7,641 research outputs found
Impurity effect as a probe for the pairing symmetry of graphene-based superconductors
The single impurity effect on the graphene-based superconductor is studied
theoretically. Four different pairing symmetries are discussed. Sharp resonance
peaks are found near the impurity site for the -wave pairing symmetry and
the -wave pairing symmetry when the chemical potential is large. As the
chemical potential decreases, the in-gap states are robust for the
pairing symmetry while they disappear for the pairing symmetry. Such
in-gap peaks are absent for the fully gapped extended -wave pairing symmetry
and the nodal -wave pairing symmetry. The existence of the in-gap resonance
peaks can be explained well based on the sign-reversal of the superconducting
gap along different Fermi pockets and by analyzing the denominator of the
-matrix. All of the features can be accessed by the experiments, which
provide a useful probe for the pairing symmetry of graphene-based
superconductors.Comment: 7 pages, 7 figure
Secure Transmission and Self-Energy Recycling for Wireless-Powered Relay Systems with Partial Eavesdropper Channel State Information
This paper focuses on the secure transmission of wireless-powered relay
systems with imperfect eavesdropper channel state information (ECSI). For
efficient energy transfer and information relaying, a novel two-phase protocol
is proposed, in which the relay operates in full-duplex (FD) mode to achieve
simultaneous wireless power and information transmission. Compared with those
existing protocols, the proposed design possesses two main advantages: 1) it
fully exploits the available hardware resource (antenna element) of relay and
can offer higher secrecy rate; 2) it enables self-energy recycling (S-ER) at
relay, in which the loopback interference (LI) generated by FD operation is
harvested and reused for information relaying. To maximize the worst-case
secrecy rate (WCSR) through jointly designing the source and relay beamformers
coupled with the power allocation ratio, an optimization problem is formulated.
This formulated problem is proved to be non-convex and the challenge to solve
it is how to concurrently solve out the beamformers and the power allocation
ratio. To cope with this difficulty, an alternative approach is proposed by
converting the original problem into three subproblems. By solving these
subproblems iteratively, the closed form solutions of robust beamformers and
power allocation ratio for the original problem are achieved. Simulations are
done and results reveal that the proposed S-ER based secure transmission scheme
outperforms the traditional time-switching based relaying (TSR) scheme at a
maximum WCSR gain of 80%.Results also demonstrate that the WCSR performance of
the scheme reusing idle antennas for information reception is much better than
that of schemes exploiting only one receive antenna.Comment: 13 pages, 9 figure
PTB-TIR: A Thermal Infrared Pedestrian Tracking Benchmark
Thermal infrared (TIR) pedestrian tracking is one of the important components
among numerous applications of computer vision, which has a major advantage: it
can track pedestrians in total darkness. The ability to evaluate the TIR
pedestrian tracker fairly, on a benchmark dataset, is significant for the
development of this field. However, there is not a benchmark dataset. In this
paper, we develop a TIR pedestrian tracking dataset for the TIR pedestrian
tracker evaluation. The dataset includes 60 thermal sequences with manual
annotations. Each sequence has nine attribute labels for the attribute based
evaluation. In addition to the dataset, we carry out the large-scale evaluation
experiments on our benchmark dataset using nine publicly available trackers.
