3,392 research outputs found
Improved surface quality of anisotropically etched silicon {111} planes for mm-scale integrated optics
We have studied the surface quality of millimeter-scale optical mirrors
produced by etching CZ and FZ silicon wafers in potassium hydroxide to expose
the planes. We find that the FZ surfaces have four times lower noise
power at spatial frequencies up to . We conclude that mirrors
made using FZ wafers have higher optical quality
A stochastic model for early placental development
In the human, placental structure is closely related to placental function and consequent pregnancy outcome. Studies have noted abnormal placental shape in small-for-gestational age infants which extends to increased lifetime risk of cardiovascular disease. The origins and determinants of placental shape are incompletely under-stood and are difficult to study in vivo. In this paper we model the early development of the placenta in the human, based on the hypothesis that this is driven by dynamics dominated by a chemo-attractant effect emanating from proximal spiral arteries in the decidua. We derive and explore a two-dimensional stochastic model for these events, and investigate the effects of loss of spiral arteries in regions near to the cord insertion on the shape of the placenta. This model demonstrates that placental shape is highly variable and disruption of spiral arteries can exert profound effects on placental shape, particularly if this disruption is close to the cord insertion. Thus, placental shape reflects the underlying maternal vascular bed. Abnormal placental shape may reflect an abnormal uterine environment, which predisposes to pregnancy complications
ICP polishing of silicon for high quality optical resonators on a chip
Miniature concave hollows, made by wet etching silicon through a circular
mask, can be used as mirror substrates for building optical micro-cavities on a
chip. In this paper we investigate how ICP polishing improves both shape and
roughness of the mirror substrates. We characterise the evolution of the
surfaces during the ICP polishing using white-light optical profilometry and
atomic force microscopy. A surface roughness of 1 nm is reached, which reduces
to 0.5 nm after coating with a high reflectivity dielectric. With such smooth
mirrors, the optical cavity finesse is now limited by the shape of the
underlying mirror
Deep spectroscopy of z~1 6C radio galaxies - II. Breaking the redshift-radio power degeneracy
The results of a spectroscopic analysis of 3CR and 6C radio galaxies at
redshift z~1 are contrasted with the properties of lower redshift radio
galaxies, chosen to be matched in radio luminosity to the 6C sources studied at
z~1, thus enabling the P-z degeneracy to be broken. Partial rank correlations
and principal component analysis have been used to determine which of z and P
are the critical parameters underlying the observed variation of the ionization
state andd kinematics of the emission line gas. [OII]/H-beta is shown to be a
useful ionization mechanism diagnostic. Statistical analysis of the data shows
that the ionization state of the emission line gas is strongly correlated with
radio power, once the effects of other parameters are removed. No dependence of
ionization state on z is observed, implying that the ionization state of the
emission line gas is solely a function of the AGN properties rather than the
hostt galaxy and/or environment. Statistical analysis of the kinematic
properties of the emission line gas shows that these are strongly correlated
independently withh both P and z. The correlation with redshift is the stronger
of the two, suggesting that host galaxy composition or environment may play a
role in producing the less extreme gas kinematics observed in the emission line
regions of low redshift galaxies. For both the ionization and kinematic
properties of thee galaxies, the independent correlations observed with radio
size are strongest. Radio source age is a determining factor for the extended
emission line regions.Comment: 10 pages, 5 figures, accepted for publication in MNRA
Structure and equation of state of interaction site models for disc-shaped lamellar colloids
We apply RISM (Reference Interaction Site Model) and PRISM (polymer-RISM)
theories to calculate the site-site pair structure and the osmotic equation of
state of suspensions of circular or hexagonal platelets (lamellar colloids)
over a range of ratios of the particle diameter over thickness. Despite the
neglect of edge effects, the simpler PRISM theory yields results in good
agreement with the more elaborate RISM calculations, provided the correct form
factor, characterizing the intramolecular structure of the platelets, is used.
