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

Cold‐pool‐driven convective initiation: using causal graph analysis to determine what convection‐permitting models are missing

Cold‐pool‐driven convective initiation is investigated in high‐resolution, convection‐permitting 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 ICON‐LEM model with grid sizes from 156 to 625 m. In these simulations, we identify cold pools, cold‐pool 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 lower‐resolution 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 multi‐linear 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 km‐scale, numerical weather prediction models. We thus expect that a better representation of cold‐pool‐driven 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 $T_c$. 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 $A_{xc}[\rho (\vec r,t)]$ in time-dependent density functional theory , where $\rho (\vec r,t)$ is the time-dependent density, is invariant under the transformation to an accelerated frame of reference $\rho (\vec r,t) \to \rho ' (\vec r,t) = \rho (\vec r + \vec x (t),t)$, where $\vec x (t)$ is an arbitrary function of time. This invariance implies that the exchange-correlation potential in the Kohn-Sham equation transforms in the following manner: $V_{xc}[\rho '; \vec r, t] = V_{xc}[\rho; \vec r + \vec x (t),t]$. Some of the approximate formulas that have been proposed for $V_{xc}$ 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

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

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

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 $CaAl$ based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature ($T$) 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. $\simeq$ $T^{-3/4}$ at low $T$, 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