The experimental results help us understand the strengths and weaknesses of
these trackers.In addition, in order to gain more insight into the TIR
pedestrian tracker, we divide its functions into three components: feature
extractor, motion model, and observation model. Then, we conduct three
comparison experiments on our benchmark dataset to validate how each component
affects the tracker's performance. The findings of these experiments provide
some guidelines for future research. The dataset and evaluation toolkit can be
downloaded at {https://github.com/QiaoLiuHit/PTB-TIR_Evaluation_toolkit}.Comment: 10 pages,IEEE Transactions on Multimedia (2019
Calculation of intrinsic spin Hall conductivity by Wannier interpolation
\textit{Ab-initio} calculation of intrinsic spin Hall conductivity (SHC)
generally requires a strict convergence criterion and a dense k-point mesh to
sample the Brillouin zone, making its convergence challenging and
time-consuming. Here we present a scheme for efficiently and accurately
calculating SHC based on maximally localized Wannier function (MLWF). The
quantities needed by the Kubo formula of SHC are derived in the space of MLWF
and it is shown that only the Hamiltonian, the overlap and the spin operator
matrices are required from the initial \textit{ab-initio} calculation. The
computation of these matrices and the interpolation of Kubo formula on a dense
k-point mesh can be easily achieved. We validate our results by prototypical
calculations on fcc Pt and GaAs, which demonstrate that the Wannier
interpolation approach is of high accuracy and efficiency. Calculations of
-Ta and -Ta show that SHC of -Ta is 2.7 times of
-Ta, while both have the opposite sign relative to fcc Pt and are an
order of magnitude smaller than Pt. The calculated spin Hall angle of ,
is quite consistent with previous experiment on -Ta, further suggesting
intrinsic contribution may dominate in -Ta. Our approach could
facilitate large-scale SHC calculations, and may benefit the discovery of
materials with high intrinsic SHC
Centrally Concentrated X-ray Radiation from an Extended Accreting Corona in Active Galactic Nuclei
The X-ray emission from bright active galactic nuclei (AGNs) is believed to
originate in a hot corona lying above a cold, geometrically thin accretion
disk. A highly concentrated corona located within gravitational radii
above the black hole is inferred from observations. Based on the accretion of
interstellar medium/wind, a disk corona model has been proposed in which the
corona is well coupled to the disk by radiation, thermal conduction, as well as
by mass exchange \citep{Liu2015, Qiao2017}. Such a model avoids artificial
energy input to the corona and has been used to interpret the spectral features
observed in AGN. In this work, it is shown that the bulk emission size of the
corona is very small for the extended accretion flow in our model. More than
80\% of the hard X-ray power is emitted from a small region confined within 10
Schwarzschild radii around a non-spinning black hole, which is expected to be
even smaller accordingly for a spinning black hole. Here, the corona emission
is more extended at higher Eddington ratios. The compactness parameter of the
corona, , is shown to be in
the range of 1-33 for Eddington ratios of 0.02 - 0.1. Combined with the
electron temperature in the corona, this indicates that electron--positron pair
production is not dominant in this regime. A positive relation between the
compactness parameter and photon index is also predicted. By comparing the
above model predictions with observational features, we find that the model is
in agreement with observations.Comment: 11 pages, 4 figure
Anisotropies of different mass compositions of cosmic rays
The spectral hardenings of cosmic ray nuclei above GV followed by
softenings around 10 TV, the knee of the all-particle spectrum around PeV
energies, as well as the pattern change of the amplitude and phase of the
large-scale anisotropies around 100 TeV indicate the complexities of the origin
and transportation of Galactic cosmic rays. It has been shown that nearby
source(s) are most likely to be the cause of such spectral features of both the
spectra and the anisotropies. In this work, we study the anisotropy features of
different mass composition (or mass groups) of cosmic rays in this nearby
source model. We show that even if the spectral features from the nearby source
component is less distinctive compared with the background component from e.g.,
the population of distant sources, the anisotropy features are more remarkable
to be identified. Measurements of the anisotropies of each mass composition
(group) of cosmic rays by the space experiments such as DAMPE and HERD and the
ground-based experiments such as LHAASO in the near future are expected to be
able to critically test this scenario.Comment: 9 pages, 5 figure
Disk corona interaction: mechanism for the disk truncation and spectrum change in low luminosity AGN
The truncation of an optically thick, geometrically thin accretion disk is
investigated in the context of low luminosity AGN (LLAGN). We generalize the
disk evaporation model used in the interpretative framework of black hole X-ray
binaries by including the effect of a magnetic field in accretion disks
surrounding supermassive black holes. The critical transition mass accretion
rate for which the disk is truncated is found to be insensitive to magnetic
effects, but its inclusion leads to a smaller truncation radius in comparison
to a model without its consideration. That is, a thin viscous disk is truncated
for LLAGN at an Eddington ratio less than 0.03 for a standard viscosity
parameter (). An increase of the viscosity parameter results in a
higher critical transition mass accretion rate and a correspondingly smaller
truncation distance, the latter accentuated by greater magnetic energy
densities in the disk. Based on these results, the truncation radii inferred
from spectral fits of LLAGN published in the literature are consistent with the
disk evaporation model. The infrared emission arising from the truncated
geometrically thin accretion disks may be responsible for the red bump seen in
such LLAGN.Comment: 21 pages, 2 figures. Accepted for publication in Ap
A Hybrid Two Component Accretion Flow Surrounding Supermassive Black Holes in AGN
It is commonly believed that the optical/UV and X-ray emissions in luminous
AGN are produced in an accretion disk and an embedded hot corona respectively.
