14,907 research outputs found
Low temperature reflectivity study of ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates
We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane
oriented ZnO substrates. The optical properties of these quantum wells are
studied by using reflectance spectroscopy. The optical spectra reveal strong
in-plane optical anisotropies, as predicted by group theory, and marked
reflectance structures, as an evidence of good interface morphologies.
Signatures ofc onfined excitons built from the spin-orbit split-off valence
band, the analog of exciton C in bulk ZnO are detected in normal incidence
reflectivity experiments using a photon polarized along the c axis of the
wurtzite lattice. Experiments performed in the context of an orthogonal photon
polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A
and B bulk excitons. Envelope function calculations which include excitonic
interaction nicely account for the experimental report
On the non-perturbative realization of QCD gauge-invariance
A few years ago the use of standard functional manipulations was demonstrated
to imply an unexpected property satisfied by the fermionic Green's functions of
QCD: effective locality. This feature of QCD is non-perturbative as it results
from a full integration of the gluonic degrees of freedom. In this paper,
previous derivations of effective locality are reviewed, corrected, and
enhanced. Focussing on the way non-abelian gauge invariance is realized in the
non-perturbative regime of QCD, the deeper meaning of effective locality is
discussed.Comment: 18 page
Chemical abundances of fast-rotating massive stars. I. Description of the methods and individual results
Aims: Recent observations have challenged our understanding of rotational
mixing in massive stars by revealing a population of fast-rotating objects with
apparently normal surface nitrogen abundances. However, several questions have
arisen because of a number of issues, which have rendered a reinvestigation
necessary; these issues include the presence of numerous upper limits for the
nitrogen abundance, unknown multiplicity status, and a mix of stars with
different physical properties, such as their mass and evolutionary state, which
are known to control the amount of rotational mixing. Methods: We have
carefully selected a large sample of bright, fast-rotating early-type stars of
our Galaxy (40 objects with spectral types between B0.5 and O4). Their
high-quality, high-resolution optical spectra were then analysed with the
stellar atmosphere modelling codes DETAIL/SURFACE or CMFGEN, depending on the
temperature of the target. Several internal and external checks were performed
to validate our methods; notably, we compared our results with literature data
for some well-known objects, studied the effect of gravity darkening, or
confronted the results provided by the two codes for stars amenable to both
analyses. Furthermore, we studied the radial velocities of the stars to assess
their binarity. Results: This first part of our study presents our methods and
provides the derived stellar parameters, He, CNO abundances, and the
multiplicity status of every star of the sample. It is the first time that He
and CNO abundances of such a large number of Galactic massive fast rotators are
determined in a homogeneous way.Comment: accepted for publication by A&
The economics of airport noise: how to manage markets for noise licenses
Noise-induced pollution constitutes a hot and topical societal problem for all major airports. This paper discusses various issues in the implementation of a market for noise licenses as a solution to solve the noise externality between the residents located around airports and the aircrafts moving in and to airports.airport, environment, noise, licenses
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