23 research outputs found
Benchmark density functional theory calculations for nano-scale conductance
We present a set of benchmark calculations for the Kohn-Sham elastic
transmission function of five representative single-molecule junctions. The
transmission functions are calculated using two different density functional
theory (DFT) methods, namely an ultrasoft pseudopotential plane wave code in
combination with maximally localized Wannier functions, and the norm-conserving
pseudopotential code Siesta which applies an atomic orbital basis set. For all
systems we find that the Siesta transmission functions converge toward the
plane-wave result as the Siesta basis is enlarged. Overall, we find that an
atomic basis with double-zeta and polarization is sufficient (and in some cases
even necessary) to ensure quantitative agreement with the plane-wave
calculation. We observe a systematic down shift of the Siesta transmission
functions relative to the plane-wave results. The effect diminishes as the
atomic orbital basis is enlarged, however, the convergence can be rather slow.Comment: 10 pages, 7 figure
Close Pairs as Probes of the Galaxy's Chemical Evolution
Understanding the galaxy in which we live is one of the great intellectual
challenges facing modern science. With the advent of high quality observational
data, the chemical evolution modeling of our galaxy has been the subject of
numerous studies in the last years. However, all these studies have one missing
element which is the evolution of close binaries. Reason: their evolution is
very complex and single stars only perhaps can do the job. (Un)Fortunately at
present we know that a significant fraction of the observed intermediate mass
and massive stars are members of a binary or multiple system and that certain
objects can only be formed through binary evolution. Therefore galactic studies
that do not account for close binaries may be far from realistic. We
implemented a detailed binary population in a galactic chemical evolutionary
model. Notice that this is not something simple like replacing chemical yields.
Here we discuss three topics: the effect of binaries on the evolution of 14N,
the evolution of the type Ia supernova rate and the effects on the G-dwarf
distribution, the link between the evolution of the r-process elements and
double neutron star mergers (candidates of short gamma-ray burst objects).Comment: 10 pages, 5 figures, Invited paper at IAUS240, IAUXXVI GA Pragu
Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature with
detailed chemical abundance information (spanning -4.2 < [Fe/H] < +0.3), and we
compute their space velocities and Galactic orbital parameters. We identify
members of the inner and outer stellar halo populations in our sample based
only on their kinematic properties and then compare the abundance ratios of
these populations as a function of [Fe/H]. In the metallicity range where the
two populations overlap (-2.5 < [Fe/H] < -1.5), the mean [Mg/Fe] of the outer
halo is lower than the inner halo by ~0.1 dex. For [Ni/Fe] and [Ba/Fe], the
star-to-star abundance scatter of the inner halo is consistently smaller than
in the outer halo. The [Na/Fe], [Y/Fe], [Ca/Fe], and [Ti/Fe] ratios of both
populations show similar means and levels of scatter. Our inner halo population
is chemically homogeneous, suggesting that a significant fraction of the Milky
Way stellar halo originated from a well-mixed ISM. In contrast, our outer halo
population is chemically diverse, suggesting that another significant fraction
of the Milky Way stellar halo formed in remote regions where chemical
enrichment was dominated by local supernova events. We find no abundance trends
with maximum radial distance from the Galactic center or maximum vertical
distance from the Galactic disk. We also find no common kinematic signature for
groups of metal-poor stars with peculiar abundance patters, such as the
alpha-poor stars or stars showing unique neutron-capture enrichment patterns.
Several stars and dSph systems with unique abundance patterns spend the
majority of their time in the distant regions of the Milky Way stellar halo,
suggesting that the true outer halo of the Galaxy may have little resemblance
to the local stellar halo.Comment: Accepted for publication in AJ. Full tables available upon reques