222 research outputs found
Electromagnetic Boundary Conditions Defined in Terms of Normal Field Components
A set of four scalar conditions involving normal components of the fields D
and B and their normal derivatives at a planar surface is introduced, among
which different pairs can be chosen to represent possible boundary conditions
for the electromagnetic fields. Four such pairs turn out to yield meaningful
boundary conditions and their responses for an incident plane wave at a planar
boundary are studied. The theory is subsequently generalized to more general
boundary surfaces defined by a coordinate function. It is found that two of the
pairs correspond to the PEC and PMC conditions while the other two correspond
to a mixture of PEC and PMC conditions for fields polarized TE or TM with
respect to the coordinate defining the surface
Inhomogeneous Big-Bang Nucleosynthesis in Light of Recent Observations
We consider inhomogeneous big bang nucleosynthesis in light of the present
observational situation. Different observations of He-4 and D disagree with
each other, and depending on which set of observations one uses, the estimated
primordial He-4 corresponds to a lower baryon density in standard big bang
nucleosynthesis than what one gets from deuterium. Recent Kamiokande results
rule out a favorite particle physics solution to this tension between He-4 and
D. Inhomogeneous nucleosynthesis can alleviate this tension, but the more
likely solution is systematics in the observations. The upper limit to Omega_b
from inhomogeneous nucleosynthesis is higher than in standard nucleosynthesis,
given that the distance scale of the inhomogeneity is near the optimal value,
which maximizes effects of neutron diffusion. Possible sources of baryon
inhomogeneity include the QCD and electroweak phase transitions. The distance
scale of the inhomogeneities arising from the electroweak transition is too
small for them to have a large effect on nucleosynthesis, but the effect may
still be larger than some of the other small corrections recently incorporated
to SBBN codes.Comment: 12 pages, 8 figures, REVTe
Analytical investigations of ground modifications assisting the detection of buried object
A ground-penetrating radar (GPR) antenna excites a perfectly electric conducting inclusion buried inside the ground. The scattering problem is solved semi-analytically via integral equation techniques. The permittivity and thickness of a superstrate deposited atop the ground are determined such that the detectability of the inclusion is significantly increased. Results from numerical simulations are presented exhibiting the effectiveness of the approach. Emphasis is given on the effects that the shape of the buried inclusion has on the scattered field
Design and analytically full-wave validation of the invisibility cloaks, concentrators, and field rotators created with a general class of transformations
We investigate a general class of electromagnetic devices created with any
continuous transformation functions by rigorously calculating the analytical
expressions of the electromagnetic field in the whole space. Some interesting
phenomena associated with these transformation devices, including the
invisibility cloaks, concentrators, and field rotators, are discussed. By
carefully choosing the transformation function, we can realize cloaks which are
insensitive to perturbations at both the inner and outer boundaries.
Furthermore, we find that when the coating layer of the concentrator is
realized with left-handed materials, energy will circulate between the coating
and the core, and the energy transmits through the core of the concentrator can
be much bigger than that transmits through the concentrator. Therefore, such
concentrator is also a power flux amplifier. Finally, we propose a spherical
field rotator, which functions as not only a wave vector rotator, but also a
polarization rotator, depending on the orientations of the spherical rotator
with respect to the incident wave direction. The functionality of these novel
transformation devices are all successfully confirmed by our analytical full
wave method, which also provides an alternate computational efficient
validation method in contrast to numerical validation methods.Comment: 22 pages, 3 figure
Electrostatic resonances of a negative-permittivity hemisphere
This article studies the electric response of an electrically small hemispherical object with negative permittivity by computing its polarizability which is determined by two orthogonal components, the axial one and the transverse one. A certain range of negative permittivity values is found where the mathematical determination of the polarizability becomes impossible due to an unlimited number of singularities. These singularities are due to surface plasmons, also referred to as electrostatic resonances, caused by the sharp edge of the hemisphere. It is also found that the planar surface of the hemisphere may support resonant surface modes. Furthermore, there exists a dipolar resonance determined by the overall geometry. In addition, it is shown that the resonances can be smoothened by introducing losses and, even more importantly, rounding the edge.Peer reviewe
Simulation and data processing of GOMOS measurements
In this paper the data simulation and data inversion studies for stellar occultation measurements are discussed. The specific application is the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument which has been proposed for the first European Platform, Polar Orbiting Earth Mission (POEM-1)
Signs, curls, and time variations: learning to appreciate Faraday’s law
In this article, we present the analysis of a study on the development of conceptual understanding of dynamic electromagnetic fields of electrical engineering students in Finland. The focus of the study was teaching and understanding of Faraday’s law. A coil with two light-emitting diodes and a strong permanent magnet were used with which the induced electromotive force could be made visible. However, the field and flux directions, temporal changes, and topological constellations within this setting determine in a subtle manner the character of the resulting electric effect. The demonstration was used on electromagnetic field theory classes at Aalto University, Finland, to assess the conceptual understanding of the students. Drawing from the Peer Instruction principle, the students were asked to fill in a questionnaire concerning this experiment, first on their own, and once again after discussing with their neighbors in the classroom. They were asked about the direction of the electric force and the confidence of their answer. The results show that the answer is not very obvious: students tend to vote for the wrong answer. The Peer Instruction discussion greatly improves the situation. Also, the confidence of the students increases thanks to the discussion period with neighbors. The results, however, seem to be somewhat sensitive to the exact constellation and the administration of the experiment
Plasmonic Cloaking of Cylinders: Finite Length, Oblique Illumination and Cross-Polarization Coupling
Metamaterial cloaking has been proposed and studied in recent years following
several interesting approaches. One of them, the scattering-cancellation
technique, or plasmonic cloaking, exploits the plasmonic effects of suitably
designed thin homogeneous metamaterial covers to drastically suppress the
scattering of moderately sized objects within specific frequency ranges of
interest. Besides its inherent simplicity, this technique also holds the
promise of isotropic response and weak polarization dependence. Its theory has
been applied extensively to symmetrical geometries and canonical 3D shapes, but
its application to elongated objects has not been explored with the same level
of detail. We derive here closed-form theoretical formulas for infinite
cylinders under arbitrary wave incidence, and validate their performance with
full-wave numerical simulations, also considering the effects of finite lengths
and truncation effects in cylindrical objects. In particular, we find that a
single isotropic (idealized) cloaking layer may successfully suppress the
dominant scattering coefficients of moderately thin elongated objects, even for
finite lengths comparable with the incident wavelength, providing a weak
dependence on the incidence angle. These results may pave the way for
application of plasmonic cloaking in a variety of practical scenarios of
interest.Comment: 17 pages, 11 figures, 2 table
Macroscopic phase segregation in superconducting K0.73Fe1.67Se2 as seen by muon spin rotation and infrared spectroscopy
Using muon spin rotation (\muSR) and infrared spectroscopy we investigated
the recently discovered superconductor K0.73Fe1.67Se2 with Tc = 32 K. We show
that the combined data can be consistently described in terms of a
macroscopically phase segregated state with a matrix of ~88% volume fraction
that is insulating and strongly magnetic and inclusions with a ~12% volume
fraction which are metallic, superconducting and non-magnetic. The electronic
properties of the latter, in terms of the normal state plasma frequency and the
superconducting condensate density, appear to be similar as in other iron
selenide or arsenide superconductors.Comment: 22 pages, 8 figures. (citation list correction.
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