12,494 research outputs found
Calculation of Dielectric Susceptibility for Complex Ionic Systems: Application to a Predicted Superlattice
The Gordon-Kim theory of interionic interactions between closed-shell ions, which has had considerable success for existing alkali halide systems, has been applied to the determination of the structure, phonon dispersion, and room-temperature dielectric properties of a proposed superlattice, RbF(NaC1)2, of alkali halides. The dielectric properties of such a hypothetical material can have an unusually strong low-frequency dependence because of the presence of Brillouin-zonefolded infrared”-active modes. In evaluating the standard expressions for the dielectric properties, in the rigid-ion approximation, we make use of standard cubic and quartic anharmonic terms in addition to the instantaneous-phonon cubic anharmonic term, which is needed because of the lack of a center of inversion. Our procedure for numerically calculating the absorption for complex materials is described in detail. Effects of off-diagonal terms in the Green\u27s function and of the long-range ionic Coulomb interactions are examined. In particular, an absorption peak at 30 cm-1 is found to be especially sensitive to these contributions. This paper demonstrates (a) the theoretical existence of metastable phases of an alkali halide superlattice and (b) the application of anharmonic perturbation theory to determine the dielectric response in the infrared and millimeter-wave region for such complex materials. The particular structure studied is just one of many possibilities in this class of materials, which offers the opportunity for joint theoretical and experimental research to fabricate structures with properties tailored for specific applications
Weakly nonlinear theory of grain boundary motion in patterns with crystalline symmetry
We study the motion of a grain boundary separating two otherwise stationary
domains of hexagonal symmetry. Starting from an order parameter equation
appropriate for hexagonal patterns, a multiple scale analysis leads to an
analytical equation of motion for the boundary that shares many properties with
that of a crystalline solid. We find that defect motion is generically opposed
by a pinning force that arises from non-adiabatic corrections to the standard
amplitude equation. The magnitude of this force depends sharply on the
mis-orientation angle between adjacent domains so that the most easily pinned
grain boundaries are those with an angle between four and eight degrees.
Although pinning effects may be small, they do not vanish asymptotically near
the onset of this subcritical bifurcation, and can be orders of magnitude
larger than those present in smectic phases that bifurcate supercritically
The effects of bioturbation on the initiation of motion of intertidal sands
The results of laboratory flume experiments on initiation of sediment motion in natural marine sediments (fine sand) are reported. Sediment cores were taken from an intertidal region during all seasons of the year over a four-year period…
On the nature of the most obscured C-rich AGB stars in the Magellanic Clouds
The stars in the Magellanic Clouds with the largest degree of obscuration are
used to probe the highly uncertain physics of stars in the asymptotic giant
branch (AGB) phase of evolution. Carbon stars in particular, provide key
information on the amount of third dredge-up (TDU) and mass loss. We use two
independent stellar evolution codes to test how a different treatment of the
physics affects the evolution on the AGB. The output from the two codes are
used to determine the rates of dust formation in the circumstellar envelope,
where the method used to determine the dust is the same for each case. The
stars with the largest degree of obscuration in the LMC and SMC are identified
as the progeny of objects of initial mass and , respectively. This difference in mass is motivated by the
difference in the star formation histories of the two galaxies, and offers a
simple explanation of the redder infrared colours of C-stars in the LMC
compared to their counterparts in the SMC. The comparison with the Spitzer
colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass
fraction must have been reached by stars of initial
mass around . Our results confirm the necessity of adopting
low-temperature opacities in stellar evolutionary models of AGB stars. These
opacities allow the stars to obtain mass-loss rates high enough () to produce the amount of dust needed to reproduce the
Spitzer coloursComment: 14 pages, 5 figures, 1 table; accepted for publication in MNRAS Main
Journa
A Dramatic Decrease in Carbon Star Formation in M31
We analyze resolved stellar near-infrared photometry of 21 HST fields in M31
to constrain the impact of metallicity on the formation of carbon stars.
