1,911 research outputs found
Gyrotropic impact upon negatively refracting surfaces
Surface wave propagation at the interface between different types of gyrotropic materials and an isotropic negatively refracting medium, in which the relative permittivity and relative permeability are, simultaneously, negative is investigated. A general approach is taken that embraces both gyroelectric and gyromagnetic materials, permitting the possibility of operating in either the low GHz, THz or the optical frequency regimes. The classical transverse Voigt configuration is adopted and a complete analysis of non-reciprocal surface wave dispersion is presented. The impact of the surface polariton modes upon the reflection of both plane waves and beams is discussed in terms of resonances and an example of the influence upon the GoosâHĂ€nchen shift is given
Nonlinear surface waves in left-handed materials
We study both linear and nonlinear surface waves localized at the interface
separating a left-handed medium (i.e. the medium with both negative dielectric
permittivity and negative magnetic permeability) and a conventional (or
right-handed) dielectric medium. We demonstrate that the interface can support
both TE- and TM-polarized surface waves - surface polaritons, and we study
their properties. We describe the intensity-dependent properties of nonlinear
surface waves in three different cases, i.e. when both the LH and RH media are
nonlinear and when either of the media is nonlinear. In the case when both
media are nonlinear, we find two types of nonlinear surface waves, one with the
maximum amplitude at the interface, and the other one with two humps. In the
case when one medium is nonlinear, only one type of surface wave exists, which
has the maximum electric field at the interface, unlike waves in right-handed
materials where the surface-wave maximum is usually shifted into a
self-focussing nonlinear medium. We discus the possibility of tuning the wave
group velocity in both the linear and nonlinear cases, and show that
group-velocity dispersion, which leads to pulse broadening, can be balanced by
the nonlinearity of the media, so resulting in soliton propagation.Comment: 9 pages, 10 figure
Stable isotope values in modern bryozoan carbonate from New Zealand and implications for paleoenvironmental interpretation
Bryozoan carbonate contains useful geochemical evidence of temperate shelf paleoenvironments. Stable isotope values were determined for 103 modern marine bryozoan skeletons representing 30 species from New Zealand. ÎŽ18O values range from -1.4 to 2.8 VPDB, while ÎŽ13C range from -4.5 to 2.8 VPDB (values uncorrected for mineralogical variation). These values are distinct from those of both tropical marine skeletons and New Zealand Tertiary fossils. Most bryozoans secrete carbonate in or near isotopic equilibrium with sea water, except for Celleporina and Steginoporella. The complex and variable mineralogies of the bryozoans reported here make correction for mineralogical effects problematic. Nevertheless, mainly aragonitic forms display higher isotope values, as anticipated. Both temperature and salinity constrain ÎŽ18O and ÎŽ13C values, and vary with latitude and water depth. Ten samples from a single branch of Cinctipora elegans from the Otago shelf cover a narrow range, although the striking difference in carbon isotope values between the endozone and exozone probably reflects different mineralisation histories. Our stable isotope results from three different laboratories on a single population from a single location are encouragingly consistent. Monomineralic bryozoans, when carefully chosen to avoid species suspected of vital fractionation, have considerable potential as geochemical paleoenvironmental indicators, particularly in temperate marine environments where bryozoans are dominant sediment producers
Hybrid propulsion for launch vehicle boosters: A program status update
Results obtained in studying the origin and suppression of large-amplitude pressure oscillations in a 24 in. diameter hybrid motor using a liquid oxygen/hydroxylterminated polybutadiene/polycyclopentadiene propellant system are discussed. Tests conducted with liquid oxygen flow rates varying from 10 to 40 lbm/sec were designed to gauge the effectiveness of various vaporization chamber flow fields, injector designs, and levels of heat addition in suppressing high-frequency longitudinal mode oscillations. Longitudinal acoustic modes did not arise in any tests. However, initial testing revealed the presence of high-amplitude, sinusoidal, nonacoustic oscillations persisting throughout the burn durations. Analysis showed this to be analogous to chug mode instability in liquid rocket engines brought about by a coupling of motor combustion processes and the liquid oxygen feed system. Analytical models were developed and verified by test data to predict the amplitude and frequency of feed-system-coupled combustion pressure oscillations. Subsequent testing showed that increasing the feed system impedance eliminated the bulk mode instability. This paper documents the work completed to date in performance of the Hybrid Propulsion Technology for Launch Vehicle Boosters Program (NAS8-39942) sponsored by NASA's George C. Marshall Space Flight Center
Information scraps: how and why information eludes our personal information management tools
