4,487 research outputs found
Energy Gaps in Graphene Nanoribbons
Based on a first-principles approach, we present scaling rules for the band
gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs
considered have either armchair or zigzag shaped edges on both sides with
hydrogen passivation. Both varieties of ribbons are shown to have band gaps.
This differs from the results of simple tight-binding calculations or solutions
of the Dirac's equation based on them. Our {\it ab initio} calculations show
that the origin of energy gaps for GNRs with armchair shaped edges arises from
both quantum confinement and the crucial effect of the edges. For GNRs with
zigzag shaped edges, gaps appear because of a staggered sublattice potential on
the hexagonal lattice due to edge magnetization. The rich gap structure for
ribbons with armchair shaped edges is further obtained analytically including
edge effects. These results reproduce our {\it ab initio} calculation results
very well
Cavitation scaling experiments with headforms : bubble dynamics
Utilizing some novel instrumentation which allowed detection and location of individual cavitation bubbles in flows around headforms. Ceccio and Brennen (1991 and 1989) recently examined the interaction between individual bubbles and the structure of the boundary layer and flow field in which the bubble is growing and collapsing. They were able to show that individual bubbles are often fissioned by the fluid shear and that this process can significantly effect the acoustic signal produced by the collapse. Furthermore they were able to demonstrate a relationship between the number of cavitation events and the nuclei number distribution measured by holographic methods in the upstream flow. More recently Kumar and Brenncn (1991-1992) have closely examined further statistical properties of the acoustical signals from individual cavitation bubbles on two different headformsm in order to learn more about the bubble/flow interactions.
However the above experiments were all conducted in the same facility with the same size of headform (5.08cm in diameter) and over a fairly narrow range of flow velocities (around 9m/s). Clearly this raises the issue of how the phenomena identified in those earlier experiments change with changes of speed, scale and facility. The present paper will describe experiments conducted in order to try to answer some of these important qucstions regarding the scaling of the cavitation phenomena. We present data from experiments conducted in the Large Cavitation Channel of the David Taylor Research Center in Memphis, Tennessee, on similar headforms which are 5.08, 25.4 and 50.8cm in diameter for speeds ranging up to 15m/s and for a range of cavitation numbers. In this paper we focus on visual observations of the cavitation patterns and changes in these patterns with speed and headform size
Cavitation Scaling Experiments with Axisymmetric Bodies
Several experiments by Ceccio and Brennen (1991, 1989) and Kumar and Brennen (1992, 1991) have closely examined the interaction between individual cavitation bubbles and the boundary layer, as well as statistical properties of the acoustical signals produced by the bubble collapse. All of these experiments were, however, conducted in the same facility with the same headform size (5.08cm in diameter) and over a fairly narrow range of flow velocities (around 9m/s). Clearly this raises the issue of how the phenomena identified change with speed, scale and facility. The present paper describes experiments conducted in order to try to answer some of these important questions regarding the scaling of the cavitation phenomena. The experiments were conducted in the Large Cavitation Channel of the David Taylor Research Center in Memphis Tennessee, on geometrically similar Schiebe headforms which are 5.08, 25.4 and 50.8cm in diameter for speeds ranging up to 15m/s and for a range of cavitation numbers
APPLICABILITY OF FOUR LOCALIZED-CALIBRATION METHODS IN UNDERWATER MOTION ANALYSIS
Four different localized-calibration methods were developed based on the DLT (direct linear transformation) algorithm in an effort to reduce the error due to refraction in underwater motion analysis. Their applicability in underwater motion analysis was assessed based on a simulated 3D calibration trial with 2 cameras and a hexahedral calibration frame. It was concluded from the analysis of the calibration results that (a) all methods substantially reduced the maximum reconstruction error and demonstrated the potential to minimize object space deformation, (b) localization methods based on overlapped control volumes/areas revealed superior performance than those based on distinct volumes/areas, and (c) the 2D DLT-based localization algorithm provided more accurate object space reconstruction than the 3D DLT-based algorithm
Electron Beam Supercollimation in Graphene Superlattices
Although electrons and photons are intrinsically different, importing useful
concepts in optics to electronics performing similar functions has been
actively pursued over the last two decades. In particular, collimation of an
electron beam is a long-standing goal. We show that ballistic propagation of an
electron beam with virtual no spatial spreading or diffraction, without a
waveguide or external magnetic field, can be achieved in graphene under an
appropriate class of experimentally feasible one-dimensional external periodic
potentials. The novel chiral quasi-one-dimensional metallic state that the
charge carriers are in originates from a collapse of the intrinsic helical
nature of the charge carriers in graphene owing to the superlattice potential.
Beyond providing a new way to constructing chiral one-dimensional states in two
dimensions, our findings should be useful in graphene-based electronic devices
(e.g., for information processing) utilizing some of the highly developed
concepts in optics.Comment: 7 pages, 4 figures (including supporting online material), published
online in Nano Letter
Long Term Financial Impacts of Drought Management Strategies
This paper analyzes the financial implications of drought management strategies for a model ranch in South Texas. The 2006 drought that affected Texas livestock producers left many cattlemen asking the age old question, "Should I cull to reduce my herd size or purchase feed to maintain my current herd".Livestock Production/Industries,
New Generation of Massless Dirac Fermions in Graphene under External Periodic Potentials
We show that new massless Dirac fermions are generated when a slowly varying
periodic potential is applied to graphene. These quasiparticles, generated near
the supercell Brillouin zone boundaries with anisotropic group velocity, are
different from the original massless Dirac fermions. The quasiparticle
wavevector (measured from the new Dirac point), the generalized pseudospin
vector, and the group velocity are not collinear. We further show that with an
appropriate periodic potential of triangular symmetry, there exists an energy
window over which the only available states are these quasiparticles, thus,
providing a good system to probe experimentally the new massless Dirac
fermions. The required parameters of external potentials are within the realm
of laboratory conditions.Comment: 4 pages, 4 figure
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