4,398 research outputs found
Tunable reflection minima of nanostructured antireflective surfaces
Broadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application
Temperature dependence of the charge carrier mobility in gated quasi-one-dimensional systems
The many-body Monte Carlo method is used to evaluate the frequency dependent
conductivity and the average mobility of a system of hopping charges,
electronic or ionic on a one-dimensional chain or channel of finite length. Two
cases are considered: the chain is connected to electrodes and in the other
case the chain is confined giving zero dc conduction. The concentration of
charge is varied using a gate electrode. At low temperatures and with the
presence of an injection barrier, the mobility is an oscillatory function of
density. This is due to the phenomenon of charge density pinning. Mobility
changes occur due to the co-operative pinning and unpinning of the
distribution. At high temperatures, we find that the electron-electron
interaction reduces the mobility monotonically with density, but perhaps not as
much as one might intuitively expect because the path summation favour the
in-phase contributions to the mobility, i.e. the sequential paths in which the
carriers have to wait for the one in front to exit and so on. The carrier
interactions produce a frequency dependent mobility which is of the same order
as the change in the dc mobility with density, i.e. it is a comparably weak
effect. However, when combined with an injection barrier or intrinsic disorder,
the interactions reduce the free volume and amplify disorder by making it
non-local and this can explain the too early onset of frequency dependence in
the conductivity of some high mobility quasi-one-dimensional organic materials.Comment: 9 pages, 8 figures, to be published in Physical Review
The Integer Valued SU(3) Casson Invariant for Brieskorn spheres
We develop techniques for computing the integer valued SU(3) Casson
invariant. Our method involves resolving the singularities in the flat moduli
space using a twisting perturbation and analyzing its effect on the topology of
the perturbed flat moduli space. These techniques, together with Bott-Morse
theory and the splitting principle for spectral flow, are applied to calculate
the invariant for all Brieskorn homology spheres.Comment: 50 pages, 3 figure
Helium ion microscopy and energy selective scanning electron microscopy – two advanced microscopy techniques with complementary applications
Both scanning electron microscopes (SEM) and helium ion microscopes (HeIM) are based on the same principle of a charged particle beam scanning across the surface and generating secondary electrons (SEs) to form images. However, there is a pronounced difference in the energy spectra of the emitted secondary electrons emitted as result of electron or helium ion impact. We have previously presented evidence that this also translates to differences in the information depth through the analysis of dopant contrast in doped silicon structures in both SEM and HeIM. Here, it is now shown how secondary electron emission spectra (SES) and their relation to depth of origin of SE can be experimentally exploited through the use of energy filtering (EF) in low voltage SEM (LV-SEM) to access bulk information from surfaces covered by damage or contamination layers. From the current understanding of the SES in HeIM it is not expected that EF will be as effective in HeIM but an alternative that can be used for some materials to access bulk information is presented
Suppression of backscattered diffraction from sub-wavelength ‘moth-eye’ arrays
The eyes and wings of some species of moth are covered with arrays of nanoscale features that dramatically reduce reflection of light. There have been multiple examples where this approach has been adapted for use in antireflection and antiglare technologies with the fabrication of artificial moth-eye surfaces. In this work, the suppression of iridescence caused by the diffraction of light from such artificial regular moth-eye arrays at high angles of incidence is achieved with the use of a new tiled domain design, inspired by the arrangement of features on natural moth-eye surfaces. This bio-mimetic pillar architecture contains high optical rotational symmetry and can achieve high levels of diffraction order power reduction. For example, a tiled design fabricated in silicon and consisting of domains with 9 different orientations of the traditional hexagonal array exhibited a ~96% reduction in the intensity of the ?1 diffraction order. It is suggested natural moth-eye surfaces have evolved a tiled domain structure as it confers efficient antireflection whilst avoiding problems with high angle diffraction. This combination of antireflection and stealth properties increases chances of survival by reducing the risk of the insect being spotted by a predator. Furthermore, the tiled domain design could lead to more effective artificial moth-eye arrays for antiglare and stealth applications
Interferometric Evidence for Resolved Warm Dust in the DQ Tau System
We report on near-infrared (IR) interferometric observations of the
double-lined pre-main sequence (PMS) binary system DQ Tau. We model these data
with a visual orbit for DQ Tau supported by the spectroscopic orbit & analysis
of \citet{Mathieu1997}. Further, DQ Tau exhibits significant near-IR excess;
modeling our data requires inclusion of near-IR light from an 'excess' source.
