45,104 research outputs found
Pattern Synthesis of Dual-band Shared Aperture Interleaved Linear Antenna Arrays
This paper presents an approach to improve the efficiency of an array aperture by interleaving two different arrays in the same aperture area. Two sub-arrays working at different frequencies are interleaved in the same linear aperture area. The available aperture area is efficiently used. The element positions of antenna array are optimized by using Invasive Weed Optimization (IWO) to reduce the peak side lobe level (PSLL) of the radiation pattern. To overcome the shortness of traditional methods which can only fulfill the design of shared aperture antenna array working at the same frequency, this method can achieve the design of dual-band antenna array with wide working frequency range. Simulation results show that the proposed method is feasible and efficient in the synthesis of dual-band shared aperture antenna array
Hot Spots on the Fermi Surface of Bi2212: Stripes versus Superstructure
In a recent paper Saini et al. have reported evidence for a pseudogap around
(pi,0) at room temperature in the optimally doped superconductor Bi2212. This
result is in contradiction with previous ARPES measurements. Furthermore they
observed at certain points on the Fermi surface hot spots of high spectral
intensity which they relate to the existence of stripes in the CuO planes. They
also claim to have identified a new electronic band along Gamma-M1 whose one
dimensional character provides further evidence for stripes. We demonstrate in
this Comment that all the measured features can be simply understood by
correctly considering the superstructure (umklapp) and shadow bands which occur
in Bi2212.Comment: 1 page, revtex, 1 encapsulated postscript figure (color
Charmonium properties in hot quenched lattice QCD
We study the properties of charmonium states at finite temperature in
quenched QCD on large and fine isotropic lattices. We perform a detailed
analysis of charmonium correlation and spectral functions both below and above
. Our analysis suggests that both S wave states ( and )
and P wave states ( and ) disappear already at about . The charm diffusion coefficient is estimated through the Kubo formula and
found to be compatible with zero below and approximately at
.Comment: 32 pages, 19 figures, typo corrected, discussions on isotropic vs
anisotropic lattices expanded, published versio
Solar flare hard X-ray spikes observed by RHESSI: a statistical study
Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales
of seconds to milliseconds in high-energy HXR emission profiles during solar
flare eruptions. Aims. We present a preliminary statistical investigation of
temporal and spectral properties of HXR spikes. Methods. Using a three-sigma
spike selection rule, we detected 184 spikes in 94 out of 322 flares with
significant counts at given photon energies, which were detected from
demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI). About one fifth of these spikes are also
detected at photon energies higher than 100 keV. Results. The statistical
properties of the spikes are as follows. (1) HXR spikes are produced in both
impulsive flares and long-duration flares with nearly the same occurrence
rates. Ninety percent of the spikes occur during the rise phase of the flares,
and about 70% occur around the peak times of the flares. (2) The time durations
of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not
dependent on photon energies. The spikes exhibit symmetric time profiles with
no significant difference between rise and decay times. (3) Among the most
energetic spikes, nearly all of them have harder count spectra than their
underlying slow-varying components. There is also a weak indication that spikes
exhibiting time lags in high-energy emissions tend to have harder spectra than
spikes with time lags in low-energy emissions.Comment: 16 pages, 13 figure
Charge and spin Hall effect in graphene with magnetic impurities
We point out the existence of finite charge and spin Hall conductivities of
graphene in the presence of a spin orbit interaction (SOI) and localized
magnetic impurities. The SOI in graphene results in different transverse forces
on the two spin channels yielding the spin Hall current. The magnetic
scatterers act as spin-dependent barriers, and in combination with the SOI
effect lead to a charge imbalance at the boundaries. As indicated here, the
charge and spin Hall effects should be observable in graphene by changing the
chemical potential close to the gap.Comment: 7 page
Localized magnetic states in biased bilayer and trilayer graphene
We study the localized magnetic states of impurity in biased bilayer and
trilayer graphene. It is found that the magnetic boundary for bilayer and
trilayer graphene presents the mixing features of Dirac and conventional
fermion. For zero gate bias, as the impurity energy approaches the Dirac point,
the impurity magnetization region diminishes for bilayer and trilayer graphene.
When a gate bias is applied, the dependence of impurity magnetic states on the
impurity energy exhibits a different behavior for bilayer and trilayer graphene
due to the opening of a gap between the valence and the conduction band in the
bilayer graphene with the gate bias applied. The magnetic moment and the
corresponding magnetic transition of the impurity in bilayer graphene are also
investigated.Comment: 16 pages,6 figure
Evolution of the Fermi surface with carrier concentration in Bi_2Sr_2CaCu_2O_{8+\delta}
We show, by use of angle-resolved photoemission spectroscopy, that underdoped
Bi_2Sr_2CaCu_2O_{8+\delta} appears to have a large Fermi surface centered at
(\pi,\pi), even for samples with a T_c as low as 15 K. No clear evidence of a
Fermi surface pocket around (\pi/2,\pi/2) has been found. These conclusions are
based on a determination of the minimum gap locus in the pseudogap regime T_c <
T < T^*, which is found to coincide with the locus of gapless excitations in
momentum space (Fermi surface) determined above T^*. These results suggest that
the pseudogap is more likely of precursor pairing rather than magnetic origin.Comment: 4 pages, revtex, 4 postscript color figure
Direct observation of particle-hole mixing in the superconducting state by angle-resolved photoemission
Particle-hole (p-h) mixing is a fundamental consequence of the existence of a
pair condensate. We present direct experimental evidence for p-h mixing in the
angle-resolved photoemission (ARPES) spectra in the superconducting state of
Bi_2Sr_2CaCu_2O_{8+\delta}. In addition to its pedagogical importance, this
establishes unambiguously that the gap observed in ARPES is associated with
superconductivity.Comment: 3 pages, revtex, 4 postscript figure
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