3,365 research outputs found
On ultrafast magnetic flux dendrite propagation into thin superconducting films
We suggest a new theoretical approach describing the velocity of magnetic
flux dendrite penetration into thin superconducting films. The key assumptions
for this approach are based upon experimental observations. We treat a dendrite
tip motion as a propagating flux jump instability. Two different regimes of
dendrite propagation are found. A fast initial stage is followed by a slow
stage, which sets in as soon as a dendrite enters into the vortex-free region.
We find that the dendrite velocity is inversely proportional to the sample
thickness. The theoretical results and experimental data obtained by a
magneto-optic pump-probe technique are compared and excellent agreement between
the calculations and measurements is found.Comment: 4 pages, 4 figure
Coldâpoolâdriven convective initiation: using causal graph analysis to determine what convectionâpermitting models are missing
Coldâpoolâdriven convective initiation is investigated in highâresolution, convectionâpermitting simulations with a focus on the diurnal cycle and organization of convection and the sensitivity to grid size. Simulations of four different days over Germany were performed using the ICONâLEM model with grid sizes from 156 to 625âm. In these simulations, we identify cold pools, coldâpool boundaries and initiated convection. Convection is triggered much more efficiently in the vicinity of cold pools than in other regions and can provide as much as 50% of total convective initiation, in particular in the late afternoon. By comparing different model resolutions, we find that cold pools are more frequent, smaller and less intense in lowerâresolution simulations. Furthermore, their gust fronts are weaker and less likely to trigger new convection. To identify how model resolution affects this triggering probability, we use a linear causal graph analysis. In doing so, we postulate a graph structure with potential causal pathways and then apply multiâlinear regression accordingly. We find a dominant, systematic effect: reducing grid sizes directly reduces upward mass flux at the gust front, which causes weaker triggering probabilities. These findings are expected to be even more relevant for kmâscale, numerical weather prediction models. We thus expect that a better representation of coldâpoolâdriven convective initiation will improve forecasts of convective precipitation
Thermodynamics and Excitations of Condensed Polaritons in Disordered Microcavities
We study the thermodynamic condensation of microcavity polaritons using a
realistic model of disorder in semiconductor quantum wells. This approach
correctly describes the polariton inhomogeneous broadening in the low density
limit, and treats scattering by disorder to all orders in the condensed regime.
While the weak disorder changes the thermodynamic properties of the transition
little, the effects of disorder in the condensed state are prominent in the
excitations and can be seen in resonant Rayleigh scattering.Comment: 5 pages, 3 eps figures (published version
Observation of a New Mechanism of Spontaneous Generation of Magnetic Flux in a Superconductor
We report the discovery of a new mechanism of spontaneous generation of a
magnetic flux in a superconductor cooled through . The sign of the
spontaneous flux changes randomly from one cooldown to the next, and follows a
Gaussian distribution. The width of the distribution increases with the size of
the temperature gradient in the sample. Our observations appear inconsistent
with the well known mechanisms of flux generation. The dependence on the
temperature gradient suggests that the flux may be generated through an
instability of the thermoelectric superconducting-normal quasiparticle
counterflow
Alloy disorder effects on the room temperature optical properties of GaInNAs quantum wells
The effect of alloy disorder on the optical density of states and the average
room temperature carrier statistics in GaInNAs quantum wells is discussed. A
red shift between the peak of the room temperature photoluminescence and the
surface photovoltage spectra, that systematically increases with the nitrogen
content within the quantum wells is observed. The relationship between this
Stokes' shift and the absorption linewidth in different samples suggests that
the photoexcited carriers undergo a continuous transition, from being in
quasi-thermal equilibrium with the lattice to being completely trapped by the
quantum dot-like potential fluctuations, as the nitrogen fraction in the alloy
is increased. The values of the 'electron temperature' inferred from the
photoluminescence spectra are found to be consistent with this interpretation.Comment: 3 figure
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
Motor Performance as Risk Factor for Lower Extremity Injuries in Children
Purpose: Physical activity related injuries in children constitute a costly public health matter. The influence of motor performance on injury risk is unclear. The purpose was to examine if motor performance was a risk factor of traumatic and overuse lower extremity injuries in a normal population of children.
Methods: This study included 1244 participants from 8 to 14-years-old at baseline, all participating in "the Childhood Health, Activity and Motor Performance School Study Denmark". The follow-up period was up to 15 months. The motor performance tests were static balance, single leg hop for distance, core stability tests, vertical jump, shuttle run, and a cardiorespiratory fitness test. Lower extremity injuries were registered by clinicians by weekly questionnaires and classified according to the ICD-10 system.
Results: Poor balance increased risk for traumatic injury in the foot region (IRR=1.09-1.15), and good performance in single leg hop for distance protected against traumatic knee injuries (IRR=0.66-0.68). Good performance in core stability tests and vertical jump increased the risk for traumatic injuries in the foot region (IRR=1.12-1.16). Poor balance increased the risk for overuse injuries in the foot region (IRR=1.65), as did good performance in core stability tests and shuttle run, especially for knee injuries (IRR=1.07-1.18).
Conclusions: Poor balance (sway) performance was a consistent predictor of traumatic injuries, in particular for traumatic ankle injuries. Good motor performance (core stability, vertical jump, shuttle run) was positively associated with traumatic and overuse injuries, and negatively (single leg hop) associated with traumatic injuries, indicating different influence on injury risk. Previous injury was a confounder affecting the effect size and the significance. More studies are needed to consolidate the findings, to clarify the influence of different performance tests on different types of injuries and to examine the influence of behaviour in relation to injury ris
Coordination Dependence of Hyperfine Fields of 5sp Impurities on Ni Surfaces
We present first-principles calculations of the magnetic hyperfine fields H
of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the
dependence of H on the coordination number by placing the impurity in the
surfaces, on top of them at the adatom positions, and in the bulk. We find a
strong coordination dependence of H, different and characteristic for each
impurity. The behavior is explained in terms of the on-site s-p hybridization
as the symmetry is reduced at the surface. Our results are in agreement with
recent experimental findings.Comment: 4 pages, 3 figure
Unusual thermoelectric behavior of packed crystalline granular metals
Loosely packed granular materials are intensively studied nowadays.
Electrical and thermal transport properties should reflect the granular
structure as well as intrinsic properties. We have compacted crystalline
based metallic grains and studied the electrical resistivity and the
thermoelectric power as a function of temperature () from 15 to 300K. Both
properties show three regimes as a function of temperature. It should be
pointed out : (i) The electrical resistivity continuously decreases between 15
and 235 K (ii) with various dependences, e.g. at low ,
while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near
60K, and (v) presents a rather unusual square root of temperature dependence at
low temperature. It is argued that these three regimes indicate a competition
between geometric and thermal processes, - for which a theory seems to be
missing in the case of TEP. The microchemical analysis results are also
reported indicating a complex microstructure inherent to the phase diagram
peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure
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