8,026 research outputs found
A conserved variable in the perturbed hydrodynamic world model
We introduce a scalar-type perturbation variable which is conserved in
the large-scale limit considering general sign of three-space curvature (),
the cosmological constant (), and time varying equation of state. In a
pressureless medium is {\it exactly conserved} in all scales.Comment: 4 pages, no figure, To appear in Phys. Rev.
Resting-state functional connectivity in mathematical expertise
To what extent are different levels of expertise reflected in the functional connectivity of the brain? We addressed this question by using resting-state functional magnetic resonance imaging (fMRI) in mathematicians versus non-mathematicians. To this end, we investigated how the two groups of participants differ in the correlation of their spontaneous blood oxygen level-dependent fluctuations across the whole brain regions during resting state. Moreover, by using the classification algorithm in machine learning, we investigated whether the resting-state fMRI networks between mathematicians and non-mathematicians were distinguished depending on features of functional connectivity. We showed diverging involvement of the frontalâthalamicâtemporal connections for mathematicians and the medialâfrontal areas to precuneus and the lateral orbital gyrus to thalamus connections for non-mathematicians. Moreover, mathematicians who had higher scores in mathematical knowledge showed a weaker connection strength between the left and right caudate nucleus, demonstrating the connectionsâ characteristics related to mathematical expertise. Separate functional networks between the two groups were validated with a maximum classification accuracy of 91.19 using the distinct resting-state fMRI-based functional connectivity features. We suggest the advantageous role of preconfigured resting-state functional connectivity, as well as the neural efficiency for expertsâ successful performance
Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu(BO)$_2
We have studied spin excitation spectra in the Shastry-Sutherland model
compound SrCu(BO) in magnetic fields using far-infrared Fourier
spectroscopy. The transitions from the ground singlet state to the triplet
state at 24 cm and to several bound triplet states are induced by the
electric field component of the far-infrared light. To explain the light
absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM)
mechanism where light couples to a phonon mode, allowing the DM interaction.
Two optical phonons couple light to the singlet to triplet transition in
SrCu(BO). One is -polarized and creates an intra-dimer dynamic
DM along the c axis. The other is -polarized and creates an intra-dimer
dynamic DM interaction, it is in the plane and perpendicular to the
dimer axis. Singlet levels at 21.5 and 28.6 cm anti-cross with the first
triplet as is seen in far-infrared spectra. We used a cluster of two dimers
with a periodic boundary condition to perform a model calculation with scaled
intra- and inter-dimer exchange interactions. Two static DM interactions are
sufficient to describe the observed triplet state spectra. The static
inter-dimer DM in the c-direction cm splits the triplet state
sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87},
167205 (2001)]. The static intra-dimer DM in the plane (perpendicular to
the dimer axis) cm, allowed by the buckling of CuBO
planes, couples the triplet state to the 28.6 cm singlet as is seen from
the avoided crossing.Comment: 12 pages with 7 figures, some references correcte
Investigation of aged aerosols in size-resolved Asian dust storm particles transported from Beijing, China, to Incheon, Korea, using low-<i>Z</i> particle EPMA
This is the first study of Asian dust storm (ADS) particles collected in
Beijing, China, and Incheon, Korea, during a spring ADS event. Using a
seven-stage May impactor and a quantitative electron probe X-ray
microanalysis (ED-EPMA, also known as low-<i>Z</i> particle EPMA), we examined the
composition and morphology of 4200 aerosol particles at stages 1â6 (with a
size cut-off of 16, 8, 4, 2, 1, and 0.5 ÎŒm in equivalent
aerodynamic diameter, respectively) collected during an ADS event on 28â29
April 2005. The results showed that there were large differences in the
chemical compositions between particles in sample S1 collected in Beijing
immediately after the peak time of the ADS and in samples S2 and S3, which
were collected in Incheon approximately 5 h and 24 h later, respectively.
