3,780 research outputs found
Transport, Aharonov-Bohm, and Topological Effects in Graphene Molecular Junctions and Graphene Nanorings
The unique ultra-relativistic, massless, nature of electron states in
two-dimensional extended graphene sheets, brought about by the honeycomb
lattice arrangement of carbon atoms in two-dimensions, provides ingress to
explorations of fundamental physical phenomena in graphene nanostructures. Here
we explore the emergence of new behavior of electrons in atomically precise
segmented graphene nanoribbons (GNRs) and graphene rings with the use of
tight-binding calculations, non-equilibrium Green's function transport theory,
and a newly developed Dirac continuum model that absorbs the
valence-to-conductance energy gaps as position-dependent masses, including
topological-in-origin mass-barriers at the contacts between segments. Through
transport investigations in variable-width segmented GNRs with armchair,
zigzag, and mixed edge terminations we uncover development of new
Fabry-Perot-like interference patterns in segmented GNRs, a crossover from the
ultra-relativistic massless regime, characteristic of extended graphene
systems, to a massive relativistic behavior in narrow armchair GNRs, and the
emergence of nonrelativistic behavior in zigzag-terminated GNRs. Evaluation of
the electronic states in a polygonal graphene nanoring under the influence of
an applied magnetic field in the Aharonov-Bohm regime, and their analysis with
the use of a relativistic quantum-field theoretical model, unveils development
of a topological-in-origin zero-energy soliton state and charge
fractionization. These results provide a unifying framework for analysis of
electronic states, coherent transport phenomena, and the interpretation of
forthcoming experiments in segmented graphene nanoribbons and polygonal rings.Comment: 5 figures. For related papers, see
http://www.prism.gatech.edu/~ph274cy/. in J. Phys. Chem. C (2015), article
ASAP. arXiv admin note: substantial text overlap with arXiv:1502.0020
Choosing effective methods for design diversity - How to progress from intuition to science
Design diversity is a popular defence against design faults in safety critical systems. Design diversity is at times pursued by simply isolating the development teams of the different versions, but it is presumably better to "force" diversity, by appropriate prescriptions to the teams. There are many ways of forcing diversity. Yet, managers who have to choose a cost-effective combination of these have little guidance except their own intuition. We argue the need for more scientifically based recommendations, and outline the problems with producing them. We focus on what we think is the standard basis for most recommendations: the belief that, in order to produce failure diversity among versions, project decisions should aim at causing "diversity" among the faults in the versions. We attempt to clarify what these beliefs mean, in which cases they may be justified and how they can be checked or disproved experimentally
Recurrence for persistent random walks in two dimensions
We discuss the question of recurrence for persistent, or Newtonian, random
walks in Z^2, i.e., random walks whose transition probabilities depend both on
the walker's position and incoming direction. We use results by Toth and
Schmidt-Conze to prove recurrence for a large class of such processes,
including all "invertible" walks in elliptic random environments. Furthermore,
rewriting our Newtonian walks as ordinary random walks in a suitable graph, we
gain a better idea of the geometric features of the problem, and obtain further
examples of recurrence.Comment: 20 pages, 7 figure
On Systematic Design of Protectors for Employing OTS Items
Off-the-shelf (OTS) components are increasingly used in application areas with stringent dependability requirements. Component wrapping is a well known structuring technique used in many areas. We propose a general approach to developing protective wrappers that assist in integrating OTS items with a focus on the overall system dependability. The wrappers are viewed as redundant software used to detect errors or suspicious activity and to execute appropriate recovery when possible; wrapper development is considered as a part of system integration activities. Wrappers are to be rigorously specified and executed at run time as a means of protecting OTS items against faults in the rest of the system, and the system against the OTS item's faults. Possible symptoms of erroneous behaviour to be detected by a protective wrapper and possible actions to be undertaken in response are listed and discussed. The information required for wrapper development is provided by traceability analysis. Possible approaches to implementing “protectors” in the standard current component technologies are briefly outline
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