5,091 research outputs found
Absorption spectrum of iron in the vacuum ultraviolet 2950 - 1588 angstrom
Absorption spectrum of iron in vacuum ultraviole
Local molecular field theory for the treatment of electrostatics
We examine in detail the theoretical underpinnings of previous successful
applications of local molecular field (LMF) theory to charged systems. LMF
theory generally accounts for the averaged effects of long-ranged components of
the intermolecular interactions by using an effective or restructured external
field. The derivation starts from the exact Yvon-Born-Green hierarchy and shows
that the approximation can be very accurate when the interactions averaged over
are slowly varying at characteristic nearest-neighbor distances. Application of
LMF theory to Coulomb interactions alone allows for great simplifications of
the governing equations. LMF theory then reduces to a single equation for a
restructured electrostatic potential that satisfies Poisson's equation defined
with a smoothed charge density. Because of this charge smoothing by a Gaussian
of width sigma, this equation may be solved more simply than the detailed
simulation geometry might suggest. Proper choice of the smoothing length sigma
plays a major role in ensuring the accuracy of this approximation. We examine
the results of a basic confinement of water between corrugated wall and justify
the simple LMF equation used in a previous publication. We further generalize
these results to confinements that include fixed charges in order to
demonstrate the broader impact of charge smoothing by sigma. The slowly-varying
part of the restructured electrostatic potential will be more symmetric than
the local details of confinements.Comment: To be published in J Phys-Cond Matt; small misprint corrected in Eq.
(12) in V
A new approach for efficient simulation of Coulomb interactions in ionic fluids
We propose a simplified version of local molecular field (LMF) theory to
treat Coulomb interactions in simulations of ionic fluids. LMF theory relies on
splitting the Coulomb potential into a short-ranged part that combines with
other short-ranged core interactions and is simulated explicitly. The averaged
effects of the remaining long-ranged part are taken into account through a
self-consistently determined effective external field. The theory contains an
adjustable length parameter sigma that specifies the cut-off distance for the
short-ranged interaction. This can be chosen to minimize the errors resulting
from the mean-field treatment of the complementary long-ranged part. Here we
suggest that in many cases an accurate approximation to the effective field can
be obtained directly from the equilibrium charge density given by the Debye
theory of screening, thus eliminating the need for a self-consistent treatment.
In the limit sigma -> 0, this assumption reduces to the classical Debye
approximation. We examine the numerical performance of this approximation for a
simple model of a symmetric ionic mixture. Our results for thermodynamic and
structural properties of uniform ionic mixtures agree well with similar results
of Ewald simulations of the full ionic system. In addition we have used the
simplified theory in a grand-canonical simulation of a nonuniform ionic mixture
where an ion has been fixed at the origin. Simulations using short-ranged
truncations of the Coulomb interactions alone do not satisfy the exact
condition of complete screening of the fixed ion, but this condition is
recovered when the effective field is taken into account. We argue that this
simplified approach can also be used in the simulations of more complex
nonuniform systems.Comment: To be published in Journal of Chemical Physic
Properties of cage rearrangements observed near the colloidal glass transition
We use confocal microscopy to study the motions of particles in concentrated
colloidal systems. Near the glass transition, diffusive motion is inhibited, as
particles spend time trapped in transient ``cages'' formed by neighboring
particles. We measure the cage sizes and lifetimes, which respectively shrink
and grow as the glass transition approaches. Cage rearrangements are more
prevalent in regions with lower local concentrations and higher disorder.
Neighboring rearranging particles typically move in parallel directions,
although a nontrivial fraction move in anti-parallel directions, usually from
pairs of particles with initial separations corresponding to the local maxima
and minima of the pair correlation function , respectively.Comment: 5 pages, 4 figures; text & figures revised in v
The United States Chiropractic Workforce: An alternative or complement to primary care?
UnlabelledBackgroundIn the United States (US) a shortage of primary care physicians has become evident. Other health care providers such as chiropractors might help address some of the nation's primary care needs simply by being located in areas of lesser primary care resources. Therefore, the purpose of this study was to examine the distribution of the chiropractic workforce across the country and compare it to that of primary care physicians.MethodsWe used nationally representative data to estimate the per 100,000 capita supply of chiropractors and primary care physicians according to the 306 predefined Hospital Referral Regions. Multiple variable Poisson regression was used to examine the influence of population characteristics on the supply of both practitioner-types.ResultsAccording to these data, there are 74,623 US chiropractors and the per capita supply of chiropractors varies more than 10-fold across the nation. Chiropractors practice in areas with greater supply of primary care physicians (Pearson's correlation 0.17, p-value < 0.001) and appear to be more responsive to market conditions (i.e. more heavily influenced by population characteristics) in regards to practice location than primary care physicians.ConclusionThese findings suggest that chiropractors practice in areas of greater primary care physician supply. Therefore chiropractors may be functioning in more complementary roles to primary care as opposed to an alternative point of access
From Discrete Hopping to Continuum Modeling on Vicinal Surfaces with Applications to Si(001) Electromigration
Coarse-grained modeling of dynamics on vicinal surfaces concentrates on the
diffusion of adatoms on terraces with boundary conditions at sharp steps, as
first studied by Burton, Cabrera and Frank (BCF). Recent electromigration
experiments on vicinal Si surfaces suggest the need for more general boundary
conditions in a BCF approach. We study a discrete 1D hopping model that takes
into account asymmetry in the hopping rates in the region around a step and the
finite probability of incorporation into the solid at the step site. By
expanding the continuous concentration field in a Taylor series evaluated at
discrete sites near the step, we relate the kinetic coefficients and
permeability rate in general sharp step models to the physically suggestive
parameters of the hopping models. In particular we find that both the kinetic
coefficients and permeability rate can be negative when diffusion is faster
near the step than on terraces. These ideas are used to provide an
understanding of recent electromigration experiment on Si(001) surfaces where
step bunching is induced by an electric field directed at various angles to the
steps.Comment: 10 pages, 4 figure
Attraction Between Like-Charged Walls: Short-Ranged Simulations Using Local Molecular Field Theory
Effective attraction between like-charged walls mediated by counterions is
studied using local molecular field (LMF) theory. Monte Carlo simulations of
the "mimic system'' given by LMF theory, with short-ranged "Coulomb core"
interactions in an effective single particle potential incorporating a
mean-field average of the long-ranged Coulomb interactions, provide a direct
test of the theory, and are in excellent agreement with more complex
simulations of the full Coulomb system by Moreira and Netz [Eur. Phys. J. E 8,
33 (2002)]. A simple, generally-applicable criterion to determine the
consistency parameter sigma_{min} needed for accurate use of the LMF theory is
presented
Circles in the Sky: Finding Topology with the Microwave Background Radiation
If the universe is finite and smaller than the distance to the surface of
last scatter, then the signature of the topology of the universe is writ large
on the microwave background sky. We show that the microwave background will be
identified at the intersections of the surface of last scattering as seen by
different ``copies'' of the observer. Since the surface of last scattering is a
two-sphere, these intersections will be circles, regardless of the background
geometry or topology. We therefore propose a statistic that is sensitive to all
small, locally homogeneous topologies. Here, small means that the distance to
the surface of last scatter is smaller than the ``topology scale'' of the
universe.Comment: 14 pages, 10 figures, IOP format. This paper is a direct descendant
of gr-qc/9602039. To appear in a special proceedings issue of Class. Quant.
Grav. covering the Cleveland Topology & Cosmology Worksho
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