1,217 research outputs found
Phonons of Metallic Vicinal Surfaces
We present an analysis of the vibrational dynamics of metal vicinal surfaces
using the embedded atom method to describe the interaction potential and both a
real space Green's function method and a slab method to calculate the phonons.
We report two main general characteristics : a global shift of the surface
vibrational density of states resulting from a softening of the force field.
The latter is a direct result of the reduction of coordination for the
different type of surface atoms; and an appearance of high frequency modes
above the bulk band, resulting from a stiffening of the force field near the
step atom. The latter is due to a rearrangement of the atomic positions during
the relaxation of the surface atoms yielding a large shortening of the nearest
neighbor distances near the step atoms.Comment: 6 figures, to appear in Sur. Sci. proceedings of VAS1
Lattice Resistance and Peierls Stress in Finite-size Atomistic Dislocation Simulations
Atomistic computations of the Peierls stress in fcc metals are relatively
scarce. By way of contrast, there are many more atomistic computations for bcc
metals, as well as mixed discrete-continuum computations of the Peierls-Nabarro
type for fcc metals. One of the reasons for this is the low Peierls stresses in
fcc metals. Because atomistic computations of the Peierls stress take place in
finite simulation cells, image forces caused by boundaries must either be
relaxed or corrected for if system size independent results are to be obtained.
One of the approaches that has been developed for treating such boundary forces
is by computing them directly and subsequently subtracting their effects, as
developed by V. B. Shenoy and R. Phillips [Phil. Mag. A, 76 (1997) 367]. That
work was primarily analytic, and limited to screw dislocations and special
symmetric geometries. We extend that work to edge and mixed dislocations, and
to arbitrary two-dimensional geometries, through a numerical finite element
computation. We also describe a method for estimating the boundary forces
directly on the basis of atomistic calculations. We apply these methods to the
numerical measurement of the Peierls stress and lattice resistance curves for a
model aluminum (fcc) system using an embedded-atom potential.Comment: LaTeX 47 pages including 20 figure
Reorganization of columnar architecture in the growing visual cortex
Many cortical areas increase in size considerably during postnatal
development, progressively displacing neuronal cell bodies from each other. At
present, little is known about how cortical growth affects the development of
neuronal circuits. Here, in acute and chronic experiments, we study the layout
of ocular dominance (OD) columns in cat primary visual cortex (V1) during a
period of substantial postnatal growth. We find that despite a considerable
size increase of V1, the spacing between columns is largely preserved. In
contrast, their spatial arrangement changes systematically over this period.
While in young animals columns are more band-like, layouts become more
isotropic in mature animals. We propose a novel mechanism of growth-induced
reorganization that is based on the `zigzag instability', a dynamical
instability observed in several inanimate pattern forming systems. We argue
that this mechanism is inherent to a wide class of models for the
activity-dependent formation of OD columns. Analyzing one member of this class,
the Elastic Network model, we show that this mechanism can account for the
preservation of column spacing and the specific mode of reorganization of OD
columns that we observe. We conclude that neurons systematically shift their
selectivities during normal development and that this reorganization is induced
by the cortical expansion during growth. Our work suggests that cortical
circuits remain plastic for an extended period in development in order to
facilitate the modification of neuronal circuits to adjust for cortical growth.Comment: 8+13 pages, 4+8 figures, paper + supplementary materia
Developing an e-infrastructure for social science
We outline the aims and progress to date of the National Centre for e-Social
Science e-Infrastructure project. We examine the challenges faced by the project, namely in
ensuring outputs are appropriate to social scientists, managing the transition from research
projects to service and embedding software and data within a wider infrastructural
framework. We also provide pointers to related work where issues which have ramifications
for this and similar initiatives are being addressed
Finite Sized Atomistic Simulations of Screw Dislocations
The interaction of screw dislocations with an applied stress is studied using
atomistic simulations in conjunction with a continuum treatment of the role
played by the far field boundary condition. A finite cell of atoms is used to
consider the response of dislocations to an applied stress and this introduces
an additional force on the dislocation due to the presence of the boundary.
Continuum mechanics is used to calculate the boundary force which is
subsequently accounted for in the equilibrium condition for the dislocation.
Using this formulation, the lattice resistance curve and the associated Peierls
stress are calculated for screw dislocations in several close packed metals. As
a concrete example of the boundary force method, we compute the bow out of a
pinned screw dislocation; the line-tension of the dislocation is calculated
from the results of the atomistic simulations using a variational principle
that explicitly accounts for the boundary force.Comment: LaTex, 20 pages, 11 figure
Cephalosporin-resistant Pneumococci and Sickle Cell Disease
Increasingly resistant bacteria in sickle cell disease patients indicate need to evaluate extendedspectrum cephalosporin therapy
The DRIFT Dark Matter Experiments
The current status of the DRIFT (Directional Recoil Identification From
Tracks) experiment at Boulby Mine is presented, including the latest limits on
the WIMP spin-dependent cross-section from 1.5 kg days of running with a
mixture of CS2 and CF4. Planned upgrades to DRIFT IId are detailed, along with
ongoing work towards DRIFT III, which aims to be the world's first 10 m3-scale
directional Dark Matter detector.Comment: Proceedings of the 3rd International conference on Directional
Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10 June 201
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