1,736 research outputs found
Stress intensity factors in bonded half planes containing inclined cracks and subjected to antiplane shear loading
The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks was considered. For the semi-infinite crack the problem was solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses were calculated. For finite cracks the problem was reduced to a pair of integral equations. Numerical results were obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles
Effect of symmetry distortions on photoelectron selection rules and spectra of Bi_2Sr_2CaCu_2O_{8+ delta}
We derive photoelectron selection rules along the glide plane in orthorhombic
Bi_2Sr_2CaCu_2O_{8+\delta} (Bi2212). These selection rules explain the reversed
intensity behavior of the shadow and the main band of the material as a natural
consequence of the variating representation of the final state as a function of
k_\parallel. Our one-step simulations strongly support the structural origin of
the shadow band but we also introduce a scenario for detecting
antiferromagnetic signatures in low doping.Comment: AMS-LaTeX, 5 pages, 4 figure
EXITE2 Observation of the SIGMA Source GRS 1227+025
We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h
observation on May 8, 1997, shows a 260 mCrab source detected at
in each of two bands (50-70 and 70-93 keV) and located 30'
from 3C273 and consistent in position with the SIGMA source GRS1227+025. The
EXITE2 spectrum is consistent with a power law with photon index 3 and large
low energy absorption, as indicated by the GRANAT/SIGMA results. No source was
detected in more sensitive followup EXITE2 observations in 2000 and 2001 with
3 upper limits of 190 and 65 mCrab, respectively. Comparison with the
flux detected by SIGMA shows the source to be highly variable, suggesting it
may be non-thermal and beamed and thus the first example of a ``type 2''
(absorbed) Blazar. Alternatively it might be (an unprecedented) very highly
absorbed binary system undergoing accretion disk instability outbursts,
possibly either a magnetic CV, or a black hole X-ray nova.Comment: 12 pages, 4 figures, accepted for publication in Ap
Unabsorbed Seyfert 2 galaxies
We present a sample of 17 type 2 Seyfert galaxies which have an X-ray column density lower than 10^{22} cm^{-2}. The Compton thin nature of these sources is strongly suggested by isotropic indicators. We estimate the fraction of these sources to be in the range of 10% - 30% of the population of type 2 Seyfert galaxies. Furthermore, this fraction appears to increase progressively at lower luminosities. The simple formulation of the Unified Model for Seyfert galaxies is not applicable in such sources since the pc-scale molecular torus is not likely to be responsible for the low column density observed, instead the absorption observed is likely to originate at larger scales. According to this hypothesis, in these objects the broad line regions are covered by some dusty obscuring material. In particular, this could occur in objects with dust lanes, patches or HII regions. However, we cannot rule out that in the lowest luminosity sources the BLR is weak, absent or has faded away. This last scenario is consistent with the predictions of some recent theoretical models for low luminosity AGNs
Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane
For atomic thin layer insulating materials we provide an exact analytic form
of the two-dimensional screened potential. In contrast to three-dimensional
systems where the macroscopic screening can be described by a static dielectric
constant in 2D systems the macroscopic screening is non local (q-dependent)
showing a logarithmic divergence for small distances and reaching the
unscreened Coulomb potential for large distances. The cross-over of these two
regimes is dictated by 2D layer polarizability that can be easily computed by
standard first-principles techniques. The present results have strong
implications for describing gap-impurity levels and also exciton binding
energies. The simple model derived here captures the main physical effects and
reproduces well, for the case of graphane, the full many-body GW plus
Bethe-Salpeter calculations. As an additional outcome we show that the impurity
hole-doping in graphane leads to strongly localized states, what hampers
applications in electronic devices. In spite of the inefficient and nonlocal
two-dimensional macroscopic screening we demonstrate that a simple
approach is capable to describe the electronic and
transport properties of confined 2D systems.Comment: 17 pages, 3 figure
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