319,853 research outputs found
Adaptive multibeam phased array design for a Spacelab experiment
The parametric tradeoff analyses and design for an Adaptive Multibeam Phased Array (AMPA) for a Spacelab experiment are described. This AMPA Experiment System was designed with particular emphasis to maximize channel capacity and minimize implementation and cost impacts for future austere maritime and aeronautical users, operating with a low gain hemispherical coverage antenna element, low effective radiated power, and low antenna gain-to-system noise temperature ratio
Free-charge carrier parameters of n-type, p-type and compensated InN:Mg determined by Infrared Spectroscopic Ellipsometry
Infrared spectroscopic ellipsometry is applied to investigate the free-charge
carrier properties of Mg-doped InN films. Two representative sets of In-polar
InN grown by molecular beam epitaxy with Mg concentrations ranging from
cm to cm are compared. P-type
conductivity is indicated for the Mg concentration range of
cm to cm from a systematic investigation of the
longitudinal optical phonon plasmon broadening and the mobility parameter in
dependence of the Mg concentration. A parameterized model that accounts for the
phonon-plasmon coupling is applied to determine the free-charge carrier
concentration and mobility parameters in the doped bulk InN layer as well as
the GaN template and undoped InN buffer layer for each sample. The free-charge
carrier properties in the second sample set are consistent with the results
determined in a comprehensive analysis of the first sample set reported earlier
[Sch\"oche et al., J. Appl. Phys. 113, 013502 (2013)]. In the second set, two
samples with Mg concentration of cm are identified as
compensated n-type InN with very low electron concentrations which are suitable
for further investigation of intrinsic material properties that are typically
governed by high electron concentrations even in undoped InN. The compensated
n-type InN samples can be clearly distinguished from the p-type conductive
material of similar plasma frequencies by strongly reduced phonon plasmon
broadening
No-Go Theorem in Spacetimes with Two Commuting Spacelike Killing Vectors
Four-dimensional Riemannian spacetimes with two commuting spacelike Killing
vectors are studied in Einstein's theory of gravity, and found that no outer
apparent horizons exist, provided that the dominant energy condition holds.Comment: latex, 1 figure, version published in Gen. Relativ. Grav., 37,
1919-1926 (2005
theory and geometric origin of the dark sector in Horava-Lifshitz gravity
Inclusion of term in the action of Horava-Lifshitz quantum gravity
with projectability but without detailed balance condition is investigated,
where denotes the 3-spatial dimensional Ricci scalar. Conditions for the
spin-0 graviton to be free of ghosts and instability are studied. The
requirement that the theory reduce to general relativity in the IR makes the
scalar mode unstable in the Minkowski background but stable in the de Sitter.
It is remarkable that the dark sector, dark matter and dark energy, of the
universe has a naturally geometric origin in such a setup. Bouncing universes
can also be constructed. Scalar perturbations in the FRW backgrounds with
non-zero curvature are presented.Comment: Mod. Phys. Lett. A26, 387-398 (2011
Exactly solvable quantum spin tubes and ladders
We find families of integrable n-leg spin-1/2 ladders and tubes with general
isotropic exchange interactions between spins. These models are equivalent to
su(N) spin chains with N=2^n. Arbitrary rung interactions in the spin tubes and
ladders induce chemical potentials in the equivalent spin chains. The
potentials are n-dependent and differ for the tube and ladder models. The
models are solvable by means of nested Bethe Ansatz.Comment: 6 pages, Latex, 1 eps fig, to appear in J. Phys.
Optical I-band Linear Polarimetry of the Magnetar 4U 0142+61 with Subaru
The magnetar 4U~0142+61 has been well studied at optical and infrared
wavelengths and is known to have a complicated broad-band spectrum over the
wavelength range. Here we report the result from our linear imaging polarimetry
of the magnetar at optical -band. From the polarimetric observation carried
out with the 8.2-m Subaru telescope, we determine the degree of linear
polarization 3.4\%, or 5.6\% (90\% confidence level).
Considering models suggested for optical emission from magnetars, we discuss
the implications of our result. The upper limit measurement indicates that
different from radio pulsars, magnetars probably would not have strongly
polarized optical emission if the emission arises from their magnetosphere as
suggested.Comment: 5 pages, 1 figure, accepted for publication on Ap
Hot spots and waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks -a study by Low Temperature Scanning Laser Microscopy
Recently, it has been shown that large stacks of intrinsic Josephson
junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization
presumably triggered by a cavity resonance. To investigate this effect we use
Low Temperature Scanning Laser Microscopy to image electric field
distributions. Apart from verifying the appearance of cavity modes at low bias
we find that, in a high input power regime, standing-wave patterns are created
through interactions with a hot spot, possibly pointing to a new mode of
generating synchronized radiation in intrinsic Josephson junction stacks.Comment: 6 pages, 5 figures, supplementary information include
Nuclear spin relaxation rates in two-leg spin ladders
Using the transfer-matrix DMRG method, we study the nuclear spin relaxation
rate 1/T_1 in the two-leg s=1/2 ladder as function of the inter-chain
(J_{\perp}) and intra-chain (J_{|}) couplings. In particular, we separate the
q_y=0 and \pi contributions and show that the later contribute significantly to
the copper relaxation rate ^{63}(1/T_1) in the experimentally relevant coupling
and temperature range. We compare our results to both theoretical predictions
and experimental measures on ladder materials.Comment: Few modifications from the previous version 4 pages, 5 figures,
accepted for publication in PR
Instability of cosmological event horizons of non-static global cosmic strings
The stability of the cosmological event horizons found recently by Gregory
[Phys. Rev. D54, 4955 (1996)] for a class of non-static global cosmic strings
is studied. It is shown that they are not stable to both test particles and
physical perturbations. In particular, the back reaction of the perturbations
of null dust fluids will turn them into spacetime singularities. The resulted
singularities are strong in the sense that the distortion of test particles
diverges logarithmically when these singular hypersurfaces are approaching.Comment: Latex, no figure
- …