39,471 research outputs found
Kinetic Monte Carlo simulation of faceted islands in heteroepitaxy using multi-state lattice model
A solid-on-solid model is generalized to study the formation of Ge pyramid
islands bounded by (105) facets on Si(100) substrates in two dimensions. Each
atomic column is not only characterized by the local surface height but also by
two deformation state variables dictating the local surface tilt and vertical
extension. These deformations phenomenologically model surface reconstructions
in (105) facets and enable the formation of islands which better resemble
faceted pyramids. We demonstrate the model by application to a kinetic limited
growth regime. We observe significantly reduced growth rates after faceting and
a continuous nucleation of new islands until overcrowding occurs.Comment: 7 pages, 5 figure
Enhanced 3-epi-25-hydroxyvitamin D3 signal leads to overestimation of its concentration and amplifies interference in 25-hydroxyvitamin D LC-MS/MS assays
Background 3-epi-25-hydroxyvitamin D3 (3-epi-25OHD3) interferes in most liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays for 25-hydroxyvitamin D (25OHD). The clinical significance of this is unclear, with concentrations from undetectable to 230 nmol/L reported. Many studies have quantified 3-epi-25OHD3 based on 25OHD3 calibrators or other indirect methods, and we speculated that this contributes to the observed variability in reported 3-epi-25OHD3 concentrations.
Methods We compared continuous MS/MS infusions of 3-epi-25OHD3 and 25OHD3 solutions, spiked both analytes into the same serum matrix and analysed patient samples to assess the effect of three different quantitation methods on 3-epi-25OHD3 concentration. Experiments were performed on an LC-MS/MS system using a phenyl column which does not resolve 3-epi-25OHD3, and a modified method utilizing a Zorbax SB-CN column that chromatographically resolves 3-epi-25OHD3 from 25OHD3.
Results A greater 3-epi-25OHD3 signal, compared with 25OHD3, was observed during equimolar post-column continuous infusion of analyte solutions, and following analysis of a serum pool spiked with both analytes. 3-epi-25OHD3 signal enhancement was dependent on mobile phase composition. Compared with 3-epi-25OHD3 calibrators, indirect quantitation methods resulted in up to 10 times as many samples having 3-epi-25OHD3 concentrations ≥ 10 nmol/L, and an approximately fourfold increase in the maximum observed 3-epi-25OHD3 concentration to 95 nmol/L.
Conclusions Enhanced 3-epi-25OHD3 signal leads to overestimation of its concentrations in the indirect quantitation methods used in many previous studies. The enhanced signal may contribute to greater interference in some 25OHD LC-MS/MS assays than others. We highlight that equimolar responses cannot be assumed in LC-MS/MS systems, even if two molecules are structurally similar
DEIMOS Observations of WISE-Selected, Optically Obscured AGNs
While there are numerous criteria for photometrically identifying active
galactic nuclei (AGNs), searches in the optical and UV tend to exclude galaxies
that are highly dust obscured. This is problematic for constraining models of
AGN evolution and estimating the AGN contribution to the cosmic X-ray and IR
backgrounds, as highly obscured objects tend to be underrepresented in
large-scale surveys. To address this, we identify potentially obscured AGNs
using mid-IR color colors from the Wide-field Infrared Survey Explorer (WISE)
catalog. This paper presents the results of optical spectroscopy of obscured
AGN candidates using Keck DEIMOS, and their physical properties derived from
these spectra. We find that a color criterion effectively selects
AGNs with a higher median level of extinction compared to the AGNs
found in the SDSS DR7 survey. This optical extinction can be measured using SED
modeling or by using as a measure of optical to IR flux. We find that
specific, targeted observations are necessary to find the most highly optically
obscured AGNs, and that additional far-IR photometry is necessary to further
constrain the dust properties of these AGNs.Comment: 20 pages, 25 figures, accepted by MNRA
Phase field modelling of surfactants in multi-phase flow
A diffuse interface model for surfactants in multi-phase flow with three or
more fluids is derived. A system of Cahn-Hilliard equations is coupled with a
Navier-Stokes system and an advection-diffusion equation for the surfactant
ensuring thermodynamic consistency. By an asymptotic analysis the model can be
related to a moving boundary problem in the sharp interface limit, which is
derived from first principles. Results from numerical simulations support the
theoretical findings. The main novelties are centred around the conditions in
the triple junctions where three fluids meet. Specifically the case of local
chemical equilibrium with respect to the surfactant is considered, which allows
for interfacial surfactant flow through the triple junctions
On Random Walks with a General Moving Barrier
Random walks with a general, nonlinear barrier have found recent applications
ranging from reionization topology to refinements in the excursion set theory
of halos. Here, we derive the first-crossing distribution of random walks with
a moving barrier of an arbitrary shape. Such a distribution is shown to satisfy
an integral equation that can be solved by a simple matrix inversion, without
the need for Monte Carlo simulations, making this useful for exploring a large
parameter space. We discuss examples in which common analytic approximations
fail, a failure which can be remedied using the method described here.Comment: 6 pages, 7 figures, submitted to Ap
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