76,112 research outputs found
Paraxial ray optics cloaking
Despite much interest and progress in optical spatial cloaking, a
three-dimensional (3D), transmitting, continuously multidirectional cloak in
the visible regime has not yet been demonstrated. Here we experimentally
demonstrate such a cloak using ray optics, albeit with some edge effects. Our
device requires no new materials, uses isotropic off-the-shelf optics, scales
easily to cloak arbitrarily large objects, and is as broadband as the choice of
optical material, all of which have been challenges for current cloaking
schemes. In addition, we provide a concise formalism that quantifies and
produces perfect optical cloaks in the small-angle (`paraxial') limit
Paraxial full-field cloaking
We complete the `paraxial' (small-angle) ray optics cloaking formalism
presented previously [Choi and Howell, Opt. Express 22, 29465 (2014)], by
extending it to the full-field of light. Omnidirectionality is then the only
relaxed parameter of what may be considered an ideal, broadband, field cloak.
We show that an isotropic plate of uniform thickness, with appropriately
designed refractive index and dispersion, can match the phase over the whole
visible spectrum. Our results support the fundamental limits on cloaking for
broadband vs. omnidirectionality, and provide insights into when anisotropy may
be required
Anti-correlated time lags in the Z source GX 5-1: Possible evidence for a truncated accretion disk
We investigate the nature of the inner accretion disk in the neutron star
source GX 5-1 by making a detailed study of time lags between X-rays of
different energies. Using the cross-correlation analysis, we found
anti-correlated hard and soft time lags of the order of a few tens to a few
hundred seconds and the corresponding intensity states were mostly the
horizontal branch (HB) and upper normal branch (NB). The model independent and
dependent spectral analysis showed that during these time lags the structure of
accretion disk significantly varied. Both eastern and western approaches were
used to unfold the X-ray continuum and systematic changes were observed in soft
and hard spectral components. These changes along with a systematic shift in
the frequency of quasi-periodic oscillations (QPOs) made it substantially
evident that the geometry of the accretion disk is truncated. Simultaneous
energy spectral and power density spectral study shows that the production of
the horizontal branch oscillations (HBOs) are closely related to the
Comptonizing region rather than the disk component in the accretion disk. We
found that as the HBO frequency decreases from the hard apex to upper HB, the
disk temperature increases along with an increase in the coronal temperature
which is in sharp contrast with the changes found in black hole binaries where
the decrease in QPO frequency is accompanied by a decrease in the disk
temperature and a simultaneous increase in the coronal temperature. We discuss
the results in the context of re-condensation of coronal material in the inner
region of the disk.Comment: 40 pages, 7 figures, accepted for publication in The Astrophysical
Journal Supplement (ApJS
Prediction of airfoil stall using Navier-Stokes equations in streamline coordinates
A Navier-Stokes procedure to calculate the flow about an airfoil at incidence was developed. The parabolized equations are solved in the streamline coordinates generated for an arbitrary airfoil shape using conformal mapping. A modified k-epsilon turbulence model is applied in the entire domain, but the eddy viscosity in the laminar region is suppressed artificially to simulate the region correctly. The procedure was applied to airfoils at various angles of attack, and the results are quite satisfactory for both laminar and turbulent flows. It is shown that the present choice of the coordinate system reduces the error due to numerical diffusion, and that the lift is accurately predicted for a wide range of incidence
Steady-state traffic flow on a ring road with up- and down- slopes
This paper studies steady-state traffic flow on a ring road with up- and
down- slopes using a semi-discrete model. By exploiting the relations between
the semi-discrete and the continuum models, a steady-state solution is uniquely
determined for a given total number of vehicles on the ring road. The solution
is exact and always stable with respect to the first-order continuum model,
whereas it is a good approximation with respect to the semi-discrete model
provided that the involved equilibrium constant states are linearly stable. In
an otherwise case, the instability of one or more equilibria could trigger
stop-and-go waves propagating in certain road sections or throughout the ring
road. The indicated results are reasonable and thus physically significant for
a better understanding of real traffic flow on an inhomogeneous road
Factorization in graviton interactions
The study of factorization in the linearized gravity is extended to the
graviton scattering processes with a massive scalar particle, with a massless
vector boson and also with a graviton. Every transition amplitude is shown to
be completely factorized and the physical implications of their common factors
are discussed.Comment: 5 pages, Revtex 3.0, SNUTP 93-7
Male Partners' Involvement Towards Prenatal Screening and Diagnostic Testing for Down Syndrome
Introduction: Now, male partners' involvement in prenatal screening and diagnostic testing for Down syndrome is becoming increasingly recognized as well to ensure that parents are well informed of the risks and benefits of screening. The aim of study was to understand the degree of male partners' involvement during pregnancy in Singapore population. Methods: A cross-sectional survey of male partners' attending prenatal counseling was performed. The instrument used to measure the level of involvement is a self-assessment questionnaire that identifies the role of male partners with a Likert scale. Descriptive statistics was used to analyze data gained. Result: A total of 107 participants completed the questionnaire. Sixty-seven percent of male partners were found to have a highlevel of involvement while 32.7% was found to have a medium level of involvement. Most of them stated that women can pursue prenatal testing without their permission. Male partners found it more important for them to accompany their spouse to amniocentesis or CVS than to the Down syndrome screening test. When participants were asked about how much information about Down syndrome they sought prior to the appointment, how much discussion they had with their spouse about Down syndrome testing, and about whether they or their spouse should be the first person to receive test results, most stated that they were undecided. Conclusion: These results revealed that male partners were very well involved in the Down syndrome testing during pregnancy and future studies should assess possible underlying factors that influence male partners' involvement
Characterisation of the dynamical quantum state of a zero temperature Bose-Einstein condensate
We describe the quantum state of a Bose-Einstein condensate at zero
temperature. By evaluating the Q-function we show that the ground state of
Bose-Einstein condensate under the Hartree approximation is squeezed. We find
that multimode Schroedinger cat states are generated as the condensate evolves
in a ballistic expansion.Comment: 13 pages, 6 figure
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