3,257 research outputs found
Happy endings for flip graphs
We show that the triangulations of a finite point set form a flip graph that
can be embedded isometrically into a hypercube, if and only if the point set
has no empty convex pentagon. Point sets of this type include convex subsets of
lattices, points on two lines, and several other infinite families. As a
consequence, flip distance in such point sets can be computed efficiently.Comment: 26 pages, 15 figures. Revised and expanded for journal publicatio
A Skyrme lattice with hexagonal symmetry
Recently it has been found that the structure of Skyrmions has a close
analogy to that of fullerene shells in carbon chemistry. In this letter we show
that this analogy continues further, by presenting a Skyrme field that
describes a lattice of Skyrmions with hexagonal symmetry. This configuration, a
novel `domain wall' in the Skyrme model, has low energy per baryon (about 6%
above the Faddeev-Bogomolny bound) and in many ways is analogous to graphite.
By comparison to the energy per baryon of other known Skyrmions and also the
Skyrme crystal, we discuss the possibility of finding Skyrmion shells of higher
charge.Comment: 12 pages, 1 figure. To appear in Phys. Lett.
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
Spherical Tiling by 12 Congruent Pentagons
The tilings of the 2-dimensional sphere by congruent triangles have been
extensively studied, and the edge-to-edge tilings have been completely
classified. However, not much is known about the tilings by other congruent
polygons. In this paper, we classify the simplest case, which is the
edge-to-edge tilings of the 2-dimensional sphere by 12 congruent pentagons. We
find one major class allowing two independent continuous parameters and four
classes of isolated examples. The classification is done by first separately
classifying the combinatorial, edge length, and angle aspects, and then
combining the respective classifications together.Comment: 53 pages, 40 figures, spherical geometr
Grain boundaries in graphene grown by chemical vapor deposition
The scientific literature on grain boundaries (GBs) in graphene
was reviewed. The review focuses mainly on the experimental findings on
graphene grown by chemical vapor deposition (CVD) under a very wide range of
experimental conditions (temperature, pressure hydrogen/hydrocarbon ratio, gas
flow velocity and substrates). Differences were found in the GBs depending on
the origin of graphene: in micro-mechanically cleaved graphene (produced using
graphite originating from high-temperature, high-pressure synthesis), rows of
non-hexagonal rings separating two perfect graphene crystallites are found more
frequently, while in graphene produced by CVD—despite the very wide range
of growth conditions used in different laboratories—GBs with more pronounced
disorder are more frequent. In connection with the observed disorder, the stability
of two-dimensional amorphous carbon is discussed and the growth conditions
that may impact on the structure of the GBs are reviewed. The most frequently
used methods for the atomic scale characterization of the GB structures, their
possibilities and limitations and the alterations of the GBs in CVD graphene
during the investigation (e.g. under e-beam irradiation) are discussed. The effects
of GB disorder on electric and thermal transport are reviewed and the relatively
scarce data available on the chemical properties of the GBs are summarized.
GBs are complex enough nanoobjects so that it may be unlikely that two experimentally produced GBs of several microns in length could be completely
identical in all of their atomic scale details. Despite this, certain generalized
conclusions may be formulated, which may be helpful for experimentalists in
interpreting the results and in planning new experiments, leading to a more
systematic picture of GBs in CVD graphene
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