1,328 research outputs found
Full-plane block Kalman filter for image restoration, A
Includes bibliographical references.A new two-dimensional (2-D) block Kalman filtering method is presented which uses a full-plane image model to generate a more accurate filtered estimate of an image that has been corrupted by additive noise and full-plane blur. Causality is maintained within the filtering process by employing multiple concurrent block estimators. In addition, true state dynamics are preserved, resulting in an accurate Kalman gain matrix. Simulation results on a test image corrupted by additive white Gaussian noise are presented for various image models and compared to those of the previous block Kalman filtering methods
Comparison between measured and predicted turbulence frequency spectra in ITG and TEM regimes
The observation of distinct peaks in tokamak core reflectometry measurements
- named quasi-coherent-modes (QCMs) - are identified as a signature of
Trapped-Electron-Mode (TEM) turbulence [H. Arnichand et al. 2016 Plasma Phys.
Control. Fusion 58 014037]. This phenomenon is investigated with detailed
linear and nonlinear gyrokinetic simulations using the \gene code. A Tore-Supra
density scan is studied, which traverses through a Linear (LOC) to Saturated
(SOC) Ohmic Confinement transition. The LOC and SOC phases are both simulated
separately. In the LOC phase, where QCMs are observed, TEMs are robustly
predicted unstable in linear studies. In the later SOC phase, where QCMs are no
longer observed, ITG modes are identified. In nonlinear simulations, in the ITG
(SOC) phase, a broadband spectrum is seen. In the TEM (LOC) phase, a clear
emergence of a peak at the TEM frequencies is seen. This is due to reduced
nonlinear frequency broadening of the underlying linear modes in the TEM regime
compared with the ITG regime. A synthetic diagnostic of the nonlinearly
simulated frequency spectra reproduces the features observed in the
reflectometry measurements. These results support the identification of core
QCMs as an experimental marker for TEM turbulenc
Optical absorption in semiconductor quantum dots: Nonlocal effects
The optical absorption of a single spherical semiconductor quantum dot in an
electrical field is studied taking into account the nonlocal coupling between
the field of the light and the polarizability of the semiconductor. These
nonlocal effects lead to a small size anf field dependent shift and broadening
of the excitonic resonance which may be of interest in future high precision
experiments.Comment: 6 pages, 4 figure
3D Architected Carbon Electrodes for Energy Storage
The ability to design a particular geometry of porous electrodes at multiple length scales in a lithium‐ion battery can significantly and positively influence battery performance because it enables control over distribution of current and potential and can enhance ion and electron transport. 3D architecturally designed carbon electrodes are developed, whose structural factors are independently controlled and whose dimensions span micrometers to centimeters, using digital light processing and pyrolysis. These free‐standing lattice electrodes are comprised of monolithic glassy carbon beams, are lightweight, with a relative density of 0.1–0.35, and mechanically robust, with a maximum precollapse stress of 27 MPa, which facilitates electrode recycling. The specific strength is 101 kN m kg⁻¹, comparable to that of 6061 aluminum alloy. These carbon electrodes can reach a mass loading of 70 mg cm⁻² and an areal capacity of 3.2 mAh cm⁻² at a current density of 2.4 mA cm⁻². It is demonstrated that this approach allows for independent design of structural factors, i.e., beam diameter, electrode thickness, and surface morphology, enabling control over Li‐ion transport length, overpotential and battery performance, not available for slurry‐based electrodes. This multiscale approach to design of electrodes may open substantial performance‐enhancing capabilities for solid‐ and liquid‐state batteries, flow batteries, and fuel cells
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire
We show that a single exciton state in an individual ordered conjugated
polymer chain exhibits macroscopic quantum spatial coherence reaching tens of
microns, limited by the chain length. The spatial coherence of the k=0 exciton
state is demonstrated by selecting two spatially separated emitting regions of
the chain and observing their interference.Comment: 12 pages with 2 figure
Carrier relaxation in GaAs v-groove quantum wires and the effects of localization
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove
quantum wires (QWRs) with a large subband separation (46 meV). Signatures of
inhibited carrier relaxation mechanisms are seen in temperature-dependent
photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we
observe strong emission from the first excited state of the QWR below ~50 K.
This is attributed to reduced inter-subband relaxation via phonon scattering
between localized states. Theoretical calculations and experimental results
indicate that the pinch-off regions, which provide additional two-dimensional
confinement for the QWR structure, have a blocking effect on relaxation
mechanisms for certain structures within the v-groove. Time-resolved PL
measurements show that efficient carrier relaxation from excited QWR states
into the ground state, occurs only at temperatures > 30 K. Values for the low
temperature radiative lifetimes of the ground- and first excited-state excitons
have been obtained (340 ps and 160 ps respectively), and their corresponding
localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct
corrupt figure
Local disorder and optical properties in V-shaped quantum wires : towards one-dimensional exciton systems
The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires
(QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of
different generations of samples shows that the localization length has been
enhanced as the growth techniques were improved. In the best samples, excitons
are delocalized in islands of length of the order of 1 micron, and form a
continuum of 1D states in each of them, as evidenced by the sqrt(T) dependence
of the radiative lifetime. On the opposite, in the previous generation of QWRs,
the localization length is typically 50 nm and the QWR behaves as a collection
of quantum boxes. These localization properties are compared to structural
properties and related to the progresses of the growth techniques. The presence
of residual disorder is evidenced in the best samples and explained by the
separation of electrons and holes due to the large in-built piezo-electric
field present in the structure.Comment: 8 figure
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