1,335 research outputs found
Spectra of comet P/Halley at R = 4 - 8 AU
Spectra of Comet Halley (lambda lambda = 3400-6500 A) were acquired at pre- and post-perihelion distances of 4.8 AU on 1985 Feb. 17 (Coma V equals 18.9 mag) and 1987 Feb. 1 (coma V = 15.9 mag) using the 4.5-m Multiple-Mirror Telescope (MMT) and the CTIO 4.0-m telescope, respectively. The CN(0,0) violet system band flux at 4.8 AU was approx. 15 times greater at the post-perhelion phase compared to pre-perihelion. Additional post-perihelion spectra, obtained on 1986 Nov. 28 to 30 with the MTT, showed CN(0,0) and very weak C3 4040 A emission. The MMT data are one-dimensional spectra (aperture: 5 arc sec diameter) obtained with an intensified Reticon while the CTIO data are two-dimensional spectra (slit length = 280 arc sec) obtained with a 2D-Frutti photon counting system. Extended CN(0,0) emission was detected in the 1987 Feb. 1 (at 4.8 AU) spectra to a distance of at least 70 arc sec in the solar and anti-solar directions. Additional CCD spectra obtained with the KPNO 2.2-meter telescope on 1988 Feb. 20 (at 7.9 AU) show scattered solar continuum approx. 32 arc sec diameter. However, no emission features were detected at 7.9 AU
Magneto-elastic coupling and unconventional magnetic ordering in triangular multiferroic AgCrS2
The temperature evolution of the crystal and magnetic structures of
ferroelectric sulfide AgCrS2 have been investigated by means of neutron
scattering. AgCrS2 undergoes at TN = 41.6 K a first-order phase transition,
from a paramagnetic rhombohedral R3m to an antiferromagnetic monoclinic
structure with a polar Cm space group. In addition to being ferroelectric below
TN, the low temperature phase of AgCrS2 exhibits an unconventional collinear
magnetic structure that can be described as double ferromagnetic stripes
coupled antiferromagnetically, with the magnetic moment of Cr+3 oriented along
b within the anisotropic triangular plane. The magnetic couplings stabilizing
this structure are discussed using inelastic neutron scattering results.
Ferroelectricity below TN in AgCrS2 can possibly be explained in terms of
atomic displacements at the magneto-elastic induced structural distortion.
These results contrast with the behavior of the parent frustrated
antiferromagnet and spin-driven ferroelectric AgCrO2
Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2
Density functional perturbation theory calculations of alpha-quartz using
extended norm conserving pseudopotentials have been used to study the elastic
properties and phonon dispersion relations along various high symmetry
directions as a function of bulk, uniaxial and non-hydrostatic pressure. The
computed equation of state, elastic constants and phonon frequencies are found
to be in good agreement with available experimental data. A zone boundary (1/3,
1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around
the same pressure, studies of the Born stability criteria reveal that the
structure is mechanically unstable. The phonon and elastic softening are
related to the high pressure phase transitions and amorphization of quartz and
these studies suggest that the mean transition pressure is lowered under
non-hydrostatic conditions. Application of uniaxial pressure, results in a
post-quartz crystalline monoclinic C2 structural transition in the vicinity of
the K-point instability. This structure, intermediate between quartz and
stishovite has two-thirds of the silicon atoms in octahedral coordination while
the remaining silicon atoms remain tetrahedrally coordinated. This novel
monoclinic C2 polymorph of silica, which is found to be metastable under
ambient conditions, is possibly one of the several competing dense forms of
silica containing octahedrally coordinated silicon. The possible role of high
pressure ferroelastic phases in causing pressure induced amorphization in
silica are discussed.Comment: 17 pages, 8 figs., 8 Table
Making HIV Prevention Programming Count: Identifying Predictors of Success in a Parent-Based HIV Prevention Program for Youth
Predictors of change in the number of sexual topics parents discussed and responsiveness during sex communication with their preadolescent after participating in a five-session sexual risk reduction intervention for parents were examined. Data were from 339 African American parents of preadolescents enrolled in the intervention arm of a randomized-controlled trial of the Parents’ Matter! Program (PMP). Four categories of predictors of success were examined: time and resource constraints, personal characteristics, the parent-child relationship, and parent perceptions of child readiness for sex communication. There were only sporadic associations between success and time and resource constraints for either outcome. Parent perception of child readiness for sex communication was positively associated with discussions of sex topics (b = 1.11, confidence interval [CI]: 0.24-1.97) and parental responsiveness (b = .68, CI: 0.22-1.15). Although parents face time and resource constraints, most attended at least four sessions, and demographics such as income had limited effects on program success
Enhancing HIV Communication between Parents and Children: Efficacy of the Parents Matter!
