8,295 research outputs found
Effect of Hexagonal Boron Nitride on Energy Band Gap of Graphene Antidot Structures
The zero band gap (Eg) graphene becomes narrow Eg semiconductor when graphene
is patterned with periodic array of hexagonal shaped antidots, the resultant is
the hexagonal Graphene Antidot Lattice (hGAL). Based on the number of atomic
chains between antidots, hGALs can be even and odd. The even hGALs (ehGAL) are
narrow Eg semiconductors and odd hGALs (ohGAL) are semi-metals. The Eg opening
up by hGALs is not sufficient to operate a realistic switching transistor. Also
hGAL transistors realized on Si/SiO2 substrate are suffering with low carrier
mobility and ON-OFF current ratio. In order to achieve a sizable Eg with good
mobility, AB Bernal stacked hGALs on hexagonal Boron Nitride (hBN), ABA Bernal
stacked hBN / hGAL / hBN sandwiched structures and AB misaligned hGAL /hBN
structures are reported here for the first time. Using the first principles
method the electronic structure calculations are performed. A sizable Eg of
about 1.04 eV (940+100 meV) is opened when smallest neck width medium radius
ehGAL supported on hBN and about 1.1 eV (940 + 200 meV) is opened when the same
is sandwiched between hBN layers. A band gap on the order of 71 meV is opened
for Bernal stacked ohGAL / hBN and nearly 142 meV opened for hBN / ohGAL /hBN
structures for smallest radius and width of nine atomic chains between
antidots. Unlike a misaligned graphene on hBN, the misaligned ohGAL/hBN
structure shows increased Eg. This study could open up new ways of band gap
engineering for graphene based nanostructures. Keywords: Graphene, graphene
antidots, hexagonal boron nitride, band structure, band gap engineeringComment: 14 pages, 5 figures, Innovative Systems Design and Engineering,Vol 3,
No 12 (2012
Age spreads in clusters and associations: the lithium test
We report the evidence that several low-mass stars (<~0.4 Msun) of the Orion
and Upper Scorpius clusters have lithium abundances well below the interstellar
value. Due to time-dependent depletion, our result implies stellar ages greater
than ~5 Myr, suggesting that star formation has been proceeding for a long time
in these systems.Comment: to appear in IMF@50: The Initial Mass Function 50 years later, eds.
E. Corbelli et al. (Kluwer Acad. Press), 2004, in pres
On the Chiral WZNW Phase Space, Exchange r-Matrices and Poisson-Lie Groupoids
This is a review of recent work on the chiral extensions of the WZNW phase
space describing both the extensions based on fields with generic monodromy as
well as those using Bloch waves with diagonal monodromy. The symplectic form on
the extended phase space is inverted in both cases and the chiral WZNW fields
are found to satisfy quadratic Poisson bracket relations characterized by
monodromy dependent exchange r-matrices. Explicit expressions for the exchange
r-matrices in terms of the arbitrary monodromy dependent 2-form appearing in
the chiral WZNW symplectic form are given. The exchange r-matrices in the
general case are shown to satisfy a new dynamical generalization of the
classical modified Yang-Baxter (YB) equation and Poisson-Lie (PL) groupoids are
constructed that encode this equation analogously as PL groups encode the
classical YB equation. For an arbitrary simple Lie group , exchange
r-matrices are exhibited that are in one-to-one correspondence with the
possible PL structures on and admit them as PL symmetries.Comment: Based on a lecture by L.F. at the Seminaire de Mathematiques
Superieures, Montreal, 1999; LaTeX, 21 page
Diffraction in the semiclassical description of mesoscopic devices
In pseudo integrable systems diffractive scattering caused by wedges and
impurities can be described within the framework of Geometric Theory of
Diffraction (GDT) in a way similar to the one used in the Periodic Orbit Theory
of Diffraction (POTD). We derive formulas expressing the reflection and
transition matrix elements for one and many diffractive points and apply it for
impurity and wedge diffraction. Diffraction can cause backscattering in
situations, where usual semiclassical backscattering is absent causing an
erodation of ideal conductance steps. The length of diffractive periodic orbits
and diffractive loops can be detected in the power spectrum of the reflection
matrix elements. The tail of the power spectrum shows decay
due to impurity scattering and decay due to wedge scattering.
