14,493 research outputs found
Large tunable photonic band gaps in nanostructured doped semiconductors
A plasmonic nanostructure conceived with periodic layers of a doped
semiconductor and passive semiconductor is shown to generate spontaneously
surface plasmon polaritons thanks to its periodic nature. The nanostructure is
demonstrated to behave as an effective material modeled by a simple dielectric
function of ionic-crystal type, and possesses a fully tunable photonic band
gap, with widths exceeding 50%, in the region extending from mid-infra-red to
Tera-Hertz.Comment: 6 pages, 4 figures, publishe
Teaching "Symmetry" in the Introductory Physics Curriculum
Modern physics is largely defined by fundamental symmetry principles and
Noether's Theorem. Yet these are not taught, or rarely mentioned, to beginning
students, thus missing an opportunity to reveal that the subject of physics is
as lively and contemporary as molecular biology, and as beautiful as the arts.
We prescribe a symmetry module to insert into the curriculum, of a week's
length.Comment: 15 pages, 4 figure
Self-similar cosmologies in 5D: spatially flat anisotropic models
In the context of theories of Kaluza-Klein type, with a large extra
dimension, we study self-similar cosmological models in 5D that are
homogeneous, anisotropic and spatially flat. The "ladder" to go between the
physics in 5D and 4D is provided by Campbell-Maagard's embedding theorems. We
show that the 5-dimensional field equations determine the form of
the similarity variable. There are three different possibilities: homothetic,
conformal and "wave-like" solutions in 5D. We derive the most general
homothetic and conformal solutions to the 5D field equations. They require the
extra dimension to be spacelike, and are given in terms of one arbitrary
function of the similarity variable and three parameters. The Riemann tensor in
5D is not zero, except in the isotropic limit, which corresponds to the case
where the parameters are equal to each other. The solutions can be used as 5D
embeddings for a great variety of 4D homogeneous cosmological models, with and
without matter, including the Kasner universe. Since the extra dimension is
spacelike, the 5D solutions are invariant under the exchange of spatial
coordinates. Therefore they also embed a family of spatially {\it
inhomogeneous} models in 4D. We show that these models can be interpreted as
vacuum solutions in braneworld theory. Our work (I) generalizes the 5D
embeddings used for the FLRW models; (II) shows that anisotropic cosmologies
are, in general, curved in 5D, in contrast with FLRW models which can always be
embedded in a 5D Riemann-flat (Minkowski) manifold; (III) reveals that
anisotropic cosmologies can be curved and devoid of matter, both in 5D and 4D,
even when the metric in 5D explicitly depends on the extra coordinate, which is
quite different from the isotropic case.Comment: Typos corrected. Minor editorial changes and additions in the
Introduction and Summary section
Gamma-rays from the compact colliding wind region in Cyg OB2 #5
In this contribution we model the non-thermal emission (from radio to
gamma-rays) produced in the compact (and recently detected) colliding wind
region in the multiple stellar system Cyg OB2 #5. We focus our study on the
detectability of the produced gamma-rays.Comment: To appear in the proceedings of the 5th International Symposium on
High-Energy Gamma-Ray Astronomy (Gamma2012), held in Heidelberg, July 9-13,
201
Large tunable photonic band gaps in nanostructured doped semiconductors
A plasmonic nanostructure conceived with periodic layers of a doped
semiconductor and passive semiconductor is shown to generate spontaneously
surface plasmon polaritons thanks to its periodic nature. The nanostructure is
demonstrated to behave as an effective material modeled by a simple dielectric
function of ionic-crystal type, and possesses a fully tunable photonic band
gap, with widths exceeding 50%, in the region extending from mid-infra-red to
Tera-Hertz.Comment: 6 pages, 4 figures, publishe
Equivalence Between Space-Time-Matter and Brane-World Theories
We study the relationship between space-time-matter (STM) and brane theories.
These two theories look very different at first sight, and have different
motivation for the introduction of a large extra dimension. However, we show
that they are equivalent to each other. First we demonstrate that STM predicts
local and non-local high-energy corrections to general relativity in 4D, which
are identical to those predicted by brane-world models. Secondly, we notice
that in brane models the usual matter in 4D is a consequence of the dependence
of five-dimensional metrics on the extra coordinate. If the 5D bulk metric is
independent of the extra dimension, then the brane is void of matter. Thus, in
brane theory matter and geometry are unified, which is exactly the paradigm
proposed in STM. Consequently, these two 5D theories share the same concepts
and predict the same physics. This is important not only from a theoretical
point of view, but also in practice. We propose to use a combination of both
methods to alleviate the difficult task of finding solutions on the brane. We
show an explicit example that illustrate the feasibility of our proposal.Comment: Typos corrected, three references added. To appear in Mod. Phys. Let
Thorium-Uranium fractionation as an indicator of petrogenetic processes
A mean Th/U ratio, ~4, seems to characterize most
terrestrial, lunar, and meteoritic igneous materials and
major patterns of lead isotopic evolution develop principally in systems with Th/U about 3.7-4.0. Some
important crustal igneous subsystems show systematic
deviations from these values. Such fractionation generally
is attributed to the geochemical behavior of the large
lithophile actinide ions in various enriched minor phases.
This probably is true in highly differentiated series with
higher Th and U levels (>8 and 2 ppm). A different
fractionation mechanism may be more important in some
lower concentration systems. Isotope dilution mass spectrometric studies of U and Th in diverse igneous feldspar separates reveal significant U and Th partitioning into them with drastic fractionation of Th/U (values 0.2- 3.0). Alkali feldspars in granitic rocks display partition coefficients (feldspar/total rock) of about 0.005-0.02 for U
and 0.001-0.004 for Th; Th/U~0.5-3.0. In plagioclase
in gabbros, tonalites, and granodiorites with lower Th and
U, the coefficients appear larger (U~0.1-0.4, Th~0.04-0.2) but Th/U appears lower (Th/U~0.2-2.0) than in K-feldspar. Limited data suggests some pyraxenes and other major minerals may also fractionate Th/U to lower ratios. Differences in ionic radius and uranium oxidation states may contribute to the undefined fractionation mechanism.
Processes of primary differentiation (fractional
crystallization, partial melting) involving gabbroic systems
seem to reflect this fractionation. Oceanic tholeiites, end
massive gabbroic complexes display generally low Th/U
values. Their lead isotopes reflect reservoirs with more
normal Th/U ratios. This suggests limits on the number of
fractionation cycles, mantle mixing, and/or reservoir dimensions for primary basaltic systems. Th and U and their
associated lead isotope systems can be used with lanthanide
R.E.E. to assist in developmet of petrogenetic models
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