We examine efficacy of the Parents Matter! Program (PMP), a program to teach African-American parents of preadolescents sexual communication and HIV-prevention skills, through a multicenter, randomized control trial. A total of 1115 parent-child participants were randomized to one of three intervention arms (enhanced, brief, control). Percentages and 95% confidence intervals compare parents’ perception of child readiness to learn about sexual issues, communication effectiveness, and dyad concordance from baseline to 12 months postintervention. Wilcoxon rank sum tests compare the changes in scores measuring communication content in HIV/ AIDS, abstinence, and condom use. Compared to control, parents in the enhanced arm increased perception of child readiness to learn about sex (16% vs. 29%; p \u3c .001), and a greater proportion of parent-child dyads reported concordant responses on communication topics: HIV/AIDS (15%, 95% CI = 8-21%; p \u3c .001), abstinence (13%, 95% CI = 7-20%; p \u3c .001), condoms (15%, 95% CI = 9-22%; p \u3c .001). Increases in communication scores in HIV/AIDS, abstinence, and condom use were greater in the enhanced arm than control (p \u3c 0.01). We conclude that the enhanced PMP can help parents educate children about HIV and prepare children to avoid sexual risk
High real-space resolution measurement of the local structure of Ga_1-xIn_xAs using x-ray diffraction
High real-space resolution atomic pair distribution functions (PDF)s from the
alloy series Ga_1-xIn_xAs have been obtained using high-energy x-ray
diffraction. The first peak in the PDF is resolved as a doublet due to the
presence of two nearest neighbor bond lengths, Ga-As and In-As, as previously
observed using XAFS. The widths of nearest, and higher, neighbor pairs are
analyzed by separating the strain broadening from the thermal motion. The
strain broadening is five times larger for distant atomic neighbors as compared
to nearest neighbors. The results are in agreement with model calculations.Comment: 4 pages, 5 figure
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4
Extensive X-ray and neutron scattering experiments and additional
transmission electron microscopy results reveal the partial decomposition of
Nd2-xCexCuO4 (NCCO) in a low-oxygen-fugacity environment such as that typically
realized during the annealing process required to create a superconducting
state. Unlike a typical situation in which a disordered secondary phase results
in diffuse powder scattering, a serendipitous match between the in-plane
lattice constant of NCCO and the lattice constant of one of the decomposition
products, (Nd,Ce)2O3, causes the secondary phase to form an oriented,
quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from
the secondary phase appear at rational positions (H,K,0) in the reciprocal
space of NCCO. Additionally, because of neodymium paramagnetism, the
application of a magnetic field increases the low-temperature intensity
observed at these positions via neutron scattering. Such effects may mimic the
formation of a structural superlattice or the strengthening of
antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified
through careful and systematic experimentation. For typical reduction
conditions, the (Nd,Ce)2O3 volume fraction is ~1%, and the secondary-phase
layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100
angstromsthick. The presence of the secondary phase should also be taken into
account in the analysis of other experiments on NCCO, such as transport
measurements.Comment: 15 pages, 17 figures, submitted to Phys. Rev.
Pulsar Jets: Implications for Neutron Star Kicks and Initial Spins
We study implications for the apparent alignment of the spin axes,
proper-motions, and polarization vectors of the Crab and Vela pulsars. The spin
axes are deduced from recent Chandra X-ray Observatory images that reveal jets
and nebular structure having definite symmetry axes. The alignments indicate
these pulsars were born either in isolation or with negligible velocity
contributions from binary motions. We examine the effects of rotation and the
conditions under which spin-kick alignment is produced for various models of
neutron star kicks. If the kick is generated when the neutron star first forms
by asymmetric mass ejection or/and neutrino emission, then the alignment
requires that the protoneutron star possesses an original spin with period
much less than the kick timescale, thus spin-averaging the kick forces.
The kick timescale ranges from 100 ms to 10 s depending on whether the kick is
hydrodynamically driven or neutrino-magnetic field driven. For hydrodynamical
models, spin-kick alignment further requires the rotation period of an
asymmetry pattern at the radius near shock breakout (>100 km) to be much less
than ~100 ms; this is difficult to satisfy unless rotation plays a dynamically
important role in the core collapse and explosion (P_s\lo 1 ms). Aligned kick
and spin vectors are inherent to the slow process of asymmetric electromagnetic
radiation from an off-centered magnetic dipole. We reassess the viability of
this effect, correcting a factor of 4 error in Harrison and Tademaru's
calculation that increases the size of the effect. To produce a kick velocity
of order a few hundred km/s requires that the neutron star be born with an
initial spin close to 1 ms and that spindown due to r-mode driven gravitational
radiation be inefficient compared to standard magnetic braking.Comment: Small changes/additions; final version to be published in ApJ,
Vol.549 (March 10, 2001
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
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