18,085 research outputs found
Dark Matter in B-L Extended MSSM Models
We analyze the dark matter problem in the context of supersymmetric
U(1)_{B-L} model. In this model, the lightest neutalino can be the B-L gaugino
widetilde {Z}_{B-L} or the extra Higgsinos widetilde{chi}_{1,2} dominated. We
compute the thermal relic abundance of these particles and show that, unlike
the LSP in MSSM, they can account for the observed relic abundance with no
conflict with other phenomenological constraints. The prospects for their
direct detection, if they are part of our galactic halo, are also discussed.Comment: 11 pages, 9 figures. Published versio
Dielectric responses of the layered cobalt oxysulfide Sr_2Cu_2CoO_2S_2 with CoO_2 square-planes
We have studied the dielectric responses of the layered cobalt oxysulfide
SrCuCoOS with the CoO square-planes. With decreasing
temperature below the N\'eel temperature, the resistivity increases like a
semiconductor, and the thermopower decreases like a metal. The dielectric
constant is highly dependent on temperature, and the dielectric relaxation is
systematically changed with temperature, which is strongly correlated to the
magnetic states. These behaviors suggest that carriers distributed
homogeneously in the paramagnetic state at high temperatures are expelled from
the antiferromagnetically ordered spin domain below the N\'eel temperature.Comment: 3 pages, 4 eps figures, to be published in J. Appl. Phy
Bifurcation analysis in an associative memory model
We previously reported the chaos induced by the frustration of interaction in
a non-monotonic sequential associative memory model, and showed the chaotic
behaviors at absolute zero. We have now analyzed bifurcation in a stochastic
system, namely a finite-temperature model of the non-monotonic sequential
associative memory model. We derived order-parameter equations from the
stochastic microscopic equations. Two-parameter bifurcation diagrams obtained
from those equations show the coexistence of attractors, which do not appear at
absolute zero, and the disappearance of chaos due to the temperature effect.Comment: 19 page
Dark Matter Annihilation and the PAMELA, FERMI and ATIC Anomalies
If dark matter (DM) annihilation accounts for the tantalizing excess of
cosmic ray electron/positrons, as reported by the PAMELA, ATIC, HESS and FERMI
observatories, then the implied annihilation cross section must be relatively
large. This results, in the context of standard cosmological models, in very
small relic DM abundances that are incompatible with astrophysical
observations. We explore possible resolutions to this apparent conflict in
terms of non-standard cosmological scenarios; plausibly allowing for large
cross sections, while maintaining relic abundances in accord with current
observations.Comment: 13 pages, 3 figures; published for publication in Physical Review
Dust in Hot Plasma of Nearby Dusty Elliptical Galaxies Observed with the Spitzer Space Telescope
We report on mid- and far-IR Spitzer observations of 7 nearby dusty
elliptical galaxies by using the Multiband Imaging Photometer (MIPS) and
Infrared Spectrograph (IRS). Our sample galaxies are known to contain an
excessive amount of interstellar dust against sputtering destruction in hot
plasma filling the interstellar space of elliptical galaxies. In order to study
the origin and the properties of the excess dust in the hot plasma, we selected
galaxies with a wide range of X-ray luminosities but similar optical luminos
ities for our Spitzer Guest Observers (GO1) program. The 7 galaxies are
detected at the MIPS 24 um, 70 um, and 160 um bands; the far- to mid-IR flux
ratios of relatively X-ray-bright elliptical galaxies are lower than those of
X-ray-faint galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon
(PAH) emission features are detected significantly from 5 of the 7 galaxies; t
he emission intensities are weaker as the X-ray luminosity of the galaxy is
larger. We have found a correlation between the far- to mid-IR flux ratio and
the equivalent width of the PAH emission feature. We have obtained apparent
spatial correspondence between mid-IR and X-ray distributions in the outer
regions for the three X-ray-brightest galaxies in our sample. Possible
interpretations for our observational results are discussed.Comment: 25 pages, 7 figures, accepted for publication in Publications of the
Astronomical Society of Japa
Origin of critical-temperature enhancement of an iron-based high-T_c superconductor, LaFeAsO_{1-x}F_{x} : NMR study under high pressure
Nuclear magnetic resonance (NMR) measurements of an iron (Fe)-based
superconductor LaFeAsO_{1-x}F_x (x = 0.08 and 0.14) were performed at ambient
pressure and under pressure. The relaxation rate 1/T_1 for the overdoped
samples (x = 0.14) shows T-linear behavior just above T_c, and pressure
application enhances 1/T_1T similar to the behavior of T_c. This implies that
1/T_1T = constant originates from the Korringa relation, and an increase in the
density of states at the Fermi energy D(E_F) leads to the enhancement of T_c.
