903 research outputs found
Inleiding tot ân uitleg van protestantwees en die betekenis daarvan by die opvoederlike inhoudgewing
Quantum Suppression of the Rayleigh Instability in Nanowires
A linear stability analysis of metallic nanowires is performed in the
free-electron model using quantum chaos techniques. It is found that the
classical instability of a long wire under surface tension can be completely
suppressed by electronic shell effects, leading to stable cylindrical
configurations whose electrical conductance is a magic number 1, 3, 5, 6,...
times the quantum of conductance. Our results are quantitatively consistent
with recent experiments with alkali metal nanowires.Comment: 10 pages, 5 eps figures, updated and expanded, accepted for
publication in "Nonlinearity
Free-electron Model for Mesoscopic Force Fluctuations in Nanowires
When two metal electrodes are separated, a nanometer sized wire (nanowire) is
formed just before the contact breaks. The electrical conduction measured
during this retraction process shows signs of quantized conductance in units of
G_0=2e^2/h. Recent experiments show that the force acting on the wire during
separation fluctuates, which has been interpreted as being due to atomic
rearrangements. In this report we use a simple free electron model, for two
simple geometries, and show that the electronic contribution to the force
fluctuations is comparable to the experimentally found values, about 2 nN.Comment: 4 pages, 3 figures, reference correcte
Instability driven fragmentation of nanoscale fractal islands
Formation and evolution of fragmentation instabilities in fractal islands,
obtained by deposition of silver clusters on graphite, are studied. The
fragmentation dynamics and subsequent relaxation to the equilibrium shapes are
controlled by the deposition conditions and cluster composition. Sharing common
features with other materials' breakup phenomena, the fragmentation instability
is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres
Defect-induced perturbations of atomic monolayers on solid surfaces
We study long-range morphological changes in atomic monolayers on solid
substrates induced by different types of defects; e.g., by monoatomic steps in
the surface, or by the tip of an atomic force microscope (AFM), placed at some
distance above the substrate. Representing the monolayer in terms of a suitably
extended Frenkel-Kontorova-type model, we calculate the defect-induced density
profiles for several possible geometries. In case of an AFM tip, we also
determine the extra force exerted on the tip due to the tip-induced
de-homogenization of the monolayer.Comment: 4 pages, 2 figure
Electronic entropy, shell structure, and size-evolutionary patterns of metal clusters
We show that electronic-entropy effects in the size-evolutionary patterns of
relatively small (as small as 20 atoms), simple-metal clusters become prominent
already at moderate temperatures. Detailed agreement between our
finite-temperature-shell-correction-method calculations and experimental
results is obtained for certain temperatures. This agreement includes a
size-dependent smearing out of fine-structure features, accompanied by a
measurable reduction of the heights of the steps marking major-shell and
subshell closings, thus allowing for a quantitative analysis of cluster
temperatures.Comment: Latex/Revtex, 4 pages with 3 Postscript figure
Emission lines of Fe XI in the 257--407 A wavelength region observed in solar spectra from EIS/Hinode and SERTS
Theoretical emission-line ratios involving Fe XI transitions in the 257-407 A
wavelength range are derived using fully relativistic calculations of radiative
rates and electron impact excitation cross sections. These are subsequently
compared with both long wavelength channel Extreme-Ultraviolet Imaging
Spectrometer (EIS) spectra from the Hinode satellite (covering 245-291 A), and
first-order observations (235-449 A) obtained by the Solar Extreme-ultraviolet
Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 A
lines of Fe XI are detected in two EIS spectra, confirming earlier
identifications of these features, and 276.36 A is found to provide an electron
density diagnostic when ratioed against the 257.55 A transition. Agreement
between theory and observation is found to be generally good for the SERTS data
sets, with discrepancies normally being due to known line blends, while the
257.55 A feature is detected for the first time in SERTS spectra. The most
useful Fe XI electron density diagnostic is found to be the 308.54/352.67
intensity ratio, which varies by a factor of 8.4 between N_e = 10^8 and 10^11
cm^-3, while showing little temperature sensitivity. However, the 349.04/352.67
ratio potentially provides a superior diagnostic, as it involves lines which
