1,240 research outputs found
Stimulated emission from semi-polar (11-22) GaN overgrown on sapphire
(11-22) semi-polar GaN is expected to exhibit major advantages compared with current c-plane polar GaN in the fabrication of long wavelength such as green and yellow emitters. However, all the III-nitride based semi-/non- polar laser diodes (LDs) reported so far have been achieved exclusively based on homo-epitaxial growth on extremely expensive free-standing GaN substrates with a very limited size. In this paper, we have observed a stimulated emission at room temperature achieved on our semi-polar (11-22) GaN overgrown on a micro-rod arrayed template with an optimized design on m-plane sapphire. This has never been achieved previously on any semi-polar GaN grown on sapphire. Furthermore, an optical gain of 130cm−1 has been measured by means of performing a standard laser stripe-length dependent optical measurement. The values of the threshold and the optical gain obtained are comparable to those of the c-plane GaN reported so far, further validating the satisfactory crystal quality of our overgrown semi-polar (11-22) GaN on sapphire. This represents a major step towards the development of III-nitride semi-polar based LDs on sapphire, especially in the long wavelength regime
Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun
We search for persistent and quasi-periodic release events of streamer blobs
during 2007 with the Large Angle Spectrometric Coronagraph on the \textit{Solar
and Heliospheric Observatory} and assess the velocity of the slow solar wind
along the plasma sheet above the corresponding streamer by measuring the
dynamic parameters of blobs. We find 10 quasi-periodic release events of
streamer blobs lasting for three to four days. In each day of these events, we
observe three-five blobs. The results are in line with previous studies using
data observed near the last solar minimum. Using the measured blob velocity as
a proxy for that of the mean flow, we suggest that the velocity of the
background slow solar wind near the Sun can vary significantly within a few
hours. This provides an observational manifestation of the large velocity
variability of the slow solar wind near the Sun.Comment: 14 pages, 5 figures, accepted by Soalr Physic
Scattering mechanism in a step-modulated subwavelength metal slit: a multi-mode multi-reflection analysis
In this paper, the scattering/transmission inside a step-modulated
subwavelength metal slit is investigated in detail. We firstly investigate the
scattering in a junction structure by two types of structural changes. The
variation of transmission and reflection coefficients depending on structural
parameters are analyzed. Then a multi-mode multi-reflection model based on ray
theory is proposed to illustrate the transmission in the step-modulated slit
explicitly. The key parts of this model are the multi-mode excitation and the
superposition procedure of the scatterings from all possible modes, which
represent the interference and energy transfer happened at interfaces. The
method we use is an improved modal expansion method (MEM), which is a more
practical and efficient version compared with the previous one [Opt. Express
19, 10073 (2011)]. In addition, some commonly used methods, FDTD, scattering
matrix method, and improved characteristic impedance method, are compared with
MEM to highlight the preciseness of these methods.Comment: 25 pages, 9 figure
A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections
In this paper, with a survey through the Large Angle and Spectrometric
Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma
blobs flowing outwards sequentially along a bright coronal ray in the wake of a
coronal mass ejection. The ray is believed to be associated with the current
sheet structure that formed as a result of solar eruption, and the blobs are
products of magnetic reconnection occurring along the current sheet. The ray
morphology and blob dynamics are investigated statistically. It is found that
the apparent angular widths of the rays at a fixed time vary in a range of
2.1-6.6 (2.0-4.4) degrees with an average of 3.5 (2.9) degrees at 3 (4) Rs,
respectively, and the observed durations of the events vary from 12 h to a few
days with an average of 27 h. It is also found, based on the analysis of blob
motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated,
and 22% (10) moved with a nearly-constant speed. Comparing the dynamics of our
blobs and those that are observed above the tip of a helmet streamer, we find
that the speeds and accelerations of the blobs in these two cases differ
significantly. It is suggested that these differences of the blob dynamics stem
from the associated magnetic reconnection involving different magnetic field
configurations and triggering processes.Comment: 12 pages, 6 figures, accepted by Solar Physic
Integrating fluctuations into distribution of resources in transportation networks
We propose a resource distribution strategy to reduce the average travel time
in a transportation network given a fixed generation rate. Suppose that there
are essential resources to avoid congestion in the network as well as some
extra resources. The strategy distributes the essential resources by the
average loads on the vertices and integrates the fluctuations of the
instantaneous loads into the distribution of the extra resources. The
fluctuations are calculated with the assumption of unlimited resources, where
the calculation is incorporated into the calculation of the average loads
without adding to the time complexity. Simulation results show that the
fluctuation-integrated strategy provides shorter average travel time than a
previous distribution strategy while keeping similar robustness. The strategy
is especially beneficial when the extra resources are scarce and the network is
heterogeneous and lowly loaded.Comment: 14 pages, 4 figure
Accurate Atmospheric Parameters at Moderate Resolution Using Spectral Indices: Preliminary Application to the MARVELS Survey
Studies of Galactic chemical and dynamical evolution in the solar
neighborhood depend on the availability of precise atmospheric parameters
(Teff, [Fe/H] and log g) for solar-type stars. Many large-scale spectroscopic
surveys operate at low to moderate spectral resolution for efficiency in
