1,027 research outputs found
30th European Congress on Obesity (ECO 2023)
This is the abstract book of 30th European Congress on Obesity (ECO 2023
A Scanning Electron Microscope for Ultracold Atoms
We propose a new technique for the detection of single atoms in ultracold
quantum gases. The technique is based on scanning electron microscopy and
employs the electron impact ionization of trapped atoms with a focussed
electron probe. Subsequent detection of the resulting ions allows for the
reconstruction of the atoms position. This technique is expected to achieve a
much better spatial resolution compared to any optical detection method. In
combination with the sensitivity to single atoms, it makes new in situ
measurements of atomic correlations possible. The detection principle is also
well suited for the addressing of individual sites in optical lattices.Comment: 5 pages, 2 figure
Derivation and verification of empirical catchment response time equations for medium to large catchments in South Africa
Published ArticleDespite uncertainties and errors in measurement, observed peak discharges are the best estimate of the true peak discharge from a
catchment. However, in ungauged catchments, the catchment response time is a fundamental input to all methods of estimating
peak discharges; hence, errors in estimated catchment response time directly impact on estimated peak discharges. In South
Africa, this is particularly the case in ungauged medium to large catchments where practitioners are limited to use empirical
methods that were calibrated on small catchments not located in South Africa. The time to peak (TP), time of concentration (TC)
and lag time (TL) are internationally the most frequently used catchment response time parameters and are normally estimated
using either hydraulic or empirical methods. Almost 95% of all the time parameter estimation methods developed internationally
are empirically based. This paper presents the derivation and verification of empirical TP equations in a pilot scale study using 74
catchments located in four climatologically different regions of South Africa, with catchment areas ranging from 20 km2 to
35 000 km2. The objective is to develop unique relationships between observed TP values and key climatological and
geomorphological catchment predictor variables in order to estimate catchment TP values at ungauged catchments. The results
show that the derived empirical TP equation(s) meet the requirement of consistency and ease of application. Independent
verification tests confirmed the consistency, while the statistically significant independent predictor variables included in the
regressions provide a good estimation of catchment response times and are also easy to determine by practitioners when required
for future applications in ungauged catchments. It is recommended that the methodology used in this study should be expanded
to other catchments to enable the development of a regional approach to improve estimation of time parameters on a
national-scale. However, such a national-scale application would not only increase the confidence in using the suggested
methodology and equation(s) in South Africa, but also highlights that a similar approach could be adopted internationally
Are estimates of catchment response time inconsistent as used in current flood hydrology practice in South Africa?
Published ArticleCatchment response time parameters are one of the primary inputs required when design floods, especially in ungauged catchments, need to be estimated. The time of concentration (TC) is the most frequently used time parameter in flood hydrology practice, and continues to find application in both event-based methods and continuous hydrological models. Despite the widespread use of the TC, a unique working definition and equation(s) are currently lacking in South Africa. This paper presents the results of the direct and indirect TCestimation for three sets of catchments, which highlight their inherent variability and inconsistencies. These case studies demonstrate that estimates of TC, using different equations, may differ from one another by up to 800%. As a consequence of this high variability and uncertainty, we recommend that, for design hydrology and calibration purposes, observed TCvalues should be estimated using both the average catchment TCvalue, which is based on the event means, and a linear catchment response function. This approach is not only practical, but also proved to be objective and consistent in the study areas investigated in this paper
All-optical formation of a Bose-Einstein condensate for applications in scanning electron microscopy
We report on the production of a F=1 spinor condensate of 87Rb atoms in a
single beam optical dipole trap formed by a focused CO2 laser. The condensate
is produced 13mm below the tip of a scanning electron microscope employing
standard all-optical techniques. The condensate fraction contains up to 100,000
atoms and we achieve a duty cycle of less than 10s.Comment: 5 pages, 4 figure
Coherent collisional spin dynamics in optical lattices
We report on the observation of coherent, purely collisionally driven spin
dynamics of neutral atoms in an optical lattice. For high lattice depths, atom
pairs confined to the same lattice site show weakly damped Rabi-type
oscillations between two-particle Zeeman states of equal magnetization, induced
by spin changing collisions. This paves the way towards the efficient creation
of robust entangled atom pairs in an optical lattice. Moreover, measurement of
the oscillation frequency allows for precise determination of the spin-changing
collisional coupling strengths, which are directly related to fundamental
scattering lengths describing interatomic collisions at ultracold temperatures.Comment: revised version; 4 pages, 5 figure
Structure of strongly coupled, multi-component plasmas
We investigate the short-range structure in strongly coupled fluidlike plasmas using the hypernetted chain approach generalized to multicomponent systems. Good agreement with numerical simulations validates this method for the parameters considered. We found a strong mutual impact on the spatial arrangement for systems with multiple ion species which is most clearly pronounced in the static structure factor. Quantum pseudopotentials were used to mimic diffraction and exchange effects in dense electron-ion systems. We demonstrate that the different kinds of pseudopotentials proposed lead to large differences in both the pair distributions and structure factors. Large discrepancies were also found in the predicted ion feature of the x-ray scattering signal, illustrating the need for comparison with full quantum calculations or experimental verification
Quantitative adsorbate structure determination under catalytic reaction conditions
Current methods allow quantitative local structure determination of adsorbate geometries on surfaces in ultrahigh vacuum (UHV) but are incompatible with the higher pressures required for a steady-state catalytic reactions. Here we show that photoelectron diffraction can be used to determine the structure of the methoxy and formate reaction intermediates during the steady-state oxidation of methanol over Cu(110) by taking advantage of recent instrumental developments to allow near-ambient pressure x-ray photoelectron spectroscopy. The local methoxy site differs from that under static UHV conditions, attributed to the increased surface mobility and dynamic nature of the surface under reaction conditions
Strong-coupling effects in the relaxation dynamics of ultracold neutral plasmas
We describe a hybrid molecular dynamics approach for the description of
ultracold neutral plasmas, based on an adiabatic treatment of the electron gas
and a full molecular dynamics simulation of the ions, which allows us to follow
the long-time evolution of the plasma including the effect of the strongly
coupled ion motion. The plasma shows a rather complex relaxation behavior,
connected with temporal as well as spatial oscillations of the ion temperature.
Furthermore, additional laser cooling of the ions during the plasma evolution
drastically modifies the expansion dynamics, so that crystallization of the ion
component can occur in this nonequilibrium system, leading to lattice-like
structures or even long-range order resulting in concentric shells
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