26,016 research outputs found
Third edge for a graphene nanoribbon: A tight-binding model calculation
The electronic and transport properties of an extended linear defect embedded
in a zigzag nanoribbon of realistic width are studied, within a tight binding
model approach. Our results suggest that such defect profoundly modify the
properties of the nanoribbon, introducing new conductance quantization values
and modifying the conductance quantization thresholds. The linear defect along
the nanoribbon behaves as an effective third edge of the system, which shows a
metallic behavior, giving rise to new conduction pathways that could be used in
nanoscale circuitry as a quantum wire.Comment: 6 pages, 6 figures. Two new figures and a few references adde
Magnetic states of linear defects in graphene monolayers: effects of strain and interaction
The combined effects of defect-defect interaction and of uniaxial or biaxial
strains of up to 10\% on the development of magnetic states on the
defect-core-localized quasi-one-dimensional electronic states generated by the
so-called 558 linear extended defect in graphene monolayers are investigated by
means of {\it ab initio} calculations. Results are analyzed on the basis of the
heuristics of the Stoner criterion. We find that conditions for the emergence
of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel
strains (along the defect direction) at both limits of isolated and interacting
558 defects. Parallel strains are shown to lead to two cooperative effects that
favor the emergence of itinerant magnetism: enhancement of the DOS of the
resonant defect states in the region of the Fermi level and tuning of the Fermi
level to the maximum of the related DOS peak. A perpendicular strain is
likewise shown to enhance the DOS of the defect states, but it also effects a
detunig of the Fermi level that shifts away from the maximum of the DOS of the
defect states, which inhibts the emergence of magnetic states. As a result,
under biaxial strains the stabilization of a magnetic state depends on the
relative magnitudes of the two components of strain.Comment: 9 pages 8 figure
Graphene kirigami as a platform for stretchable and tunable quantum dot arrays
The quantum transport properties of a graphene kirigami similar to those
studied in recent experiments are calculated in the regime of elastic,
reversible deformations. Our results show that, at low electronic densities,
the conductance profile of such structures replicates that of a system of
coupled quantum dots, characterized by a sequence of minibands and stop-gaps.
The conductance and I-V curves have different characteristics in the distinct
stages of elastic deformation that characterize the elongation of these
structures. Notably, the effective coupling between localized states is
strongly reduced in the small elongation stage, whereas in the large elongation
regime the development of strong, localized pseudomagnetic field barriers can
reinforce the coupling and reestablish resonant tunneling across the kirigami.
This provides an interesting example of interplay between geometry and
pseudomagnetic field-induced confinement. The alternating miniband and
stop-gaps in the transmission lead to I-V characteristics with negative
differential conductance in well defined energy/doping ranges. These effects
should be stable in a realistic scenario that includes edge roughness and
Coulomb interactions, as these are expected to further promote localization of
states at low energies in narrow segments of graphene nanostructures.Comment: 10 pages, 10 figure
Electrostatically confined Quantum Rings in bilayer Graphene
We propose a new system where electron and hole states are electrostatically
confined into a quantum ring in bilayer graphene. These structures can be
created by tuning the gap of the graphene bilayer using nanostructured gates or
by position-dependent doping. The energy levels have a magnetic field ()
dependence that is strikingly distinct from that of usual semiconductor quantum
rings. In particular, the eigenvalues are not invariant under a
transformation and, for a fixed total angular momentum index , their field
dependence is not parabolic, but displays two minima separated by a saddle
point. The spectra also display several anti-crossings, which arise due to the
overlap of gate-confined and magnetically-confined states.Comment: 5 pages, 6 figures, to appear in Nano Letter
Splitting of critical energies in the =0 Landau level of graphene
The lifting of the degeneracy of the states from the graphene =0 Landau
level (LL) is investigated through a non-interacting tight-binding model with
random hoppings. A disorder-driven splitting of two bands and of two critical
energies is observed by means of density of states and participation ratio
calculations. The analysis of the probability densities of the states within
the =0 LL provides some insights into the interplay of lattice and disorder
