52 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
Silent cerebral infarct after cardiac catheterization as detected by diffusion weighted Magnetic Resonance Imaging: a randomized comparison of radial and femoral arterial approaches
Background and objective: Cerebral microembolism detected by transcranial Doppler (TCD) occurs systematically
during cardiac catheterization, but its clinical relevance, remains unknown. Studies suggest that asymptomatic embolic
cerebral infarction detectable by diffusion-weighted (DW) MRI might exist after percutaneous cardiac interventions with
a frequency as high as 15 to 22% of cases. We have set up, for the first time, a prospective multicenter trial to assess the
rate of silent cerebral infarction after cardiac catheterization and to compare the impact of the arterial access site,
comparing radial and femoral access, on this phenomenon.
Study design: This prospective study will be performed in patients with severe aortic valve stenosis. To assess the
occurrence of cerebral infarction, all patients will undergo cerebral DW-MRI and neurological assessment within 24
hours before, and 48 hours after cardiac catheterization and retrograde catheterization of the aortic valve.
Randomization for the access site will be performed before coronary angiography. A subgroup will be monitored by
transcranial power M-mode Doppler during cardiac catheterization to observe cerebral blood flow and track emboli.
Neuropsychological tests will also be recorded in a subgroup of patients before and after the interventional procedures
to assess the impact of silent brain injury on potential cognitive decline. The primary end-point of the study is a direct
comparison of ischemic cerebral lesions as detected by serial cerebral DW-MRI between patients explored by radial
access and patients explored by femoral access. Secondary end-points include comparison of neuropsychological test
performance and number of microembolism signals observed in the two groups.
Implications: Using serial DW-MRI, silent cerebral infarction rate will be defined and the potential influence of vascular
access site will be evaluated. Silent cerebral infarction might be a major concern during cardiac catheterization and its
potential relationship to cognitive decline needs to be assessed.
Study registration: The SCIPION study is registered through National Institutes of Health-sponsored clinical trials
registry and has been assigned the Identifier: NCT 00329979
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