55,982 research outputs found
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
Errors in Hellmann-Feynman Forces due to occupation number broadening, and how they can be corrected
In ab initio calculations of electronic structures, total energies, and
forces, it is convenient and often even necessary to employ a broadening of the
occupation numbers. If done carefully, this improves the accuracy of the
calculated electron densities and total energies and stabilizes the convergence
of the iterative approach towards self-consistency. However, such a boardening
may lead to an error in the calculation of the forces. Accurate forces are
needed for an efficient geometry optimization of polyatomic systems and for ab
initio molecular dynamics (MD) calculations. The relevance of this error and
possible ways to correct it will be discussed in this paper. The first approach
is computationally very simple and in fact exact for small MD time steps. This
is demonstrated for the example of the vibration of a carbon dimer and for the
relaxation of the top layer of the (111)-surfaces of aluminium and platinum.
The second, more general, scheme employs linear-response theory and is applied
to the calculation of the surface relaxation of Al(111). We will show that the
quadratic dependence of the forces on the broadening width enables an efficient
extrapolation to the correct result. Finally the results of these correction
methods will be compared to the forces obtained by using the smearing scheme,
which has been proposed by Methfessel and Paxton.Comment: 6 pages, 5 figures, Scheduled tentatively for the issue of Phys. Rev.
B 15 15 Dec 97 Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Direct and Simultaneous Observation of Ultrafast Electron and Hole Dynamics in Germanium
Understanding excited carrier dynamics in semiconductors is crucial for the
development of photovoltaics and efficient photonic devices. However,
overlapping spectral features in optical/NIR pump-probe spectroscopy often
render assignments of separate electron and hole carrier dynamics ambiguous.
Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin
films are directly and simultaneously observed by attosecond transient
absorption spectroscopy (ATAS) in the extreme ultraviolet at the germanium
M_{4,5}-edge (~30 eV). We decompose the ATAS spectra into contributions of
electronic state blocking and photo-induced band shifts at a carrier density of
8*10^{20}cm^{-3}. Separate electron and hole relaxation times are observed as a
function of hot carrier energies. A first order electron and hole decay of ~1
ps suggests a Shockley-Read-Hall recombination mechanism. The simultaneous
observation of electrons and holes with ATAS paves the way for investigating
few to sub-femtosecond dynamics of both holes and electrons in complex
semiconductor materials and across junctions.Comment: Includes Supplementary Informatio
The dark matter halo shape of edge-on disk galaxies - II. Modelling the HI observations: methods
This is the second paper of a series in which we attempt to put constraints
on the flattening of dark halos in disk galaxies. For this purpose, we observe
the HI in edge-on galaxies, where it is in principle possible to measure the
force field in the halo vertically and radially from gas layer flaring and
rotation curve decomposition respectively. To calculate the force fields, we
need to analyse the observed XV diagrams to accurately measure all three
functions that describe the planar kinematics and distribution of a galaxy: the
radial HI surface density, the rotation curve and the HI velocity dispersion.
In this paper, we discuss the improvements and limitations of the methods
previously used to measure these HI properties. We extend the constant velocity
dispersion method to include determination of the HI velocity dispersion as a
function of galactocentric radius and perform extensive tests on the quality of
the fits. We will apply this 'radial decomposition XV modelling method' to our
HI observations of 8 HI-rich, late-type, edge-on galaxies in the third paper of
this series.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12566.pd
Numerical Renormalization Group Analysis of Interacting Quantum Dots
Wilson's Numerical Renormalization Group (NRG) is so far the only
nonperturbative technique that can reliably access low-energy properties of
quantum impurity systems. We present a recent extension of the method, the
DM-NRG, which yields highly accurate results for dynamical quantities at
arbitrary frequencies and temperatures. As an application, we determine the
spectrum of a quantum dot in an external magnetic field. Furthermore, we
discuss magnetic impurities with orbital degeneracy, which have been inferred
in recent experiments on quantum dots in an Aharonov-Bohm geometry. It is
demonstrated that for spinless electrons, interference between neighbouring
levels sets the low-energy scale of the system. Switching on an external field
leads to a remarkable crossover into a regime dominated by orbital Kondo
screening. We predict that the broadening-induced level splitting should be
clearly visible in measurements of the optical absorption power. A more general
model including the electron spin is studied within an extended two-band NRG
procedure. We observe competition between interference and Kondo screening,
similar to the situation in two-impurity models (RKKY).Comment: Invited talk at the DPG spring meeting 2001, to appear in Advances in
Solid State Physics 4
Decomposition Based Search - A theoretical and experimental evaluation
In this paper we present and evaluate a search strategy called Decomposition
Based Search (DBS) which is based on two steps: subproblem generation and
subproblem solution. The generation of subproblems is done through value
ranking and domain splitting. Subdomains are explored so as to generate,
according to the heuristic chosen, promising subproblems first.
We show that two well known search strategies, Limited Discrepancy Search
(LDS) and Iterative Broadening (IB), can be seen as special cases of DBS. First
we present a tuning of DBS that visits the same search nodes as IB, but avoids
restarts. Then we compare both theoretically and computationally DBS and LDS
using the same heuristic. We prove that DBS has a higher probability of being
successful than LDS on a comparable number of nodes, under realistic
assumptions. Experiments on a constraint satisfaction problem and an
optimization problem show that DBS is indeed very effective if compared to LDS.Comment: 16 pages, 8 figures. LIA Technical Report LIA00203, University of
Bologna, 200
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