effects on the splitting process. An uneven spatial distribution of the wave
function amplitudes between the two graphene sublattices is found for the
states in between the two split peaks. It is shown that as the splitting is
increased (linear increasing with disorder and square root increasing with
magnetic field), the two split levels also get increasingly broadened, in such
a way that the proportion of the overlapped states keeps approximately constant
for a wide range of disorder or magnetic field variation.Comment: 6 figure
Form factor approach to dynamical correlation functions in critical models
We develop a form factor approach to the study of dynamical correlation
functions of quantum integrable models in the critical regime. As an example,
we consider the quantum non-linear Schr\"odinger model. We derive
long-distance/long-time asymptotic behavior of various two-point functions of
this model. We also compute edge exponents and amplitudes characterizing the
power-law behavior of dynamical response functions on the particle/hole
excitation thresholds. These last results confirm predictions based on the
non-linear Luttinger liquid method. Our results rely on a first principles
derivation, based on the microscopic analysis of the model, without invoking,
at any stage, some correspondence with a continuous field theory. Furthermore,
our approach only makes use of certain general properties of the model, so that
it should be applicable, with possibly minor modifications, to a wide class of
(not necessarily integrable) gapless one dimensional Hamiltonians.Comment: 33 page
The Possibility of Thermal Instability in Early-Type Stars Due to Alfven Waves
It was shown by dos Santos et al. the importance of Alfv\'en waves to explain
the winds of Wolf-Rayet stars. We investigate here the possible importance of
Alfv\'en waves in the creation of inhomogeneities in the winds of early-type
stars. The observed infrared emission (at the base of the wind) of early-type
stars is often larger than expected. The clumping explains this characteristic
in the wind, increasing the mean density and hence the emission measure, making
possible to understand the observed infrared, as well as the observed
enhancement in the blue wing of the line. In this study, we
investigate the formation of these clumps a via thermal instability. The
heat-loss function used, , includes physical processes such as:
emission of (continuous and line) recombination radiation; resonance line
emission excited by electron collisions; thermal bremsstrahlung; Compton
heating and cooling; and damping of Alfv\'en waves. As a result of this
heat-loss function we show the existence of two stable equilibrium regions. The
stable equilibrium region at high temperature is the diffuse medium and at low
temperature the clumps. Using this reasonable heat-loss function, we show that
the two stable equilibrium regions can coexist over a narrow range of pressures
describing the diffuse medium and the clumps.Comment: 21 pages (psfig.sty), 5 figures (included), ApJ accepted. Also
available at http://www.iagusp.usp.br/preprints/preprint.htm
Coherent states, constraint classes, and area operators in the new spin-foam models
Recently, two new spin-foam models have appeared in the literature, both
motivated by a desire to modify the Barrett-Crane model in such a way that the
imposition of certain second class constraints, called cross-simplicity
constraints, are weakened. We refer to these two models as the FKLS model, and
the flipped model. Both of these models are based on a reformulation of the
cross-simplicity constraints. This paper has two main parts. First, we clarify
the structure of the reformulated cross-simplicity constraints and the nature
of their quantum imposition in the new models. In particular we show that in
the FKLS model, quantum cross-simplicity implies no restriction on states. The
deeper reason for this is that, with the symplectic structure relevant for
FKLS, the reformulated cross-simplicity constraints, in a certain relevant
sense, are now \emph{first class}, and this causes the coherent state method of
imposing the constraints, key in the FKLS model, to fail to give any
restriction on states. Nevertheless, the cross-simplicity can still be seen as
implemented via suppression of intertwiner degrees of freedom in the dynamical
propagation. In the second part of the paper, we investigate area spectra in
the models. The results of these two investigations will highlight how, in the
flipped model, the Hilbert space of states, as well as the spectra of area
operators exactly match those of loop quantum gravity, whereas in the FKLS (and
Barrett-Crane) models, the boundary Hilbert spaces and area spectra are
different.Comment: 21 pages; statements about gamma limits made more precise, and minor
phrasing change
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