observing large samples, which makes the stellar characterization difficult due
to the high degree of blending of spectral features. While most surveys use
spectral synthesis, in this work we employ an alternative method based on
spectral indices to determine the atmospheric parameters of a sample of nearby
FGK dwarfs and subgiants observed by the MARVELS survey at moderate resolving
power (R~12,000). We have developed three codes to automatically normalize the
observed spectra, measure the equivalent widths of the indices and, through the
comparison of those with values calculated with pre-determined calibrations,
derive the atmospheric parameters of the stars. The calibrations were built
using a sample of 309 stars with precise stellar parameters obtained from the
analysis of high-resolution FEROS spectra. A validation test of the method was
conducted with a sample of 30 MARVELS targets that also have reliable
atmospheric parameters from high-resolution spectroscopic analysis. Our
approach was able to recover the parameters within 80 K for Teff, 0.05 dex for
[Fe/H] and 0.15 dex for log g, values that are lower or equal to the typical
external uncertainties found between different high-resolution analyzes. An
additional test was performed with a subsample of 138 stars from the ELODIE
stellar library and the literature atmospheric parameters were recovered within
125 K for Teff, 0.10 dex for [Fe/H] and 0.29 dex for log g. These results show
that the spectral indices are a competitive tool to characterize stars with the
intermediate resolution spectra.Comment: Accepted for publication in AJ. Abstract edited to comply with arXiv
standards regarding the number of character
Correlation between centrality metrics and their application to the opinion model
In recent decades, a number of centrality metrics describing network
properties of nodes have been proposed to rank the importance of nodes. In
order to understand the correlations between centrality metrics and to
approximate a high-complexity centrality metric by a strongly correlated
low-complexity metric, we first study the correlation between centrality
metrics in terms of their Pearson correlation coefficient and their similarity
in ranking of nodes. In addition to considering the widely used centrality
metrics, we introduce a new centrality measure, the degree mass. The m order
degree mass of a node is the sum of the weighted degree of the node and its
neighbors no further than m hops away. We find that the B_{n}, the closeness,
and the components of x_{1} are strongly correlated with the degree, the
1st-order degree mass and the 2nd-order degree mass, respectively, in both
network models and real-world networks. We then theoretically prove that the
Pearson correlation coefficient between x_{1} and the 2nd-order degree mass is
larger than that between x_{1} and a lower order degree mass. Finally, we
investigate the effect of the inflexible antagonists selected based on
different centrality metrics in helping one opinion to compete with another in
the inflexible antagonists opinion model. Interestingly, we find that selecting
the inflexible antagonists based on the leverage, the B_{n}, or the degree is
more effective in opinion-competition than using other centrality metrics in
all types of networks. This observation is supported by our previous
observations, i.e., that there is a strong linear correlation between the
degree and the B_{n}, as well as a high centrality similarity between the
leverage and the degree.Comment: 20 page
Dynamics of multipartite quantum correlations under decoherence
Quantum discord is an optimal resource for the quantification of classical
and non-classical correlations as compared to other related measures. Geometric
measure of quantum discord is another measure of quantum correlations.
Recently, the geometric quantum discord for multipartite states has been
introduced by Jianwei Xu [arxiv:quant/ph.1205.0330]. Motivated from the recent
study [Ann. Phys. 327 (2012) 851] for the bipartite systems, I have
investigated global quantum discord (QD) and geometric quantum discord (GQD)
under the influence of external environments for different multipartite states.
Werner-GHZ type three-qubit and six-qubit states are considered in inertial and
non-inertial settings. The dynamics of QD and GQD is investigated under
amplitude damping, phase damping, depolarizing and flipping channels. It is
seen that the quantum discord vanishes for p>0.75 in case of three-qubit GHZ
states and for p>0.5 for six qubit GHZ states. This implies that multipartite
states are more fragile to decoherence for higher values of N. Surprisingly, a
rapid sudden death of discord occurs in case of phase flip channel. However,
for bit flip channel, no sudden death happens for the six-qubit states. On the
other hand, depolarizing channel heavily influences the QD and GQD as compared
to the amplitude damping channel. It means that the depolarizing channel has
the most destructive influence on the discords for multipartite states. From
the perspective of accelerated observers, it is seen that effect of environment
on QD and GQD is much stronger than that of the acceleration of non-inertial
frames. The degradation of QD and GQD happens due to Unruh effect. Furthermore,
QD exhibits more robustness than GQD when the multipartite systems are exposed
to environment.Comment: 15 pages, 4 figures, 4 table
Active Galaxies in the UV
In this article we present different aspects of AGN studies demonstrating the
importance of the UV spectral range. Most important diagnostic lines for
studying the general physical conditions as well as the metalicities in the
central broad line region in AGN are emitted in the UV. The UV/FUV continuum in
AGN excites not only the emission lines in the immediate surrounding but it is
responsible for the ionization of the intergalactic medium in the early stages
of the universe. Variability studies of the emission line profiles of AGN in
the UV give us information on the structure and kinematics of the immediate
surrounding of the central supermassive black hole as well as on its mass
itself.Comment: 29 pages, 13 figures, Ap&SS in pres
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
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