16,349 research outputs found
Coherent XUV generation driven by sharp metal tips photoemission
It was already experimentally demonstrated that high-energy electrons can be
generated using metal nanotips as active media. In addition, it has been
theoretically proven that the high-energy tail of the photoemitted electrons is
intrinsically linked to the recollision phenomenon. Through this recollision
process it is also possible to convert the energy gained by the laser-emitted
electron in the continuum in a coherent XUV photon. It means the emission of
harmonic radiation appears to be feasible, although it has not been
experimentally demonstrated hitherto till now. In this paper, we employ a
quantum mechanical approach to model the electron dipole moment including both
the laser experimental conditions and the bulk matter properties and predict is
possible to generate coherent UV and XUV radiation using metal nanotips as
sources. Our quantum mechanical results are fully supported by their classical
counterparts.Comment: arXiv admin note: substantial text overlap with arXiv:1309.034
N-Delta(1232) axial form factors from weak pion production
The N-Delta axial form factors are determined from neutrino induced pion
production ANL & BNL data by using a state of the art theoretical model, which
accounts both for background mechanisms and deuteron effects. We find
violations of the off diagonal Goldberger-Treiman relation at the level of 2
sigma which might have an impact in background calculations for T2K and
MiniBooNE low energy neutrino oscillation precision experiments.Comment: 4 pages, 1 figur
Kinematic study of planetary nebulae in NGC 6822
By measuring precise radial velocities of planetary nebulae (which belong to
the intermediate age population), H II regions, and A-type supergiant stars
(which are members of the young population) in NGC 6822, we aim to determine if
both types of population share the kinematics of the disk of H I found in this
galaxy.
Spectroscopic data for four planetary nebulae were obtained with the high
spectral resolution spectrograph Magellan Inamori Kyocera Echelle (MIKE) on the
Magellan telescope at Las Campanas Observatory. Data for other three PNe and
one H II region were obtained from the SPM Catalog of Extragalactic Planetary
Nebulae which employed the Manchester Echelle Spectrometer attached to the 2.1m
telescope at the Observatorio Astron\'omico Nacional, M\'exico. In the
wavelength calibrated spectra, the heliocentric radial velocities were measured
with a precision better than 5-6 km s. Data for three additional H II
regions and a couple of A-type supergiant stars were collected from the
literature. The heliocentric radial velocities of the different objects were
compared to the velocities of the H i disk at the same position.
From the analysis of radial velocities it is found that H II regions and
A-type supergiants do share the kinematics of the H I disk at the same
position, as expected for these young objects. On the contrary, planetary
nebula velocities differ significantly from that of the H I at the same
position. The kinematics of planetary nebulae is independent from the young
population kinematics and it is closer to the behavior shown by carbon stars,
which are intermediate-age members of the stellar spheroid existing in this
galaxy. Our results are confirming that there are at least two very different
kinematical systems in NGC 6822
Weak Pion Production off the Nucleon
We develop a model for the weak pion production off the nucleon, which
besides the Delta pole mechanism (weak excitation of the
resonance and its subsequent decay into ), includes also some background
terms required by chiral symmetry. We re-fit the form factor to
the flux averaged ANL differential cross
section data, finding a substantially smaller contribution of the Delta pole
mechanism than traditionally assumed in the literature. Within this scheme, we
calculate several differential and integrated cross sections, including pion
angular distributions, induced by neutrinos and antineutrinos and driven both
by charged and neutral currents. In all cases we find that the background terms
produce quite significant effects and that they lead to an overall improved
description of the data, as compared to the case where only the Delta pole
mechanism is considered. We also show that the interference between the Delta
pole and the background terms produces parity-violating contributions to the
pion angular differential cross section, which are intimately linked to odd
correlations in the contraction between the leptonic and hadronic tensors.
However, these latter correlations do not imply a genuine violation of time
reversal invariance because of the existence of strong final state interaction
effects.Comment: Typos corrected; comments adde
Influence of Disorder Strength on Phase Field Models of Interfacial Growth
We study the influence of disorder strength on the interface roughening
process in a phase-field model with locally conserved dynamics. We consider two
cases where the mobility coefficient multiplying the locally conserved current
is either constant throughout the system (the two-sided model) or becomes zero
in the phase into which the interface advances (one-sided model). In the limit
of weak disorder, both models are completely equivalent and can reproduce the
physical process of a fluid diffusively invading a porous media, where
super-rough scaling of the interface fluctuations occurs. On the other hand,
increasing disorder causes the scaling properties to change to intrinsic
anomalous scaling. In the limit of strong disorder this behavior prevails for
the one-sided model, whereas for the two-sided case, nucleation of domains in
front of the invading front are observed.Comment: Accepted for publication in PR
Above threshold ionization by few-cycle spatially inhomogeneous fields
We present theoretical studies of above threshold ionization (ATI) produced
by spatially inhomogeneous fields. This kind of field appears as a result of
the illumination of plasmonic nanostructures and metal nanoparticles with a
short laser pulse. We use the time-dependent Schr\"odinger equation (TDSE) in
reduced dimensions to understand and characterize the ATI features in these
fields. It is demonstrated that the inhomogeneity of the laser electric field
plays an important role in the ATI process and it produces appreciable
modifications to the energy-resolved photoelectron spectra. In fact, our
numerical simulations reveal that high energy electrons can be generated.
Specifically, using a linear approximation for the spatial dependence of the
enhanced plasmonic field and with a near infrared laser with intensities in the
mid- 10^{14} W/cm^{2} range, we show it is possible to drive electrons with
energies in the near-keV regime. Furthermore, we study how the carrier envelope
phase influences the emission of ATI photoelectrons for few-cycle pulses. Our
quantum mechanical calculations are supported by their classical counterparts
Intruder States and their Local Effect on Spectral Statistics
The effect on spectral statistics and on the revival probability of intruder
states in a random background is analysed numerically and with perturbative
methods. For random coupling the intruder does not affect the GOE spectral
statistics of the background significantly, while a constant coupling causes
very strong correlations at short range with a fourth power dependence of the
spectral two-point function at the origin.The revival probability is
significantly depressed for constant coupling as compared to random coupling.Comment: 18 pages, 10 Postscript figure
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