17,801 research outputs found
Evidence for core-hole-mediated inelastic x-ray scattering from metallic FeTe
We present a detailed analysis of resonant inelastic scattering (RIXS) from
FeTe with unprecedented energy resolution. In contrast to the sharp
peaks typically seen in insulating systems at the transition metal edge,
we observe spectra which show different characteristic features. For low energy
transfer, we experimentally observe theoretically predicted many-body effects
of resonant Raman scattering from a non-interacting gas of fermions.
Furthermore, we find that limitations to this many-body electron-only theory
are realized at high Raman shift, where an exponential lineshape reveals an
energy scale not present in these considerations. This regime, identified as
emission, requires considerations of lattice degrees of freedom to understand
the lineshape. We argue that both observations are intrinsic general features
of many-body physics of metals.Comment: 4 pages, 4 figure
An efficient high-order algorithm for acoustic scattering from penetrable thin structures in three dimensions
This paper presents a high-order accelerated algorithm for the solution of the integral-equation formulation of volumetric scattering problems. The scheme is particularly well suited to the analysis of “thin” structures as they arise in certain applications (e.g., material coatings); in addition, it is also designed to be used in conjunction with existing low-order FFT-based codes to upgrade their order of accuracy through a suitable treatment of material interfaces. The high-order convergence of the new procedure is attained through a combination of changes of parametric variables (to resolve the singularities of the Green function) and “partitions of unity” (to allow for a simple implementation of spectrally accurate quadratures away from singular points). Accelerated evaluations of the interaction between degrees of freedom, on the other hand, are accomplished by incorporating (two-face) equivalent source approximations on Cartesian grids. A detailed account of the main algorithmic components of the scheme are presented, together with a brief review of the corresponding error and performance analyses which are exemplified with a variety of numerical results
On Absorption by Circumstellar Dust, With the Progenitor of SN2012aw as a Case Study
We use the progenitor of SN2012aw to illustrate the consequences of modeling
circumstellar dust using Galactic (interstellar) extinction laws that (1)
ignore dust emission in the near-IR and beyond; (2) average over dust
compositions, and (3) mis-characterize the optical/UV absorption by assuming
that scattered photons are lost to the observer. The primary consequences for
the progenitor of SN2012aw are that both the luminosity and the absorption are
significantly over-estimated. In particular, the stellar luminosity is most
likely in the range 10^4.8 < L/Lsun < 10^5.0 and the star was not extremely
massive for a Type IIP progenitor, with M < 15Msun. Given the properties of the
circumstellar dust and the early X-ray/radio detections of SN2012aw, the star
was probably obscured by an on-going wind with Mdot ~ 10^-5.5 to 10^-5.0
Msun/year at the time of the explosion, roughly consistent with the expected
mass loss rates for a star of its temperature (T_* ~ 3600K) and luminosity. In
the spirit of Galactic extinction laws, we supply simple interpolation formulas
for circumstellar extinction by dusty graphitic and silicate shells as a
function of wavelength (>0.3 micron) and total (absorption plus scattering)
V-band optical depth (tau < 20). These do not include the contributions of dust
emission, but provide a simple, physical alternative to incorrectly using
interstellar extinction laws.Comment: Submitted to Ap
High-resolution ab initio three-dimensional X-ray diffraction microscopy
Coherent X-ray diffraction microscopy is a method of imaging non-periodic
isolated objects at resolutions only limited, in principle, by the largest
scattering angles recorded. We demonstrate X-ray diffraction imaging with high
resolution in all three dimensions, as determined by a quantitative analysis of
the reconstructed volume images. These images are retrieved from the 3D
diffraction data using no a priori knowledge about the shape or composition of
the object, which has never before been demonstrated on a non-periodic object.
We also construct 2D images of thick objects with infinite depth of focus
(without loss of transverse spatial resolution). These methods can be used to
image biological and materials science samples at high resolution using X-ray
undulator radiation, and establishes the techniques to be used in
atomic-resolution ultrafast imaging at X-ray free-electron laser sources.Comment: 22 pages, 11 figures, submitte
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