20,254 research outputs found
Dirac model of electronic transport in graphene antidot barriers
In order to use graphene for semiconductor applications, such as transistors
with high on/off ratios, a band gap must be introduced into this otherwise
semimetallic material. A promising method of achieving a band gap is by
introducing nanoscale perforations (antidots) in a periodic pattern, known as a
graphene antidot lattice (GAL). A graphene antidot barrier (GAB) can be made by
introducing a 1D GAL strip in an otherwise pristine sheet of graphene. In this
paper, we will use the Dirac equation (DE) with a spatially varying mass term
to calculate the electronic transport through such structures. Our approach is
much more general than previous attempts to use the Dirac equation to calculate
scattering of Dirac electrons on antidots. The advantage of using the DE is
that the computational time is scale invariant and our method may therefore be
used to calculate properties of arbitrarily large structures. We show that the
results of our Dirac model are in quantitative agreement with tight-binding for
hexagonal antidots with armchair edges. Furthermore, for a wide range of
structures, we verify that a relatively narrow GAB, with only a few antidots in
the unit cell, is sufficient to give rise to a transport gap
Electronic and optical properties of graphene antidot lattices: Comparison of Dirac and tight-binding models
The electronic properties of graphene may be changed from semimetallic to
semiconducting by introducing perforations (antidots) in a periodic pattern.
The properties of such graphene antidot lattices (GALs) have previously been
studied using atomistic models, which are very time consuming for large
structures. We present a continuum model that uses the Dirac equation (DE) to
describe the electronic and optical properties of GALs. The advantages of the
Dirac model are that the calculation time does not depend on the size of the
structures and that the results are scalable. In addition, an approximation of
the band gap using the DE is presented. The Dirac model is compared with
nearest-neighbour tight-binding (TB) in order to assess its accuracy. Extended
zigzag regions give rise to localized edge states, whereas armchair edges do
not. We find that the Dirac model is in quantitative agreement with TB for GALs
without edge states, but deviates for antidots with large zigzag regions.Comment: 15 pages, 7 figures. Accepted by Journal of Physics: Condensed matte
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Sand Penetration By High-Speed Projectiles
Tungsten projectiles were shot into sand at velocities between 600 and 2200 m/s. Penetration was maximum at about 775 m/s. Below that velocity, projectiles were apparently stabilized by a fin set. Above that velocity, projectiles were broken by transverse loads. High-speed penetration resulted in comminution of sand particles, reducing their size by about 1000 times.Mechanical Engineerin
Synthesis and analysis of jet fuel from shale oil and coal syncrudes
Thirty-two jet fuel samples of varying properties were produced from shale oil and coal syncrudes, and analyzed to assess their suitability for use. TOSCO II shale oil and H-COAL and COED syncrudes were used as starting materials. The processes used were among those commonly in use in petroleum processing-distillation, hydrogenation and catalytic hydrocracking. The processing conditions required to meet two levels of specifications regarding aromatic, hydrogen, sulfur and nitrogen contents at two yield levels were determined and found to be more demanding than normally required in petroleum processing. Analysis of the samples produced indicated that if the more stringent specifications of 13.5% hydrogen (min.) and 0.02% nitrogen (max.) were met, products similar in properties to conventional jet fuels were obtained. In general, shale oil was easier to process (catalyst deactivation was seen when processing coal syncrudes), consumed less hydrogen and yielded superior products. Based on these considerations, shale oil appears to be preferred to coal as a petroleum substitute for jet fuel production
Bubble coalescence in breathing DNA: Two vicious walkers in opposite potentials
We investigate the coalescence of two DNA-bubbles initially located at weak
segments and separated by a more stable barrier region in a designed construct
of double-stranded DNA. The characteristic time for bubble coalescence and the
corresponding distribution are derived, as well as the distribution of
coalescence positions along the barrier. Below the melting temperature, we find
a Kramers-type barrier crossing behaviour, while at high temperatures, the
bubble corners perform drift-diffusion towards coalescence. The results are
obtained by mapping the bubble dynamics on the problem of two vicious walkers
in opposite potentials.Comment: 7 pages, 4 figure
Weak Gravitational Lensing by a Sample of X-ray Luminous Clusters of Galaxies -- III. Serendipitous Weak Lensing Detections of Dark and Luminous Mass Concentrations
In the course of a weak gravitational lensing survey of 39 clusters of
galaxies,covering a total sky area of ~1 square degree, we have serendipitously
discovered mass concentrations in the fields of A1705 and A1722 which are most
probably not associated with the main cluster target. By combining weak lensing
information with two-color galaxy photometry in fields centered on our sample
clusters, we identify a new cluster candidate at z~0.5 in the field of A1705.
This cluster candidate also displays strong lensing in the form of a giant
luminous arc. The new mass concentration in the field of A1722 also seems to be
associated with an optically luminous cluster of galaxies at z~0.5, but in this
case there is some evidence for additional structures along the line of sight
that may contribute to the lensing signal. A third cluster, A959, has a dark
sub-clump which shows interesting morphological evidence in the mass map for
being associated with the main cluster. This is the first case where there is
any significant evidence for a physical association between a dark sub-clump
(discovered from weak lensing) and a normal cluster. Analysis of archival X-ray
data shows that the three new mass concentrations are not firmly detected in
X-rays and that they are X-ray underluminous.Comment: 14 pages, 10 figures, version accepted by ApJ. See
http://www.nordita.dk/~dahle/paper3.ps.gz for a version with high-resolution
figures and Fig.5 in colo
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