194,621 research outputs found
On the Profiles and Polarization of Raman Scattered Emission Lines in Symbiotic Stars:II. Numerical Simulations
A Monte Carlo method is used to calculate the profiles and the polarization
of the Raman scattered O VI lines(lambda lambda 6827,7088) in symbiotic stars.
A point-like isotropic UV radiation source is assumed and a simple spherical
wind model is adopted for the kinematics of the scattering material from the
cool giant. We first investigate the case where the incident line photons are
described by a Gaussian profile having a width of 10^4 K. We subsequently
investigate the effects of the extended ionized region and non-spherical wind
models including a disk-type wind and a bipolar wind. The cases where the
emissin source is described by non-Gaussian profiles are briefly studied.
Finally, as an additional component for the kinematics of symbiotic stars the
orbital motion of the hot component around the cool giant is included and the
effect on the spectropolarimetry is investigated. In this case the polarization
direction changes around the red part of the Raman-scattered emission lines,
when the observer's line of sight is perpendicular to the orbital plane, and no
such effect is seen when the line of sight lies in the orbital plane.
Furthermore, complex peak structures are seen in the degree of polarization and
polarized flux, which have often been observed in several symbiotic systems
including RR Tel. Brief observational consequences and preditions are discussed
in relation to the present and future spectropolarimetry for symbiotic stars.
It is concluded that spectropolarimetry may provide a powerful diagnostic of
the physical conditions of symbiotic stars.Comment: 22 pages, Tex, 15 postscript figuer
Compensated Half-metallicity in the Trigonally Distorted Perovskite-type NiCrO
Using first principles calculations, we investigate the electronic and
magnetic properties of the trigonally distorted (R-3c) perovskite-derived
NiCrO. Within the local spin density approximation (LSDA), our calculations
show that this system is an exactly compensated half-metal (CHM). The local
spin moments of Cr 2.04, and antialigned Ni -1.41 and three oxygens -0.63 (in
the units of ), indicate high spin S=3/2 Cr and S=3/2
(NiO) units. Considering reasonable values of the on-site Coulomb
repulsion U on both Ni and Cr ions with LDA+U approach, this system becomes an
insulator (as reported by Chamberland and Cloud) having a narrow gap in the
spin-up channel, whereas the other channel has a large gap of ~3 eV. Although
inclusion of U seemingly leads to the transition Ni^{3+}_3$ and provide another route to search for
CHM, which is a property sought by many.Comment: 5 pages, 5 embedded figures, (To be published in PRB rapid Commun.
Charge and Spin Ordering in Insulating NaCoO: Effects of Correlation and Symmetry
Ab initio band theory including correlations due to intra-atomic repulsion is
applied to study charge disproportionation and charge- and spin-ordering in
insulating NaCoO. Various ordering patterns (zigzag and two
striped) for four-Co supercells are analyzed before focusing on the observed
"out-of-phase stripe" pattern of antiferromagnetic Co spins along
charge-ordered stripes. This pattern relieves frustration and shows distinct
analogies with the cuprate layers: a bipartite lattice of antialigned spins,
with axes at 90 degree angles. Substantial distinctions with cuprates are also
discussed, including the tiny gap of a new variant of "charge transfer" type
within the Co 3d system.Comment: 5 pages, 3 figure
Charge Disproportionation and Spin Ordering Tendencies in Na(x)CoO2
The strength and effect of Coulomb correlations in the (superconducting when
hydrated) x~1/3 and ``enhanced'' x~2/3 regimes of Na(x)CoO2 are evaluated using
the correlated band theory LDA+U method. Our results, neglecting quantum
fluctuations, are: (1) allowing only ferromagnetic order, there is a critical
U_c = 3 eV, above which charge disproportionation occurs for both x=1/3 and
x=2/3, (2) allowing antiferromagnetic order at x=1/3, U_c drops to 1 eV for
disproportionation, (3) disproportionation and gap opening occur
simultaneously, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic
coupling is favored over ferromagnetic, while below U_c ferromagnetism is
favored. Comparison of the calculated Fermi level density of states compared to
reported linear specific heat coefficients indicates enhancement of the order
of five for x~0.7, but negligible enhancement for x~0.3. This trend is
