17,591 research outputs found
Graphene-Dielectric Composite Metamaterials: Evolution from Elliptic to Hyperbolic Wavevector Dispersion and The Transverse Epsilon-Near-Zero Condition
We investigated a multilayer graphene-dielectric composite material,
comprising graphene sheets separated by subwavelength-thick dielectric spacer,
and found it to exhibit hyperbolic isofrequency wavevector dispersion at far-
and mid-infrared frequencies allowing propagation of waves that would be
otherwise evanescent in a dielectric. Electrostatic biasing was considered for
tunable and controllable transition from hyperbolic to elliptic dispersion. We
explored the validity and limitation of the effective medium approximation
(EMA) for modeling wave propagation and cutoff of the propagating spatial
spectrum due to the Brillouin zone edge. We found that EMA is capable of
predicting the transition of the isofrequency dispersion diagram under certain
conditions. The graphene-based composite material allows propagation of
backward waves under the hyperbolic dispersion regime and of forward waves
under the elliptic regime. Transition from hyperbolic to elliptic dispersion
regimes is governed by the transverse epsilon-near-zero (TENZ) condition, which
implies a flatter and wider propagating spectrum with higher attenuation, when
compared to the hyperbolic regime. We also investigate the tunable transparency
of the multilayer at that condition in contrast to other materials exhibiting
ENZ phenomena.Comment: to be published in Journal of Nanophotonic
Determinants and characteristics of voluntary Internet disclosures in GCC countries
The purpose of this paper is to examine the determinants and characteristics of voluntary Internet disclosures by listed companies in three Gulf Cooperation Council countries. This paper uses archival data from 192 listed companies in Qatar Stock Exchange, Muscat Securities Market, and Bahrain Bourse. Binary Logistic Regression analysis is used to examine the determinants of Internet financial reporting. Kruskal-Wallis test is used to examine the differences in disclosure characteristics among the three countries. The results reveal that firm size is the major influencing factor that impacts Internet financial reporting in the GCC. A number of disclosure characteristics differ significantly between the three countries. The paper provides insights into corporate Internet disclosure in the GCC that will benefit all stakeholders with an interest in corporate reporting in this important region of the world. The results are consistent with previous literature that corporate size is a major determinant of Internet financial reporting
14-Bromo-12-chloro-2,16-dioxapentacyclohenicosa-3(8),10,12,14-tetraene-7,20-dione
In the title compound, C19H16BrClO4, both the fused xanthene rings and one of the cyclohexane rings adopt envelope conformations, while the other cyclohexane ring is in a chair conformation. In the crystal, molecules are linked by C-H...O hydrogen bonds, forming infinite chains running along [10-1] incorporating R22(16) ring motifs. In addition, C-H...[pi] interactions and weak [pi]-[pi] stacking interactions [centroid-centroid distance = 3.768 (3) Ã…] help to consolidate the packing
Virtual integration platform for computational fluid dynamics
Computational Fluid Dynamics (CFD) tools used in shipbuilding industry involve multiple disciplines, such as resistance, manoeuvring, and cavitation. Traditionally, the analysis was performed separately and sequentially in each discipline, which often resulted in conflict and inconsistency of hydrodynamic prediction. In an effort to solve such problems for future CFD computations, a Virtual Integration Platform (VIP) has been developed in the University of Strathclyde within two EU FP6 projects - VIRTUE and SAFEDOR1. The VIP provides a holistic collaborative environment for designers with features such as Project/Process Management, Distributed Tools Integration, Global Optimisation, Version Management, and Knowledge Management. These features enhance collaboration among customers, ship design companies, shipyards, and consultancies not least because they bring together the best expertise and resources around the world. The platform has been tested in seven European ship design companies including consultancies. Its main functionalities along with advances are presented in this paper with two industrial applications
International ownership and firm performance in Arab economies
This paper aims at filling existing research by examining the impact of corporate governance and ownership structure on firm performance using cross-sectional data from companies in the MENA region for the years 2009-2013. The results indicate that higher ownership concentration is associated with higher returns. Furthermore, firms with higher international ownership share tend to perform better than those with only local private and/or state ownership. The results suggest some prevalent features with respect to ownership and performance of firms in the MENA region. Due to the volatile social and business environment, these firms operate in, they may be particularly dependent on effective ownership structures and support which may be provided by international, institutional, and large shareholders
A New Amplification Regime for Traveling Wave Tubes with Third Order Modal Degeneracy
Engineering of the eigenmode dispersion of slow-wave structures (SWSs) to
achieve desired modal characteristics, is an effective approach to enhance the
performance of high power traveling wave tube (TWT) amplifiers or oscillators.
We investigate here for the first time a new synchronization regime in TWTs
based on SWSs operating near a third order degeneracy condition in their
dispersion. This special three-eigenmode synchronization is associated with a
stationary inflection point (SIP) that is manifested by the coalescence of
three Floquet-Bloch eigenmodes in the SWS. We demonstrate the special features
of "cold" (without electron beam) periodic SWSs with SIP modeled as coupled
transmission lines (CTLs) and investigate resonances of SWSs of finite length.
We also show that by tuning parameters of a periodic SWS one can achieve an SIP
with nearly ideal flat dispersion relationship with zero group velocity or a
slightly slanted one with a very small (positive or negative) group velocity
leading to different operating schemes. When the SIP structure is synchronized
with the electron beam potential benefits for amplification include (i) gain
enhancement, (ii) gain-bandwidth product improvement, and (iii) higher power
efficiency, when compared to conventional Pierce-like TWTs. The proposed theory
paves the way for a new approach for potential improvements in gain, power
efficiency and gain-bandwidth product in high power microwave amplifiers.Comment: 15 pages, 11 figure
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