7,827 research outputs found
Optical imaging of resonant electrical carrier injection into individual quantum dots
We image the micro-electroluminescence (EL) spectra of self-assembled InAs
quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and
demonstrate optical detection of resonant carrier injection into a single QD.
Resonant tunneling of electrons and holes into the QDs at bias voltages below
the flat-band condition leads to sharp EL lines characteristic of individual
QDs, accompanied by a spatial fragmentation of the surface EL emission into
small and discrete light- emitting areas, each with its own spectral
fingerprint and Stark shift. We explain this behavior in terms of Coulomb
interaction effects and the selective excitation of a small number of QDs
within the ensemble due to preferential resonant tunneling paths for carriers.Comment: 4 page
PAHs and star formation in the HII regions of nearby galaxies M83 and M33
We present mid-infrared (MIR) spectra of HII regions within star-forming
galaxies M83 and M33. Their emission features are compared with Galactic and
extragalactic HII regions, HII-type galaxies, starburst galaxies, and
Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and
M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in
the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting
different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic
database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH
size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 m PAH
band depends on object type; however, there is no dependence on metallicity for
both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio
decreases with the hardness of the radiation field and galactocentric radius
(Rg), yet the ionization alone cannot account for the variation seen in all of
our sources; (v) the relative strength of PAH features does not change
significantly with increasing radiation hardness, as measured through the
[NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR
calibrations based on the tight correlation between the 6.2, 7.7, and 11.3
m PAH luminosities with the 24 m luminosity and the combination of
the 24 m and H luminosity; (vii) Based on the total luminosity
from PAH and FIR emission, we argue that extragalactic HII regions are more
suitable templates in modeling and interpreting the large scale properties of
galaxies compared to Galactic HII regions.Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRA
Herschel PACS and SPIRE spectroscopy of the Photodissociation Regions associated with S 106 and IRAS 23133+6050
Photodissociation regions (PDRs) contain a large fraction of all of the
interstellar matter in galaxies. Classical examples include the boundaries
between ionized regions and molecular clouds in regions of massive star
formation, marking the point where all of the photons energetic enough to
ionize hydrogen have been absorbed. In this paper we determine the physical
properties of the PDRs associated with the star forming regions IRAS 23133+6050
and S 106 and present them in the context of other Galactic PDRs associated
with massive star forming regions. We employ Herschel PACS and SPIRE
spectroscopic observations to construct a full 55-650 {\mu}m spectrum of each
object from which we measure the PDR cooling lines, other fine- structure
lines, CO lines and the total far-infrared flux. These measurements are then
compared to standard PDR models. Subsequently detailed numerical PDR models are
compared to these predictions, yielding additional insights into the dominant
thermal processes in the PDRs and their structures. We find that the PDRs of
each object are very similar, and can be characterized by a two-phase PDR model
with a very dense, highly UV irradiated phase (n 10^6 cm^(-3), G
10^5) interspersed within a lower density, weaker radiation field phase
(n 10^4 cm^(-3), G 10^4). We employed two different numerical
models to investigate the data, firstly we used RADEX models to fit the peak of
the CO ladder, which in conjunction with the properties derived yielded
a temperature of around 300 K. Subsequent numerical modeling with a full PDR
model revealed that the dense phase has a filling factor of around 0.6 in both
objects. The shape of the CO ladder was consistent with these components
with heating dominated by grain photoelectric heating. An extra excitation
component for the highest J lines (J > 20) is required for S 106.Comment: 20 pages, 10 figures, A&A Accepte
Measurement of Both Gas and Particle Velocity in Turbulent Two-Phase Flow
A laser-Doppler anemometer was used to measure the velocity of both the gas and particles in a turbulent two-phase flow for conditions when the distribution of the velocities of the two phases overlaps. The velocities from the two phases are separated by comparing the Doppler amplitude to the pedestal amplitude. Results of the measure of the gas-particle flow downstream of a nozzle mounted in a circular pipe are presented
Long term cognitive outcomes of early term (37-38 weeks) and late preterm (34-36 weeks) births: a systematic review
Background: There is a paucity of evidence regarding long-term outcomes of late preterm (34-36 weeks) and early term (37-38 weeks) delivery. The objective of this systematic review was to assess long-term cognitive outcomes of children born at these gestations. Methods: Four electronic databases (Medline, Embase, clinicaltrials.gov and PsycINFO) were searched. Last search was 5 th August 2016. Studies were included if they reported gestational age, IQ measure and the ages assessed. The protocol was registered with the International prospective register of systematic reviews (PROSPERO Record CRD42015015472). Two independent reviewers assessed the studies. Data were abstracted and critical appraisal performed of eligible papers. Results: Of 11,905 potential articles, seven studies reporting on 41,344 children were included. For early term births, four studies (n = 35,711) consistently showed an increase in cognitive scores for infants born at full term (39-41 weeks) compared to those born at early term (37-38 weeks) with increases for each week of term (difference between 37 and 40 weeks of around 3 IQ points), despite differences in age of testing and method of IQ/cognitive testing. Four studies (n = 5644) reporting childhood cognitive outcomes of late preterm births (34 - 36 weeks) also differed in study design (cohort and case control); age of testing; and method of IQ testing, and found no differences in outcomes between late preterm and term births, although risk of bias was high in included studies. Conclusion: Children born at 39-41 weeks have higher cognitive outcome scores compared to those born at early term (37-38 weeks). This should be considered when discussing timing of delivery. For children born late preterm, the data is scarce and when compared to full term (37-42 weeks) did not show any difference in IQ scores
Collision Tumour of the Ampulla of Vater: Carcinoid and Adenocarcinoma
Obstructive jaundice is most commonly due to luminal stones or lesions of the head of the pancreas and more rarely ampullary and primary common bile duct lesions. Obstruction due to lesions of the ampulla of Vater may be due to adenocarcinoma which has a significantly better long term prognosis than carcinomas located in the head of the pancreas. A case is presented where two tumours were identified at the ampulla of Vater of the resected specimen one an adenocarcinoma and the other a carcinoid tumour representing a collision tumour
Correlation measurements in high-multiplicity events
Requirements for correlation measurements in high--multiplicity events are
discussed. Attention is focussed on detection of so--called hot spots,
two--particle rapidity correlations, two--particle momentum correlations (for
quantum interferometry) and higher--order correlations. The signal--to--noise
ratio may become large in the high--multiplicity limit, allowing meaningful
single--event measurements, only if the correlations are due to collective
behavior.Comment: MN 55455, 20 pages, KSUCNR-011-92 and TPI-MINN-92/47-T (revised).
Revised to correct typo in equation (30), and to fill in a few steps in
calculations. Now published as Phys. Rev. C 47 (1993) 232
Combination Forecasts of Bond and Stock Returns: An Asset Allocation Perspective
We investigate the out-of-sample forecasting ability of the HML, SMB, momentum, short-term and long-term reversal factors along with their size and value decompositions on U.S. bond and stock returns for a variety of horizons ranging from the short run (1 month) to the long run (2 years). Our findings suggest that these factors contain significantly more information for future bond and stock market returns than the typically employed financial variables. Combination of forecasts of the empirical factors turns out to be particularly successful, especially from an an asset allocation perspective. Similar findings pertain to the European and Japanese markets
- …