Se contents. Possible reasons for this are
discussed.Comment: 14 pages, Revtex, 14 Postscript figures embedded in the tex
How do surgeons decide? Conduit choice in coronary artery bypass graft surgery in the UK
OBJECTIVES
Conduits used in coronary artery bypass grafting may have significant impact on outcomes, but evidence is mixed and there is large variation in practice. This study provides insights into the opinions of the UK surgeons on conduit use and their decision-making processes.
METHODS
A questionnaire was created using the Ottawa Decision Support Framework to elicit the importance that surgeons placed on bilateral internal mammary artery grafting, skeletonization, total arterial revascularization and sequential anastomoses on a scale of 1â10. Scores âĽ8 were deemed âimportantâ and â¤3 ânot importantâ. Surgeons were asked to specify changes to practice in frail patients or emergencies. Additional questions included conduit type used, factors affecting decision-making and vein harvesting methods. Questionnaires were administered in person with data analysed centrally.
RESULTS
Ninety-seven consultant cardiac surgeons from 25 centres responded. Thirty-two percent surgeons routinely used radial arteries and 36% used right internal mammary artery. High-quality evidence contributed most to decision-making receiving a total of 328/960 points, with consultant experience being the second (255/960 points). There was a bimodal distribution of perceived importance of bilateral internal mammary artery use, with 29 (30%) âimportantâ and ânot importantâ scores each. 23% of surgeons found total arterial revascularization important. Most surgeons (64%) preferred pedicled mammary arteries. Twenty-six percent of surgeons considered sequential grafting to be important.
CONCLUSIONS
Low uptake of total arterial revascularization and bilateral internal mammary artery among the UK consultants may be due to the lack of high-quality evidence demonstrating a significant benefit. It is also possible that reluctance to use certain conduits may stem from low levels of exposure to conduits or inadequate training, particularly given the importance of consultant experience on decision-making
First principles electronic structure of spinel LiCr2O4: A possible half-metal?
We have employed first-principles electronic structure calculations to
examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5}
electronic configuration. The cell (cubic) and internal (oxygen position)
structural parameters have been obtained for this compound through structural
relaxation in the first-principles framework. Within the one-electron band
picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The
electronic structure is substantially different from the closely related and
well known rutile half-metal CrO2. In particular, we find a smaller conduction
band width in the spinel compound, perhaps as a result of the distinct topology
of the spinel crystal structure, and the reduced oxidation state. The magnetism
and half-metallicity of LiCr2O4 has been mapped in the parameter space of its
cubic crystal structure. Comparisons with superconducting LiTi2O4 (d^{0.5}),
heavy-fermion LiV2O4 (d^{1.5}) and charge-ordering LiMn2O4 (d^{3.5}) suggest
the effectiveness of a nearly-rigid band picture involving simple shifts of the
position of E_F in these very different materials. Comparisons are also made
with the electronic structure of ZnV2O4 (d^{2}), a correlated insulator that
undergoes a structural and antiferromagnetic phase transition.Comment: 9 pages, 7 Figures, version as published in PR
Learning from Jerusalem: Rethinking urban conflicts in the 21st century Introduction
Jerusalem - its past, present and future - will serve as the foundation of our understanding of the geographies of cities within contemporary urban theory and practice in the 21st century. The underlying theoretical supposition in this special feature1 is that what have been labelled con- tested cities have growing similarities to less polarized cities similarities found in the expansion of ethnic, racial and class conflicts that revolve around issues of housing, infrastructure, participation and identity. In this sense, Jerusalem represents a rather exceptional case study and demonstrates a powerful spatio-political urban pattern in the field of urban studies. The examination of Jerusalem can advance our understanding of the relationship between planning conflicts and urban geopolitics in a growing number of cities worldwide
Gene Expression Atlas updateâa value-added database of microarray and sequencing-based functional genomics experiments
Gene Expression Atlas (http://www.ebi.ac.uk/gxa) is an added-value database providing information about gene expression in different cell types, organism parts, developmental stages, disease states, sample treatments and other biological/experimental conditions. The content of this database derives from curation, re-annotation and statistical analysis of selected data from the ArrayExpress Archive and the European Nucleotide Archive. A simple interface allows the user to query for differential gene expression either by gene names or attributes or by biological conditions, e.g. diseases, organism parts or cell types. Since our previous report we made 20 monthly releases and, as of Release 11.08 (August 2011), the database supports 19 species, which contains expression data measured for 19â014 biological conditions in 136â551 assays from 5598 independent studies
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