consistent with strong magnetic behavior and local moments (Curie-Weiss
susceptibility) for x>0.5 while there no magnetic behavior or local moments
reported for x<0.5. We suggest that the phase diagram is characterized by a
crossover from effective single-band character with U >> W for x>0.5 into a
three-band regime for x U_eff <= U/\sqrt(3) ~ W and
correlation effects are substantially reduced.Comment: 10 pages, 8 figures, corrected a few typos and changed reference
Chemical Differences between K and Na in Alkali Cobaltates
KCoO shares many similarities with NaCoO, as well as some
important differences (no hydration-induced superconductivity has been
reported). At =20 K, KCoO becomes an insulator with a tiny
optical gap as happens in NaCoO at 52 K. This similarity, with a
known common structure, enables direct comparisons to be made. Using the
K-zigzag structure recently reported and the local density approximation, we
compare and contrast these cobaltates at x=0.5. Although the electronic
structures are quite similar as expected, substantial differences are observed
near the Fermi level. These differences are found to be attributable mostly to
the chemical, rather than structural difference: although Na is normally
considered to be fully ion, K has somewhat more highly ionic character than
does Na in these cobaltates.Comment: 5 paper
Boron Spectral Density and Disorder Broadening in B-doped Diamond
Comparison of periodic B dopants with a random alloy of substitional boron in
diamond is carried out using several supercells and the coherent potential
approximation (CPA) for the random alloy case. The main peak in the B local
density of states is shifted to lower binding energy compared to the
corresponding C peak in intrinsic diamond. In supercells, this shows up as
strongly B-character bands split from bulk C bands away from the zone center,in
an energy region around -1 eV. Even for a 4*4*4 supercell (BC), effects
of the dopant order are evident in the form of primarily B-character bands just
below the Fermi level at the supercell zone boundary. The bands resulting from
the CPA are of continuous mixed C-B character. They resemble virtual crystal
bands, but broadened somewhat reflecting the disorder-induced lifetime, and are
consistent with angle-resolved photoemission band maps. The B character is 1.7
times larger than for C (per atom) near the top of the valence bands for CPA,
and roughly the same for supercells. CPA results are particularly useful since
they characterize the wavevector and energy dependence of disorder broadening.Comment: 8 pages and 9 embedded figures (To appear in PRB
"Personality and Earnings"
This paper studies personality as a potential explanation for wage differentials between apparently similar workers. This follows initial studies by Jencks (1979) that suggest that certain personality traits, such as industriousness and leadership, have an impact on earnings. The paper aims to provide a theoretical framework within which these effects may be analyzed. The study begins by outlining four issues as a backdrop to the model: rationality, the industry, firms, and workers. A crucial factor to the model is the memeÑa mental gene that affects personality. Taking these four factors into consideration, the Contested Exchange model from Bowles and Gintis (1990) is used. Then, it is adapted to study memetic effects on the wage rate. This is followed by an analysis of how memes may affect personality and thus earnings. The issues that require further study and resolution are 1) which traits create wage differentials, and 2) two-way causality: does personality affect the wage, or does a wage premium become an incentive for a person to adopt new memes?
Upper stage in-flight retargeting to enhance geosynchronous satellite operations
Real time utilization of propellant reserves that are not needed is available with the implementation of the in-flight retargeting capability for the Centaur Upper Stage. Application to a performance critical, geosynchronous mission is discussed. The operational duration of the satellite may be increased by selectively choosing the appropriate final orbit injection conditions. During ascent Centaur evaluates the amount of propellant excess available and adjusts the final orbit target to consume the excess. Typical satellite mission requirements are introduced to illustrate the mission analysis process to determine the pre-flight nominal target and the in-flight retarget function
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