are closer in wavelength, and varies by a factor of 14.7 between N_e = 10^8 and
10^11 cm^-3. Unfortunately, the 349.04 A line is relatively weak, and also
blended with the second-order Fe X 174.52 A feature, unless the first-order
instrument response is enhanced.Comment: 9 pages, 5 figures, 13 tables; MNRAS in pres
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Convective initiation and storm lifeâcycles in convectionâpermitting simulations of the Met Office Unified Model over South Africa
Convective initiation is a challenge for convectionâpermitting models due to its sensitivity to subâkm processes. We evaluate the representation of convective storms and their initiation over South Africa during four summer months in Met Office Unified Model simulations at 1.5âkm horizontal grid length. Storm size distributions from the model compare well against radar observations, but rainfall in the model is predominantly produced by large storms (50 km in diameter or larger) in the evening, whereas radar observations show most rainfall occurs throughout the afternoon, from storms 10â50 km in diameter. In all months, modelled maximum number of storm initiations occurs at least 2 hours prior to the radarâobserved maximum. However, the diurnal cycle of rainfall compares well between model and observations, suggesting the numerous storm initiations in the simulations do not produce much rainfall. Modelled storms are generally less intense than in the radar observations, especially in early summer. In February, when tropical influences dominate, the simulated storms are of similar intensity to observed storms. Simulated storms tend to reach their peak intensity in the first 15 minutes after initiation, then gradually become less intense as they grow. In radar observations, storms reach their peak intensity 15â30 minutes into their life cycle, stay intense as they grow larger, then gradually weaken after they have reached their maximum diameter. Two November case studies of severe convection are analysed in detail. Higher resolution grid length initiates convection slightly earlier (300 m cf. 1.5 km) with the same science settings. Two 1.5âkm simulations that apply more subâgrid mixing have delayed convective initiation. Analysis of soundings indicates little difference in convective indices, suggesting that differences in convection may be attributed to choices in subâgrid mixing parameters
Structure and stability of finite gold nanowires
Finite gold nanowires containing less than 1000 atoms are studied using the
molecular dynamics simulation method and embedded atom potential. Nanowires
with the face-centered cubic structure and the (111) oriented cross-section are
prepared at T=0 K. After annealing and quenching the structure and vibrational
properties of nanowires are studied at room temperature. Several of these
nanowires form multi-walled structures of lasting stability. They consist of
concentrical cylindrical sheets and resemble multi-walled carbon nanotubes.
Vibrations are investigated by diagonalization of the dynamical matrix. It was
found that several percents of vibrational modes are unstable because of
uncompleted restructuring of initial fcc nanowires.Comment: 4 figures in gif forma
Force, charge, and conductance of an ideal metallic nanowire
The conducting and mechanical properties of a metallic nanowire formed at the
junction between two macroscopic metallic electrodes are investigated. Both
two- and three-dimensional wires with a W(ide)-N(arrow)-W(ide) geometry are
modelled in the free-electron approximation with hard-wall boundary conditions.
Tunneling and quantum-size effects are treated exactly using the scattering
matrix formalism. Oscillations of order E_F/lambda_F in the tensile force are
found when the wire is stretched to the breaking point, which are synchronized
with quantized jumps in the conductance. The force and conductance are shown to
be essentially independent of the width of the wide sections (electrodes). The
exact results are compared with an adiabatic approximation; the later is found
to overestimate the effects of tunneling, but still gives qualitatively
reasonable results for nanowires of length L>>lambda_F, even for this abrupt
geometry. In addition to the force and conductance, the net charge of the
nanowire is calculated and the effects of screening are included within linear
response theory. Mesoscopic charge fluctuations of order e are predicted which
are strongly correlated with the mesoscopic force fluctuations. The local
density of states at the Fermi energy exhibits nontrivial behavior which is
correlated with fine structure in the force and conductance, showing the
importance of treating the whole wire as a mesoscopic system rather than
treating only the narrow part.Comment: 23 pages, 8 figure
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