2,446 research outputs found
Collisionless Hydrodynamics of Doped Graphene in a Magnetic Field
The electrodynamics of a two-dimensional gas of massless fermions in graphene
is studied by a collisionless hydrodynamic approach. A low-energy dispersion
relation for the collective modes (plasmons) is derived both in the absence and
in the presence of a perpendicular magnetic field. The results for graphene are
compared to those for a standard two-dimensional gas of massive electrons. We
further compare the results within the classical hydrodynamic approach to the
full quantum mechanical calculation in the random phase approximation. The
low-energy dispersion relation is shown to be a good approximation at small
wave vectors. The limitations of this approach at higher order is also
discussed.Comment: 7 pages, 1 figur
Effect of Point Defects on the Optical and Transport Properties of MoS2 and WS2
Imperfections in the crystal structure, such as point defects, can strongly
modify the optical and transport properties of materials. Here, we study the
effect of point defects on the optical and DC conductivities of single layers
of semiconducting transition metal dichalcogenides with the form S,
where =Mo or W. The electronic structure is considered within a six bands
tight-binding model, which accounts for the relevant combination of
orbitals of the metal and orbitals of the chalcogen . We use the
Kubo formula for the calculation of the conductivity in samples with different
distributions of disorder. We find that and/or S defects create mid-gap
states that localize charge carriers around the defects and which modify the
optical and transport properties of the material, in agreement with recent
experiments. Furthermore, our results indicate a much higher mobility for
-doped WS in comparison to MoS
A Convolutional Neural Network for the Automatic Diagnosis of Collagen VI related Muscular Dystrophies
The development of machine learning systems for the diagnosis of rare
diseases is challenging mainly due the lack of data to study them. Despite this
challenge, this paper proposes a system for the Computer Aided Diagnosis (CAD)
of low-prevalence, congenital muscular dystrophies from confocal microscopy
images. The proposed CAD system relies on a Convolutional Neural Network (CNN)
which performs an independent classification for non-overlapping patches tiling
the input image, and generates an overall decision summarizing the individual
decisions for the patches on the query image. This decision scheme points to
the possibly problematic areas in the input images and provides a global
quantitative evaluation of the state of the patients, which is fundamental for
diagnosis and to monitor the efficiency of therapies.Comment: Submitted for review to Expert Systems With Application
Do tolerant societies demand better institutions?
The increasing ethnic heterogeneity that many societies are experiencing could be interpreted as a detrimental phenomenon, since empirical literature exists that indicates that higher levels of ethnic fractionalization induce higher levels of corruption. This paper aims to show the role of tolerance in overcoming this harmful effect of ethnic heterogeneity. To this end, a sample of 86 countries is tested for a positive association between ethnic fractionalization and corruption. It is then shown that tolerance offsets
this effect through both direct and indirect effects on corruption. In order to analyse the
indirect effects, the level of income and the freedom of the press are selected as channels, since these represent two determinants of corruption that are linked to tolerance.
Moreover, tolerance and corruption have been modelled as composites. Consequently,
Partial Least Squares path modelling (PLS-PM) has been used. For our sample, an index of tolerance towards immigrants and people of different race and an index of corruption are constructed, for which several sources are jointly utilised.
Our results appear to indicate that the adverse effect of ethnic fractionalization on corruption is offset by tolerance, which reduces corruption not only directly but also indirectly through the level of income and the freedom of the press
Self-Consistent Screening Approximation for Flexible Membranes: Application to Graphene
Crystalline membranes at finite temperatures have an anomalous behavior of
the bending rigidity that makes them more rigid in the long wavelength limit.
This issue is particularly relevant for applications of graphene in nano- and
micro-electromechanical systems. We calculate numerically the height-height
correlation function of crystalline two-dimensional membranes,
determining the renormalized bending rigidity, in the range of wavevectors
from \AA till 10 \AA in the self-consistent screening
approximation (SCSA). For parameters appropriate to graphene, the calculated
correlation function agrees reasonably with the results of atomistic Monte
Carlo simulations for this material within the range of from
\AA till 1 \AA. In the limit our data for the
exponent of the renormalized bending rigidity is compatible with the previously known analytical results for the
SCSA . However, this limit appears to be reached only for
\AA whereas at intermediate the behavior of
cannot be described by a single exponent.Comment: 5 pages, 4 figure
Thermodynamics of quantum crystalline membranes
We investigate the thermodynamic properties and the lattice stability of
two-dimensional crystalline membranes, such as graphene and related compounds,
in the low temperature quantum regime . A key role is played by
the anharmonic coupling between in-plane and out-of plane lattice modes that,
in the quantum limit, has very different consequences than in the classical
regime. The role of retardation, namely of the frequency dependence, in the
effective anharmonic interactions turns out to be crucial in the quantum
regime. We identify a crossover temperature, , between classical and
quantum regimes, which is K for graphene. Below , the
heat capacity and thermal expansion coefficient decrease as power laws with
decreasing temperature, tending to zero for as required by the
third law of thermodynamics.Comment: 13 pages, 1 figur
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