856 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
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
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
Strongly exchange-coupled triplet pairs in an organic semiconductor
From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie Universität Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908
Quantifying, displaying and accounting for heterogeneity in the meta-analysis of RCTs using standard and generalised Q statistics
RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are.Abstract Background Clinical researchers have often preferred to use a fixed effects model for the primary interpretation of a meta-analysis. Heterogeneity is usually assessed via the well known Q and I2 statistics, along with the random effects estimate they imply. In recent years, alternative methods for quantifying heterogeneity have been proposed, that are based on a 'generalised' Q statistic. Methods We review 18 IPD meta-analyses of RCTs into treatments for cancer, in order to quantify the amount of heterogeneity present and also to discuss practical methods for explaining heterogeneity. Results Differing results were obtained when the standard Q and I2 statistics were used to test for the presence of heterogeneity. The two meta-analyses with the largest amount of heterogeneity were investigated further, and on inspection the straightforward application of a random effects model was not deemed appropriate. Compared to the standard Q statistic, the generalised Q statistic provided a more accurate platform for estimating the amount of heterogeneity in the 18 meta-analyses. Conclusions Explaining heterogeneity via the pre-specification of trial subgroups, graphical diagnostic tools and sensitivity analyses produced a more desirable outcome than an automatic application of the random effects model. Generalised Q statistic methods for quantifying and adjusting for heterogeneity should be incorporated as standard into statistical software. Software is provided to help achieve this aim.Published versio
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