25,092 research outputs found
Theory of Transmission of Light by Sub-wavelength Cylindrical Holes in Metallic Films
This paper presents theory and finite-difference time-domain (FDTD)
calculations for a single and arrays of sub-wavelength cylindrical holes in
metallic films presenting large transmission. These calculations are in
excellent agreement with experimental measurements. This effect has to be
understood in terms of the properties exhibited by the dielectric constant of
metals which cannot be treated as ideal metals for the purpose of transmission
and diffraction of light. We discuss the cases of well-differentiated metals
silver and tungsten. It is found that the effect of surface plasmons or other
surface wave excitations due to a periodical set of holes or other roughness at
the surface is marginal. The effect can enhance but also can depress the
transmission of the arrays as shown by theory and experiments. The peak
structure observed in experiments is a consequence of the interference of the
wavefronts transmitted by each hole and is determined by the surface array
period independently of the material. Without large transmission through a
single hole there is no large transmission through the array. We found that in
the case of Ag which at the discussed frequencies is a metal there are
cylindrical plasmons at the wall of the hole, as reported by Economu et al 30
years ago, that enhanced the transmission. But it turns out, as will be
explained, that for the case of W which behaves as a dielectric, there is also
a large transmission when compared with that of an ideal metal waveguide. To
deal with this problem one has to use the measured dielectric function of the
metals. We discuss thoroughly all these cases and compare with the data.Comment: 13 pages and 9 figure
Hybrid preconditioning for iterative diagonalization of ill-conditioned generalized eigenvalue problems in electronic structure calculations
The iterative diagonalization of a sequence of large ill-conditioned
generalized eigenvalue problems is a computational bottleneck in quantum
mechanical methods employing a nonorthogonal basis for {\em ab initio}
electronic structure calculations. We propose a hybrid preconditioning scheme
to effectively combine global and locally accelerated preconditioners for rapid
iterative diagonalization of such eigenvalue problems. In partition-of-unity
finite-element (PUFE) pseudopotential density-functional calculations,
employing a nonorthogonal basis, we show that the hybrid preconditioned block
steepest descent method is a cost-effective eigensolver, outperforming current
state-of-the-art global preconditioning schemes, and comparably efficient for
the ill-conditioned generalized eigenvalue problems produced by PUFE as the
locally optimal block preconditioned conjugate-gradient method for the
well-conditioned standard eigenvalue problems produced by planewave methods
Nonuniqueness in spin-density-functional theory on lattices
In electronic many-particle systems, the mapping between densities and spin
magnetizations, {n(r), m(r)}, and potentials and magnetic fields, {v(r), B(r)},
is known to be nonunique, which has fundamental and practical implications for
spin-density-functional theory (SDFT). This paper studies the nonuniqueness
(NU) in SDFT on arbitrary lattices. Two new, non-trivial cases are discovered,
here called local saturation and global noncollinear NU, and their properties
are discussed and illustrated. In the continuum limit, only some well-known
special cases of NU survive.Comment: 4 pages, 1 figur
Chemoviscosity modeling for thermosetting resins
A chemoviscosity model, which describes viscosity rise profiles accurately under various cure cycles, and correlates viscosity data to the changes of physical properties associated with structural transformations of the thermosetting resin system during cure, was established. Work completed on chemoviscosity modeling for thermosetting resins is reported
Studies on chemoviscosity modeling for thermosetting resins
A new analytical model for simulating chemoviscosity of thermosetting resins has been formulated. The model is developed by modifying the well-established Williams-Landel-Ferry (WLF) theory in polymer rheology for thermoplastic materials. By introducing a relationship between the glass transition temperature Tg(t) and the degree of cure alpha(t) of the resin system under cure, the WLF theory can be modified to account for the factor of reaction time. Temperature dependent functions of the modified WLF theory constants C sub 1 (t) and C sub 2 (t) were determined from the isothermal cure data. Theoretical predictions of the model for the resin under dynamic heating cure cycles were shown to compare favorably with the experimental data. This work represents progress toward establishing a chemoviscosity model which is capable of not only describing viscosity profiles accurately under various cure cycles, but also correlating viscosity data to the changes of physical properties associated with the structural transformation of the thermosetting resin systems during cure
Learning associations between clinical information and motion-based descriptors using a large scale MR-derived cardiac motion atlas
The availability of large scale databases containing imaging and non-imaging
data, such as the UK Biobank, represents an opportunity to improve our
understanding of healthy and diseased bodily function. Cardiac motion atlases
provide a space of reference in which the motion fields of a cohort of subjects
can be directly compared. In this work, a cardiac motion atlas is built from
cine MR data from the UK Biobank (~ 6000 subjects). Two automated quality
control strategies are proposed to reject subjects with insufficient image
quality. Based on the atlas, three dimensionality reduction algorithms are
evaluated to learn data-driven cardiac motion descriptors, and statistical
methods used to study the association between these descriptors and non-imaging
data. Results show a positive correlation between the atlas motion descriptors
and body fat percentage, basal metabolic rate, hypertension, smoking status and
alcohol intake frequency. The proposed method outperforms the ability to
identify changes in cardiac function due to these known cardiovascular risk
factors compared to ejection fraction, the most commonly used descriptor of
cardiac function. In conclusion, this work represents a framework for further
investigation of the factors influencing cardiac health.Comment: 2018 International Workshop on Statistical Atlases and Computational
Modeling of the Hear
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