26,213 research outputs found
Properties of nano-graphite ribbons with zigzag edges -- Difference between odd and even legs --
Persistent currents and transport properties are investigated for the
nano-graphite ribbons with zigzag shaped edges with paying attention to system
length dependence. It is found that both the persistent current in the
isolated ring and the conductance of the system connected to the perfect leads
show the remarkable dependences. In addition, the dependences for the
systems with odd legs and those with even legs are different from each other.
On the persistent current, the amplitude for the cases with odd legs shows
power-low behavior as with being the number of legs, whereas the
maximum of it decreases exponentially for the cases with even legs. The
conductance per one spin normalized by behaves as follows. In the even
legs cases, it decays as , whereas it reaches to unity for in the odd legs cases. Thus, the material is shown to have a remarkable
property that there is the qualitative difference between the systems with odd
legs and those with even legs even in the absence of the electron-electron
interaction.Comment: 4 pagaes, 8 figures, LT25 conference proceeding, accepted for
publication in Journal of Physics: Conference Serie
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
Unparticle Physics in the Moller Scattering
We investigate the virtual effects of vector unparticles in the Moller
scattering. We derive the analytic expression for scattering amplitudes with
unpolarized beams. We obtain 95% confidence level limits on the unparticle
couplings and with integrated luminosity of
and and 500 GeV energies. We show
that limits on are more sensitive than .Comment: 10 pages, 5 figures, 4 table
Confocal microscopic analysis of optical crosstalk in GaN micro-pixel light-emitting diodes
© 2015 AIP Publishing LLC. The optical crosstalk phenomenon in GaN micro-pixel light-emitting diodes (LED) has been investigated by confocal microscopy. Depth-resolved confocal emission images indicate light channeling along the GaN and sapphire layers as the source of crosstalk. Thin-film micro-pixel devices are proposed, whereby the light-trapping sapphire layers are removed by laser lift-off. Optical crosstalk is significantly reduced but not eliminated due to the remaining GaN layer. Another design involving micro-pixels which are completely isolated is further proposed; such devices exhibited low-noise and enhanced optical performances, which are important attributes for high-density micro-pixel LED applications including micro-displays and multi-channel optical communications.published_or_final_versio
Science communication in the media and human mobility during the COVID-19 pandemic: a time series and content analysis
Objectives
The relationship between human mobility and nature of science (NOS) salience in the UK news media was examined.
Study design
This is a mixed-method study.
Methods
A time series NOS salience data set was established from the content analysis of 1520 news articles related to non-pharmaceutical interventions of COVID-19. Data were taken from articles published between November 2021 and February 2022, which correlates with period of the change from pandemic to endemic status. Vector autoregressive model fitting with human mobility took place.
Results
The findings suggest that it was not the number of COVID-19 news articles nor the actual number of cases/deaths, but the specific NOS content that was associated with mobility change during the pandemic. Data indicate a Granger causal negative direction (P 0.1).
Conclusions
The findings of the study suggest that the ways in which the news media discuss epidemics can influence changes in human mobility. It is therefore essential that public health communicators emphasise the basis of scientific evidence to eliminate potential media bias in health and science communication for the promotion of public health policy. The present study approach, which combines time series and content analysis and uses an interdisciplinary lens from science communication, could also be adopted to other interdisciplinary health-related topics
Tensor Rank Estimation and Completion via CP-based Nuclear Norm
Tensor completion (TC) is a challenging problem of recovering
missing entries of a tensor from its partial observation. One main
TC approach is based on CP/Tucker decomposition. However, this
approach often requires the determination of a tensor rank a priori.
This rank estimation problem is difficult in practice. Several
Bayesian solutions have been proposed but they often under/overestimate
the tensor rank while being quite slow. To address this
problem of rank estimation with missing entries, we view the weight
vector of the orthogonal CP decomposition of a tensor to be analogous
to the vector of singular values of a matrix. Subsequently,
we define a new CP-based tensor nuclear norm as the L1-norm of
this weight vector. We then propose Tensor Rank Estimation based
on L1-regularized orthogonal CP decomposition (TREL1) for both
CP-rank and Tucker-rank. Specifically, we incorporate a regularization
with CP-based tensor nuclear norm when minimizing the
reconstruction error in TC to automatically determine the rank of
an incomplete tensor. Experimental results on both synthetic and
real data show that: 1) Given sufficient observed entries, TREL1 can
estimate the true rank (both CP-rank and Tucker-rank) of incomplete
tensors well; 2) The rank estimated by TREL1 can consistently
improve recovery accuracy of decomposition-based TC methods;
3) TREL1 is not sensitive to its parameters in general and more
efficient than existing rank estimation methods
Randomized observation times for the compound Poisson risk model: The discounted penalty function
In the framework of collective risk theory, we consider a compound Poisson risk model for the surplus process where the process (and hence ruin) can only be observed at random observation times. For Erlang(n) distributed inter-observation times, explicit expressions for the discounted penalty function at ruin are derived. The resulting model contains both the usual continuous-time and the discrete-time risk model as limiting cases, and can be used as an effective approximation scheme for the latter. Numerical examples are given that illustrate the effect of random observation times on various ruin-related quantities
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