17,822 research outputs found
Evidence from scanning tunneling microscopy in support of a structural model for the InSb(001)-c(8Ă—2) surface
In this letter we present evidence from scanning tunneling microscopy studies in support of a recently proposed structural model for the indium-terminated c(8×2) surface of InSb(001). This structural model, by Norris and co-workers, is based on a surface x-ray diffraction study [Surf. Sci. 409, 27 (1998)], and represents a significant departure from previously suggested models for the c(8×2) reconstruction on any (001) surface of the common III–V semiconductor materials. Although filled state images of the InSb(001)-c(8×2) surface have previously been published, empty states image of sufficient quality to extract any meaningful information have not previously been reported. The observations are in excellent agreement with the recently proposed model for this surface reconstruction
FeH Absorption in the Near-Infrared Spectra of Late M and L Dwarfs
We present medium-resolution z-, J-, and H-band spectra of four late-type
dwarfs with spectral types ranging from M8 to L7.5. In an attempt to determine
the origin of numerous weak absorption features throughout their near-infrared
spectra, and motivated by the recent tentative identification of the E 4\Pi- A
^4\Pi system of FeH near 1.6 microns in umbral and cool star spectra, we have
compared the dwarf spectra to a laboratory FeH emission spectrum. We have
identified nearly 100 FeH absorption features in the z-, J-, and H-band spectra
of the dwarfs. In particular, we have identified 34 features which dominate the
appearance of the H-band spectra of the dwarfs and which appear in the
laboratory FeH spectrum. Finally, all of the features are either weaker or
absent in the spectrum of the L7.5 dwarf which is consistent with the weakening
of the known FeH bandheads in the spectra of the latest L dwarfs.Comment: accepted by Ap
Spectral reflectances of natural targets for use in remote sensing studies
A collection of spectral reflectances of 156 natural targets is presented in a uniform format. For each target both a graphical plot and a digital tabulation of reflectance is given. The data were taken from the literature and include laboratory, field, and aircraft measurements. A discussion of the different measurements of reflectance is given, along with the changes in apparent reflectance when targets are viewed through the atmosphere. The salient features of the reflectance curves of common target types are presented and discussed
Fermionic Zero Modes of Supergravity Cosmic Strings
Recent developments in string theory suggest that cosmic strings could be
formed at the end of brane inflation. Supergravity provides a realistic model
to study the properties of strings arising in brane inflation. Whilst the
properties of cosmic strings in flat space-time have been extensively studied
there are significant complications in the presence of gravity. We study the
effects of gravitation on cosmic strings arising in supergravity. Fermion zero
modes are a common feature of cosmic strings, and generically occur in
supersymmetric models. The corresponding massless currents can give rise to
stable string loops (vortons). The vorton density in our universe is strongly
constrained, allowing many theories with cosmic strings to be ruled out. We
investigate the existence of fermion zero modes on cosmic strings in
supergravity theories. A general index theorem for the number of zero modes is
derived. We show that by including the gravitino, some (but not all) zero modes
disappear. This weakens the constraints on cosmic string models. In particular,
winding number one cosmic D-strings in models of brane inflation are not
subject to vorton constraints. We also discuss the effects of supersymmetry
breaking on cosmic D-strings.Comment: 33 page
Determining Damping Trends from a Range of Cable Harness Assemblies on a Launch Vehicle Panel from Test Measurements
The team of authors at Marshall Space Flight Center (MSFC) has been investigating estimating techniques for the vibration response of launch vehicle panels excited by acoustics and/or aero-fluctuating pressures. Validation of the approaches used to estimate these environments based on ground tests of flight like hardware is of major importance to new vehicle programs. The team at MSFC has recently expanded upon the first series of ground test cases completed in December 2010. The follow on tests recently completed are intended to illustrate differences in damping that might be expected when cable harnesses are added to the configurations under test. This validation study examines the effect on vibroacoustic response resulting from the installation of cable bundles on a curved orthogrid panel. Of interest is the level of damping provided by the installation of the cable bundles and whether this damping could be potentially leveraged in launch vehicle design. The results of this test are compared with baseline acoustic response tests without cables. Damping estimates from the measured response data are made using a new software tool that employs a finite element model (FEM) of the panel in conjunction with advanced optimization techniques. This paper will report on the \damping trend differences. observed from response measurements for several different configurations of cable harnesses. The data should assist vibroacoustics engineers to make more informed damping assumptions when calculating vibration response estimates when using model based analysis approach. Achieving conservative estimates that have more flight like accuracy is desired. The paper may also assist analysts in determining how ground test data may relate to expected flight response levels. Empirical response estimates may also need to be adjusted if the measured response used as an input to the study came from a test article without flight like cable harnesses
Live-Fly, Large-Scale Field Experimentation for Large Numbers of Fixed-Wing UAVs
In this paper, we present extensive advances in
live-fly field experimentation capabilities of large numbers
of fixed-wing aerial robots, and highlight both the enabling
technologies as well as the challenges addressed in such large-scale
flight operations. We showcase results from recent field
tests, including the autonomous launch, flight, and landing of 50
UAVs, which illuminate numerous operational lessons learned
and generate rich multi-UAV datasets. We detail the design and
open architecture of the testbed, which intentionally leverages
low-cost and open-source components, aimed at promoting continued
advances and alignment of multi-robot systems research
and practice
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