657 research outputs found
X-ray Absorption Fine Structure in Embedded Atoms
Oscillatory structure is found in the atomic background absorption in
x-ray-absorption fine structure (XAFS). This atomic-XAFS or AXAFS arises from
scattering within an embedded atom, and is analogous to the Ramsauer-Townsend
effect. Calculations and measurements confirm the existence of AXAFS and show
that it can dominate contributions such as multi-electron excitations. The
structure is sensitive to chemical effects and thus provides a new probe of
bonding and exchange effects on the scattering potential.Comment: 4 pages plus 2 postscript figures, REVTEX 3.
Cumulant expansion for phonon contributions to the electron spectral function
We describe an approach for calculations of phonon contributions to the
electron spectral function, including both quasiparticle properties and
satellites. The method is based on a cumulant expansion for the retarded
one-electron Green's function and a many-pole model for the electron
self-energy. The electron-phonon couplings are calculated from the Eliashberg
functions, and the phonon density of states is obtained from a Lanczos
representation of the phonon Green's function. Our calculations incorporate ab
initio dynamical matrices and electron-phonon couplings from the density
functional theory code ABINIT. Illustrative results are presented for several
elemental metals and for Einstein and Debye models with a range of coupling
constants. These are compared with experiment and other theoretical models.
Estimates of corrections to Migdal's theorem are obtained by comparing with
leading order contributions to the self-energy, and are found to be significant
only for large electron-phonon couplings at low temperatures
Critical Point Field Mixing in an Asymmetric Lattice Gas Model
The field mixing that manifests broken particle-hole symmetry is studied for
a 2-D asymmetric lattice gas model having tunable field mixing properties.
Monte Carlo simulations within the grand canonical ensemble are used to obtain
the critical density distribution for different degrees of particle-hole
asymmetry. Except in the special case when this asymmetry vanishes, the density
distributions exhibit an antisymmetric correction to the limiting
scale-invariant form. The presence of this correction reflects the mixing of
the critical energy density into the ordering operator. Its functional form is
found to be in excellent agreement with that predicted by the mixed-field
finite-size-scaling theory of Bruce and Wilding. A computational procedure for
measuring the significant field mixing parameter is also described, and its
accuracy gauged by comparing the results with exact values obtained
analytically.Comment: 10 Pages, LaTeX + 8 figures available from author on request, To
appear in Z. Phys.
Exciton spectroscopy of hexagonal boron nitride using non-resonant x-ray Raman scattering
We report non-resonant x-ray Raman scattering (XRS) measurements from
hexagonal boron nitride for transferred momentum from 2 to 9
along directions both in and out of the basal plane. A
symmetry-based argument, together with real-space full multiple scattering
calculations of the projected density of states in the spherical harmonics
basis, reveals that a strong pre-edge feature is a dominantly -type
Frenkel exciton with no other \textit{s}-, \textit{p}-, or \textit{d}-
components. This conclusion is supported by a second, independent calculation
of the \textbf{q}-dependent XRS cross-section based on the Bethe-Salpeter
equation
Deconvolving Instrumental and Intrinsic Broadening in Excited State X-ray Spectroscopies
Intrinsic and experimental mechanisms frequently lead to broadening of
spectral features in excited-state spectroscopies. For example, intrinsic
broadening occurs in x-ray absorption spectroscopy (XAS) measurements of heavy
elements where the core-hole lifetime is very short. On the other hand,
nonresonant x-ray Raman scattering (XRS) and other energy loss measurements are
more limited by instrumental resolution. Here, we demonstrate that the
Richardson-Lucy (RL) iterative algorithm provides a robust method for
deconvolving instrumental and intrinsic resolutions from typical XAS and XRS
data. For the K-edge XAS of Ag, we find nearly complete removal of ~9.3 eV FWHM
broadening from the combined effects of the short core-hole lifetime and
instrumental resolution. We are also able to remove nearly all instrumental
broadening in an XRS measurement of diamond, with the resulting improved
spectrum comparing favorably with prior soft x-ray XAS measurements. We present
a practical methodology for implementing the RL algorithm to these problems,
emphasizing the importance of testing for stability of the deconvolution
process against noise amplification, perturbations in the initial spectra, and
uncertainties in the core-hole lifetime.Comment: 35 pages, 13 figure
- …