4,808 research outputs found
Observation of the Meissner effect with ultracold atoms in bosonic ladders
We report on the observation of the Meissner effect in bosonic flux ladders
of ultracold atoms. Using artificial gauge fields induced by laser-assisted
tunneling, we realize arrays of decoupled ladder systems that are exposed to a
uniform magnetic field. By suddenly decoupling the ladders and projecting into
isolated double wells, we are able to measure the currents on each side of the
ladder. For large coupling strengths along the rungs of the ladder, we find a
saturated maximum chiral current corresponding to a full screening of the
artificial magnetic field. For lower coupling strengths, the chiral current
decreases in good agreement with expectations of a vortex lattice phase. Our
work marks the first realization of a low-dimensional Meissner effect and,
furthermore, it opens the path to exploring interacting particles in low
dimensions exposed to a uniform magnetic field
Optical receiver characterizations and corrections for ground-based and airborne measurements of spectral actinic flux densities
Solar actinic radiation in the ultraviolet and visible range (UV/VIS) perpetuates atmospheric photochemistry by inducing photolysis processes which form reactive radical species. Photolysis frequencies are rate constants that quantify the rates of photolysis reactions and therefore constitute important parameters for quantitative analyses. Photolysis frequencies are usually calculated from modelled or measured solar spectral actinic flux densities. Suitable measurement techniques are available, but measurement accuracy can suffer from non-ideal 2π or 4π solid-angle reception characteristics of the usually employed 2π optical receivers or receiver combinations. These imperfections, i.e. deviations from an angle-independent response, should be compensated for by corrections of the measured data. In this work, the relative angular sensitivities of four commonly used 2π quartz receivers were determined in the laboratory in a range 280–660 nm. Based on this information, the influence of the non-ideal responses on measured spectral actinic flux densities for ground-based and airborne applications was investigated for a wide range of atmospheric conditions. Spectral radiance distributions and contributions of direct, diffuse downward and diffuse upward spectral actinic flux densities were calculated with a radiative transfer model to derive the corrections. The intention was to determine the ranges of possible corrections under realistic measurement conditions and to derive simple parametrizations with reasonable uncertainties. For ground-based 2π measurements of downward spectral actinic flux densities, corrections typically range <10 % dependent on wavelength and solar zenith angle, with 2 %–8 % uncertainties covering all atmospheric conditions. Corrections for 4π airborne measurements were determined for the platforms Zeppelin NT (New Technology) and HALO (High Altitude and Long Range Research Aircraft) in altitude ranges 0.05–2 and 0.2–15 km, respectively. Total, downward and upward spectral actinic flux densities were treated separately. In addition to various atmospheric conditions, different ground albedos and small (<5∘) aircraft attitude variations were considered in the uncertainties, as well as aircraft headings with respect to the sun in the case of HALO. Corrections for total and downward spectral actinic flux densities again typically range <10 % dependent on wavelength, solar zenith angle and altitude, with 2 %–10 % uncertainties covering all atmospheric conditions for solar zenith angles below 80∘. For upward spectral actinic flux densities, corrections were more variable and significantly greater, up to about −50 % at low altitudes and low ground albedos. A parametrization for corrections and uncertainties was derived using uncorrected ratios of upward / downward spectral actinic flux densities as input, applicable independent of atmospheric conditions for a given wavelength, solar zenith angle and altitude. The use was limited to conditions with solar zenith angles <80∘ when direct sun radiation cannot strike upward- and downward-looking receivers simultaneously. Examples of research flights with the Zeppelin and HALO are discussed, as well as other approaches described in the literature.</p
Concurrent TNFRSF1A R92Q and pyrin E230K mutations in a child with multiple sclerosis
We report a 16-year-old female patient with a severe course of multiple sclerosis and concomitant symptoms suggestive of a hereditary autoinflammatory disease. Genetic analyses revealed that she inherited a TNFRSF1A R92Q mutation from her mother and a pyrin E230K mutation from her father. To our knowledge, this is the first report of a patient with severe childhood multiple sclerosis and mutations in two genes which predispose to hereditary autoinflammatory disorders. We speculate that these mutations contribute to early multiple sclerosis manifestation and enhance the inflammatory damage inflicted by the autoimmune response
Fractal dimension crossovers in turbulent passive scalar signals
The fractal dimension of turbulent passive scalar signals is
calculated from the fluid dynamical equation. depends on the