The inverse Compton scattering of disk photons by hot electrons in the corona
can effectively cool the coronal gas if the mass supply is predominantly via a
cool disk like flow as in BHXRBs. Thus, the application of such a model to AGNs
fails to produce their observed X-ray emission. As a consequence, a fraction of
disk accretion energy is usually assumed to be transferred to the corona. To
avoid this assumption, we propose that gas in a vertically extended
distribution is supplied to a supermassive black hole by the gravitational
capture of interstellar medium or stellar wind material. In this picture, the
gas partially condenses to an underlying cool disk as it flows toward the black
hole, releasing accretion energy as X-ray emission and supplying mass for the
disk accretion. Detailed numerical calculations reveal that the X-ray
luminosity can reach a few tens of percent of the bolometric luminosity. The
value of varies from 0.9 to 1.2 for the mass supply rate
ranging from 0.03 to 0.1 times the Eddington value. The corresponding photon
index in the 2-10 keV energy band varies from 1.9 to 2.3. Such a picture
provides a natural extension of the model for low luminosity AGN where
condensation is absent at low mass accretion rates and no optically thick disk
exists in the inner region.Comment: 21 pages,4 figures, accepted for publication in Ap
A realistic phase screen model for forward multiple-scattering media
Existing random phase screen (RPS) models for forward multiple-scattering
media fail to incorporate ballistic light. In this letter, we redesign the
angular spectrum of the screen by means of Monte-Carlo simulation based on an
assumption that a single screen should represent all the scattering events a
photon experiences between two adjacent screens. Three examples demonstrate
that the proposed model exhibits more realistic optical properties than
conventional RPS models in terms of attenuation of ballistic light, evolution
of beam profile and angular memory effect. The proposed model also provides the
flexibility to balance the computing accuracy, speed and memory usage by tuning
the screen spacing.Comment: 4 pages, 4 figure
Improved Thermometer from Intermediate Mass Fragments in Heavy-Ion Collisions with Isobaric Yield Ratio Difference
\item[Background] Temperature is an important parameter in studying many
important questions in heavy-ion collisions. A thermometer based on the
isobaric yield ratio (IYR) has been proposed [Ma \textit{et al.}, Phys. Rev. C
\textbf{86}, 054611 (2012) and Ma \textit{et al.}, \textit{ibid.}, Phys. Rev. C
\textbf{88}, 014609 (2013)]. \item[Purpose] An improved thermometer ()
is proposed based on the difference between IYRs. obtained from
isobars in different reactions will be compared. \item[Methods] The yields of
three isobars are employed in . The residual free energy of the three
isobars are replaced by that of the binding energy. No secondary decay
modification for odd fragment is used in . \item[Results] The
measured fragment yields in the 140 MeV Ca + Be
(Ta) and Ni + Be (Ta), the 1 GeV Xe + Pb, and the Sn + Sn reactions have been
analyzed to obtain from IMFs. from most of the fragments in
the Ca and Ni reactions is in the range of 0.6 MeV 3.5 MeV. from most of the fragments in the Xe and
Sn reactions is in the range of 0.5 MeV 2.5 MeV,
while the range is 0.5 MeV 4 MeV from most of the fragments in
the Xe reaction. In general, for most of the fragments in the
Ca and Ni reactions are very similar (except in the very
neutron-rich fragments), and from IMFs in the Xe and
Sn reactions is also similar. A slightly dependence of on
is found. \item[Conclusions] Using the binding energy of the nucleus,
can be obtained without the knowledge of the free energies of
fragments. In the investigated reactions, from most of the IMFs is
low.Comment: 7 pages, 9 figures. To appear on Physical Review
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