5,655 research outputs found
An omnidirectional retroreflector based on the transmutation of dielectric singularities
In the field of transformation optics, metamaterials mimic the effect of
coordinate transformations on electromagnetic waves, creating the illusion that
the waves are propagating through a virtual space. Transforming space by
appropriately designed materials makes devices possible that have been deemed
impossible. In particular, transformation optics has led to the demonstration
of invisibility cloaking for microwaves, surface plasmons and infrared light.
Here we report the achievement of another "impossible task". We implement, for
microwaves, a device that would normally require a dielectric singularity, an
infinity in the refractive index. We transmute a singularity in virtual space
into a mere topological defect in a real metamaterial. In particular, we
demonstrate an omnidirectional retroreflector, a device for faithfully
reflecting images and for creating high visibility, from all directions. Our
method is robust, potentially broadband and similar techniques could be applied
for visible light
On the equivalence of Eulerian and Lagrangian variables for the two-component Camassa-Holm system
The Camassa-Holm equation and its two-component Camassa-Holm system
generalization both experience wave breaking in finite time. To analyze this,
and to obtain solutions past wave breaking, it is common to reformulate the
original equation given in Eulerian coordinates, into a system of ordinary
differential equations in Lagrangian coordinates. It is of considerable
interest to study the stability of solutions and how this is manifested in
Eulerian and Lagrangian variables. We identify criteria of convergence, such
that convergence in Eulerian coordinates is equivalent to convergence in
Lagrangian coordinates. In addition, we show how one can approximate global
conservative solutions of the scalar Camassa-Holm equation by smooth solutions
of the two-component Camassa-Holm system that do not experience wave breaking
Proper Matter Collineations of Plane Symmetric Spacetimes
We investigate matter collineations of plane symmetric spacetimes when the
energy-momentum tensor is degenerate. There exists three interesting cases
where the group of matter collineations is finite-dimensional. The matter
collineations in these cases are either four, six or ten in which four are
isometries and the rest are proper.Comment: 10 pages, LaTex, accepted for publication in Modern Physics Letters
The unique rapid variabilities of the iron K line profiles in NGC 4151
We present a detailed analysis of the iron K line variabilities in
NGC 4151 by using long ASCA observation data obtained in May 1995. Despite the
relatively small amplitude variations in the continuum flux, the iron K
line flux and profile show dramatic variations. Particularly, the line profile
changes from single peak to seeming double peaks and back in time scales of a
few 10 sec. The seemingly double-peaked profiles can be well interpreted as
line emission from a Keplerian ring around a massive black hole. An absorption
line at around 5.9 keV is also marginnaly detected. We discussed current Fe K
line models, but none of them can well explain the observed line and continuum
variations.Comment: 18 pages, latex, 3 figures, ApJ accepte
Synthesis of Carbon Nanotubes by Catalytic Chemical Vapor Deposition
As a new carbon material in the twenty-first century, carbon nanotubes (CNTs) have excellent optical, electrical, magnetic, thermal, chemical, and mechanical properties. There are many synthesis methods to produce CNTs. Compared with other methods, chemical vapor deposition (CVD) is the most effective method that has broad prospects for large-scale control of CNTs in recent years due to its simple equipment, simple operation, and lower cost. In order to gain a comprehensive understanding of the controlling parameters about the formation of CNTs, this chapter reviews the latest progress in the preparation of CNTs by CVD from three of the most important influencing factors: carbon sources, catalysts, and substrates. Among them, the catalyst is the most influential factor for the morphology, structure, and properties of CNTs. It should be pointed out that many growth factors can control the particle size distribution, composition, and structure of the catalysts, such as catalyst substrate, metal transition components added, calcination temperature, etc
Trends in immediate postmastectomy breast reconstruction in the United Kingdom
The study aimed to evaluate local and national trends in immediate breast reconstruction (IBR) using the national English administrative records, Hospital Episode Statistics. Our prediction was an increase in implant-only and free flap procedures and a decline in latissimus flap reconstructions.Data from an oncoplastic center were interrogated to derive numbers of implant-only, autologous latissimus dorsi (LD), LD-assisted, and autologous pedicled or free flap IBR procedures performed between 2004 and 2013. Similarly, Hospital Episode Statistics data were used to quantify national trends in these procedures from 1996 to 2012 using a curve fitting analysis.National data suggest an increase in LD procedures between 1996 (n = 250) and 2002 (n = 958), a gradual rise until 2008 (n = 1398) followed by a decline until 2012 (n = 1090). As a percentage of total IBR, trends in LD flap reconstruction better fit a quadratic (R(2) = 0.97) than a linear function (R(2) = 0.63), confirming a proportional recent decline in LD flap procedures. Conversely, autologous (non-LD) flap reconstructions have increased (1996 = 0.44%; 2012 = 2.76%), whereas implant-only reconstructions have declined (1996 = 95.42%; 2012 = 84.92%). Locally, 70 implant-assisted LD procedures were performed in 2003 -2004, but only 2 were performed in 2012 to 2013.Implants are the most common IBR technique; autologous free flap procedures have increased, and pedicled LD flap procedures are in decline
Efficient Multi-Party Quantum Secret Sharing Schemes
In this work, we generalize the quantum secret sharing scheme of Hillary,
Bu\v{z}ek and Berthiaume[Phys. Rev. A59, 1829(1999)] into arbitrary
multi-parties. Explicit expressions for the shared secret bit is given. It is
shown that in the Hillery-Bu\v{z}ek-Berthiaume quantum secret sharing scheme
the secret information is shared in the parity of binary strings formed by the
measured outcomes of the participants. In addition, we have increased the
efficiency of the quantum secret sharing scheme by generalizing two techniques
from quantum key distribution. The favored-measuring-basis Quantum secret
sharing scheme is developed from the Lo-Chau-Ardehali technique[H. K. Lo, H. F.
Chau and M. Ardehali, quant-ph/0011056] where all the participants choose their
measuring-basis asymmetrically, and the measuring-basis-encrypted Quantum
secret sharing scheme is developed from the Hwang-Koh-Han technique [W. Y.
Hwang, I. G. Koh and Y. D. Han, Phys. Lett. A244, 489 (1998)] where all
participants choose their measuring-basis according to a control key. Both
schemes are asymptotically 100% in efficiency, hence nearly all the GHZ-states
in a quantum secret sharing process are used to generate shared secret
information.Comment: 7 page
Overcoming Language Dichotomies: Toward Effective Program Comprehension for Mobile App Development
Mobile devices and platforms have become an established target for modern
software developers due to performant hardware and a large and growing user
base numbering in the billions. Despite their popularity, the software
development process for mobile apps comes with a set of unique, domain-specific
challenges rooted in program comprehension. Many of these challenges stem from
developer difficulties in reasoning about different representations of a
program, a phenomenon we define as a "language dichotomy". In this paper, we
reflect upon the various language dichotomies that contribute to open problems
in program comprehension and development for mobile apps. Furthermore, to help
guide the research community towards effective solutions for these problems, we
provide a roadmap of directions for future work.Comment: Invited Keynote Paper for the 26th IEEE/ACM International Conference
on Program Comprehension (ICPC'18
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