152 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
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
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
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.
Determination of Omega_b From Big Bang Nucleosynthesis in the Presence of Regions of Antimatter
Production of regions of antimatter in the early universe is predicted in
many baryogenesis models. Small scale antimatter regions would annihilate
during or soon after nucleosynthesis, affecting the abundances of the light
elements. In this paper we study how the acceptable range in Omega_b changes in
the presence of antimatter regions, as compared to the standard big bang
nucleosynthesis. It turns out that it is possible to produce at the same time
both a low 4He value (Y_p < 0.240) and a low D/H value (D/H < 4e-5), but
overproduction of 7Li is unavoidable at large Omega_b.Comment: 9 pages, PRD version, ref. 6 correcte
Negative Refraction in Perspective
The concept of negative refraction is attracting a lot of attention. The
initial ideas and the misconceptions that have arisen are discussed in
sufficient detail to understand the conceptual structure that binds negative
refraction to the existence of backward wave and forward wave phenomena. A
presentation of the properties of isotropic media supporting backward waves is
followed by a discussion of negative phase velocity media, causality,
anisotropic crystals and some connections to photonic crystals. The historical
background is always coupled to a detailed presentation of all the issues. The
paper is driven numerically and is illustrated with the outcomes of original
FDTD simulations
Analytical, numerical, and experimental investigation of a Luneburg lens system for directional cloaking
In this study, the design of a directional cloaking based on the Luneburg lens system is proposed and its operating principle is experimentally verified. The cloaking concept is analytically investigated via geometrical optics and numerically realized with the help of the finite-difference time-domain method. In order to benefit from its unique focusing and/or collimating characteristics of light, the Luneburg lens is used. We show that by the proper combination of Luneburg lenses in an array form, incident light bypasses the region between junctions of the lenses, i.e., the "dark zone." Hence, direct interaction of an object with propagating light is prevented if one places the object to be cloaked inside that dark zone. This effect is used for hiding an object which is made of a perfectly electric conductor material. In order to design an implementable cloaking device, the Luneburg lens is discretized into a photonic crystal structure having gradually varying air cylindrical holes in a dielectric material by using Maxwell Garnett effective medium approximations. Experimental verifications of the designed cloaking structure are performed at microwave frequencies of around 8 GHz. The proposed structure is fabricated by three-dimensional printing of dielectric polylactide material and a brass metallic alloy is utilized in place of the perfectly electric conductor material in microwave experiments. Good agreement between numerical and experimental results is found. © 2019 American Physical Society
The Barents area changes – How will Finland adapt? (Barentsin alue muuttuu – miten Suomi sopeutuu?)
The cumulative impacts of environmental, climatic and societal changes and their consequences will affect the development of the Arctic region in the coming decades. Adaptation to these changes will require measures of all the actors in the region. Finland, part of the Euro-Arctic region, will adapt to these changes in a variety of ways. The Barents area is unique in the Arctic in being a multicultural, relatively densely populated area with well-developed industries and infrastructure.
This report examines adaptation to changes and their consequences in the Barents area in terms of governance and Finland’s capacities to adapt. The aim has been to produce comprehensive information from the Finnish perspective for local and national decision-makers about long-term changes in the region, their expected impacts and adaptation options, and to support decision-making that will advance adaptation. The report includes recommendations.
This report is based on the contribution of Finnish experts to an Arctic Council and Arctic Monitoring and Assessment Programme (AMAP) project titled ”Adaptation Actions for a Changing Arctic” (AACA). The project has prepared a pilot report by Nordic and Russian experts on the Barents area in English on changes, their impacts and adaptation options. The report will be published in 2017 (AMAP 2017)
- …