253 research outputs found
Photovoltage Detection of Edge Magnetoplasmon Oscillations and Giant Magnetoplasmon Resonances in A Two-Dimensional Hole System
In our high mobility p-type AlGaAs/GaAs two-dimensional hole samples, we
originally observe the B-periodic oscillation induced by microwave (MW) in
photovoltage (PV) measurements. In the frequency range of our measurements (5 -
40 GHz), the period ({\Delta}B) is inversely proportional to the microwave
frequency (f). The distinct oscillations come from the edge magnetoplasmon
(EMP) in the high quality heavy hole system. In our hole sample with a very
large effective mass, the observation of the EMP oscillations is in neither the
low frequency limit nor the high frequency limit, and the damping of the EMP
oscillations is very weak under high magnetic fields. Simultaneously, we
observe the giant plasmon resonance signals in our measurements on the shallow
two-dimensional hole system (2DHS)
Highly Stable and Conductive Microcapsules for Enhancement of Joule Heating Performance
Nanocarbons show great promise for establishing the next generation of Joule heating systems, but suffer from the limited maximum temperature due to precociously convective heat dissipation from electrothermal system to surrounding environment. Here we introduce a strategy to eliminate such convective heat transfer by inserting highly stable and conductive microcapsules into the electrothermal structures. The microcapsule is composed of encapsulated long-chain alkanes and graphene oxide/carbon nanotube hybrids as core and shell material, respectively. Multiform carbon nanotubes in the microspheres stabilize the capsule shell to resist volume-change-induced rupture during repeated heating/cooling process, and meanwhile enhance the thermal conductance of encapsulated alkanes which facilitates an expeditious heat exchange. The resulting microcapsules can be homogeneously incorporated in the nanocarbon-based electrothermal structures. At a dopant of 5%, the working temperature can be enhanced by 30% even at a low voltage and moderate temperature, which indicates a great value in daily household applications. Therefore, the stable and conductive microcapsule may serve as a versatile and valuable dopant for varieties of heat generation systems
A study on the sharp knee and fine structures of cosmic ray spectra
The paper investigates the overall and detailed features of cosmic ray (CR)
spectra in the knee region using the scenario of nuclei-photon interactions
around the acceleration sources. Young supernova remnants can be the physical
realities of such kind of CR acceleration sites. The results show that the
model can well explain the following problems simultaneously with one set of
source parameters: the knee of CR spectra and the sharpness of the knee, the
detailed irregular structures of CR spectra, the so-called "component B" of
Galactic CRs, and the electron/positron excesses reported by recent
observations. The coherent explanation serves as evidence that at least a
portion of CRs might be accelerated at the sources similar to young supernova
remnants, and one set of source parameters indicates that this portion mainly
comes from standard sources or from a single source.Comment: 13 pages, 4 figures, accepted for publication in SCIENCE CHINA
Physics, Mechanics & Astronomy
Imaging nigral pathology and clinical progression in Parkinson's disease
The pattern of dopamine cell loss in Parkinson's disease is known to be prominent in the ventrolateral and caudal substantia nigra, but less severe in the dorsal and rostral region. Both diffusion tensor imaging and R2* relaxometry of the substantia nigra have been reported as potential markers for Parkinson's disease, but their relative ability to mark disease progression and differences in pathophysiological bases remains unclear
Observation of TeV gamma rays from the Cygnus region with the ARGO-YBJ experiment
We report the observation of TeV gamma-rays from the Cygnus region using the
ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources
are located in this region including the two bright extended MGRO J2019+37 and
MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is
the most significant source apart from the Crab Nebula. No signal from MGRO
J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper
limits at 90% confidence level for all the events above 600 GeV with medium
energy of 3 TeV are lower than the Milagro flux, implying that the source might
be variable and hard to be identified as a pulsar wind nebula. The only
statistically significant (6.4 standard deviations) gamma-ray signal is found
from MGRO J2031+41, with a flux consistent with the measurement by Milagro.Comment: 14 pages, 4 figure
Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment
We report the observation of a very high energy \gamma-ray source, whose
position is coincident with HESS J1841-055. This source has been observed for
4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its
emission is detected with a statistical significance of 5.3 standard
deviations. Parameterizing the source shape with a two-dimensional Gaussian
function we estimate an extension \sigma=(0.40(+0.32,-0.22}) degree, consistent
with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x
10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy
range 0.9-50 TeV. The integral \gamma-ray flux above 1 TeV is 1.3-+0.4 Crab
units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the
flux determination between HESS and ARGO-YBJ, and possible counterparts at
other wavelengths are discussed.Comment: 17 pages, 4 figures, have been accepted for publication in Ap
Mean Interplanetary Magnetic Field Measurement Using the ARGO-YBJ Experiment
The sun blocks cosmic ray particles from outside the solar system, forming a
detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ
experiment in Tibet. Because the cosmic ray particles are positive charged, the
magnetic field between the sun and the earth deflects them from straight
trajectories and results in a shift of the shadow from the true location of the
sun. Here we show that the shift measures the intensity of the field which is
transported by the solar wind from the sun to the earth.Comment: 6 papges,3 figure
Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment
ARGO-YBJ is an air shower detector array with a fully covered layer of
resistive plate chambers. It is operated with a high duty cycle and a large
field of view. It continuously monitors the northern sky at energies above 0.3
TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period
from 2007 November to 2010 February. This source was observed by the
satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray
band. Mrk 421 was especially active in the first half of 2008. Many flares are
observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux
observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag
between the X-ray and gamma-ray photons longer than 1 day is found. The
evolution of the spectral energy distribution is investigated by measuring
spectral indices at four different flux levels. Hardening of the spectra is
observed in both X-ray and gamma-ray bands. The gamma-ray flux increases
quadratically with the simultaneously measured X-ray flux. All these
observational results strongly favor the synchrotron self-Compton process as
the underlying radiative mechanism.Comment: 30 pages, 8 figure
Measurement of the cosmic ray antiproton/proton flux ratio at TeV energies with the ARGO-YBJ detector
Cosmic ray antiprotons provide an important probe to study the cosmic ray
propagation in the interstellar space and to investigate the existence of dark
matter. Acting the Earth-Moon system as a magnetic spectrometer, paths of
primary antiprotons are deflected in the opposite sense with respect to those
of the protons in their way to the Earth. This effect allows, in principle, the
search for antiparticles in the direction opposite to the observed deficit of
cosmic rays due to the Moon (the so-called `Moon shadow').
The ARGO-YBJ experiment, located at the Yangbajing Cosmic Ray Laboratory
(Tibet, P.R. China, 4300 m a.s.l., 606 g/cm), is particularly effective in
measuring the cosmic ray antimatter content via the observation of the cosmic
rays shadowing effect due to: (1) good angular resolution, pointing accuracy
and long-term stability; (2) low energy threshold; (3) real sensitivity to the
geomagnetic field.
Based on all the data recorded during the period from July 2006 through
November 2009 and on a full Monte Carlo simulation, we searched for the
existence of the shadow cast by antiprotons in the TeV energy region. No
evidence of the existence of antiprotons is found in this energy region. Upper
limits to the flux ratio are set to 5 % at a median energy of 1.4
TeV and 6 % at 5 TeV with a confidence level of 90%. In the TeV energy range
these limits are the lowest available.Comment: Contact authors: G. Di Sciascio ([email protected]) and R.
Iuppa ([email protected]), INFN Sezione di Roma Tor Vergata, Roma, Ital
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