The RISM equation of state is sensitive to the number of sites used to model
the platelets, but saturates when the hard spheres, associated with the
interaction sites, nearly touch; the limiting equation of state agrees
reasonably well with available simulation data for all densities up to the
isotropic-nematic transition. When properly scaled with the second virial
coefficient, the equations of state of platelets with different aspect ratios
nearly collapse on a single master curve.Comment: 10 Pages, 11 Figures, Typesetted using RevTeX
A Complete Sample of Megaparsec Size Double Radio Sources from SUMSS
We present a complete sample of megaparsec-size double radio sources compiled
from the Sydney University Molonglo Sky Survey (SUMSS). Almost complete
redshift information has been obtained for the sample. The sample has the
following defining criteria: Galactic latitude |b| > 12.5 deg, declination <
-50 deg and angular size > 5 arcmin. All the sources have projected linear size
larger than 0.7 Mpc (assuming H_o = 71 km/s/Mpc). The sample is chosen from a
region of the sky covering 2100 square degrees. In this paper, we present
843-MHz radio images of the extended radio morphologies made using the Molonglo
Observatory Synthesis Telescope (MOST), higher resolution radio observations of
any compact radio structures using the Australia Telescope Compact Array
(ATCA), and low resolution optical spectra of the host galaxies from the 2.3-m
Australian National University (ANU) telescope at Siding Spring Observatory.
The sample presented here is the first in the southern hemisphere and
significantly enhances the database of known giant radio sources. The giant
radio sources with linear size exceeding 0.7 Mpc have an abundance of (215
Mpc)^(-3) at the sensitivity of the survey. In the low redshift universe, the
survey may be suggesting the possibility that giant radio sources with relict
lobes are more numerous than giant sources in which beams from the centre
currently energize the lobes.Comment: 67 pages, 29 figures, for full resolution figures see
http://www.astrop.physics.usyd.edu.au/SUMSS/PAPERS/Submit-May11-ms.pd
Un-reduction
This paper provides a full geometric development of a new technique called
un-reduction, for dealing with dynamics and optimal control problems posed on
spaces that are unwieldy for numerical implementation. The technique, which was
originally concieved for an application to image dynamics, uses Lagrangian
reduction by symmetry in reverse. A deeper understanding of un-reduction leads
to new developments in image matching which serve to illustrate the
mathematical power of the technique.Comment: 25 pages, revised versio
A framework for mimetic discretization of the rotating shallow-water equations on arbitrary polygonal grids
Copyright © 2012 Society for Industrial and Applied MathematicsAccurate simulation of atmospheric flow in weather and climate prediction models requires the discretization of the governing equations to have a number of desirable properties. Although these properties can be achieved relatively straightforwardly on a latitude-longitude grid, they are much more challenging on the various quasi-uniform spherical grids that are now under consideration. A recently developed scheme—called TRiSK—has these desirable properties on grids that have an orthogonal dual. The present work extends the TRiSK scheme into a more general framework suitable for grids that have a nonorthogonal dual, such as the equiangular cubed sphere. We also show that this framework fits within the wider framework of mimetic discretizations and discrete exterior calculus. One key ingredient is the definition of certain mapping operators that are discrete analogues of the Hodge star operator, enabling the definition of a compatible inner product. Discrete Coriolis terms are also included within the mimetic framework, and in such a way as to conserve energy and ensure that discrete geostrophic balance can be maintained; this requires the definition of a further mapping operator, with special properties, that transfers the discrete velocity field from the primal to the dual grid
Variational water-wave model with accurate dispersion and vertical vorticity
A new water-wave model has been derived which is based on variational techniques and combines a depth-averaged vertical (component of) vorticity with depth-dependent potential flow. The model facilitates the further restriction of the vertical profile of the velocity potential to n-th order polynomials or a finite-element profile with a small number of elements (say), leading to a framework for efficient modeling of the interaction of steepening and breaking waves near the shore with a large-scale horizontal flow. The equations are derived from a constrained variational formulation which leads to conservation laws for energy, mass, momentum and vertical vorticity. It is shown that the potential-flow water-wave equations and the shallow-water equations are recovered in the relevant limits. Approximate shock relations are provided, which can be used in numerical schemes to model breaking waves
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