2,863 research outputs found
An anomalous alloy: Y_x Si_{1-x}
We study via density functional-based molecular dynamics the structural and
dynamical properties of the rare earth silicon amorphous alloy Y_xSi_{1-x} for
x=0.093 and x=0.156. The Si network forms cavities in which a Y^{3+} cation is
entrapped. Its electrons are transferred to the Si network and are located in
the dangling bonds of the Si atoms that line the Y cavities. This leads to the
presence of low coordinated Si atoms that can be described as monovalent or
divalent anions. For x=0.156, the cavities touch each other and share Si atoms
that have two dangling bonds. The vibrational spectrum is similar to that of
amorphous Si. However, doping induces a shoulder at 70 cm^{-1} and a pronounced
peak at 180 cm^{-1} due to low coordinated Si.Comment: 4 pages, 4 figure
First principles study of adsorbed Cu_n (n=1-4) microclusters on MgO(100): structural and electronic properties
We present a density functional study of the structural and electronic
properties of small Cu_n (n=1,4) aggregates on defect-free MgO(100). The
calculations employ a slab geometry with periodic boundary conditions,
supercells with up to 76 atoms, and include full relaxation of the surface
layer and of all adsorbed atoms. The preferred adsorption site for a single Cu
adatom is on top of an oxygen atom. The adsorption energy and Cu-O distance are
E_S-A = 0.99 eV and d_S-A = 2.04 Angstroems using the Perdew-Wang gradient
corrected exchange correlation functional. The saddle point for surface
diffusion is at the "hollow" site, with a diffusion barrier of around 0.45 eV.
For the adsorbed copper dimer, two geometries, one parallel and one
perpendicular to the surface, are very close in energy. For the adsorbed Cu_3,
a linear configuration is preferred to the triangular geometry. As for the
tetramer, the most stable adsorbed geometry for Cu_4 is a rhombus. The
adsorption energy per Cu atom decreases with increasing the size of the
cluster, while the Cu-Cu cohesive energy increases, rapidly becoming more
important than the adsorption energy.Comment: Major revision, Latex(2e) document, 23 pages, 11 figures, accepted
for publication in J. of Chem. Phys., paper available at
http://irrmawww.epfl.ch/vm/vm_wor
Are primary care factors associated with hospital episodes for adverse drug reactions? A national observational study.
Development of the CHARIOT Research Register for the Prevention of Alzheimer’s Dementia and Other Late Onset Neurodegenerative Diseases
Identifying cognitively healthy people at high risk of developing dementia is an ever-increasing focus. These individuals are essential for inclusion in observational studies into the natural history of the prodromal and early disease stages and for interventional studies aimed at prevention or disease modification. The success of this research is dependent on having access to a well characterised, representative and sufficiently large population of individuals. Access to such a population remains challenging as clinical research has, historically, focussed on patients with dementia referred to secondary and tertiary services. The primary care system in the United Kingdom allows access to a true prodromal population prior to symptoms emerging and specialist referral. We report the development and recruitment rates of the CHARIOT register, a primary care-based recruitment register for research into the prevention of dementia. The CHARIOT register was designed specifically to support recruitment into observational natural history studies of pre-symptomatic or prodromal dementia stages, and primary or secondary prevention pharmaceutical trials or other prevention strategies for dementia and other cognitive problems associated with ageing.Participants were recruited through searches of general practice lists across the west and central London regions. Invitations were posted to individuals aged between 60 and 85 years, without a diagnosis of dementia. Upon consent, a minimum data set of demographic and contact details was extracted from the patient's electronic health record.To date, 123 surgeries participated in the register, recruiting a total of 24,509 participants-a response rate of 22.3%. The age, gender and ethnicity profiles of participants closely match that of the overall eligible population. Higher response rates tended to be associated with larger practices (r = 0.34), practices with a larger older population (r = 0.27), less socioeconomically disadvantaged practices (r = 0.68), and practices with a higher proportion of White patients (r = 0.82).Response rates are comparable to other registers reported in the literature, and indicate good interest and support for a research register and for participation in research for the prevention of age-related neurodegenerative diseases and dementia. We consider that the simplicity of the approach means that this system is easily scalable and replicable across the UK and internationally
Hybrid Superconductor-Quantum Point Contact Devices using InSb Nanowires
Proposals for studying topological superconductivity and Majorana bound
states in nanowires proximity coupled to superconductors require that transport
in the nanowire is ballistic. Previous work on hybrid nanowire-superconductor
systems has shown evidence for Majorana bound states, but these experiments
were also marked by disorder, which disrupts ballistic transport. In this
letter, we demonstrate ballistic transport in InSb nanowires interfaced
directly with superconducting Al by observing quantized conductance at
zero-magnetic field. Additionally, we demonstrate that the nanowire is
proximity coupled to the superconducting contacts by observing Andreev
reflection. These results are important steps for robustly establishing
topological superconductivity in InSb nanowires
Tunneling and delocalization in hydrogen bonded systems: a study in position and momentum space
Novel experimental and computational studies have uncovered the proton
momentum distribution in hydrogen bonded systems. In this work, we utilize
recently developed open path integral Car-Parrinello molecular dynamics
methodology in order to study the momentum distribution in phases of high
pressure ice. Some of these phases exhibit symmetric hydrogen bonds and quantum
tunneling. We find that the symmetric hydrogen bonded phase possesses a
narrowed momentum distribution as compared with a covalently bonded phase, in
agreement with recent experimental findings. The signatures of tunneling that
we observe are a narrowed distribution in the low-to-intermediate momentum
region, with a tail that extends to match the result of the covalently bonded
state. The transition to tunneling behavior shows similarity to features
observed in recent experiments performed on confined water. We corroborate our
ice simulations with a study of a particle in a model one-dimensional double
well potential that mimics some of the effects observed in bulk simulations.
The temperature dependence of the momentum distribution in the one-dimensional
model allows for the differentiation between ground state and mixed state
tunneling effects.Comment: 14 pages, 13 figure
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