19,854 research outputs found
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Charge and Orbital Ordering and Spin State Transition Driven by Structural Distortion in YBaCo_2O_5
We have investigated electronic structures of antiferromagnetic YBaCo_2O_5
using the local spin-density approximation (LSDA) + U method. The charge and
orbital ordered insulating ground state is correctly obtained with the strong
on-site Coulomb interaction. Co^{2+} and Co^{3+} ions are found to be in the
high spin (HS) and intermediate spin (IS) state, respectively. It is considered
that the tetragonal to orthorhombic structural transition is responsible for
the ordering phenomena and the spin states of Co ions. The large contribution
of the orbital moment to the total magnetic moment indicates that the
spin-orbit coupling is also important in YBaCo_2O_5.Comment: 4 pages including 4 figures, Submitted to Phys. Rev. Let
SPH Simulations of Galactic Gaseous Disk with Bar: Distribution and Kinematic Structure of Molecular Clouds toward the Galactic Center
We have performed Smoothed Particle Hydrodynamic (SPH) simulations to study
the response of molecular clouds in the Galactic disk to a rotating bar and
their subsequent evolution in the Galactic Center (GC) region. The Galactic
potential in our models is contributed by three axisymmetric components
(massive halo, exponential disk, compact bulge) and a non-axisymmetric bar.
These components are assumed to be invariant in time in the frame corotating
with the bar. Some noticeable features such as an elliptical outer ring, spiral
arms, a gas-depletion region, and a central concentration have been developed
due to the influence of the bar. The rotating bar induces non-circular motions
of the SPH particles, but hydrodynamic collisions tend to suppress the random
components of the velocity. The velocity field of the SPH particles is
consistent with the kinematics of molecular clouds observed in HCN (1-0)
transition; these clouds are thought to be very dense clouds. However, the l-v
diagram of the clouds traced by CO is quite different from that of our SPH
simulation, being more similar to that obtained from simulations using
collisionless particles. The diagram of a mixture of collisional and
collisionless particles gives better reproduction of the kinematic structures
of the GC clouds observed in the CO line. The fact that the kinematics of HCN
clouds can be reproduced by the SPH particles suggests that the dense clouds in
the GC are formed via cloud collisions induced by rotating bar.Comment: 31 pages, 10 pigures, accepted for publication in Ap
Vaccine effectiveness against influenza A(H3N2)-associated hospitalized illness: United States, 2022
BACKGROUND: The COVID-19 pandemic was associated with historically low influenza circulation during the 2020-2021 season, followed by an increase in influenza circulation during the 2021-2022 US season. The 2a.2 subgroup of the influenza A(H3N2) 3C.2a1b subclade that predominated was antigenically different from the vaccine strain.
METHODS: To understand the effectiveness of the 2021-2022 vaccine against hospitalized influenza illness, a multistate sentinel surveillance network enrolled adults aged ≥18 years hospitalized with acute respiratory illness and tested for influenza by a molecular assay. Using the test-negative design, vaccine effectiveness (VE) was measured by comparing the odds of current-season influenza vaccination in influenza-positive case-patients and influenza-negative, SARS-CoV-2-negative controls, adjusting for confounders. A separate analysis was performed to illustrate bias introduced by including SARS-CoV-2-positive controls.
RESULTS: A total of 2334 patients, including 295 influenza cases (47% vaccinated), 1175 influenza- and SARS-CoV-2-negative controls (53% vaccinated), and 864 influenza-negative and SARS-CoV-2-positive controls (49% vaccinated), were analyzed. Influenza VE was 26% (95% CI: -14% to 52%) among adults aged 18-64 years, -3% (-54% to 31%) among adults aged ≥65 years, and 50% (15-71%) among adults aged 18-64 years without immunocompromising conditions. Estimated VE decreased with inclusion of SARS-CoV-2-positive controls.
CONCLUSIONS: During a season where influenza A(H3N2) was antigenically different from the vaccine virus, vaccination was associated with a reduced risk of influenza hospitalization in younger immunocompetent adults. However, vaccination did not provide protection in adults ≥65 years of age. Improvements in vaccines, antivirals, and prevention strategies are warranted
Emergence of skew distributions in controlled growth processes
Starting from a master equation, we derive the evolution equation for the
size distribution of elements in an evolving system, where each element can
grow, divide into two, and produce new elements. We then probe general
solutions of the evolution quation, to obtain such skew distributions as
power-law, log-normal, and Weibull distributions, depending on the growth or
division and production. Specifically, repeated production of elements of
uniform size leads to power-law distributions, whereas production of elements
with the size distributed according to the current distribution as well as no
production of new elements results in log-normal distributions. Finally,
division into two, or binary fission, bears Weibull distributions. Numerical
simulations are also carried out, confirming the validity of the obtained
solutions.Comment: 9 pages, 3 figure
Compatible pairs of orthogonal polynomials
19 pages, no figures.-- MSC1991 code: 33C45.MR#: MR1736624 (2001a:33009)Zbl#: Zbl 0944.33012We find necessary and sufficient conditions for an orthogonal polynomial system to be compatible with another orthogonal polynomial system. As applications, we find new characterizations of semi-classical and classical orthogonal polynomials.The work of D. H. Kim and K. H. Kwon was
partially supported by KOSEF (98-0701-03-01-5) and GARC at Seoul National University. The work of F. Marcellán was partially supported by Dirección General de Enseñanza Superior (DGES) of Spain under grant PB96-0120-C03-0l.Publicad
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Ultrahigh power and energy density in partially ordered lithium-ion cathode materials
The rapid market growth of rechargeable batteries requires electrode materials that combine high power and energy and are made from earth-abundant elements. Here we show that combining a partial spinel-like cation order and substantial lithium excess enables both dense and fast energy storage. Cation overstoichiometry and the resulting partial order is used to eliminate the phase transitions typical of ordered spinels and enable a larger practical capacity, while lithium excess is synergistically used with fluorine substitution to create a high lithium mobility. With this strategy, we achieved specific energies greater than 1,100 Wh kg–1 and discharge rates up to 20 A g–1. Remarkably, the cathode materials thus obtained from inexpensive manganese present a rare case wherein an excellent rate capability coexists with a reversible oxygen redox activity. Our work shows the potential for designing cathode materials in the vast space between fully ordered and disordered compounds
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