12,976 research outputs found
Improved version of the eikonal model for absorbing spherical particles
We present a new expression of the scattering amplitude, valid for spherical
absorbing objects, which leads to an improved version of the eikonal method
outside the diffraction region. Limitations of this method are discussed and
numerical results are presented and compared successfully with the Mie theory.Comment: 7 pages, postscript figures available on cpt.univ-mrs.fr, to appear
in J. Mod. Optic
Sub-Terahertz Monochromatic Transduction with Semiconductor Acoustic Nanodevices
We demonstrate semiconductor superlattices or nanocavities as narrow band
acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics
experiments in the transmission geometry with pump and probe incident on
opposite sides of the thick substrate, phonon generation and detection
processes are fully decoupled. Generating with the semiconductor device and
probing on the metal, we show that both superlattices and nanocavities generate
spectrally narrow wavepackets of coherent phonons with frequencies in the
vicinity of the zone center and time durations in the nanosecond range,
qualitatively different from picosecond broadband pulses usually involved in
picosecond acoustics with metal generators. Generating in the metal and probing
on the nanoacoustic device, we furthermore evidence that both nanostructured
semiconductor devices may be used as very sensitive and spectrally selective
detectors
Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152
We present visible and near IR images of the compact HII region Sh 152. Some
of these images reveal the presence of Extended Red Emission (ERE) around 698
nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2
micron. Other images show the near infrared (7-12 micron) continuous emission
of the nebula. The ERE emission is found to coincide with the ionized region
and significantly differ from the UIRBs location. Also some evidence is found
in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the
colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ;
available in html format at http://www.obs-hp.fr/preprints.htm
Collective Atomic Recoil Laser as a synchronization transition
We consider here a model previously introduced to describe the collective
behavior of an ensemble of cold atoms interacting with a coherent
electromagnetic field. The atomic motion along the self-generated
spatially-periodic force field can be interpreted as the rotation of a phase
oscillator. This suggests a relationship with synchronization transitions
occurring in globally coupled rotators. In fact, we show that whenever the
field dynamics can be adiabatically eliminated, the model reduces to a
self-consistent equation for the probability distribution of the atomic
"phases". In this limit, there exists a formal equivalence with the Kuramoto
model, though with important differences in the self-consistency conditions.
Depending on the field-cavity detuning, we show that the onset of synchronized
behavior may occur through either a first- or second-order phase transition.
Furthermore, we find a secondary threshold, above which a periodic self-pulsing
regime sets in, that is immediately followed by the unlocking of the
forward-field frequency. At yet higher, but still experimentally meaningful,
input intensities, irregular, chaotic oscillations may eventually appear.
Finally, we derive a simpler model, involving only five scalar variables, which
is able to reproduce the entire phenomenology exhibited by the original model
Work distribution functions for hysteresis loops in a single-spin system
We compute the distribution of the work done in driving a single Ising spin
with a time-dependent magnetic field. Using Glauber dynamics we perform
Monte-Carlo simulations to find the work distributions at different driving
rates. We find that in general the work-distributions are broad with a
significant probability for processes with negative dissipated work. The
special cases of slow and fast driving rates are studied analytically. We
verify that various work fluctuation theorems corresponding to equilibrium
initial states are satisfied while a steady state version is not.Comment: 9 pages, 15 figure
Hanbury Brown Twiss effect for ultracold quantum gases
We have studied 2-body correlations of atoms in an expanding cloud above and
below the Bose-Einstein condensation threshold. The observed correlation
function for a thermal cloud shows a bunching behavior, while the correlation
is flat for a coherent sample. These quantum correlations are the atomic
analogue of the Hanbury Brown Twiss effect. We observe the effect in three
dimensions and study its dependence on cloud size.Comment: Figure 1 availabl
Image effects in transport at metal-molecule interfaces
We present a method for incorporating image-charge effects into the
description of charge transport through molecular devices. A simple model
allows us to calculate the adjustment of the transport levels, due to the
polarization of the electrodes as charge is added to and removed from the
molecule. For this, we use the charge distributions of the molecule between two
metal electrodes in several charge states, rather than in gas phase, as
obtained from a density-functional theory-based transport code. This enables us
to efficiently model level shifts and gap renormalization caused by
image-charge effects, which are essential for understanding molecular transport
experiments. We apply the method to benzene di-amine molecules and compare our
results with the standard approach based on gas phase charges. Finally, we give
a detailed account of the application of our approach to porphyrin-derivative
devices recently studied experimentally by Perrin et al. [Nat. Nanotechnol. 8,
282 (2013)], which demonstrates the importance of accounting for image-charge
effects when modeling transport through molecular junctions
High resolution observations of friction-induced oxide and its interaction with the worn surface
A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron,an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni3Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni3Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear
Costs borne by families of children hospitalized in a pediatric intensive care unit: a pilot study.
Hospitalisation of a child in a paediatric intensive care unit (PICU) involves major stress for parents. They wish to stay at their child's bedside while at the same time giving the usual attention to their other children. The resultant out-of-pocket expenses have rarely been studied.
Over a 6-month period all the families of children hospitalised in our PICU for more than 4 days, speaking French and insured by our social security system, were eligible for inclusion. Participation was proposed only after diagnosis, treatment and prognosis had been determined. Costs were retrieved from a diary list of customised items and computed as the amount in excess of usual expenses until the end of the hospital stay.
117 children were hospitalised in our PICU for a total of 131 stays. The families of 16 fulfilled the inclusion criteria. One dropped out after a week at the parents' request. The children's age was 2.9 +/- 3.8 years and 67% were male. The majority had malformations (53%) or infections (33%). The total length of stay was 49 +/- 51 days, of which 24 +/- 41 were spent in the PICU. On average, parents spent CHF 86 +/- 31 every day, mainly on travel and meals. Over the entire hospital stay their expenses amounted to CHF 4,078 +/- 4,552.
Direct out-of-pocket expenses for parents of children hospitalised in the PICU are considerable. Improvement in the social security system may be necessary to address this issue
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