4,823 research outputs found
Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study
A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented
for the concentration range that spans the ferromagnetic-insulator (FMI) to
ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are
insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22.
All samples are ferromagnetic although the saturation magnetization for the 16%
Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI
samples have similar correlations between changes in the local Mn-O distortions
and the magnetization as observed previously for the colossal magnetoresistance
(CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully
magnetized. The data show that there are at least two distinct types of
distortions. The initial distortions removed as the insulating sample becomes
magnetized are small and provides direct evidence that roughly 50% of the Mn
sites have a small distortion/site and are magnetized first. The large
remaining Mn-O distortions at low T are attributed to a small fraction of
Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or
unmagnetized. Thus the insulating samples are very similar to the behavior of
the CMR samples up to the point at which the M/I transition occurs for the CMR
materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of
the sample is magnetic, implies that there must be preferred magnetized Mn
sites and that such sites do not percolate at these concentrations.Comment: 27 pages, 8 figures, to be submitted to Phys. Rev.
Electromagnetic Casimir piston in higher dimensional spacetimes
We consider the Casimir effect of the electromagnetic field in a higher
dimensional spacetime of the form , where is the
4-dimensional Minkowski spacetime and is an -dimensional
compact manifold. The Casimir force acting on a planar piston that can move
freely inside a closed cylinder with the same cross section is investigated.
Different combinations of perfectly conducting boundary conditions and
infinitely permeable boundary conditions are imposed on the cylinder and the
piston. It is verified that if the piston and the cylinder have the same
boundary conditions, the piston is always going to be pulled towards the closer
end of the cylinder. However, if the piston and the cylinder have different
boundary conditions, the piston is always going to be pushed to the middle of
the cylinder. By taking the limit where one end of the cylinder tends to
infinity, one obtains the Casimir force acting between two parallel plates
inside an infinitely long cylinder. The asymptotic behavior of this Casimir
force in the high temperature regime and the low temperature regime are
investigated for the case where the cross section of the cylinder in is
large. It is found that if the separation between the plates is much smaller
than the size of , the leading term of the Casimir force is the
same as the Casimir force on a pair of large parallel plates in the
-dimensional Minkowski spacetime. However, if the size of
is much smaller than the separation between the plates, the leading term of the
Casimir force is times the Casimir force on a pair of large parallel
plates in the 4-dimensional Minkowski spacetime, where is the first Betti
number of . In the limit the manifold vanishes, one
does not obtain the Casimir force in the 4-dimensional Minkowski spacetime if
is nonzero.Comment: 22 pages, 4 figure
Optimal control problems arising in the zinc sulphate electrolyte purification process
Before zinc electrolysis, zinc powder is added to the zinc sulphate electrolyte solution to facilitate the removal of harmful metallic ions. This purification process can be modeled by a time delay differential equation. Since some of the parameters in this model are unknown, zinc powder is normally added excessively. We use an optimization technique to estimate the unknown parameters from experimental data. Then, we formulate an optimal control problem to minimize the amount of zinc powder added. We solve this optimal control problem numerically by using the control parametrization method. The results indicate that the amount of zinc powder added can be decreased, on average, by approximately 7%
Finite Temperature Casimir Effect in Randall-Sundrum Models
The finite temperature Casimir effect for a scalar field in the bulk region
of the two Randall-Sundrum models, RSI and RSII, is studied. We calculate the
Casimir energy and the Casimir force for two parallel plates with separation
on the visible brane in the RSI model. High-temperature and low-temperature
cases are covered. Attractiveness versus repulsiveness of the temperature
correction to the force is discussed in the typical special cases of
Dirichlet-Dirichlet, Neumann-Neumann, and Dirichlet-Neumann boundary conditions
at low temperature. The Abel-Plana summation formula is made use of, as this
turns out to be most convenient. Some comments are made on the related
contemporary literature.Comment: 33 pages latex, 2 figures. Some changes in the discussion. To appear
in New J. Phy
Grad's Distribution Function for 13 Moments based Moment Gas Kinetic Solver for Steady and Unsteady Rarefied flows: Discrete and Explicit Forms
Efficient modeling of rarefied flow has drawn widespread interest for
practical engineering applications. In the present work, we proposed the Grad's
distribution function for 13 moments-based moment gas kinetic solver (G13-MGKS)
and the macroscopic governing equations are derived based on the moment
integral of discrete Boltzmann equation in the finite volume framework.
