7,811 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
The Nappi-Witten string in the light-cone gauge
Some of the motivations for as well as the main points of the quantization of
the Nappi Witten string in the light cone gauge are reviewed.Comment: 21 pages, Plain Tex, to appear in the E.P. Wigner memorial volume of
Acta Physica Hungaric
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
Spectrum of stochastic evolution operators: Local matrix representation approach
A matrix representation of the evolution operator associated with a nonlinear
stochastic flow with additive noise is used to compute its spectrum. In the
weak noise limit a perturbative expansion for the spectrum is formulated in
terms of local matrix representations of the evolution operator centered on
classical periodic orbits. The evaluation of perturbative corrections is easier
to implement in this framework than in the standard Feynman diagram
perturbation theory. The result are perturbative corrections to a stochastic
analog of the Gutzwiller semiclassical spectral determinant computed to several
orders beyond what has so far been attainable in stochastic and
quantum-mechanical applications.Comment: 7 pages, 2 figures, Third approach to a problem we considered in
chao-dyn/9807034 and chao-dyn/981100
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
An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Accurate measurements of lithium abundances in young low-mass stars provide
an independent and reliable age diagnostics. Previous studies of nearby star
forming regions have identified significant numbers of Li-depleted stars,often
at levels inconsistent with the ages indicated by their luminosity. We aim at a
new and accurate analysis of Li abundances in a sample of ~100 pre-main
sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar
parameters and model atmospheres appropriate for the spectral types of the
sample stars.We compute Li abundances using published values of the equivalent
widths of the Li 6708 A doublet obtained from medium/high resolution spectra.
We find that the number of significantly Li-depleted stars in Taurus-Auriga is
greatly reduced with respect to earlier results. Only 13 stars have abundances
lower than the interstellar value by a factor of 5 or greater. All of them are
weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four
stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars
belong to the class of dispersed low-mass stars, distributed around the main
sites of current star formation. If located at the distance of Taurus-Auriga,
the stellar ages implied by the derived Li abundances are in the range 3-30
Myr, greater than the bulk of the Li-rich population with implication on the
star formation history of the region. In order to derive firm conclusions about
the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the
remaining members of the association should be obtained, in particular of the
group of stars that fall in the Li-burning region of the HR diagram.Comment: Accepted for publication in Astronomy & Astrophysics. 20 pages, 5
figure
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
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