In this paper we describe information scraps -- a class of personal information whose content is scribbled on Post-it notes, scrawled on corners of random sheets of paper, buried inside the bodies of e-mail messages sent to ourselves, or typed haphazardly into text files. Information scraps hold our great ideas, sketches, notes, reminders, driving directions, and even our poetry. We define information scraps to be the body of personal information that is held outside of its natural or We have much still to learn about these loose forms of information capture. Why are they so often held outside of our traditional PIM locations and instead on Post-its or in text files? Why must we sometimes go around our traditional PIM applications to hold on to our scraps, such as by e-mailing ourselves? What are information scraps' role in the larger space of personal information management, and what do they uniquely offer that we find so appealing? If these unorganized bits truly indicate the failure of our PIM tools, how might we begin to build better tools? We have pursued these questions by undertaking a study of 27 knowledge workers. In our findings we describe information scraps from several angles: their content, their location, and the factors that lead to their use, which we identify as ease of capture, flexibility of content and organization, and avilability at the time of need. We also consider the personal emotive responses around scrap management. We present a set of design considerations that we have derived from the analysis of our study results. We present our work on an application platform, jourknow, to test some of these design and usability findings
Assessing the number of users who are excluded by domestic heating controls
This is the pre-print version of the Article. This Article is also referred to as: "Assessing the 'Design Exclusion' of Heating Controls at a Low-Cost, Low-Carbon Housing Development". - Copyright @ 2011 Taylor & FrancisSpace heating accounts for almost 60% of the energy delivered to housing which in turn accounts for nearly 27% of the total UK's carbon emissions. This study was conducted to investigate the influence of heating control design on the degree of âuser exclusionâ. This was calculated using the Design Exclusion Calculator, developed by the Engineering Design Centre at the University of Cambridge. To elucidate the capability requirements of the system, a detailed hierarchical task analysis was produced, due to the complexity of the overall task. The Exclusion Calculation found that the current design placed excessive demands upon the capabilities of at least 9.5% of the UK population over 16 years old, particularly in terms of âvisionâ, âthinkingâ and âdexterityâ requirements. This increased to 20.7% for users over 60 years old. The method does not account for the level of numeracy and literacy and so the true exclusion may be higher. Usability testing was conducted to help validate the results which indicated that 66% of users at a low-carbon housing development could not programme their controls as desired. Therefore, more detailed analysis of the cognitive demands placed upon the users is required to understand where problems within the programming process occur. Further research focusing on this cognitive interaction will work towards a solution that may allow users to behave easily in a more sustainable manner
Observation of surface gap solitons in semi-infinite waveguide arrays
We report on the first observation of surface gap solitons, recently
predicted to exist at the interface between uniform and periodic dielectric
media with defocusing nonlinearity [Ya.V. Kartashov et al., Phys. Rev. Lett.
96, 073901 (2006). We demonstrate strong self-trapping at the edge of a LiNbO_3
waveguide array and the formation of staggered surface solitons with
propagation constant inside the first photonic band gap. We study the crossover
between linear repulsion and nonlinear attraction at the surface, revealing the
mechanism of nonlinearity-mediated stabilization of the surface gap modes.Comment: 4 pages, 5 figure
The potential for gamma-emitting radionuclides to contribute to an understanding of erosion processes in South Africa
Several research projects undertaken by the authors and others over the last 14 years have used fallout and geogenic radionuclides for understanding erosion processes and sediment yield dynamics in South Africa over the last 100â200 years as European settlers colonised the interior plains and plateaux of the country and imported new livestock and farming techniques to the region. These projects have used two fallout radionuclides (210Pb and 137Cs) to date sediments accumulating in reservoirs, farm dams, wetlands, alluvial fans and floodouts and have used other fallout nuclides (7Be) and long-lived geogenic radionuclides (e.g. 40K, 235U) as part of a composite fingerprint exploring contemporary sediment sources and changes to sources through time. While successful in many parts of the world, applying these techniques in Southern Africa has posed a number of challenges often not encountered elsewhere. Here we explore some of the benefits and challenges in using gamma-emitting radionuclides, especially 137Cs, in these landscapes. Benefits include the potential for discriminating gully sidewall from topsoil sources, which has helped to identify contemporary gully systems as sediment conduits, rather than sources, and for providing a time-synchronous marker horizon in a range of sedimentary environments that has helped to develop robust chronologies. Challenges include the spatial variability in soil cover on steep rocky hillslopes, which is likely to challenge assumptions about the uniformity of initial fallout nuclide distribution, the paucity of stable (non-eroding) sites in order to estimate atmospheric fallout inventories, and the limited success of 210Pb dating in some rapidly accumulating high altitude catchments where sediments often comprise significant amounts of sand and gravel. Despite these challenges we present evidence suggesting that the use of gamma-emitting radionuclides can make a significant contribution to our understanding of erosion processes and sediment yield dynamics. Future research highlighted in the conclusion will try to address current challenges and outline new projects established to address them more fully
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
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