43,086 research outputs found
Metamaterials for light rays: ray optics without wave-optical analog in the ray-optics limit
Volumes of sub-wavelength electromagnetic elements can act like homogeneous
materials: metamaterials. In analogy, sheets of optical elements such as prisms
can act ray-optically like homogeneous sheet materials. In this sense, such
sheets can be considered to be metamaterials for light rays (METATOYs).
METATOYs realize new and unusual transformations of the directions of
transmitted light rays. We study here, in the ray-optics and scalar-wave
limits, the wave-optical analog of such transformations, and we show that such
an analog does not always exist. Perhaps, this is the reason why many of the
ray-optical possibilities offered by METATOYs have never before been
considered.Comment: 10 pages, 3 figures, references update
A comparison of plastic collapse and limit loads for single mitred pipe bends under in-plane bending
This paper presents a comparison of the plastic collapse loads from experimental in-plane bending tests on three 90 degree single un-reinforced mitred pipe bends, with the results from various 3D solid finite element models. The bending load applied reduced the bend angle and in turn, the resulting cross-sectional ovalisation led to a recognised weakening mechanism, which is only observable by testing or by including large displacement effects in the plastic finite element solution. A small displacement limit solution with an elastic-perfectly-plastic material model overestimated the collapse load by 40%. The plastic collapse finite element solution produced excellent agreement with experiment
Elastic stress concentration at radial crossholes in pressurised thick cylinders
Results of a parametric finite element analysis investigation of stress concentration at radial crossholes in pressurized cylinders are presented in numerical and graphical form. The analysis shows that the location of maximum stress does not generally occur at the junction between the bores, as is commonly supposed, but at some small distance up the crosshole from the junction. Maximum stress concentration factors (SCFs) are defined on the basis of the maximum principal stress, von Mises equivalent stress, and stress intensity. Three-dimensional plots of the SCF against the cylinder radius ratio b/a and the crosshole-to-main-bore-radius ratio c/a are presented. The SCFs were found to vary across the range of geometries considered with local minima identified within the parameter range in most cases. The results therefore allow designers to select optimum b/a and c/a ratios to minimize stress concentration in real problems
Power Spectrum Correlations Induced by Non-Linear Clustering
Gravitational clustering is an intrinsically non-linear process that
generates significant non-Gaussian signatures in the density field. We consider
how these affect power spectrum determinations from galaxy and weak-lensing
surveys. Non-Gaussian effects not only increase the individual error bars
compared to the Gaussian case but, most importantly, lead to non-trivial
cross-correlations between different band-powers. We calculate the
power-spectrum covariance matrix in non-linear perturbation theory (weakly
non-linear regime), in the hierarchical model (strongly non-linear regime), and
from numerical simulations in real and redshift space. We discuss the impact of
these results on parameter estimation from power spectrum measurements and
their dependence on the size of the survey and the choice of band-powers. We
show that the non-Gaussian terms in the covariance matrix become dominant for
scales smaller than the non-linear scale, depending somewhat on power
normalization. Furthermore, we find that cross-correlations mostly deteriorate
the determination of the amplitude of a rescaled power spectrum, whereas its
shape is less affected. In weak lensing surveys the projection tends to reduce
the importance of non-Gaussian effects. Even so, for background galaxies at
redshift z=1, the non-Gaussian contribution rises significantly around l=1000,
and could become comparable to the Gaussian terms depending upon the power
spectrum normalization and cosmology. The projection has another interesting
effect: the ratio between non-Gaussian and Gaussian contributions saturates and
can even decrease at small enough angular scales if the power spectrum of the
3D field falls faster than 1/k^2.Comment: 34 pages, 15 figures. Revised version, includes a clearer explanation
of why the hierarchical ansatz does not provide a good model of the
covariance matrix in the non-linear regime, and new constraints on the
amplitudes Ra and Rb for general 4-pt function configurations in the
non-linear regim
Electric field induced charge noise in doped silicon: ionization of phosphorus donors
We report low frequency charge noise measurement on silicon substrates with
different phosphorus doping densities. The measurements are performed with
aluminum single electron transistors (SETs) at millikelvin temperatures where
the substrates are in the insulating regime. By measuring the SET Coulomb
oscillations, we find a gate voltage dependent charge noise on the more heavily
doped substrate. This charge noise, which is seen to have a 1/f spectrum, is
attributed to the electric field induced tunneling of electrons from their
phosphorus donor potentials.Comment: 4 page, 3 figure
Bandwidth in bolometric interferometry
Bolometric Interferometry is a technology currently under development that
will be first dedicated to the detection of B-mode polarization fluctuations in
the Cosmic Microwave Background. A bolometric interferometer will have to take
advantage of the wide spectral detection band of its bolometers in order to be
competitive with imaging experiments. A crucial concern is that interferometers
are presumed to be importantly affected by a spoiling effect known as bandwidth
smearing. In this paper, we investigate how the bandwidth modifies the work
principle of a bolometric interferometer and how it affects its sensitivity to
the CMB angular power spectra. We obtain analytical expressions for the
broadband visibilities measured by broadband heterodyne and bolometric
interferometers. We investigate how the visibilities must be reconstructed in a
broadband bolometric interferometer and show that this critically depends on
hardware properties of the modulation phase shifters. Using an angular power
spectrum estimator accounting for the bandwidth, we finally calculate the
sensitivity of a broadband bolometric interferometer. A numerical simulation
has been performed and confirms the analytical results. We conclude (i) that
broadband bolometric interferometers allow broadband visibilities to be
reconstructed whatever the kind of phase shifters used and (ii) that for
dedicated B-mode bolometric interferometers, the sensitivity loss due to
bandwidth smearing is quite acceptable, even for wideband instruments (a factor
2 loss for a typical 20% bandwidth experiment).Comment: 13 pages, 14 figures, submitted to A&
Structural evaluation of candidate designs for the large space telescope primary mirror
Structural performance analyses were conducted on two candidate designs (Itek and Perkin-Elmer designs) for the large space telescope three-meter mirror. The mirror designs and the finite-element models used in the analyses evaluation are described. The results of the structural analyses for several different types of loading are presented in tabular and graphic forms. Several additional analyses are also reported: the evaluation of a mirror design concept proposed by the Boeing Co., a study of the global effects of local cell plate deflections, and an investigation of the fracture mechanics problems likely to occur with Cervit and ULE. Flexibility matrices were obtained for the Itek and Perkin-Elmer mirrors to be used in active figure control studies. Summary, conclusions, and recommendations are included
An obstruction based approach to the Kochen-Specker theorem
In [1] it was shown that the Kochen Specker theorem can be written in terms
of the non-existence of global elements of a certain varying set over the
partially ordered set of boolean subalgebras of projection operators on some
Hilbert space. In this paper, we show how obstructions to the construction of
such global elements arise, and how this provides a new way of looking at
proofs of the theorem.Comment: 14 pages, 6 figure
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