25,150 research outputs found
KN and KbarN Elastic Scattering in the Quark Potential Model
The KN and KbarN low-energy elastic scattering is consistently studied in the
framework of the QCD-inspired quark potential model. The model is composed of
the t-channel one-gluon exchange potential, the s-channel one-gluon exchange
potential and the harmonic oscillator confinement potential. By means of the
resonating group method, nonlocal effective interaction potentials for the KN
and KbarN systems are derived and used to calculate the KN and KbarN elastic
scattering phase shifts. By considering the effect of QCD renormalization, the
contribution of the color octet of the clusters (qqbar) and (qqq) and the
suppression of the spin-orbital coupling, the numerical results are in fairly
good agreement with the experimental data.Comment: 20 pages, 8 figure
On indecomposable modules over the Virasoro algebra
It is proved that an indecomposable Harish-Chandra module over the Virasoro
algebra must be (i) a uniformly bounded module, or (ii) a module in Category
, or (iii) a module in Category , or (iv) a module which
contains the trivial module as one of its composition factors.Comment: 5 pages, Latex, to appear in Science in China
Infrared Emission by Dust Around lambda Bootis Stars: Debris Disks or Thermally Emitting Nebulae?
We present a model that describes stellar infrared excesses due to heating of
the interstellar (IS) dust by a hot star passing through a diffuse IS cloud.
This model is applied to six lambda Bootis stars with infrared excesses.
Plausible values for the IS medium (ISM) density and relative velocity between
the cloud and the star yield fits to the excess emission. This result is
consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A-
to F-type stars with large underabundances of Fe-peak elements) owe their
characteristics to interactions with the ISM. This proposal invokes radiation
pressure from the star to repel the IS dust and excavate a paraboloidal dust
cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar
photosphere. However, the measurements of the infrared excesses can also be fit
by planetary debris disk models. A more detailed consideration of the
conditions to produce lambda Bootis characteristics indicates that the majority
of infrared-excess stars within the Local Bubble probably have debris disks.
Nevertheless, more distant stars may often have excesses due to heating of
interstellar material such as in our model.Comment: 10 pages, 5 figures, 4 tables, accepted by ApJ, emulateap
Precise and ultrafast molecular sieving through graphene oxide membranes
There has been intense interest in filtration and separation properties of
graphene-based materials that can have well-defined nanometer pores and exhibit
low frictional water flow inside them. Here we investigate molecular permeation
through graphene oxide laminates. They are vacuum-tight in the dry state but,
if immersed in water, act as molecular sieves blocking all solutes with
hydrated radii larger than 4.5A. Smaller ions permeate through the membranes
with little impedance, many orders of magnitude faster than the diffusion
mechanism can account for. We explain this behavior by a network of
nanocapillaries that open up in the hydrated state and accept only species that
fit in. The ultrafast separation of small salts is attributed to an 'ion
sponge' effect that results in highly concentrated salt solutions inside
graphene capillaries
Superconductivity in Ca-doped graphene
Graphene, a zero-gap semimetal, can be transformed into a metallic,
semiconducting or insulating state by either physical or chemical modification.
