620 research outputs found
Depletion of density of states near Fermi energy induced by disorder and electron correlation in alloys
We have performed high resolution photoemission study of substitutionally
disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy
spectra and pure metal spectra are found to have dips at the Fermi level when
the residual resistivity is high and when rather strong repulsive
electron-electron interaction is expected. This is in accordance with Altshuler
and Aronov's model which predicts depletion of density of states at the Fermi
level when both disorder and electron correlation are present.Comment: 1 tex file and 4 ps file
Resonant Auger spectroscopy at the L2,3 shake-up thresholds as a probe of electron correlation effects in nickel
The excitation energy dependence of the three-hole satellites in the
L3-M4,5M4,5 and L2-M4,5M4,5 Auger spectra of nickel metal has been measured
using synchrotron radiation. The satellite behavior in the non-radiative
emission spectra at the L3 and L2 thresholds is compared and the influence of
the Coster-Kronig channel explored. The three-hole satellite intensity at the
L3 Auger emission line reveals a peak structure at 5 eV above the L3 threshold
attributed to resonant processes at the 2p53d9 shake-up threshold. This is
discussed in connection with the 6-eV feature in the x-ray absorption spectrum.Comment: 8 pages, 4 figures; http://prb.aps.org/abstract/PRB/v58/i7/p3677_
Performance Bounds of Model Predictive Control for Unconstrained and Constrained Linear Quadratic Problems and Beyond
We study unconstrained and constrained linear quadratic problems and
investigate the suboptimality of the model predictive control (MPC) method
applied to such problems. Considering MPC as an approximate scheme for solving
the related fixed point equations, we derive performance bounds for the
closed-loop system under MPC. Our analysis, as well as numerical examples,
suggests new ways of choosing the terminal cost and terminal constraints, which
are \emph{not} related to the solution of the Riccati equation of the original
problem. The resulting method can have a larger feasible region, and cause
hardly any loss of performance in terms of the closed-loop cost over an
infinite horizon
Isotope shift in the electron affinity of chlorine
The specific mass shift in the electron affinity between ^{35}Cl and ^{37}Cl
has been determined by tunable laser photodetachment spectroscopy to be
-0.51(14) GHz. The isotope shift was observed as a difference in the onset of
the photodetachment process for the two isotopes. In addition, the electron
affinity of Cl was found to be 29138.59(22) cm^{-1}, giving a factor of 2
improvement in the accuracy over earlier measurements. Many-body calculations
including lowest-order correlation effects demonstrates the sensitivity of the
specific mass shift and show that the inclusion of higher-order correlation
effects would be necessary for a quantitative description.Comment: 16 pages, 6 figures, LaTeX2e, amsmat
Multiagent Rollout with Reshuffling for Warehouse Robots Path Planning
Efficiently solving path planning problems for a large number of robots is
critical to the successful operation of modern warehouses. The existing
approaches adopt classical shortest path algorithms to plan in environments
whose cells are associated with both space and time in order to avoid collision
between robots. In this work, we achieve the same goal by means of simulation
in a smaller static environment. Built upon the new framework introduced in
(Bertsekas, 2021a), we propose multiagent rollout with reshuffling algorithm,
and apply it to address the warehouse robots path planning problem. The
proposed scheme has a solid theoretical guarantee and exhibits consistent
performance in our numerical studies. Moreover, it inherits from the generic
rollout methods the ability to adapt to a changing environment by online
replanning, which we demonstrate through examples where some robots
malfunction
Evaluating research: A multidisciplinary approach to assessing research practice and quality
AbstractThere are few widely acknowledged quality standards for research practice, and few definitions of what constitutes good research. The overall aim was therefore to describe what constitutes research, and then to use this description to develop a model of research practice and to define concepts related to its quality. The primary objective was to explore such a model and to create a multidisciplinary understanding of the generic dimensions of the quality of research practice. Eight concept modelling working seminars were conducted. A graphic representation of concepts and their relationships was developed to bridge the gap between different disciplines. A concept model of research as a phenomenon was created, which included a total of 18 defined concepts and their relationships. In a second phase four main areas were distilled, describing research practice in a multidisciplinary context: Credible, Contributory, Communicable, and Conforming. Each of these was further specified in a concept hierarchy together with a defined terminology. A comprehensive quality model including 32 concepts, based on the four main areas, was developed for describing quality issues of research practice, where the model of research as a phenomenon was used to define the quality concepts. The quality model may be used for further development of elements, weights and operationalizations related to the quality of research practice in different academic fields
Correction of non-linearity effects in detectors for electron spectroscopy
Using photoemission intensities and a detection system employed by many
groups in the electron spectroscopy community as an example, we have
quantitatively characterized and corrected detector non-linearity effects over
the full dynamic range of the system. Non-linearity effects are found to be
important whenever measuring relative peak intensities accurately is important,
even in the low-countrate regime. This includes, for example, performing
quantitative analyses for surface contaminants or sample bulk stoichiometries,
where the peak intensities involved can differ by one or two orders of
magnitude, and thus could occupy a significant portion of the detector dynamic
range. Two successful procedures for correcting non-linearity effects are
presented. The first one yields directly the detector efficiency by measuring a
flat-background reference intensity as a function of incident x-ray flux, while
the second one determines the detector response from a least-squares analysis
of broad-scan survey spectra at different incident x-ray fluxes. Although we
have used one spectrometer and detection system as an example, these
methodologies should be useful for many other cases.Comment: 13 pages, 12 figure
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