5,603 research outputs found
AdS4 backgrounds with N>16 supersymmetries in 10 and 11 dimensions
We explore all warped backgrounds with the most
general allowed fluxes that preserve more than 16 supersymmetries in -
and -dimensional supergravities. After imposing the assumption that either
the internal space is compact without boundary or the isometry
algebra of the background decomposes into that of AdS and that of
, we find that there are no such backgrounds in IIB supergravity.
Similarly in IIA supergravity, there is a unique such background with 24
supersymmetries locally isometric to , and in
supergravity all such backgrounds are locally isometric to the maximally
supersymmetric solution.Comment: 53 pages. v2: minor changes and references added. v3: typos corrected
and minor footnote added, published versio
RFI mitigation with phase-only adaptive beamforming
Connected radio interferometers are sometimes used in the tied-array mode:
signals from antenna elements are coherently added and the sum signal applied
to a VLBI backend or pulsar processing machine. Usually there is no
computer-controlled amplitude weighting in the existing radio interferometer
facilities. Radio frequency interference (RFI) mitigation with phase-only
adaptive beamforming is proposed for this mode of observation. Small phase
perturbations are introduced in each of the antenna's signal. The values of
these perturbations are optimized in such a way that the signal from a radio
source of interest is preserved and RFI signals suppressed. An evolutionary
programming algorithm is used for this task. Computer simulations, made for
both one-dimensional and two-dimensional array set-ups, show considerable
suppression of RFI and acceptable changes to the main array beam in the radio
source direction.Comment: 7 pages, 11 figure
Wave Propagation in the Ground and Isolation Measures
The first part of this report deals with wave propagation in the ground. Special attention is given to the radiation of waves from a source and on the effect of layering or continuous variation of the shear modulus in the soil on travelling waves. In the second part, vibration isolation measures are described, such as open or infilled trenches and rows of bore holes or piles. The results of different theoretical and experimental investigations are compared. Finally, recently developed isolation measures are presented
Chern-Simons flows on Aloff-Wallach spaces and Spin(7)-instantons
Due to their explicit construction, Aloff-Wallach spaces are prominent in
flux compactifications. They carry G_2-structures and admit the G_2-instanton
equations, which are natural BPS equations for Yang-Mills instantons on
seven-manifolds and extremize a Chern-Simons-type functional. We consider the
Chern-Simons flow between different G_2-instantons on Aloff-Wallach spaces,
which is equivalent to Spin(7)-instantons on a cylinder over them. For a
general SU(3)-equivariant gauge connection, the generalized instanton equations
turn into gradient-flow equations on C^3 x R^2, with a particular cubic
superpotential. For the simplest member of the Aloff-Wallach family (with
3-Sasakian structure) we present an explicit instanton solution of tanh-like
shape.Comment: 1+17 pages, 1 figur
Optimal design of nanoplasmonic materials using genetic algorithms as a multi-parameter optimization tool
An optimal control approach based on multiple parameter genetic algorithms is
applied to the design of plasmonic nanoconstructs with pre-determined optical
properties and functionalities. We first develop nanoscale metallic lenses that
focus an incident plane wave onto a pre-specified, spatially confined spot. Our
results illustrate the role of symmetry breaking and unravel the principles
that favor dimeric constructs for optimal light localization. Next we design a
periodic array of silver particles to modify the polarization of an incident,
linearly-polarized plane wave in a desired fashion while localizing the light
in space. The results provide insight into the structural features that
determine the birefringence properties of metal nanoparticles and their arrays.
Of the variety of potential applications that may be envisioned, we note the
design of nanoscale light sources with controllable coherence and polarization
properties that could serve for coherent control of molecular or electronic
dynamics in the nanoscale.Comment: 13 pages, 6 figures. submitted to J. Chem. Phy
A non-existence theorem for N>16 supersymmetric AdS3 backgrounds
We show that there are no smooth warped AdS solutions in 10- and
11-dimensional supergravities which preserve strictly more than 16
supersymmetries and have internal space a compact without boundary manifold.Comment: 50 page
Anomalous suppression of the shot noise in a nanoelectromechanical system
In this paper we report a relaxation-induced suppression of the noise for a
single level quantum dot coupled to an oscillator with incoherent dynamics in
the sequential tunneling regime. It is shown that relaxation induces
qualitative changes in the transport properties of the dot, depending on the
strength of the electron-phonon coupling and on the applied voltage. In
particular, critical thresholds in voltage and relaxation are found such that a
suppression below 1/2 of the Fano factor is possible. Additionally, the current
is either enhanced or suppressed by increasing relaxation, depending on bias
being greater or smaller than the above threshold. These results exist for any
strength of the electron-phonon coupling and are confirmed by a four states toy
model.Comment: 7 pages, 7 eps figures, submitted to PRB; minor changes in the
introductio
STRAIN GAUGE VALIDATION OF FINITE ELEMENT ANALYSIS OF A TI6AL4V (ELI) MANDIBULAR IMPLANT PRODUCED THROUGH ADDITIVE MANUFACTURING
Published Conference ProceedingsMedical implants created by Ti6Al4V (ELI) through Additive Manufacturing (AM) processes have a very positive impact on the quality of life of patients who have undergone bone reconstructive surgery. The effectiveness of medical implant design for AM processes would be significantly improved if finite element analysis (FEA) could be established as an accepted design tool. This study is aimed at validating FEA as a tool for predicting the strain distribution in a Ti6Al4V (ELI) medical implant produced through a selective laser melting (SLM) process by comparing the FEA results with strain gauge measurements.
The approach followed was to demonstrate the correlation between an FEA model and strain gauge measurements performed on a human mandibular implant. For the design of the mandibular implant the geometrical representation of an adult human mandible obtained from a computerized tomography (CT) scan was used. This CAD model was then submitted to FEA when subjected to typical static mastication load conditions. Through this simulation the distribution of strain in the implant was determined. Using the same CAD model, an implant was manufactured through SLM and strain gauges were mounted on the implant at locations corresponding to the areas of highest strain as determined on the FEA model.
The results obtained from both FEA and strain gauge measurements are presented in this paper, and a correlation within an error margin of less than 10 % for most of the gauge was obtained. Requirements for achieving this level of correlation are discussed. It is concluded that FEA is indeed a powerful tool for improving the effectiveness of design for AM of medical implants
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