In sample S1, mineral dust particles accounted for more than 88% in
relative number abundance at stages 1â5; and organic carbon (OC) and reacted
NaCl-containing particles accounted for 24% and 32%, respectively,
at stage 6. On the other hand, in samples S2 and S3, in addition to
approximately 60% mineral dust, many sea spray aerosol (SSA) particles
reacted with airborne SO<sub>2</sub> and NO<sub>x</sub> (accounting for 24%
and 14% on average in samples S2 and S3, respectively), often mixed with
mineral dust, were encountered at stages 1â5, and (C, N, O, S)-rich
particles (likely a mixture of water-soluble organic carbon with
(NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and NH<sub>4</sub>NO<sub>3</sub>) were abundantly observed at
stage 6 (accounting for 68% and 51% in samples S2 and S3,
respectively). This suggests that an accumulation of sea-salt components on
individual ADS particles larger than 1 ÎŒm in diameter occurred and
many secondary aerosols smaller than 1 ÎŒm in diameter were formed
when the ADS particles passed over the Yellow Sea. In the reacted or aged
mineral dust and SSA particles, nitrate-containing and both nitrate- and
sulfate-containing species vastly outnumbered the sulfate-containing species,
implying that ambient NO<sub>x</sub> had a greater influence on the
atmospheric particles than SO<sub>2</sub> during this ADS episode. In addition to
partially- or totally-reacted CaCO<sub>3</sub>, reacted or aged Mg-containing
aluminosilicates were observed frequently in samples S2 and S3;
furthermore, a student's <i>t</i> test showed that both their atomic concentration
ratios of [Mg] / [Al] and [Mg] / [Si] were significantly elevated
(<i>P</i> < 0.05) compared to those in samples S1 (for [Mg] / [Al],
0.34 ± 0.09 and 0.40 ± 0.03 in samples S2 and S3, respectively,
vs. 0.24 ± 0.01 in sample S1; for [Mg] / [Si], 0.21 ± 0.05 and
0.22 ± 0.01 in samples S2 and S3, respectively, vs. 0.12 ± 0.02 in
sample S1). The significant increase of [Mg] / [Al] and [Mg] / [Si]
ratios in Mg-containing aluminosilicates indicates that a significant evolution or
aging must have occurred on the ADS particles in the marine atmosphere during
transport from China to Korea
Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition
Temperature-dependent resistivity of graphene grown by chemical vapor
deposition (CVD) is investigated. We observe in low mobility CVD graphene
device a strong insulating behavior at low temperatures and a metallic behavior
at high temperatures manifesting a non-monotonic in the temperature dependent
resistivity.This feature is strongly affected by carrier density modulation. To
understand this anomalous temperature dependence, we introduce thermal
activation of charge carriers in electron-hole puddles induced by randomly
distributed charged impurities. Observed temperature evolution of resistivity
is then understood from the competition among thermal activation of charge
carriers, temperature-dependent screening and phonon scattering effects. Our
results imply that the transport property of transferred CVD-grown graphene is
strongly influenced by the details of the environmentComment: 7 pages, 3 figure
Equilibrium orbit analysis in a free-electron laser with a coaxial wiggler
An analysis of single-electron orbits in combined coaxial wiggler and axial
guide magnetic fields is presented. Solutions of the equations of motion are
developed in a form convenient for computing orbital velocity components and
trajectories in the radially dependent wiggler. Simple analytical solutions are
obtained in the radially-uniform-wiggler approximation and a formula for the
derivative of the axial velocity with respect to Lorentz factor
is derived. Results of numerical computations are presented and the
characteristics of the equilibrium orbits are discussed. The third spatial
harmonic of the coaxial wiggler field gives rise to group orbits which
are characterized by a strong negative mass regime.Comment: 13 pages, 9 figures, to appear in phys. rev.
COBE constraints on inflation models with a massive non-minimal scalar field
We derive power spectra of the scalar- and tensor-type structures generated
in an inflation model based on a massive non-minimally coupled scalar field
with the strong coupling assumption. We make analyses in both the
original-frame and the conformally transformed Einstein-frame. We derive
contributions of both structures to the anisotropy of the cosmic microwave
background radiation, and compare the contributions with the four-year COBE-DMR
data. Previous study showed that sufficient amount of inflation requires a
small coupling parameter. In such a case the spectra become near Zeldovich
spectra, and the gravitational wave contribution becomes negligible compared
with the scalar-type contribution which is testable in future CMBR experiments.Comment: 4 pages, no figure, To appear in Phys. Rev.
Chandra Detection of the Forward and Reverse Shocks in Cassiopeia-A
We report the localization of the forward and reversed shock fronts in the
young supernova remnant Cas-A using X-ray data obtained with the Chandra
Observatory. High resolution X-ray maps resolve a previously unseen X-ray
feature encompassing the extremity of the remnant. This feature consists of
thin, tangential wisps of emission bordering the outer edge of the thermal
X-ray and radio remnant, forming a circular rim, approx. 2.7 in radius. Radio
images show a sharp rise in brightness at this X-ray rim, along with a large
jump in the synchrotron polarization angle. These characteristics suggest that
these wisps are the previously unresolved signature of the forward, or outer,
shock. Similarly, we identify the sharp rise in emissivity of the bright shell
for both the radio and X-ray line emission associated with the reverse shock.
The derived ratio of the averaged forward and reverse shock radii of approx.
3:2 constrains the remnant to have swept up roughly the same amount of mass as
was ejected; this suggests that Cas-A is just entering the Sedov phase.
Comparison of the X-ray spectra from the two shock regions shows that the
equivalent widths of prominent emission lines are significantly lower exterior
to the bright shell, as expected if they are respectively identified with the
shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of
the outer rim itself is dominated by power-law emission, likely the counterpart
of the non-thermal component previously seen at energies above 10 keV.Comment: 7 pages with 5 figures, LaTex, emulateapj.sty. To appear in the
Astrophysical Journal Letter
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