4,147 research outputs found
Analysis of an SIR Epidemic Model with Pulse Vaccination and Distributed Time Delay
Pulse vaccination, the repeated application of
vaccine over a defined age range, is gaining prominence as an
effective strategy for the elimination of infectious diseases. An
SIR epidemic model with pulse vaccination and distributed time delay is proposed in this paper. Using the discrete dynamical
system determined by the stroboscopic map, we obtain the exact
infection-free periodic solution of the impulsive epidemic system
and prove that the infection-free periodic solution is globally attractive if the vaccination rate is larger enough. Moreover, we
show that the disease is uniformly persistent if the vaccination
rate is less than some critical value. The permanence of the model is
investigated analytically. Our results indicate that a large pulse
vaccination rate is sufficient for the eradication of the disease
Learning attentions: residual attentional Siamese Network for high performance online visual tracking
Offline training for object tracking has recently shown
great potentials in balancing tracking accuracy and speed.
However, it is still difficult to adapt an offline trained model
to a target tracked online. This work presents a Residual Attentional
Siamese Network (RASNet) for high performance
object tracking. The RASNet model reformulates the correlation
filter within a Siamese tracking framework, and introduces
different kinds of the attention mechanisms to adapt
the model without updating the model online. In particular,
by exploiting the offline trained general attention, the target
adapted residual attention, and the channel favored feature
attention, the RASNet not only mitigates the over-fitting
problem in deep network training, but also enhances its discriminative
capacity and adaptability due to the separation
of representation learning and discriminator learning. The
proposed deep architecture is trained from end to end and
takes full advantage of the rich spatial temporal information
to achieve robust visual tracking. Experimental results
on two latest benchmarks, OTB-2015 and VOT2017, show
that the RASNet tracker has the state-of-the-art tracking accuracy
while runs at more than 80 frames per second
A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond
Since the first successful synthesis of graphene just over a decade ago, a
variety of two-dimensional (2D) materials (e.g., transition
metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered.
Among the many unique and attractive properties of 2D materials, mechanical
properties play important roles in manufacturing, integration and performance
for their potential applications. Mechanics is indispensable in the study of
mechanical properties, both experimentally and theoretically. The coupling
between the mechanical and other physical properties (thermal, electronic,
optical) is also of great interest in exploring novel applications, where
mechanics has to be combined with condensed matter physics to establish a
scalable theoretical framework. Moreover, mechanical interactions between 2D
materials and various substrate materials are essential for integrated device
applications of 2D materials, for which the mechanics of interfaces (adhesion
and friction) has to be developed for the 2D materials. Here we review recent
theoretical and experimental works related to mechanics and mechanical
properties of 2D materials. While graphene is the most studied 2D material to
date, we expect continual growth of interest in the mechanics of other 2D
materials beyond graphene
Intertwined magnetism and charge density wave order in kagome FeGe
Electron correlations often lead to emergent orders in quantum materials.
Kagome lattice materials are emerging as an exciting platform for realizing
quantum topology in the presence of electron correlations. This proposal stems
from the key signatures of electronic structures associated with its lattice
geometry: flat band induced by destructive interference of the electronic
wavefunctions, topological Dirac crossing, and a pair of van Hove singularities
(vHSs). A plethora of correlated electronic phases have been discovered amongst
kagome lattice materials, including magnetism, charge density wave (CDW),
nematicity, and superconductivity. These materials can be largely organized
into two types: those that host magnetism and those that host CDW order.
Recently, a CDW order has been discovered in the magnetic kagome FeGe,
providing a new platform for understanding the interplay between CDW and
magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we
observe all three types of electronic signatures of the kagome lattice: flat
bands, Dirac crossings, and vHSs. From both the observation of a
temperature-dependent shift of the vHSs towards the Fermi level as well as
guidance via first-principle calculations, we identify the presence of the vHSs
near the Fermi level (EF) to be driven by the development of underlying
magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW
order as gaps that open on the near-EF vHS bands, as well as evidence of
electron-phonon coupling from a kink on the vHS band together with phonon
hardening observed by inelastic neutron scattering. Our observation points to
the magnetic interaction-driven band modification resulting in the formation of
the CDW order, indicating an intertwined connection between the emergent
magnetism and vHS charge order in this moderately-correlated kagome metal.Comment: submitted on April 22, 202
The DArk Matter Particle Explorer mission
The DArk Matter Particle Explorer (DAMPE), one of the four scientific space
science missions within the framework of the Strategic Pioneer Program on Space
Science of the Chinese Academy of Sciences, is a general purpose high energy
cosmic-ray and gamma-ray observatory, which was successfully launched on
December 17th, 2015 from the Jiuquan Satellite Launch Center. The DAMPE
scientific objectives include the study of galactic cosmic rays up to
TeV and hundreds of TeV for electrons/gammas and nuclei respectively, and the
search for dark matter signatures in their spectra. In this paper we illustrate
the layout of the DAMPE instrument, and discuss the results of beam tests and
calibrations performed on ground. Finally we present the expected performance
in space and give an overview of the mission key scientific goals.Comment: 45 pages, including 29 figures and 6 tables. Published in Astropart.
