7,819 research outputs found
Next-nearest-neighbor Tight-binding Model of Plasmons in Graphene
In this paper we investigate the influence of the next-nearest-neighbor
coupling of tight-binding model of graphene on the spectrum of plasmon
excitations. The nearest-neighbor tight-binding model was previously used to
calculate plasmon spectrum in the next paper [1]. We expand the previous
results of the paper by the next-nearest-neighbor tight-binding model. Both
methods are based on the numerical calculation of the dielectric function of
graphene and loss function. Here we compare plasmon spectrum of the
next-nearest and nearest-neighbor tight-binding models and find differences
between plasmon dispersion of two models.Comment: LaTeX, 4 pages, 4 Fig
Frequency splitting of intervalley phonons in graphene
We study the thermal distribution of intervalley phonons in a graphene sheet.
These phonons have two components with the same frequency. The degeneracy of
the two modes is preserved by weak electron-phonon coupling. A sufficiently
strong electron-phonon coupling, however, can result in a splitting into an
optical and an acoustic phonon branch, which creates a fluctuating gap in the
electronic spectrum. We describe these effects by treating the phonon
distribution within a saddle-point approximation. Fluctuations around the
saddle point indicate a Berezinskii-Kosterlitz-Thouless transition of the
acoustic branch. This transition might be observable in the polarization of
Raman scattered light.Comment: 5 pages, 1 figur
Gaps and tails in graphene and graphane
We study the density of states in monolayer and bilayer graphene in the
presence of a random potential that breaks sublattice symmetries. While a
uniform symmetry-breaking potential opens a uniform gap, a random
symmetry-breaking potential also creates tails in the density of states. The
latter can close the gap again, preventing the system to become an insulator.
However, for a sufficiently large gap the tails contain localized states with
nonzero density of states. These localized states allow the system to conduct
at nonzero temperature via variable-range hopping. This result is in agreement
with recent experimental observations in graphane by Elias {\it et al.}.Comment: 16 pages, 7 figure
The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian-lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform
An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and
magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallowmarine
carbonates of the northwestern margin of the Siberian Platform is reported. The interval was
analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary
of the Enisej River. It comprises the upper Ust’-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower
Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here
in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the
excursion, showing δ13C positive values as high as+4.6‰and least-altered 87Sr/86Sr ratios of 0.70909,
is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position
of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and
Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia,
South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are
known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary.
The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk
Formation, in which carbon isotope values show a local maximum below a decrease in the upper
part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity
sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian
times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic
intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian
on chemostratigraphic grounds contains only 10–11 magnetic intervals
Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation
Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, capping, and spacer layers is observed.
© 2010 American Institute of PhysicsFCT-PTDC/CTM/100756/2008program PESSOA EGIDE/GRICESFCT-SFRH/BD/45774/2008FCT-SFRH/BD/44635/200
Polytopality and Cartesian products of graphs
We study the question of polytopality of graphs: when is a given graph the
graph of a polytope? We first review the known necessary conditions for a graph
to be polytopal, and we provide several families of graphs which satisfy all
these conditions, but which nonetheless are not graphs of polytopes. Our main
contribution concerns the polytopality of Cartesian products of non-polytopal
graphs. On the one hand, we show that products of simple polytopes are the only
simple polytopes whose graph is a product. On the other hand, we provide a
general method to construct (non-simple) polytopal products whose factors are
not polytopal.Comment: 21 pages, 10 figure
The Casimir Force in Randall Sundrum Models
We discuss and compare the effects of one extra dimension in the Randall
Sundrum models on the evaluation of the Casimir force between two parallel
plates. We impose the condition that the result reproduce the experimental
measurements within the known uncertainties in the force and the plate
separation, and get an upper bound kR < 20 if the curvature parameter k of
AdS_5 is equal to the Planck scale. Although the upper bound decreases as k
decreases, kR ~ 12, which is the required value for solving the hierarchy
problem, is consistent with the Casimir force measurements. For the case where
the 5th dimension is infinite, the correction to the Casimir force is very
small and negligible.Comment: 16 pages, 2 figures, references added, text improved, accepted for
publication in PR
Bose-Einstein Condensation in a Trap: the Case of a Dense Condensate
We consider the Bose-Einstein condensation of atoms in a trap where the
density of particles is so high that the low density approach of Gross and
Pitaevskii will not be applicable. For this purpose we use the slave boson
representation which is valid for hard-core bosons at any density. This
description leads to the same results as the Gross-Pitaevskii approach in the
low density limit, but for higher densities, it predicts the depletion of the
order parameter field condensate in the regions where the density of the atomic
cloud is high.Comment: 6 pages RevTeX, 3 eps-figure
Single-kernel ionomic profiles are highly heritable indicators of genetic and environmental influences on elemental accumulation in maize grain (Zea mays)
Peer reviewedPublisher PD
Conductivity in Two-Dimensional Disordered Model with Anisotropic Long-Range Hopping
We consider two-dimensional system of particles localized on randomly
distributed sites of squared lattice with anisotropic transfer matrix elements
between localized sites. By summing of "diffusion ladder" and "cooperon ladder"
type vertices we calculated the conductivity for various sites and particles
densities.Comment: Latex, 10 page
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