5,930 research outputs found
Absorption in dipole-lattice models of dielectrics
We develop a classical microscopic model of a dielectric. The model features
nonlinear interaction terms between polarizable dipoles and lattice vibrations.
The lattice vibrations are found to act as a pseudo-reservoir, giving broadband
absorption of electromagnetic radiation without the addition of damping terms
in the dynamics. The effective permittivity is calculated using a perturbative
iteration method and is found to have the form associated with real
dielectrics. Spatial dispersion is naturally included in the model and we also
calculate the wavevector dependence of the permittivity.Comment: 13 pages, 9 figures; references added to section
Puncture discharges in surface dielectrics as contaminant sources in spacecraft environments
Spacecraft in geosynchronous orbits are known to become charged to large negative potentials during the local midnight region of the satellite orbit. Such discharges have been studied by the electron beam irradiation of dielectric samples in a vacuum environment. In addition to static measurements and photographic examination of the puncture discharges in Teflon samples, the transient characteristics of the electrical discharges are determined from oscillographs of voltage and current and by charged particle measurements employing a biased Faraday cup and a retarding potential analyzer. Using these latter techniques, studies of angular and energy distributions of charged particles have indicated an initial burst of high energy electrons (5 x 10 to the 13th power per discharge at energies greater than 300 eV) followed by a less intense burst of lower energy negative particles. Positive ions are emitted from the discharge site in an initial high velocity burst followed by a lower velocity burst tentatively identified as carbon
Optimal traps in graphene
We transform the two-dimensional Dirac-Weyl equation, which governs the
charge carriers in graphene, into a non-linear first-order differential
equation for scattering phase shift, using the so-called variable phase method.
This allows us to utilize the Levinson Theorem to find zero-energy bound states
created electrostatically in realistic structures. These confined states are
formed at critical potential strengths, which leads to us posit the use of
`optimal traps' to combat the chiral tunneling found in graphene, which could
be explored experimentally with an artificial network of point charges held
above the graphene layer. We also discuss scattering on these states and find
the zero angular momentum states create a dominant peak in scattering
cross-section as energy tends towards the Dirac point energy, suggesting a
dominant contribution to resistivity.Comment: 11 pages, 5 figure
QSO Absorption Line Constraints on Intragroup High-Velocity Clouds
We show that the number statistics of moderate redshift MgII and Lyman limit
absorbers may rule out the hypothesis that high velocity clouds are infalling
intragroup material.Comment: 4 pages, no figures; submitted to Astrophysical Journal Letters;
revised version, more general and includes more about Braun and Burton CHVC
Discovery of Multi-Phase Cold Accretion in a Massive Galaxy at z=0.7
We present detailed photo+collisional ionization models and kinematic models
of the multi-phase absorbing gas, detected within the HST/COS, HST/STIS, and
Keck/HIRES spectra of the background quasar TON 153, at 104 kpc along the
projected minor axis of a star-forming spiral galaxy (z=0.6610). Complementary
g'r'i'Ks photometry and stellar population models indicate that the host galaxy
is dominated by a 4 Gyr stellar population with slightly greater than solar
metallicity and has an estimated log(M*)=11 and a log(Mvir)=13. Photoionization
models of the low ionization absorption, (MgI, SiII, MgII and CIII) which trace
the bulk of the hydrogen, constrain the multi-component gas to be cold
(logT=3.8-5.2) and metal poor (-1.68<[X/H]<-1.64). A lagging halo model
reproduces the low ionization absorption kinematics, suggesting gas coupled to
the disk angular momentum, consistent with cold accretion mode material in
simulations. The CIV and OVI absorption is best modeled in a separate
collisionally ionized metal-poor (-2.50<[X/H]<-1.93) warm phase with logT=5.3.