Phy
Discovery of charge density wave in a correlated kagome lattice antiferromagnet
A hallmark of strongly correlated quantum materials is the rich phase diagram
resulting from competing and intertwined phases with nearly degenerate ground
state energies. A well-known example is the copper oxides, where a charge
density wave (CDW) is ordered well above and strongly coupled to the magnetic
order to form spin-charge separated stripes that compete with
superconductivity. Recently, such rich phase diagrams have also been revealed
in correlated topological materials. In two-dimensional kagome lattice metals
consisting of corner-sharing triangles, the geometry of the lattice can produce
flat bands with localized electrons, non-trivial topology, chiral magnetic
order, superconductivity and CDW order. While CDW has been found in weakly
electron correlated nonmagnetic AV3Sb5 (A = K, Rb, Cs), it has not yet been
observed in correlated magnetic ordered kagome lattice metals. Here we report
the discovery of CDW within the antiferromagnetic (AFM) ordered phase of kagome
lattice FeGe. The CDW in FeGe occurs at wavevectors identical to that of
AV3Sb5, enhances the AFM ordered moment, and induces an emergent anomalous Hall
effect. Our findings suggest that CDW in FeGe arises from the combination of
electron correlations-driven AFM order and van Hove singularities-driven
instability possibly associated with a chiral flux phase, in stark contrast to
strongly correlated copper oxides and nickelates, where the CDW precedes or
accompanies the magnetic order.Comment: 36 pages, 4 figures in main tex
Direct detection of a break in the teraelectronvolt cosmic-ray spectrum of electrons and positrons
High energy cosmic ray electrons plus positrons (CREs), which lose energy
quickly during their propagation, provide an ideal probe of Galactic
high-energy processes and may enable the observation of phenomena such as
dark-matter particle annihilation or decay. The CRE spectrum has been directly
measured up to TeV in previous balloon- or space-borne experiments,
and indirectly up to TeV by ground-based Cherenkov -ray
telescope arrays. Evidence for a spectral break in the TeV energy range has
been provided by indirect measurements of H.E.S.S., although the results were
qualified by sizeable systematic uncertainties. Here we report a direct
measurement of CREs in the energy range by the
DArk Matter Particle Explorer (DAMPE) with unprecedentedly high energy
resolution and low background. The majority of the spectrum can be properly
fitted by a smoothly broken power-law model rather than a single power-law
model. The direct detection of a spectral break at TeV confirms the
evidence found by H.E.S.S., clarifies the behavior of the CRE spectrum at
energies above 1 TeV and sheds light on the physical origin of the sub-TeV
CREs.Comment: 18 pages, 6 figures, Nature in press, doi:10.1038/nature2447
Transient Focal Cerebral Ischemia/Reperfusion Induces Early and Chronic Axonal Changes in Rats: Its Importance for the Risk of Alzheimer's Disease
The dementia of Alzheimer's type and brain ischemia are known to increase at comparable rates with age. Recent advances suggest that cerebral ischemia may contribute to the pathogenesis of Alzheimer's disease (AD), however, the neuropathological relationship between these two disorders is largely unclear. It has been demonstrated that axonopathy, mainly manifesting as impairment of axonal transport and swelling of the axon and varicosity, is a prominent feature in AD and may play an important role in the neuropathological mechanisms in AD. In this study, we investigated the early and chronic changes of the axons of neurons in the different brain areas (cortex, hippocampus and striatum) using in vivo tracing technique and grading analysis method in a rat model of transient focal cerebral ischemia/reperfusion (middle cerebral artery occlusion, MCAO). In addition, the relationship between the changes of axons and the expression of β-amyloid 42 (Aβ42) and hyperphosphorylated Tau, which have been considered as the key neuropathological processes of AD, was analyzed by combining tracing technique with immunohistochemistry or western blotting. Subsequently, we found that transient cerebral ischemia/reperfusion produced obvious swelling of the axons and varicosities, from 6 hours after transient cerebral ischemia/reperfusion even up to 4 weeks. We could not observe Aβ plaques or overexpression of Aβ42 in the ischemic brain areas, however, the site-specific hyperphosphorylated Tau could be detected in the ischemic cortex. These results suggest that transient cerebral ischemia/reperfusion induce early and chronic axonal changes, which may be an important mechanism affecting the clinical outcome and possibly contributing to the development of AD after stroke
A Survey of Air-to-Ground Propagation Channel Modeling for Unmanned Aerial Vehicles
In recent years, there has been a dramatic increase in the use of unmanned
aerial vehicles (UAVs), particularly for small UAVs, due to their affordable
prices, ease of availability, and ease of operability. Existing and future
applications of UAVs include remote surveillance and monitoring, relief
operations, package delivery, and communication backhaul infrastructure.
Additionally, UAVs are envisioned as an important component of 5G wireless
technology and beyond. The unique application scenarios for UAVs necessitate
accurate air-to-ground (AG) propagation channel models for designing and
evaluating UAV communication links for control/non-payload as well as payload
data transmissions. These AG propagation models have not been investigated in
detail when compared to terrestrial propagation models. In this paper, a
comprehensive survey is provided on available AG channel measurement campaigns,
large and small scale fading channel models, their limitations, and future
research directions for UAV communication scenarios
- …