63,440 research outputs found
Room-temperature ballistic transport in narrow graphene strips
We investigate electron-phonon couplings, scattering rates, and mean free
paths in zigzag-edge graphene strips with widths of the order of 10 nm. Our
calculations for these graphene nanostrips show both the expected similarity
with single-wall carbon nanotubes (SWNTs) and the suppression of the
electron-phonon scattering due to a Dirichlet boundary condition that prohibits
one major backscattering channel present in SWNTs. Low-energy acoustic phonon
scattering is exponentially small at room temperature due to the large phonon
wave vector required for backscattering. We find within our model that the
electron-phonon mean free path is proportional to the width of the nanostrip
and is approximately 70 m for an 11-nm-wide nanostrip.Comment: 5 pages and 5 figure
Observing the sky at extremely high energies with the Cherenkov Telescope Array: Status of the GCT project
The Cherenkov Telescope Array is the main global project of ground-based
gamma-ray astronomy for the coming decades. Performance will be significantly
improved relative to present instruments, allowing a new insight into the
high-energy Universe [1]. The nominal CTA southern array will include a
sub-array of seventy 4 m telescopes spread over a few square kilometers to
study the sky at extremely high energies, with the opening of a new window in
the multi-TeV energy range. The Gamma-ray Cherenkov Telescope (GCT) is one of
the proposed telescope designs for that sub-array. The GCT prototype recorded
its first Cherenkov light on sky in 2015. After an assessment phase in 2016,
new observations have been performed successfully in 2017. The GCT
collaboration plans to install its first telescopes and cameras on the CTA site
in Chile in 2018-2019 and to contribute a number of telescopes to the
subsequent CTA production phase.Comment: 8 pages, 7 figures, ICRC201
Ground State Properties of the Doped 3-Leg t-J Ladder
Results for a doped 3-leg t-J ladder obtained using the density matrix
renormalization group are reported. At low hole doping, the holes form a dilute
gas with a uniform density. The momentum occupation of the odd band shows a
sharp decrease at a large value of k_F similar to the behavior of a lightly
doped t-J chain, while the even modes appear gapped. The spin-spin correlations
decay as a power law consistent with the absence of a spin gap, but the pair
field correlations are negligible. At larger doping we find evidence for a spin
gap and as x increases further we find 3-hole diagonal domain walls. In this
regime there are pair field correlations and the internal pair orbital has
d_x^2-y^2 - like symmetry. However, the pair field correlations appear to fall
exponentially at large distances.Comment: 14 pages, 11 postscript figure
Some speculations on the sudden occurrence of floods in the history of lake Magadi
Volume: XXI
The X-ray absorption spectrum of 4U1700-37 and its implications for the stellar wind of the companion HD153919
The first high resolution non-dispersive 2-60 KeV X-ray spectra of 4U1700-37 is presented. The continuum is typical of that found from X-ray pulsars; that is a flat power law between 2 and 10 keV and, beyond 10 keV, an exponential decay of characteristic energy varying between 10 and 20 keV. No X-ray pulsations were detected between 160 ms and 6 min with an amplitude greater than approximately 2%. The absorption measured at binary phases approximately 0.72 is comparable to that expected from the stellar wind of the primary. The gravitational capture of material in the wind is found to be more than enough to power the X-ray source. The increase in the average absorption after phi o approximately 0.5 is confirmed. The minimum level of adsorption is a factor of 2 or 3 lower than that reported by previous observers, which may be related to a factor of approximately 10 decline in the average X-ray luminosity over the same interval. Short term approximately 50% variations in adsorption are seen for the first time which appear to be loosely correlated with approximately 10 min flickering activity in the X-ray flux. These most likely originate from inhomogeneities in the stellar wind of the primary
Effects of Ram-Pressure from Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies
Using a simple model of molecular cloud evolution, we have quantitatively
estimated the change of star formation rate (SFR) of a disk galaxy falling
radially into the potential well of a cluster of galaxies. The SFR is affected
by the ram-pressure from the intracluster medium (ICM). As the galaxy
approaches the cluster center, the SFR increases to twice the initial value, at
most, in a cluster with high gas density and deep potential well, or with a
central pressure of because the ram-pressure
compresses the molecular gas of the galaxy. However, this increase does not
affect the color of the galaxy significantly. Further into the central region
of the cluster ( Mpc from the center), the SFR of the disk
component drops rapidly due to the effect of ram-pressure stripping. This makes
the color of the galaxy redder and makes the disk dark. These effects may
explain the observed color, morphology distribution and evolution of galaxies
in high-redshift clusters. By contrast, in a cluster with low gas density and
shallow potential well, or the central pressure of ,
the SFR of a radially infalling galaxy changes less significantly, because
neither ram-pressure compression nor stripping is effective. Therefore, the
color of galaxies in poor clusters is as blue as that of field galaxies, if
other environmental effects such as galaxy-galaxy interaction are not
effective. The predictions of the model are compared with observations.Comment: 19 pages, 9 figures, to appear in Ap
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