In the underdoped samples (x = 0.08), 1/T_1T measured at ambient pressure also
shows T-independent behavior in a wide temperature range above T_c. However, it
shows Curie-Weiss-like T dependence at 3.0 GPa accompanied by a small increase
in T_c, suggesting that predominant antiferromagnetic fluctuation suppresses
development of superconductivity or remarkable enhancement of T_c. The
qualitatively different features between underdoped and overdoped samples are
systematically explained by a band calculation with hole and electron pockets
Deformation and fracture of single-crystal and sintered polycrystalline silicon carbide produced by cavitation
An investigation was conducted to examine the deformation and fracture behavior of single-crystal and sintered polycrystalline SiC surfaces exposed to cavitation. Cavitation erosion experiments were conducted in distilled water at 25 C by using a magnetostrictive oscillator in close proximity (1 mm) to the surface of SiC. The horn frequency was 20 kHz, and the double amplitude of the vibrating disk was 50 microns. The results of the investigation indicate that the SiC (0001) surface could be deformed in a plastic manner during cavitation. Dislocation etch pits were formed when the surface was chemically etched. The number of defects, including dislocations in the SiC (0001) surface, increased with increasing exposure time to cavitation. The presence of intrinsic defects such as voids in the surficial layers of the sintered polycrystalline SiC determined the zones at which fractured grains and fracture pits (pores) were generated. Single-crystal SiC had superior erosion resistance to that of sintered polycrystalline SiC
Momentum dependence of the energy gap in the superconducting state of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu)
The energy gap of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu) was probed by
angle resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet
laser (photon energy 6.994 eV) or He I resonance line (21.218 eV) as photon
source. The results show that the gap around the node at sufficiently low
temperatures can be well described by a monotonic d-wave gap function for both
samples and the gap of the R=La sample is larger reflecting the higher Tc.
However, an abrupt deviation from the d-wave gap function and an opposite R
dependence for the gap size were observed around the antinode, which represent
a clear disentanglement between the antinodal pseudogap and the nodal
superconducting gap.Comment: Submitted as the proceedings of LT2
Spatial Distributions of Cold and Warm Interstellar Dust in M101 Resolved with AKARI/Far-Infrared Surveyor (FIS)
The nearby face-on spiral galaxy M101 has been observed with the Far-Infrared
Surveyor (FIS) onboard AKARI. The far-infrared four-band images reveal fine
spatial structures of M101, which include global spiral patterns, giant HII
regions embedded in outer spiral arms, and a bar-like feature crossing the
center. The spectral energy distribution of the whole galaxy shows the presence
of the cold dust component (18 K) in addition to the warm dust component (55
K). The distribution of the cold dust is mostly concentrated near the center,
and exhibits smoothly distributed over the entire extent of the galaxy, whereas
the distribution of the warm dust indicates some correlation with the spiral
arms, and has spotty structures such as four distinctive bright spots in the
outer disk in addition to a bar-like feature near the center tracing the CO
intensity map. The star-formation activity of the giant HII regions that
spatially correspond to the former bright spots is found to be significantly
higher than that of the rest of the galaxy. The latter warm dust distribution
implies that there are significant star-formation activities in the entire bar
filled with molecular clouds. Unlike our Galaxy, M101 is a peculiar normal
galaxy with extraordinary active star-forming regions.Comment: 18 pages, 9 figures, accepted for publication in PASJ AKARI special
issu
Low-dimensional chaos induced by frustration in a non-monotonic system
We report a novel mechanism for the occurrence of chaos at the macroscopic
level induced by the frustration of interaction, namely frustration-induced
chaos, in a non-monotonic sequential associative memory model. We succeed in
deriving exact macroscopic dynamical equations from the microscopic dynamics in
the case of the thermodynamic limit and prove that two order parameters
dominate this large-degree-of-freedom system. Two-parameter bifurcation
diagrams are obtained from the order-parameter equations. Then we analytically
show that the chaos is low-dimensional at the macroscopic level when the system
has some degree of frustration, but that the chaos definitely does not occur
without the frustration.Comment: 2 figure
- …