22,880 research outputs found
Phase diversity restoration of sunspot images I. Relations between penumbral and photospheric features
We investigate the dynamics of and the relations between small-scale
penumbral and photospheric features near the outer penumbral boundary:
penumbral grains (PGs), dark penumbral fibrils, granules, and photospheric
G-band bright points. The analysis is based on a 2 h time sequence of a sunspot
close to disc center, taken simultaneously in the G-band and in the blue
continuum at 450.7 nm. Observations were performed at the Swedish Vacuum Solar
Telescope (La Palma) in July 1999. A total of 2564 images (46 arcsec x 75
arcsec) were corrected for telescope aberrations and turbulence perturbations
by applying the inversion method of phase diversity. Our findings can by
summarized as follows: (a) One third of the outward-moving PGs pass through the
outer penumbral boundary and then either continue moving as small bright
features or expand and develop into granules. (b) Former PGs and G-band bright
points next to the spot reveal a different nature. The latter have not been
identified as a continuation of PGs escaping from the penumbra. The G-band
bright points are mostly born close to dark penumbral fibrils where the
magnetic field is strong, whereas PGs stem from the less-magnetized penumbral
component and evolve presumably to non-magnetic granules or small bright
features.Comment: Accepted by A&A, 9 pages and 5 figure
Effect of quantum interference in the nonlinear conductance of microconstrictions
The influence of the interference of electron waves, which were scattered by
single impurities, on nonlinear quantum conductance of metallic
microconstrictions (as was recently investigated experimentally) is studied
theoretically. The dependence of the interference pattern in the conductance
on the contact diameter and the spatial distribution of impurities is
analyzed. It is shown that the amplitude of conductance oscillation is strongly
depended on the position of impurities inside the constriction.Comment: 6 pages, 4 figures, To appear in PR
Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires
After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.Comment: 4 page
Connective neck evolution and conductance steps in hot point contacts
Dynamic evolution of the connective neck in Al and Pb mechanically
controllable break junctions was studied during continuous approach of
electrodes at bias voltages V_b up to a few hundred mV. A high level of power
dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in
the constriction lead to overheating of the contact area, electromigration and
current-enhanced diffusion of atoms out of the "hot spot". At a low electrode
approach rate (10 - 50 pm/s) the transverse dimension of the neck and the
conductance of the junction depend on V_b and remain nearly constant over the
approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists
of a few atoms only and the quantum nature of conductance manifests itself in
abrupt steps and reversible jumps between two or more levels. These features
are related to an ever changing number of individual conductance channels due
to the continuous rearrangement in atomic configuration of the neck, the
recurring motion of atoms between metastable states, the formation and breaking
of isolated one-atom contacts and the switching between energetically
preferable neck geometries.Comment: 21 pages 10 figure
Phase conversion in a weakly first-order quark-hadron transition
We investigate the process of phase conversion in a thermally-driven {\it
weakly} first-order quark-hadron transition. This scenario is physically
appealing even if the nature of this transition in equilibrium proves to be a
smooth crossover for vanishing baryonic chemical potential. We construct an
effective potential by combining the equation of state obtained within Lattice
QCD for the partonic sector with that of a gas of resonances in the hadronic
phase, and present numerical results on bubble profiles, nucleation rates and
time evolution, including the effects from reheating on the dynamics for
different expansion scenarios. Our findings confirm the standard picture of a
cosmological first-order transition, in which the process of phase conversion
is entirely dominated by nucleation, also in the case of a weakly first-order
transition. On the other hand, we show that, even for expansion rates much
lower than those expected in high-energy heavy ion collisions, nucleation is
very unlikely, indicating that the main mechanism of phase conversion is
spinodal decomposition. Our results are compared to those obtained for a
strongly first-order transition, as the one provided by the MIT bag model.Comment: 12 pages, 10 figures; v2: 1 reference added, minor modifications,
matches published versio
Viewing the Proton Through "Color"-Filters
While the form factors and parton distributions provide separately the shape
of the proton in coordinate and momentum spaces, a more powerful imaging of the
proton structure can be obtained through phase-space distributions. Here we
introduce the Wigner-type quark and gluon distributions which depict a full-3D
proton at every fixed light-cone momentum, like what seen through
momentum("color")-filters. After appropriate phase-space reductions, the Wigner
distributions are related to the generalized parton distributions (GPD's) and
transverse-momentum dependent parton distributions which are measurable in
high-energy experiments. The new interpretation of GPD's provides a classical
way to visualize the orbital motion of the quarks which is known to be the key
to the spin and magnetic moment of the proton.Comment: 4 page
Purifying two-bit quantum gates and joint measurements in cavity QED
Using a cavity QED setup we show how to implement a particular joint
measurement on two atoms in a fault tolerant way. Based on this scheme, we
illustrate how to realize quantum communication over a noisy channel when local
operations are subject to errors. We also present a scheme to perform and
purify a universal two-bit gate.Comment: 4 pages RevTeX, 2 figures include
Climate Variability in the Woodbush Granite Grasslands of South Africa: Effects on Grassland Diversity
South African old-growth grasslands are hyper-diverse ecosystems which evolved under naturally occurring rainfall variability. It is predicted that future precipitation patterns will become more variable, which could lead to increased frequencies of extreme and prolonged drought events. This study aimed to investigate the effects of climate variability on plant diversity of the fragmented pristine, mistbelt grasslands of the Woodbush Granite Grasslands (WGG) at Haenertsburg, South Africa. It has been reported that species composition has changed substantially in this area, as disturbance-tolerant species enter these systems or existing competitor species become more dominant. A Temporal Beta-diversity Index (TBI) was used to determine the gains and losses in taxonomic and functional diversity, and of endemic and threatened species since 2009. Results show that there was a gain in species from before (2009) to onset of drought (2015) and a loss in species from the onset of the drought (2015) to after the drought (2019), while the overall effect of the drought on species occurrence from 2009 to 2019 was non-significant suggesting ecosystem resilience to drought
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