46,275 research outputs found
On unusual narrow transmission bands for a multi-layered periodic structure containing left-handed materials
A multi-layered structure consisting of alternate right-handed material (RHM)
and left-handed material (LHM)is considered and the unusual narrow transmission
bands are explained as the competitive results of the Bragg condition and the
transparent condition. These unusual narrow transmission bands may exist
regardless whether the optical length of the LHM layer cancels the optical
length of the RHM layer or not. This unusual transmission property may
disappear when the reflection coefficient for each interface is small and the
optical length of the LHM layer does not cancel the optical length of the RHM
layer. An non-ideal model when the LHM is dispersive and lossy is also employed
to confirm the unusual transmission phenomenon.Comment: 9 pages, 4 figure
Unusual behavior of sound velocity of a Bose gas in an optical superlattice at quasi-one-dimension
A Bose gas trapped in a one-dimensional optical superlattice has emerged as a
novel superfluid characterized by tunable lattice topologies and tailored band
structures. In this work, we focus on the propagation of sound in such a novel
system and have found new features on sound velocity, which arises from the
interplay between the two lattices with different periodicity and is not
present in the case of a condensate in a monochromatic optical lattice.
Particularly, this is the first time that the sound velocity is found to first
increase and then decrease as the superlattice strength increases even at one
dimension. Such unusual behavior can be analytically understood in terms of the
competition between the decreasing compressibility and the increasing effective
mass due to the increasing superlattice strength. This result suggests a new
route to engineer the sound velocity by manipulating the superlattice's
parameters. All the calculations based on the mean-field theory are justified
by checking the exponent of the off-diagonal one-body density matrix
that is much smaller than 1. Finally, the conditions for possible experimental
realization of our scenario are also discussed.Comment: 10 pages, 5 figure
Why are some BL Lacs detected by \fermi, but others not ?
By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat
AGN sample, we have compiled a sample of 100 BL Lacs with \fermi detection
(FBLs), and a sample of 70 non-\fermi BL Lacs (NFBLs). We compared various
parameters of FBLs with those of NFBLs, including the redshift, the low
frequency radio luminosity at 408 MHz (), the absolute
magnitude of host galaxies (), the polarization fraction from
NVSS survey (), the observed arcsecond scale radio core flux at 5
GHz () and jet Doppler factor; all the parameters are directly
\textbf{measured} or derived from available data from literatures. We found
that the Doppler factor is on average larger in FBLs than in NFBLs, and the
-ray detection rate is higher in sources with higher Doppler
factor. In contrast, there are no significant differences in terms of the
intrinsic parameters of redshift, , and . FBLs seem to have a higher probability of exhibiting measurable
proper motion. These results strongly indicate a higher beaming effect in FBLs
compared to NFBLs. The radio core flux is found to be strongly correlated with
-ray flux, which remains after excluding the common dependence of the
Doppler factor. At the fixed Doppler factor, FBLs have systematically larger
radio core flux than NFBLs, implying lower -ray emission in NFBLs since
the radio and -ray flux are significantly correlated. Our results
indicate that the Doppler factor is an important parameter of -ray
detection, the non-detection of -ray emission in NFBLs is likely due to
low beaming effect, and/or low intrinsic -ray flux, and the gamma-rays
are likely produced co-spatially with the arcsecond-scale radio core radiation
and mainly through the SSC process.Comment: 6 pages, 6 figures, accepted by A&
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