4,739 research outputs found
The Dirac point electron in zero-gravity Kerr--Newman spacetime
Dirac's wave equation for a point electron in the topologically nontrivial
maximal analytically extended electromagnetic Kerr--Newman spacetime is studied
in a zero-gravity limit; here, "zero-gravity" means , where is
Newton's constant of universal gravitation. The following results are obtained:
the formal Dirac Hamiltonian on the static spacelike slices is essentially
self-adjoint; the spectrum of the self-adjoint extension is symmetric about
zero, featuring a continuum with a gap about zero that, under two smallness
conditions, contains a point spectrum. Some of our results extend to a
generalization of the zero- Kerr--Newman spacetime with different
electric-monopole-to-magnetic-dipole-moment ratio.Comment: 49 pages, 17 figures; referee's comments implemented; the endnotes in
the published version appear as footnotes in this preprin
A Note on Tsallis Holographic Dark Energy
We explore the effects of considering various infrared (IR) cutoffs,
including the particle horizon, Ricci horizon and Granda-Oliveros (GO) cutoffs,
on the properties of Tsallis holographic dark energy (THDE) model, proposed
inspired by Tsallis generalized entropy formalism \cite{THDE}. Interestingly
enough, we find that for the particle horizon as IR cutoff, the obtained THDE
model can describe the accelerated universe. This is in contrast to the usual
HDE model which cannot lead to an accelerated universe, if one consider the
particle horizon as IR cutoff. We also investigate the cosmological
consequences of THDE under the assumption of a mutual interaction between the
dark sectors of the Universe. It is shown that the evolution history of the
Universe can be described by these IR cutoffs and thus the current cosmic
acceleration can also been realized. The sound instability of THDE models for
each cutoff are also explored, separately.Comment: 12 pages, 31 figure
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
Comparison of Soil Classification Methods Using CPT Results
A series of cone penetration test were conducted in the southeast of Tehran to assess the liquefaction potential in this area. At the same time, after sounding of each cone penetration test, soil samples were also taken from different depths of boreholes to visually verify the soil classification. Seventy four samples from twenty boreholes were taken and their soil characteristics were obtained. To classify the soil layers, using recorded data, two various soil behaviour classification charts proposed by Robertson and Wride (1988), and Marr (1981) were examined which for some cases different results were obtained. In this paper validity of these procedures are investigated and discussed in details. These soil classification methods in some cases give a good results but there is a different between those charts and observed soil classification, particularly when the soil contain fines and therefore some modification must be applied
Superconducting Instability in the Periodic Anderson Model
Employing a quantum Monte Carlo simulation we find a pairing instability in
the normal state of the infinite dimensional periodic Anderson model.
Superconductivity arises from a normal state in which the screening is
protracted and which is clearly not a Fermi liquid. The phase diagram is
reentrant reflecting competition between superconductivity and Fermi liquid
formation. The estimated superconducting order parameter is even, but has nodes
as a function of frequency. This opens the possibility of a temporal node and
an effective order parameter composed of charge pairs and spin excitations.Comment: one postscript file, 6 pages including 6 figures. To appear in Phil.
Mag.
HST Paschen alpha and 1.9 micron imaging of Sgr A West
We present HST/NICMOS images at 0.2" resolution of the HI Paschen Alpha (PaA)
emission line in a 70" x 90" region of the Galactic center centered on the
non-thermal radio source Sgr A*. The majority of the emission arises from
ionized gas in the mini-spiral in the central parsec. PaA emission is also seen
from 26 stellar sources, presumably early-type stars with mass-loss winds. The
new data reveal significant small-scale structure (<1"~0.04pc) in the ionized
gas of the mini-spiral; low surface brightness emission features are also seen
for the first time. Extinction, estimated from the ratio of observed PaA
emission to 6-cm continuum emission, varies from 20 to 50 mag with a median
Av=31.1 mag, in excellent agreement with earlier estimates for the stellar
sources and indepedent measurements derived using H92alpha recombination line
data. Large increases in extinction are seen along the periphery of the ionized
gas, suggesting that the ionized gas is partially extincted by dust in the
molecular clouds at the outside of the ionized regions. The small-scale,
filamentary structures in the ionized gas have a free thermal expansion time of
only ~ 3000 yrs; either magnetic fields or mass-loss winds from the hot
emission line stars may contain the ionized filaments. For both the ionized gas
and the stellar continuum, the centroids of the emission remain within ~+/- 1"
from a radius of 2" out to 40", providing further evidence that Sgr A* is
indeed at or extremely close to the dynamical center of the Galactic nucleus
stellar distribution. The 1.9 micron surface brightness increases inwards to
0.9" and then decreases or levels off closer to Sgr A*, possibly indicating the
core radius of the central stellar distribution or depletion of the late-type
stars by stellar collisions near the central black hole.Comment: 43 pages, 15 figures, 2 tables; Accepted to ApJ (9/1/03 issue
UV-induced wettability change of teflon-modified ZnO nanorod arrays on LiNbO3 substrate
Aligned ZnO nanorod arrays films were grown on LiNbO3 substrates by aqueous growth, and subsequently rendered superhydrophobic with RF sputtered coated Teflon. The as-prepared surface exhibits superhydrophobicity with a water contact angle (CA) of 154.5deg. After 2 hours of UV irradiation on the surface, the surface wettability was approaching hydrophilic state; CA was measured to be 113deg. This study provides insights into the methodology of a low cost, efficient technique that has great potential for preparing nanostructured surface with tunable wettability
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