We think this is a universal sign of the presence of diffractive scattering in
pseudo integrable waveguides.Comment: 18 pages, Latex , ep
Quantum corrections of Abelian Duality Transformations
A modification of the Abelian Duality transformations is proposed
guaranteeing that a (not necessarily conformally invariant) -model be
quantum equivalent (at least up to two loops in perturbation theory) to its
dual. This requires a somewhat non standard perturbative treatment of the {\sl
dual} -model. Explicit formulae of the modified duality transformation
are presented for a special class of block diagonal purely metric
-models.Comment: Latex 11 pages; remarks on a free model and references adde
Scaling function in AdS/CFT from the O(6) sigma model
Asymptotic behavior of the anomalous dimensions of Wilson operators with high
spin and twist is governed in planar N=4 SYM theory by the scaling function
which coincides at strong coupling with the energy density of a two-dimensional
bosonic O(6) sigma model. We calculate this function by combining the two-loop
correction to the energy density for the O(n) model with two-loop correction to
the mass gap determined by the all-loop Bethe ansatz in N=4 SYM theory. The
result is in agreement with the prediction coming from the thermodynamical
limit of the quantum string Bethe ansatz equations, but disagrees with the
two-loop stringy corrections to the folded spinning string solution.Comment: 25 pages, 2 figure
Coadjoint orbits of the Virasoro algebra and the global Liouville equation
The classification of the coadjoint orbits of the Virasoro algebra is
reviewed and is then applied to analyze the so-called global Liouville
equation. The review is self-contained, elementary and is tailor-made for the
application. It is well-known that the Liouville equation for a smooth, real
field under periodic boundary condition is a reduction of the SL(2,R)
WZNW model on the cylinder, where the WZNW field g in SL(2,R) is restricted to
be Gauss decomposable. If one drops this restriction, the Hamiltonian reduction
yields, for the field where is a constant,
what we call the global Liouville equation. Corresponding to the winding number
of the SL(2,R) WZNW model there is a topological invariant in the reduced
theory, given by the number of zeros of Q over a period. By the substitution
, the Liouville theory for a smooth is recovered in
the trivial topological sector. The nontrivial topological sectors can be
viewed as singular sectors of the Liouville theory that contain blowing-up
solutions in terms of . Since the global Liouville equation is
conformally invariant, its solutions can be described by explicitly listing
those solutions for which the stress-energy tensor belongs to a set of
representatives of the Virasoro coadjoint orbits chosen by convention. This
direct method permits to study the `coadjoint orbit content' of the topological
sectors as well as the behaviour of the energy in the sectors. The analysis
confirms that the trivial topological sector contains special orbits with
hyperbolic monodromy and shows that the energy is bounded from below in this
sector only.Comment: Plain TEX, 48 pages, final version to appear in IJMP
NGC 3603 - a Local Template for Massive Young Clusters
We present a study of the star cluster associated with the massive Galactic
HII region NGC3603 based on near-IR broad-- and narrowband observations taken
with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss
color-color diagrams and address the impact of the high UV flux on the disk
evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt
An Introduction to Community Detection in Multi-layered Social Network
Social communities extraction and their dynamics are one of the most
important problems in today's social network analysis. During last few years,
many researchers have proposed their own methods for group discovery in social
networks. However, almost none of them have noticed that modern social networks
are much more complex than few years ago. Due to vast amount of different data
about various user activities available in IT systems, it is possible to
distinguish the new class of social networks called multi-layered social
network. For that reason, the new approach to community detection in the
multi-layered social network, which utilizes multi-layered edge clustering
coefficient is proposed in the paper.Comment: M.D. Lytras et al. (Eds.): WSKS 2011, CCIS 278, pp. 185-190, 201
Local modularity measure for network clusterizations
Many complex networks have an underlying modular structure, i.e., structural
subunits (communities or clusters) characterized by highly interconnected
nodes. The modularity has been introduced as a measure to assess the
quality of clusterizations. has a global view, while in many real-world
networks clusters are linked mainly \emph{locally} among each other
(\emph{local cluster-connectivity}). Here, we introduce a new measure,
localized modularity , which reflects local cluster structure. Optimization
of and on the clusterization of two biological networks shows that the
localized modularity identifies more cohesive clusters, yielding a
complementary view of higher granularity.Comment: 5 pages, 4 figures, RevTex4; Changed conten
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