1,938 research outputs found
Numerical simulations on the motion of atoms travelling through a standing-wave light field
The motion of metastable helium atoms travelling through a standing light
wave is investigated with a semi-classical numerical model. The results of a
calculation including the velocity dependence of the dipole force are compared
with those of the commonly used approach, which assumes a conservative dipole
force. The comparison is made for two atom guiding regimes that can be used for
the production of nanostructure arrays; a low power regime, where the atoms are
focused in a standing wave by the dipole force, and a higher power regime, in
which the atoms channel along the potential minima of the light field. In the
low power regime the differences between the two models are negligible and both
models show that, for lithography purposes, pattern widths of 150 nm can be
achieved. In the high power channelling regime the conservative force model,
predicting 100 nm features, is shown to break down. The model that incorporates
velocity dependence, resulting in a structure size of 40 nm, remains valid, as
demonstrated by a comparison with quantum Monte-Carlo wavefunction
calculations.Comment: 9 pages, 4 figure
Communicating Uncertainty in Economic Evaluations:Verifying Optimal Strategies
Background. In cost-effectiveness analysis (CEA), it is common to compare a single, new intervention with 1 or more existing interventions representing current practice ignoring other, unrelated interventions. Sectoral CEAs, in contrast, take a perspective in which the costs and effectiveness of all possible interventions within a certain disease area or health care sector are compared to maximize health in a society given resource constraints. Stochastic league tables (SLT) have been developed to represent uncertainty in sectoral CEAs but have 2 shortcomings: 1) the probabilities reflect inclusion of individual interventions and not strategies and 2) data on robustness are lacking. The authors developed an extension of SLT that addresses these shortcomings. Methods. Analogous to non-probabilistic MAXIMIN decision rules, the uncertainty of the performance of strategies in sectoral CEAs may be judged with respect to worst possible outcomes, in terms of health effects obtainable within a given budget. Therefore, the authors assessed robustness of strategies likely to be optimal by performing optimization separately on all samples and on samples yielding worse than expected health benefits. The approach was tested on 2 examples, 1 with independent and 1 with correlated cost and effect data. Results. The method was applicable to the original SLT example and to a new example and provided clear and easily interpretable results. Identification of interventions with robust performance as well as the best performing strategies was straightforward. Furthermore, the robustness of strategies was assessed with a MAXIMIN decision rule. Conclusion. The SLT extension improves the comprehensibility and extends the usefulness of outcomes of SLT for decision makers. Its use is recommended whenever an SLT approach is considered
Self-organized current transport through low angle grain boundaries in YBaCuO thin films, studied magnetometrically
The critical current density flowing across low angle grain boundaries in
YBaCuO thin films has been studied magnetometrically.
Films (200 nm thickness) were deposited on SrTiO bicrystal substrates
containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees,
and the films were patterned into rings. Their magnetic moments were measured
in applied magnetic fields up to 30 kOe at temperatures of 5 - 95 K; current
densities of rings with or without grain boundaries were obtained from a
modified critical state model. For rings containing 5 and 7 degree boundaries,
the magnetic response depends strongly on the field history, which arises in
large part from self-field effects acting on the grain boundary.Comment: 8 pages, including 7 figure
Sub-nanosecond, time-resolved, broadband infrared spectroscopy using synchrotron radiation
A facility for sub-nanosecond time-resolved (pump-probe) infrared
spectroscopy has been developed at the National Synchrotron Light Source of
Brookhaven National Laboratory. A mode-locked Ti:sapphire laser produces 2 ps
duration, tunable near-IR pump pulses synchronized to probe pulses from a
synchrotron storage ring. The facility is unique on account of the broadband
infrared from the synchrotron, which allows the entire spectral range from 2
cm-1 (0.25 meV) to 20,000 cm-1 (2.5 eV) to be probed. A temporal resolution of
200 ps, limited by the infrared synchrotron-pulse duration, is achieved. A
maximum time delay of 170 ns is available without gating the infrared detector.
To illustrate the performance of the facility, a measurement of electron-hole
recombination dynamics for an HgCdTe semiconductor film in the far- and mid
infrared range is presented.Comment: 11 pages with 9 figures include
A short note on Italy’s main economic developments and monetary history since the mid 19th century
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Puzzled by GRB 060218
We study the optical-UV/X-ray spectral energy distribution of GRB 060218
during the prompt phase and during what seems to be the afterglow phase. The
results are puzzling, since if the opt-UV and the X-ray emission belong to a
single backbody (BB), then its luminosity is too large, and this BB cannot be
interpreted as the signature of the shock breakout of the supernova. There are
also serious problems in associating the emission expected by the supernova
shock breakout with either the opt-UV or the X-ray emission. In the former case
we derive too small ejecta velocities; in the latter case, on the contrary, the
required velocity is too large, corresponding to the large radius of a BB
required to peak close to the UV band. We then present what we think is the
most conservative alternative explanation, namely a synchrotron spectrum,
self-absorbed in the opt-UV and extending up to the X-ray band, where we
observe the emission of the most energetic electrons, which are responsible for
the exponential roll-over of the spectrum. The obtained fit can explain the
entire spectrum except the BB observed in the X-rays, which must be a separate
component. The puzzling feature of this interpretation is that the same model
is required to explain the spectrum also at later times, up to 1e5 s, because
the opt-UV emission remains constant in shape and also (approximately) in
normalisation. In this case the observed X-ray flux is produced by self-Compton
emission. Thus the prompt emission phase should last for ~1e5 s or more.
Finally, we show that the BB observed in X-rays, up to 7000 seconds, can be
photospheric emission from the cocoon or stellar material, energized by the GRB
jet at radii comparable to the stellar radius (i.e. 1e10-1e11 cm), not very far
from where this material becomes transparent (e.g. 1e12 cm).Comment: revised version accepted for publication in MNRAS (Letters
International Trade and Polarization in the Labor Market
The paper builds an argument that international trade can be one explanation behind polarization of employment in the labor market observed in developed countries such as U.K. and U.S. It considers a small open economy, having production sectors which use three types of labor: high-skill, middle-skill and low-skill. The economy faces an increase in the relative price of the high-skill intensive sector. Using decision rules for choosing middle-skill and low-skill education, it is shown that such a terms of trade shock can lead to higher shares of high-skill as well as low-skill workers in the total workforce. The effects off-shoring on wages and job composition are also studied. That of low-skill and high-skill tasks, not middle-skill tasks, is shown to contribute towards polarization in job composition
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