8,719 research outputs found
Step energies and equilibrium shape of strained monolayer islands
Using a simple atomistic model of anharmonic nearest-neighbors interaction,
we have calculated the step energies of strained hexagonal monolayer islands.
These have been found to decrease with the absolute value of the misfit due to
the strain relaxation at steps. The effect is significantly more pronounced in
the case of positive misfit owing to the stronger repulsive interatomic forces.
Furthermore, (111)-faceted steps are favored at positive misfit (compressed
islands) and, to a lesser extent, (100)-faceted steps at negative misfits
(tensile islands). The result is rationalized in terms of the different bonding
geometries at step edges and a comparison with experiments is included. Thus,
the equilibrium shape transforms from regular hexagons at zero misfit to
threefold symmetric hexagons with increasing misfit.Comment: 6 pages, 5 figures, 1 table. Improved, finally accepted version
including a new figure, a new table and several minor modifications resulting
from discussions with referee
Seeing, Wind and Outer Scale Effects on Image Quality at the Magellan Telescopes
We present an analysis of the science image quality obtained on the twin 6.5
metre Magellan telescopes over a 1.5 year period, using images of ~10^5 stars.
We find that the telescopes generally obtain significantly better image quality
than the DIMM-measured seeing. This is qualitatively consistent with
expectations for large telescopes, where the wavefront outer scale of the
turbulence spectrum plays a significant role. However, the dominant effect is
found to be wind speed with Magellan outperforming the DIMMs most markedly when
the wind is strongest. Excluding data taken during strong wind conditions (>10
m/s), we find that the Magellan telescopes still significantly outperform the
DIMM seeing, and we estimate the site to have L_0 ~ 25 m on average. We also
report on the first detection of a negative bias in DIMM data. This is found to
occur, as predicted, when the DIMM is affected by certain optical aberrations
and the turbulence profile is dominated by the upper layers of the atmosphere.Comment: Accepted for publication in PASP. 10 pages, 12 figures
Stability of strained heteroepitaxial systems in (1+1) dimensions
We present a simple analytical model for the determination of the stable
phases of strained heteroepitaxial systems in (1+1) dimensions. In order for
this model to be consistent with a subsequent dynamic treatment, all
expressions are adjusted to an atomistic Lennard-Jones system. Good agreement
is obtained when the total energy is assumed to consist of two contributions:
the surface energy and the elastic energy. As a result, we determine the stable
phases as a function of the main ``control parameters'' (binding energies,
coverage and lattice mismatch). We find that there exists no set of parameters
leading to an array of islands as a stable configuration. We however show that
a slight modification of the model can lead to the formation of stable arrays
of islands.Comment: 11 pages, 14 figures, submitted to Physical Review
Nesting Induced Peierls-type Instability for Compressed Li-CI16
Alkalies are considered to be simple metals at ambient conditions. However,
recently reported theoretical and experimental results have shown an unexpected
and intriguing correlation between complex structures and an enhanced
superconducting transition temperature in lithium under pressure. In this
article we analyze the pressure induced Fermi surface deformation in bcc
lithium, and its relation to the observed cI16 structure. According to our
calculations, the Fermi surface becomes increasingly anisotropic with pressure
and develops an extended nesting along the bcc [121] direction. This nesting
induces a phonon instability of both transverse modes at N, so that a
Peierls-type mechanism is proposed to explain the stability of Li-cI16.Comment: Proceedings of Fukuoka 2006 Conference on Novel Pressure-induced
Phenomena in Condensed Matter Systems. To be published in J. Phys. Soc. Jpn.
2 pages and 3 figure
The initial-final mass relationship of white dwarfs in common proper motion pairs
A promising approach to decrease the uncertainties in the initial-final mass relationship, which is still poorly constrained, is to study white dwarfs for which external constraints are available, for instance, white dwarfs in common proper motion pairs (CPMPs). Important information of the white dwarf can be inferred from the study of the companion, since they were born at the same time and with the same initial chemical composition. In this contribution, we report new results obtained from spectroscopic observations of both members of several CPMPs composed of a F, G or K type star and a DA white dwarf
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