scale. For small Prandtl (or Schmidt) number one gets two ranges,
for small scale r and =5/3 for large r, both
as expected. But for large one gets a third, intermediate range in
which the signal is extremely wrinkled and has . In that
range the passive scalar structure function has a plateau. We
calculate the -dependence of the crossovers. Comparison with a numerical
reduced wave vector set calculation gives good agreement with our predictions.Comment: 7 pages, Revtex, 3 figures (postscript file on request
Crystal Nucleation by Laser-Induced Cavitation\ud
High-speed and high-resolution photography have been used to investigate the relationship between creation, expansion, and collapse of a vapor cavity induced by a 6 ns laser pulse and the subsequent nucleation of crystals. A thin layer of supersaturated aqueous solutions of (NH4)2SO4 and KMnO4 was confined between two glass plates with a separation of 50 and 100 μm. The expansion and collapse of the laser-induced vapor bubble occurred over a total time scale of 200 μs, while the first identifiable crystal appears one second after the laser pulse. Crystals were observed to form on a ring with a diameter of 70 μm centered in the focal point of the laser. The ring is preceded by an optical disturbance observed through the cavity around 30–50 μs after the laser pulse and vapor cavity formation. This ring-shaped optical disturbance originates from changes in refractive index induced by crystal nuclei formation. The formation of the nuclei most probably coincides with the formation of the bubble, when the rate of evaporation and the supersaturation are at their maxima. Apparently, it takes the nuclei around 30–50 μs to grow to a particle size with a visible optical disturbanc
Statistics of turbulence in the energy-containing range of Taylor-Couette compared to canonical wall-bounded flows
Considering structure functions of the streamwise velocity component in a
framework akin to the extended self-similarity hypothesis (ESS), de Silva
\textit{et al.} (\textit{J. Fluid Mech.}, vol. 823,2017, pp. 498-510) observed
that remarkably the \textit{large-scale} (energy-containing range) statistics
in canonical wall bounded flows exhibit universal behaviour. In the present
study, we extend this universality, which was seen to encompass also flows at
moderate Reynolds number, to Taylor-Couette flow. In doing so, we find that
also the transversal structure function of the spanwise velocity component
exhibits the same universal behaviour across all flow types considered. We
further demonstrate that these observations are consistent with predictions
developed based on an attached-eddy hypothesis. These considerations also yield
a possible explanation for the efficacy of the ESS framework by showing that it
relaxes the self-similarity assumption for the attached eddy contributions. By
taking the effect of streamwise alignment into account, the attached eddy model
predicts different behaviour for structure functions in the streamwise and in
the spanwise directions and that this effect cancels in the ESS-framework ---
both consistent with the data. Moreover, it is demonstrated here that also the
additive constants, which were previously believed to be flow dependent, are
indeed universal at least in turbulent boundary layers and pipe flow where
high-Reynolds number data are currently available.Comment: accepted in J. Fluid Mec
Non-continuous Froude number scaling for the closure depth of a cylindrical cavity
A long, smooth cylinder is dragged through a water surface to create a cavity
with an initially cylindrical shape. This surface void then collapses due to
the hydrostatic pressure, leading to a rapid and axisymmetric pinch-off in a
single point. Surprisingly, the depth at which this pinch-off takes place does
not follow the expected Froude power-law. Instead, it displays two
distinct scaling regimes separated by discrete jumps, both in experiment and in
numerical simulations (employing a boundary integral code). We quantitatively
explain the above behavior as a capillary waves effect. These waves are created
when the top of the cylinder passes the water surface. Our work thus gives
further evidence for the non-universality of the void collapse
Transport properties of dense dissipitive hard-sphere fluids for arbitrary energy loss models
The revised Enskog approximation for a fluid of hard spheres which lose
energy upon collision is discussed for the case that the energy is lost from
the normal component of the velocity at collision but is otherwise arbitrary.
Granular fluids with a velocity-dependent coefficient of restitution are an
important special case covered by this model. A normal solution to the Enskog
equation is developed using the Chapman-Enskog expansion. The lowest order
solution describes the general homogeneous cooling state and a generating
function formalism is introduced for the determination of the distribution
function. The first order solution, evaluated in the lowest Sonine
approximation, provides estimates for the transport coefficients for the
Navier-Stokes hydrodynamic description. All calculations are performed in an
arbitrary number of dimensions.Comment: 27 pages + 1 figur
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