1,527 research outputs found
Results of a self-triggered prototype system for radio-detection of extensive air showers at the Pierre Auger Observatory
We describe the experimental setup and the results of RAuger, a small
radio-antenna array, consisting of three fully autonomous and self-triggered
radio-detection stations, installed close to the center of the Surface Detector
(SD) of the Pierre Auger Observatory in Argentina. The setup has been designed
for the detection of the electric field strength of air showers initiated by
ultra-high energy cosmic rays, without using an auxiliary trigger from another
detection system. Installed in December 2006, RAuger was terminated in May 2010
after 65 registered coincidences with the SD. The sky map in local angular
coordinates (i.e., zenith and azimuth angles) of these events reveals a strong
azimuthal asymmetry which is in agreement with a mechanism dominated by a
geomagnetic emission process. The correlation between the electric field and
the energy of the primary cosmic ray is presented for the first time, in an
energy range covering two orders of magnitude between 0.1 EeV and 10 EeV. It is
demonstrated that this setup is relatively more sensitive to inclined showers,
with respect to the SD. In addition to these results, which underline the
potential of the radio-detection technique, important information about the
general behavior of self-triggering radio-detection systems has been obtained.
In particular, we will discuss radio self-triggering under varying local
electric-field conditions.Comment: accepted for publication in JINS
Even harmonic generation in isotropic media of dissociating homonuclear molecules
Isotropic gases irradiated by long pulses of intense IR light can generate
very high harmonics of the incident field. It is generally accepted that, due
to the symmetry of the generating medium, be it an atomic or an isotropic
molecular gas, only odd harmonics of the driving field can be produced. Here we
show how the interplay of electronic and nuclear dynamics can lead to a marked
breakdown of this standard picture: a substantial part of the harmonic spectrum
can consist of even rather than odd harmonics. We demonstrate the effect using
ab-initio solutions of the time-dependent Schr\"odinger equation for
and its isotopes in full dimensionality. By means of a simple
analytical model, we identify its physical origin, which is the appearance of a
permanent dipole moment in dissociating homonuclear molecules, caused by
light-induced localization of the electric charge during dissociation. The
effect arises for sufficiently long laser pulses and the region of the spectrum
where even harmonics are produced is controlled by pulse duration. Our results
(i) show how the interplay of femtosecond nuclear and attosecond electronic
dynamics, which affects the charge flow inside the dissociating molecule, is
reflected in the nonlinear response, and (ii) force one to augment standard
selection rules found in nonlinear optics textbooks by considering
light-induced modifications of the medium during the generation process.Comment: 7 pages, 6 figure
Searching for Very High Energy Emission from Pulsars Using the High Altitude Water Cherenkov (HAWC) Observatory
There are currently over 160 known gamma-ray pulsars. While most of them are
detected only from space, at least two are now seen also from the ground. MAGIC
and VERITAS have measured the gamma ray pulsed emission of the Crab pulsar up
to hundreds of GeV and more recently MAGIC has reported emission at
TeV. Furthermore, in the Southern Hemisphere, H.E.S.S. has detected the Vela
pulsar above 30 GeV. In addition, non-pulsed TeV emission coincident with
pulsars has been detected by many groups, including the Milagro Collaboration.
These GeV-TeV observations open the possibility of searching for
very-high-energy (VHE, > 100GeV) pulsations from gamma-rays pulsars in the HAWC
field of view.Comment: Presented at the 34th International Cosmic Ray Conference (ICRC2015),
The Hague, The Netherlands. See arXiv:1508.03327 for all HAWC contribution
Delocalization of slowly damped eigenmodes on Anosov manifolds
We look at the properties of high frequency eigenmodes for the damped wave
equation on a compact manifold with an Anosov geodesic flow. We study
eigenmodes with spectral parameters which are asymptotically close enough to
the real axis. We prove that such modes cannot be completely localized on
subsets satisfying a condition of negative topological pressure. As an
application, one can deduce the existence of a "strip" of logarithmic size
without eigenvalues below the real axis under this dynamical assumption on the
set of undamped trajectories.Comment: 28 pages; compared with version 1, minor modifications, add two
reference
Atomic environment and interfacial structural order of TiAlN/Mo multilayers
Multilayered TiAlN/Mo coatings were deposited by dc reactive magnetron sputtering in a custom-made chamber. In order to assess the composition of these coatings, a combined study of Extended X-ray Absorption Fine Structure (EXAFS) and Rutherford Backscattering Spectrometry (RBS) experiments were performed. Through the simulation of the EXAFS spectra, giving the local environment of the titanium atoms inside the nitride (TiAlN), a cubic phase has been evidenced with aluminium atoms occupying titanium sites. For modulation
periods in the range of 3.6–11.8 nm, RBS simulations on these multilayers also enabled the determination of the level of intermixing that occurs at the interfaces as a function of the negative bias voltage and number of layers. It was observed that the intermixing width could be as high as 2.1 nm for the roughest samples (larger periods) or as low as 0.4 nm for those with the sharpest interfaces (smaller periods).Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER) -Ministério da Ciência e Tecnologia.Cooperação CientÃfica e Tecnológica Internacional (ICCTI) - Ambassade de France in Portugal - Project no. 543 B3/2001.Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional “Ciência, Tecnologia, Inovação - POCTI/32670/CTM/2000
Entropy of semiclassical measures for nonpositively curved surfaces
We study the asymptotic properties of eigenfunctions of the Laplacian in the
case of a compact Riemannian surface of nonpositive sectional curvature. We
show that the Kolmogorov-Sinai entropy of a semiclassical measure for the
geodesic flow is bounded from below by half of the Ruelle upper bound. We
follow the same main strategy as in the Anosov case (arXiv:0809.0230). We focus
on the main differences and refer the reader to (arXiv:0809.0230) for the
details of analogous lemmas.Comment: 20 pages. This note provides a detailed proof of a result announced
in appendix A of a previous work (arXiv:0809.0230, version 2
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
Structural evolution of Ti-Al-Si-N nanocomposite coating
Ti–Si–Al–N films were prepared by rf reactive magnetron sputtering, in static and rotation modes, using
a wide range of different deposition conditions, which created conditions to obtain Ti–Al–Si–N coatings
with different structural arrangements.
Films prepared below a critical nitrogen flow, under conditions out of thermodynamic equilibrium,
revealed a preferential growth of an fcc (Ti,Al,Si)Nx compound with a small N deficiency. With nitrogen
flow above that critical value, the reduction of the lattice parameter was no longer detected. However,
a thermal annealing showed that a complete thermodynamically driven segregation of the TiN and Si3N4
phases was not yet obtained. The segregation upon annealing induced a self-hardening and showed
a multiphase system, where the crystalline TiN, (Ti,Al)N and (Ti,Al,Si)Nx phases were identified by X-ray
diffraction. This behavior is due to the de-mixing of the solid solution associated to a small N deficiency
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