Numerical fluxes at the cell interface related to the macroscopic variables,
stress and heat flux can be reconstructed from the Boltzmann integration
equation at surrounding points of the cell interface directly, so the
complicated partial differential equations with tedious implementation of
boundary conditions in the moment method can be avoided. Meanwhile, the
explicit expression of numerical fluxes is proposed, which could release the
present solver the from the discretization and numerical summation in molecular
velocity space. To evaluate the Grad's distribution function for 13 moments in
the present framework, the G13-MGKS with the discrete and explicit form of
numerical fluxes are examined by several test cases covering the steady and
unsteady rarefied flows. Numerical results indicate that the G13-MGKS could
simulate continuum flows accurately and present reasonable prediction for
rarefied flows at moderate Knudsen number. Moreover, the tests of computations
and memory costs demonstrate that the present framework could preserve the
highly efficient feature
Monitoring oxide quality using the spread of the dC/dV peak in scanning capacitance microscopy measurements
This article proposes a method for evaluating the quality of the overlying oxide on samples used in scanning capacitance microscopy (SCM) dopant profile extraction. The method can also be used generally as a convenient in-process method for monitoring oxide quality directly after the oxidation process without prior metallization of the oxide-semiconductor sample. The spread of the differential capacitance characteristic (dC/dV versus V plot), characterized using its full width at half maximum (FWHM), was found to be strongly dependent on the interface trap density as a consequence of the stretch-out effect of interface traps on the capacitance-voltage (C-V) curve. Results show that the FWHM of the dC/dV characteristic is a sensitive monitor of oxide quality (in terms of interface trap density) as it is not complicated by localized oxide charging effects as in the case of the SCM probe tip voltage corresponding to maximum dC/dV. The magnitude of the dC/dV peak, at any given surface potential, was also found to be independent of the interface traps and only dependent on the substrate dopant concentration, which makes SCM dopant profile extraction possible
Habitat use and movement of the mummichog (Fundulus heteroclitus) in a restored salt marsh.
ABSTRACT: The mummichog, Fundulus heteroclitus, is one of the most abundant macrofaunal components of salt marsh ecosystems along the east coast of the United States. During April-November 1998, we determined the habitat use and movement patterns of young-of-the-year (YOY) and adult mummichogs in a restored marsh, formerly a salt hay farm, and an adjacent creek in order to expand our understanding of the ecology of the species and evaluate the success of the restoration. Four major fish habitat types (large first-order natural creek, second-order created creek, linear drainage ditch, and marsh surface) were identified within the study site. Patterns of relative abundance and mark and recapture using coded wire tags were used to determine the habitat use, tidal movements, home range, and site fidelity of the species within these habitat types. A total of 14,784 fish, ranging from 20-100 mm SL, were captured with wire mesh traps and tagged, and 1,521 (10.3%) fish were recaptured. A variety of gears were used to attempt to recapture fish across all habitat types, including wire mesh traps, push nets, and otter trawls. Based on abundance and recaptures of tagged fish, the YOY and adults primarily used the shallow subtidal and intertidal areas of the created creek, the intertidal drainage ditches, and the marsh surface of the restored marsh but not the larger, first-order natural creek. At low tide, large numbers were found in the subtidal areas of the created creek; these then moved onto the marsh surface on the flooding tide. Elevation, and thus hydroperiod, appeared to influence the microscale use of the marsh surface. We estimated the home range of adults and large YOY (20-100 mm SL) to be 15 ha at high tide, which was much larger than previously quantified. There was strong site fidelity to the created creek at low tide. The habitat use and movement patterns of the mummichog appeared similar to that reported for natural marshes. Coupled with the results of other studies on the feeding, growth, and production of this species in this restored marsh, the species appeared to have responded well to the restoration
A life-long approach to physical activity for brain health
It is well established that engaging in lifelong Physical activity (PA) can help delay the onset of many chronic lifestyle related and non-communicable diseases such as cardiovascular disease, type two diabetes, cancer and chronic respiratory diseases. Additionally, growing evidence also documents the importance of PA for brain health, with numerous studies indicating regular engagement in physical activities may be protective against cognitive decline and dementia in late life. Indeed, the link between PA and brain health may be different at each stage of life from childhood, mid-life and late life. Building on this emerging body of multidisciplinary research, this review aims to summarize the current body of evidence linking regular PA and brain health across the lifespan. Specifically, we will focus on the relationship between PA and brain health at three distinct stages of life: childhood and adolescence, mid-life, late life in cognitively healthy adults and later life in adults living with age-related neurodegenerative disorders such as Parkinson's disease (PD) and Alzheimer's disease (AD).Helen Macpherson, Wei-P. Teo, Luke A. Schneider and Ashleigh E. Smit
Optimal control strategies for tuberculosis treatment: a case study in Angola
We apply optimal control theory to a tuberculosis model given by a system of
ordinary differential equations. Optimal control strategies are proposed to
minimize the cost of interventions. Numerical simulations are given using data
from Angola.Comment: This is a preprint of a paper whose final and definite form will
appear in the international journal Numerical Algebra, Control and
Optimization (NACO). Paper accepted for publication 15-March-201
Control parameterization for optimal control problems with continuous inequality constraints: New convergence results
Control parameterization is a powerful numerical technique for solving optimal control problems with general nonlinear constraints. The main idea of control parameterization is to discretize the control space by approximating the control by a piecewise-constant or piecewise-linear function, thereby yielding an approximate nonlinear programming problem. This approximate problem can then be solved using standard gradient-based optimization techniques. In this paper, we consider the control parameterization method for a class of optimal control problems in which the admissible controls are functions of bounded variation and the state and control are subject to continuous inequality constraints. We show that control parameterization generates a sequence of suboptimal controls whose costs converge to the true optimal cost. This result has previously only been proved for the case when the admissible controls are restricted to piecewise continuous functions
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