Remarkably the excess source is resolved in our data, similar in scale to the
binary itself ( 0.2 AU at apastron), rather than the larger circumbinary
disk ( 0.4 AU radius). Our observations support the \citet{Mathieu1997}
and \citet{Carr2001} inference of significant warm material near the DQ Tau
binary.Comment: 14 pgs, 3 figures, ApJL in pres
The circumbinary disk of HD 98800B: Evidence for disk warping
The quadruple young stellar system HD 98800 consists of two spectroscopic binary pairs with a circumbinary disk around the B component. Recent work by Boden and collaborators using infrared interferometry and radial velocity data resulted in a determination of the physical orbit for HD 98800B. We use the resulting inclination of the binary and the measured extinction toward the B component stars to constrain the distribution of circumbinary material. Although a standard optically and geometrically thick disk model can reproduce the spectral energy distribution, it cannot account for the observed extinction if the binary and the disk are coplanar. We next constructed a dynamical model to investigate the influence of the A component, which is not in the Ba‐Bb orbital plane, on the B disk. We find that these interactions have a substantial impact on the inclination of the B circumbinary disk with respect to the Ba‐Bb orbital plane. The resulting warp would be sufficient to place material into the line of sight and the noncoplanar disk orientation may also cause the upper layers of the disk to intersect the line of sight if the disk is geometrically thick. These simulations also support that the dynamics of the Ba‐Bb orbit clear the inner region to a radius of~3 AU. We then discuss whether the somewhat unusual properties of the HD 98800B disk are consistent with material remnant from the star formation process or with more recent creation by collisions from larger bodies
The Gould's Belt distance survey
Very Long Baseline Interferometry (VLBI) observations can provide the
position of compact radio sources with an accuracy of order 50
micro-arcseconds. This is sufficient to measure the trigonometric parallax and
proper motions of any object within 500 pc of the Sun to better than a few
percent. Because they are magnetically active, young stars are often associated
with compact radio emission detectable using VLBI techniques. Here we will show
how VLBI observations have already constrained the distance to the most often
studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and
have started to provide information on their internal structure and kinematics.
We will then briefly describe a large project (called The Gould's Belt Distance
Survey) designed to provide a detailed view of star-formation in the Solar
neighborhood using VLBI observations.Comment: To be published in the Revista Mexicana de Astronomia y Astrofisica
(Serie de Conferencias
Gauge Theoretic Invariants of, Dehn Surgeries on Knots
New methods for computing a variety of gauge theoretic invariants for
homology 3-spheres are developed. These invariants include the Chern-Simons
invariants, the spectral flow of the odd signature operator, and the rho
invariants of irreducible SU(2) representations. These quantities are
calculated for flat SU(2) connections on homology 3-spheres obtained by 1/k
Dehn surgery on (2,q) torus knots. The methods are then applied to compute the
SU(3) gauge theoretic Casson invariant (introduced in [H U Boden and C M
Herald, The SU(3) Casson invariant for integral homology 3--spheres, J. Diff.
Geom. 50 (1998) 147-206]) for Dehn surgeries on (2,q) torus knots for q=3,5,7
and 9.Comment: Version 3: minor corrections from version 2. Published by Geometry
and Topology at http://www.maths.warwick.ac.uk/gt/GTVol5/paper6.abs.htm
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