Localization of massless Dirac particles via spatial modulations of the Fermi velocity

The electrons found in Dirac materials are notorious for being difficult to manipulate due to the Klein phenomenon and absence of backscattering. Here we investigate how spatial modulations of the Fermi velocity in two-dimensional Dirac materials can give rise to localization effects, with either full (zero-dimensional) confinement or partial (one-dimensional) confinement possible depending on the geometry of the velocity modulation. We present several exactly solvable models illustrating the nature of the bound states which arise, revealing how the gradient of the Fermi velocity is crucial for determining fundamental properties of the bound states such as the zero-point energy. We discuss the implications for guiding electronic waves in few-mode waveguides formed by Fermi velocity modulation.Comment: 9 pages, 6 figure

One-dimensional Coulomb problem in Dirac materials

We investigate the one-dimensional Coulomb potential with application to a class of quasirelativistic systems, so-called Dirac-Weyl materials, described by matrix Hamiltonians. We obtain the exact solution of the shifted and truncated Coulomb problems, with the wavefunctions expressed in terms of special functions (namely Whittaker functions), whilst the energy spectrum must be determined via solutions to transcendental equations. Most notably, there are critical bandgaps below which certain low-lying quantum states are missing in a manifestation of atomic collapse.Comment: 7 pages, 5 figure

Bielectron vortices in two-dimensional Dirac semimetals

Searching for new states of matter and unusual quasiparticles in emerging materials and especially low-dimensional systems is one of the major trends in contemporary condensed matter physics. Dirac materials, which host quasiparticles which are described by ultrarelativistic Dirac-like equations, are of a significant current interest from both a fundamental and applied physics perspective. Here we show that a pair of two-dimensional massless Dirac-Weyl fermions can form a bound state independently of the sign of the inter-particle interaction potential, as long as this potential decays at large distances faster than Kepler's inverse distance law. This leads to the emergence of a new type of energetically-favourable quasiparticle: bielectron vortices, which are double-charged and reside at zero-energy. Their bosonic nature allows for condensation and may give rise to Majorana physics without invoking a superconductor. These novel quasiparticles arguably explain a range of poorly understood experiments in gated graphene structures at low doping.Comment: 9 pages, 2 figure

Massless Dirac fermions in two dimensions: Confinement in nonuniform magnetic fields

We show how it is possible to trap two-dimensional massless Dirac fermions in spatially inhomogeneous magnetic fields, as long as the formed magnetic quantum dot (or ring) is of a slowly decaying nature. It is found that a modulation of the depth of the magnetic quantum dot leads to successive confinement-deconfinement transitions of vortexlike states with a certain angular momentum, until a regime is reached where only states with one sign of angular momentum are supported. We illustrate these characteristics with both exact solutions and a hitherto unknown quasi-exactly solvable model utilizing confluent Heun functions.Comment: 7 pages, 3 figure

Radiation effects in silicon and gallium arsenide solar cells using isotropic and normally incident radiation

Several types of silicon and gallium arsenide solar cells were irradiated with protons with energies between 50 keV and 10 MeV at both normal and isotropic incidence. Damage coefficients for maximum power relative to 10 MeV were derived for these cells for both cases of omni-directional and normal incidence. The damage coefficients for the silicon cells were found to be somewhat lower than those quoted in the Solar Cell Radiation Handbook. These values were used to compute omni-directional damage coefficients suitable for solar cells protected by coverglasses of practical thickness, which in turn were used to compute solar cell degradation in two proton-dominated orbits. In spite of the difference in the low energy proton damage coefficients, the difference between the handbook prediction and the prediction using the newly derived values was negligible. Damage coefficients for GaAs solar cells for short circuit current, open circuit voltage, and maximum power were also computed relative to 10 MeV protons. They were used to predict cell degradation in the same two orbits and in a 5600 nmi orbit. Results show the performance of the GaAs solar cells in these orbits to be superior to that of the Si cells

Examining the Present and Looking to the Future of DSS and Intelligent Systems

In many respects, the decision making capability/promise of information technology has gone unfulfilled. In fact, many organizations have not advanced much past spreadsheets when it comes to computerized decision making assistance. This research attempts to examine why this is the case, and looks to the future by asking the questions “What’s the next spreadsheet? Is there a next killer app for intelligent systems/DSS?” Fifty-eight business professionals were surveyed to assist in answering these questions. Results suggest that while the spreadsheet is still by far the most used intelligent system, continuing improvements in the ease of use of information technology are helping to allow some organizations to begin to easily test and use newer DSS technologies. As additional organizations then begin to understand the purpose and usefulness of these newer technologies, their long term impact could be substantial. Statistical results suggest that Knowledge Management and GDSS technologies have the best chance in the near term to equal the impact of spreadsheets

Quadruplex digital flight control system assessment

Described are the development and validation of a double fail-operational digital flight control system architecture for critical pitch axis functions. Architectural tradeoffs are assessed, system simulator modifications are described, and demonstration testing results are critiqued. Assessment tools and their application are also illustrated. Ultimately, the vital role of system simulation, tailored to digital mechanization attributes, is shown to be essential to validating the airworthiness of full-time critical functions such as augmented fly-by-wire systems for relaxed static stability airplanes

Does Web Site Usability Correlate with Web Site Usage?

This study compares usability ratings of Web sites, based on academic research, to actual usage data, from Nielsen Online. This is accomplished by consulting academic usability research, and using those guidelines to rate eight retail Web sites on usability. Then, this study uses “real world” data, the usage data from Nielsen Online, to longitudinally validate the academic findings. Results show that, over time, the calculated usability ratings move in the same direction and in the same magnitude as the usage data

Characterization of solar cells for space applications. Volume 14: Electrical characteristics of Hughes liquid phase epitaxy gallium arsenide solar cells as a function of intensity, temperature and irradiation

Electrical characteristics of liquid phase epitaxy, P/N gallium aluminum arsenide solar cells are presented in graphical and tabular format as a function of solar illumination intensity and temperature. The solar cells were exposed to 1 MeV electron fluences of, respectively, 0, one hundred trillion, one quadrillion, and ten quadrillion e/sq cm

Bryophytes and their distribution in the Blue Mountains region of New South Wales

The bryophytes (mosses, liverworts and hornworts) that occur in the Blue Mountains region of New South Wales (latitude 33˚–34˚ S, longitude 151˚–151˚40’ E) are listed and information is provided on their distribution in the region. Species lists are based on herbarium specimens and field collections. 348 bryophyte taxa have been recorded from 70 families, including 225 moss taxa (in 108 genera from 45 families), 120 liverwort taxa (in 51 genera from 24 families) and 3 hornwort taxa (in 3 genera from one family). The moss families with most taxa are the Pottiaceae (with 23 taxa in 13 genera), Bryaceae (with 15 taxa in 3 genera) and Fissidentaceae (with 13 taxa). The largest genera are Fissidens (13 taxa), Campylopus (9) and Macromitrium (8). The liverwort family with the most taxa is Lepidoziaceae, with 29 taxa in 10 genera. The largest liverwort genera are Frullania (11 taxa) and Riccardia (8). The species lists include collections from both bushland and urban areas. Natural features of the Blue Mountains, including topography, altitude, climate and vegetation appear to be important factors influencing the number of bryophyte species recorded from each location. The number of collections from particular locations has been considerably influenced by ease of access, particularly proximity to roads, public transport and railway stations. The species lists include many records from areas that were not accessible to the early collectors of the late 19th and early 20th centuries such as Wollemi National Park, Gardens of Stone National Park, Newnes Plateau and Kanangra-Boyd National Park