Although their kinematics are consistent with a wind model, given the 2-2.5dex
difference between the galaxy stellar metallicity and the absorption
metallicity indicates the gas cannot arise from galactic winds. We discuss and
conclude that although the quasar sight-line passes along the galaxy minor axis
at projected distance of 0.3 virial radii, well inside its virial shock radius,
the combination of the relative kinematics, temperatures, and relative
metallicities indicated that the multi-phase absorbing gas arises from cold
accretion around this massive galaxy. Our results appear to contradict recent
interpretations that absorption probing the projected minor axis of a galaxy is
sampling winds.Comment: 16 pages, 11 figures, accepted for publication in MNRA
A Catalog of Absorption Lines in Eight HST/STIS E230M 1.0 < z < 1.7 Quasar Spectra
We have produced a catalog of line identifications and equivalent width
measurements for all absorption features in eight ultraviolet echelle quasar
spectra. These spectra were selected as having the highest signal-to-noise
among the HST/STIS spectra obtained with the E230M grating. We identify 56
metal-line systems toward the eight quasars, and present plots of detected
transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the
features in the Lya forest region, redward of the incidence of the Lyb forest,
are metal lines. High ionization transitions are common. We see both O VI and C
IV in 88 - 90% of the metal-line systems for which the spectra cover the
expected wavelength. Si III is seen in 58%, while low ionization absorption in
C II, Si II, and/or Al II is detected in 50% of the systems for which they are
covered. This catalog will facilitate future studies of the Lya forest and of
metal-line systems of various types.Comment: 13 pages, 2 figures, submitted to Monthly Notices of the Royal
Astronomical Society, a complete version with the appendix and all figures is
available at http://www.astro.psu.edu/users/misawa/pub/Paper/qalcat.pdf.g
Laboratory simulation of irradiation-induced dielectric breakdown in spacecraft charging
The discharging of dielectric samples irradiated by a beam of monoenergetic electrons is investigated. The development of a model, or models, which describe the discharge phenomena occuring on the irradiated dielectric targets is discussed. The electrical discharge characteristics of irradiated dielectric samples are discussed and the electrical discharge paths along dielectric surfaces and within the dielectric material are determined. The origin and destination of the surface emitted particles is examined and the charge and energy balance in the system is evaluated
Ultrasonic Inspection of Graphite-Epoxy Solid Rocket Motor Canisters
Thick filament-wound composite materials are particularly attractive for use in solid rocket motor structures. However, these materials are difficult to inspect because of the scattering losses associated with multiple fiber layers. Damage caused by either low or high velocity impact which results in matrix cracking, delaminations and broken fibers cannot be tolerated because of the possibility of catastrophic system failure. American Research Corporation of Virginia has performed a 2-year Phase II Small Business Innovation Research contract to develop an ultrasonic inspection system for graphite/epoxy rocket motor canisters [1]. This paper details the experimental apparatus and testing of rocket motor canisters, presents testing results and discusses observations and detection thresholds
Afterslip Moment Scaling and Variability from a Global Compilation of Estimates
Aseismic afterslip is postseismic fault sliding that may significantly redistribute crustal stresses and drive aftershock sequences. Afterslip is typically modeled through geodetic observations of surface deformation on a case-by-case basis, thus questions of how and why the afterslip moment varies between earthquakes remain largely unaddressed. We compile 148 afterslip studies following 53 Mw6.0–9.1 earthquakes, and formally analyze a subset of 88 well-constrained kinematic models. Afterslip and coseismic moments scale near-linearly, with a median Spearman's rank correlation coefficient (CC) of 0.91 after bootstrapping (95% range: 0.89–0.93). We infer that afterslip area and average slip scale with coseismic moment as urn:x-wiley:21699313:media:jgrb55593:jgrb55593-math-0001 and urn:x-wiley:21699313:media:jgrb55593:jgrb55593-math-0002, respectively. The ratio of afterslip to coseismic moment (Mrel) varies from 300% (interquartile range: 9%–32%). Mrel weakly correlates with Mo (CC: −0.21, attributed to a publication bias), rupture aspect ratio (CC: −0.31), and fault slip rate (CC: 0.26, treated as a proxy for fault maturity), indicating that these factors affect afterslip. Mrel does not correlate with mainshock dip, rake, or depth. Given the power-law decay of afterslip, we expected studies that started earlier and spanned longer timescales to capture more afterslip, but Mrel does not correlate with observation start time or duration. Because Mrel estimates for a single earthquake can vary by an order of magnitude, we propose that modeling uncertainty currently presents a challenge for systematic afterslip analysis. Standardizing modeling practices may improve model comparability, and eventually allow for predictive afterslip models that account for mainshock and fault zone factors to be incorporated into aftershock hazard models
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