6,199 research outputs found
On geometry-dependent vortex stability and topological spin excitations on curved surfaces with cylindrical symmetry
We study the Heisenberg Model on cylindrically symmetric curved surfaces. Two
kinds of excitations are considered. The first is given by the isotropic
regime, yielding the sine-Gordon equation and -solitons are predicted. The
second one is given by the XY model, leading to a vortex turning around the
surface. Helical states are also considered, however, topological arguments can
not be used to ensure its stability. The energy and the anisotropy parameter
which stabilizes the vortex state are explicitly calculated for two surfaces:
catenoid and hyperboloid. The results show that the anisotropy and the vortex
energy depends on the underlying geometry.Comment: 10 pages, 2 figures, Accepted for publication in Phys. Lett A (2013
Podridão-de-esclerócio do melão no Estado do Ceará.
Podridao-de-esclerocio do melao no Estado do Ceara.bitstream/CNPAT-2010/8614/1/Ct-096.pd
Magnetic exchange mechanism for electronic gap opening in graphene
We show within a local self-consistent mean-field treatment that a random
distribution of magnetic adatoms can open a robust gap in the electronic
spectrum of graphene. The electronic gap results from the interplay between the
nature of the graphene sublattice structure and the exchange interaction
between adatoms.The size of the gap depends on the strength of the exchange
interaction between carriers and localized spins and can be controlled by both
temperature and external magnetic field. Furthermore, we show that an external
magnetic field creates an imbalance of spin-up and spin-down carriers at the
Fermi level, making doped graphene suitable for spin injection and other
spintronic applications.Comment: 5 pages, 5 figure
Bilayer graphene: gap tunability and edge properties
Bilayer graphene -- two coupled single graphene layers stacked as in graphite
-- provides the only known semiconductor with a gap that can be tuned
externally through electric field effect. Here we use a tight binding approach
to study how the gap changes with the applied electric field. Within a parallel
plate capacitor model and taking into account screening of the external field,
we describe real back gated and/or chemically doped bilayer devices. We show
that a gap between zero and midinfrared energies can be induced and externally
tuned in these devices, making bilayer graphene very appealing from the point
of view of applications. However, applications to nanotechnology require
careful treatment of the effect of sample boundaries. This being particularly
true in graphene, where the presence of edge states at zero energy -- the Fermi
level of the undoped system -- has been extensively reported. Here we show that
also bilayer graphene supports surface states localized at zigzag edges. The
presence of two layers, however, allows for a new type of edge state which
shows an enhanced penetration into the bulk and gives rise to band crossing
phenomenon inside the gap of the biased bilayer system.Comment: 8 pages, 3 fugures, Proceedings of the International Conference on
Theoretical Physics: Dubna-Nano200
Modeling disorder in graphene
We present a study of different models of local disorder in graphene. Our
focus is on the main effects that vacancies -- random, compensated and
uncompensated --, local impurities and substitutional impurities bring into the
electronic structure of graphene. By exploring these types of disorder and
their connections, we show that they introduce dramatic changes in the low
energy spectrum of graphene, viz. localized zero modes, strong resonances, gap
and pseudogap behavior, and non-dispersive midgap zero modes.Comment: 16 pages, lower resolution figure
Theoretical study of the competition between folding and contact interactions on the properties of polymers using self-avoid random walk algorithm
The self-avoid random walk algorithm has been extensively used in the study
of polymers. In this work we study the basic properties of the trajectories
generated with this algorithm when two interactions are added to it: contact
and folding interaction. These interactions represent the internal forces of
the polymer as well as the effect of the solvent. When independently added to
the algorithm, the contact interaction creates the compact phase while the
folding one creates the extended phase. These are the consequences of the
typical event of each interaction. On the other hand, when this typical event
is avoided there is no established phase on the system. When simultaneously
added, there is a competition between the interactions and the folding one is
dominant over the contact one. The resulting phase is always the extended one
with and without the contact interaction.Comment: 8 pages, 7 figure
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