Observations of nearby galaxies show that the carbon stars are increasingly
rare at higher metallicity. Models indicate that carbon star formation
efficiency drops due to the decrease in dredge-up efficiency in metal-rich
thermally-pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher
initial abundance of oxygen. However, while models predict a metallicity
ceiling above which carbon stars cannot form, previous observations have not
yet pinpointed this limit. Our new observations reliably separate carbon stars
from M-type TP-AGB stars across 2.6-13.7 kpc of M31's metal-rich disk using HST
WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M)
decreases more rapidly than extrapolations of observations in more metal-poor
galaxies, resulting in a C/M that is too low by more than a factor of 10 in the
innermost fields and indicating a dramatic decline in C star formation
efficiency at metallicities higher than [M/H] -0.1 dex. The
metallicity ceiling remains undetected, but must occur at metallicities higher
than what is measured in M31's inner disk ([M/H] +0.06 dex).Comment: 16 pages, 13 Figures; text clarifications in response to the referee.
Results are unchanged; accepted for publication in Ap
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African religious ministers’ transition from expatriation to migration: the role of world-view
Purpose - This article explores the contextual determinants of the move from expatriation into
migration among ministers of religion originating from the developing world.
Design/methodology/approach - We used in-depth analysis of narratives of four African religious
ministers working in France, plus interviews with their five superiors and three host country national
colleagues.
Findings - The findings point to personal-level, organisational-level and country-level contextual
determinants, which come into play as levers or barriers in the ‘expatriation into migration’ process.
Originality/value - Our study develops a theoretical framework which points to the positive and
negative influence of three-layered contextual determinants on how expatriated low-status church
ministers from the developing world become migrants. We found a so far unreported determinant of
the personal context: the role of a world-view: very visible as ‘God centrality’ in our participants.
Results also shed new light on the international careers of this overlooked category of ‘non-traditional
expatriates’ from Africa
The Large Magellanic Cloud as a laboratory for Hot Bottom Burning in massive Asymptotic Giant Branch stars
We use Spitzer observations of the rich population of Asymptotic Giant Branch
stars in the Large Magellanic Cloud (LMC) to test models describing the
internal structure and nucleosynthesis of the most massive of these stars, i.e.
those with initial mass above . To this aim, we compare
Spitzer observations of LMC stars with the theoretical tracks of Asymptotic
Giant Branch models, calculated with two of the most popular evolution codes,
that are known to differ in particular for the treatment of convection.
Although the physical evolution of the two models are significantly different,
the properties of dust formed in their winds are surprisingly similar, as is
their position in the colour-colour (CCD) and colour-magnitude (CMD) diagrams
obtained with the Spitzer bands. This model independent result allows us to
select a well defined region in the () plane,
populated by AGB stars experiencing Hot Bottom Burning, the progeny of stars
with mass . This result opens up an important test of the
strength hot bottom burning using detailed near-IR (H and K bands)
spectroscopic analysis of the oxygen-rich, high luminosity candidates found in
the well defined region of the colour-colour plane. This test is possible
because the two stellar evolution codes we use predict very different results
for the surface chemistry, and the C/O ratio in particular, owing to their
treatment of convection in the envelope and of convective boundaries during
third dredge-up. The differences in surface chemistry are most apparent when
the model stars reach the phase with the largest infrared emission.Comment: 11 pages, 14 figures, accepted for publication in MNRA
Grover's Quantum Search Algorithm for an Arbitrary Initial Mixed State
The Grover quantum search algorithm is generalized to deal with an arbitrary
mixed initial state. The probability to measure a marked state as a function of
time is calculated, and found to depend strongly on the specific initial state.
The form of the function, though, remains as it is in the case of initial pure
state. We study the role of the von Neumann entropy of the initial state, and
show that the entropy cannot be a measure for the usefulness of the algorithm.
We give few examples and show that for some extremely mixed initial states
carrying high entropy, the generalized Grover algorithm is considerably faster
than any classical algorithm.Comment: 4 pages. See http://www.cs.technion.ac.il/~danken/MSc-thesis.pdf for
extended discussio
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