Superconductivity is conspicuously missing among these states despite
considerable experimental efforts as well as many theoretical proposals. Here,
we report superconductivity in calcium-decorated graphene achieved by
intercalation of graphene laminates that consist of well separated and
electronically decoupled graphene crystals. In contrast to intercalated
graphite, we find that Ca is the only dopant that induces superconductivity in
graphene laminates above 1.8 K among intercalants used in our experiments such
as potassium, caesium and lithium. Ca-decorated graphene becomes
superconducting at ~ 6 K and the transition temperature is found to be strongly
dependent on the confinement of the Ca layer and the induced charge carrier
concentration. In addition to the first evidence for superconducting graphene,
our work shows a possibility of inducing and studying superconductivity in
other 2D materials using their laminates
Debris Disks around Solar-Type Stars: Observations of the Pleiades with Spitzer Space Telescope
We present Spitzer MIPS observations at 24 um of 37 solar-type stars in the
Pleiades and combine them with previous observations to obtain a sample of 71
stars. We report that 23 stars, or 32 +/- 6.8%, have excesses at 24 um at least
10% above their photospheric emission. We compare our results with studies of
debris disks in other open clusters and with a study of A stars to show that
debris disks around solar-type stars at 115 Myr occur at nearly the same rate
as around A-type stars. We analyze the effects of binarity and X-ray activity
on the excess flux. Stars with warm excesses tend not to be in equal-mass
binary systems, possibly due to clearing of planetesimals by binary companions
in similar orbits. We find that the apparent anti-correlations in the incidence
of excess and both the rate of stellar rotation and also the level of activity
as judged by X-ray emission are statistically weak.Comment: 34 pages; accepted for publication in ApJ; new version included
corrections of typos, etc to match published versio
Diffuse Neutron Scattering Study of Magnetic Correlations in half-doped La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) Manganites
The short range ordered magnetic correlations have been studied in half doped
La0.5Ca0.5-xSrxMnO3 (x = 0.1, 0.3 and 0.4) compounds by polarized neutron
scattering technique. On doping Sr2+ for Ca2+ ion, these compounds with x =
0.1, 0.3, and 0.4 exhibit CE-type, mixture of CE-type and A-type, and A-type
antiferromagnetic ordering, respectively. Magnetic diffuse scattering is
observed in all the compounds above and below their respective magnetic
ordering temperatures and is attributed to magnetic polarons. The correlations
are primarily ferromagnetic in nature above T\_N, although a small
antiferromagnetic contribution is also evident. Additionally, in samples x =
0.1 and 0.3 with CE-type antiferromagnetic ordering, superlattice diffuse
reflections are observed indicating correlations between magnetic polarons. On
lowering temperature below T\_N the diffuse scattering corresponding to
ferromagnetic correlations is suppressed and the long range ordered
antiferromagnetic state is established. However, the short range ordered
correlations indicated by enhanced spin flip scattering at low Q coexist with
long range ordered state down to 3K. In x = 0.4 sample with A-type
antiferromagnetic ordering, superlattice diffuse reflections are absent.
Additionally, in comparison to x = 0.1 and 0.3 sample, the enhanced spin flip
scattering at low Q is reduced at 310K, and as temperature is reduced below
200K, it becomes negligibly low. The variation of radial correlation function,
g(r) with temperature indicates rapid suppression of ferromagnetic correlations
at the first nearest neighbor on approaching TN. Sample x = 0.4 exhibits growth
of ferromagnetic phase at intermediate temperatures (~ 200K). This has been
further explored using SANS and neutron depolarization techniques.Comment: 13 pages, 12 figures, To appear in Physical Review
Understanding mechanisms of genetic risk for adolescent internalizing and externalizing problems: The mediating role of parenting and personality
Genetic predispositions play an important role in the development of internalizing and externalizing behaviors. Understanding the mechanisms through which genetic risk unfolds to influence these developmental outcomes is critical for developing prevention and intervention efforts, capturing key elements of Irv's research agenda and scientific legacy. In this study, we examined the role of parenting and personality in mediating the effect of genetic risk on adolescents' major depressive disorder and conduct disorder symptoms. Longitudinal data were drawn from a sample of 709 European American adolescents and their mothers from the Collaborative Studies on Genetics of Alcoholism. Results from multivariate path analysis indicated that adolescents' depressive symptoms genome-wide polygenic scores (DS_GPS) predicted lower parental knowledge, which in turn was associated with more subsequent major depressive disorder and conduct disorder symptoms. Adolescents' DS_GPS also had indirect effects on these outcomes via personality, with a mediating effect via agreeableness but not via other dimensions of personality. Findings revealed that the pattern of associations was similar across adolescent gender. Our findings emphasize the important role of evocative gene-environment correlation processes and intermediate phenotypes in the pathways of risk from genetic predispositions to complex adolescent outcomes
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