3,411 research outputs found
First-principles study on dielectric properties of NaCl crystal and ultrathin NaCl films under finite external electric field
We present a first-principles study on the dielectric properties of an NaCl
crystal and ultrathin NaCl films under a finite external electric field. Our
results show that the high-frequency dielectric constant of the films is not
affected by the finite size effect from crystal surfaces and is close to that
of the crystal, whereas the static one is sensitive to the thickness of the
film due to the difference in the atomic configurations between the surface and
inside of the film.Comment: 11 pages and 4 figure
Bipolar-Hyper-Shell Galactic Center Statrburst Model: Further Evidence from ROSAT Data and New Radio and X-ray Simulations
Using the all-sky ROSAT soft X-ray and 408-MHz radio continuum data, we show
that the North Polar Spur and its western and southern counter-spurs draw a
giant dumbbell-shape necked at the galactic plane. We interpret these features
as due to a shock front originating from a starburst 15 million years ago with
a total energy of the order of ergs or type II
supernovae. We simulate all-sky distributions of radio continuum and soft X-ray
intensities based on the bipolar-hyper-shell galactic center starburst model.
The simulations can well reproduce the radio NPS and related spurs, as well as
radio spurs in the tangential directions of spiral arms. Simulated X-ray maps
in 0.25, 0.75 and 1.5 keV bands reproduce the ROSAT X-ray NPS, its western and
southern counter-spurs, and the absorption layer along the galactic plane. We
propose to use the ROSAT all-sky maps to probe the physics of gas in the
halo-intergalactic interface, and to directly date and measure the energy of a
recent Galactic Center starburst.Comment: To appear in ApJ, Latex MS in ApJ macro, 8 figures in jpg (original
quality ps figs available on request
Smooth Approximation of Lipschitz functions on Riemannian manifolds
We show that for every Lipschitz function defined on a separable
Riemannian manifold (possibly of infinite dimension), for every continuous
, and for every positive number , there exists
a smooth Lipschitz function such that
for every and
. Consequently, every separable
Riemannian manifold is uniformly bumpable. We also present some applications of
this result, such as a general version for separable Riemannian manifolds of
Deville-Godefroy-Zizler's smooth variational principle.Comment: 10 page
Discovery of Molecular Gas in the Outflow and Tidal Arms around M82
We present the first fully sampled map of 12CO (1-0) emission from M82
covering the entire galaxy. Our map contains a 12 x 15 kpc^2 area. We find that
extraplanar CO emission, previously reported at short distances above the
galactic plane, extends to heights of up to 6 kpc above the disk. Some of this
emission is associated with tidal arms seen in HI, implying either that M82
contained substantial amounts of molecular gas in the outer disk, or that
molecular gas formed after the tidal features. CO emission along the direction
of the outflow extends to distances of 3 kpc above and below the disk. At this
distance, the line is shifted in velocity about 100 km/s, and has the same
sense as the galactic outflow from the central starburst. This implies that
molecular gas may be entrained into the outflow.Comment: 4 pages, 6 figures. Uses emulateapj5. Accepted by ApJ Letter
Kinematics of Spiral Arm Streaming in M51
We use CO and H alpha velocity fields to study the gas kinematics in the
spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to
estimate the radial and tangential velocity components as a function of spiral
phase (arm distance). We find large radial and tangential streaming velocities,
which are qualitatively consistent with the predictions of density wave theory
and support the existence of shocks. The streaming motions are complex, varying
significantly across the galaxy as well as along and between arms. Aberrations
in the velocity field indicate that the disk is not coplanar, perhaps as far in
as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H
alpha are typically similar, suggesting that most of the H alpha emission
originates from regions of recent star formation. We also explore vortensity
and mass conservation conditions. Vortensity conservation, which does not
require a steady state, is empirically verified. The velocity and density
profiles show large and varying mass fluxes, which are inconsistent with a
steady flow for a single dominant global spiral mode. We thus conclude that the
spiral arms cannot be in a quasi-steady state in any rotating frame, and/or
that out of plane motions may be significant.Comment: 50 pages, including 20 figures; Accepted for publication in ApJ. PDF
version with high resolution figures available at
http://www.astro.umd.edu/~shetty/Research
Anisotropic Superconducting Properties of Optimally Doped BaFe(AsP) under Pressure
Magnetic measurements on optimally doped single crystals of
BaFe(AsP) () with magnetic fields applied
along different crystallographic axes were performed under pressure, enabling
the pressure evolution of coherence lengths and the anisotropy factor to be
followed. Despite a decrease in the superconducting critical temperature, our
studies reveal that the superconducting properties become more anisotropic
under pressure. With appropriate scaling, we directly compare these properties
with the values obtained for BaFe(AsP) as a function of
phosphorus content.Comment: 5 pages, 3 figure
Magnetic field distribution and characteristic fields of the vortex lattice for a clean superconducting niobium sample in an external field applied along a three-fold axis
The field distribution in the vortex lattice of a pure niobium single crystal
with an external field applied along a three-fold axis has been investigated by
the transverse-field muon-spin-rotation (TF-SR) technique over a wide
range of temperatures and fields. The experimental data have been analyzed with
the Delrieu's solution for the form factor supplemented by phenomenological
formulas for the parameters. This has enabled us to experimentally establish
the temperatures and fields for the Delrieu's, Ginzburg-Landau's, and Klein's
regions of the vortex lattice. Using the numerical solution of the
quasiclassical Eilenberger's equation the experimental results have been
reasonably understood. They should apply to all clean BCS superconductors. The
analytical Delrieu's model supplemented by phenomenological formulas for its
parameters is found to be reliable for analyzing TF-SR experimental data
for a substantial part of the mixed phase. The Abrikosov's limit is contained
in it.Comment: 12 pages, 15 figure
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