25,576 research outputs found
Magnetization profile for impurities in graphene nanoribbons
The magnetic properties of graphene-related materials and in particular the
spin-polarised edge states predicted for pristine graphene nanoribbons (GNRs)
with certain edge geometries have received much attention recently due to a
range of possible technological applications. However, the magnetic properties
of pristine GNRs are not predicted to be particularly robust in the presence of
edge disorder. In this work, we examine the magnetic properties of GNRs doped
with transition-metal atoms using a combination of mean-field Hubbard and
Density Functional Theory techniques. The effect of impurity location on the
magnetic moment of such dopants in GNRs is investigated for the two principal
GNR edge geometries - armchair and zigzag. Moment profiles are calculated
across the width of the ribbon for both substitutional and adsorbed impurities
and regular features are observed for zigzag-edged GNRs in particular. Unlike
the case of edge-state induced magnetisation, the moments of magnetic
impurities embedded in GNRs are found to be particularly stable in the presence
of edge disorder. Our results suggest that the magnetic properties of
transition-metal doped GNRs are far more robust than those with moments arising
intrinsically due to edge geometry.Comment: submitte
Impurity segregation in graphene nanoribbons
The electronic properties of low-dimensional materials can be engineered by
doping, but in the case of graphene nanoribbons (GNR) the proximity of two
symmetry-breaking edges introduces an additional dependence on the location of
an impurity across the width of the ribbon. This introduces energetically
favorable locations for impurities, leading to a degree of spatial segregation
in the impurity concentration. We develop a simple model to calculate the
change in energy of a GNR system with an arbitrary impurity as that impurity is
moved across the ribbon and validate its findings by comparison with ab initio
calculations. Although our results agree with previous works predicting the
dominance of edge disorder in GNR, we argue that the distribution of adsorbed
impurities across a ribbon may be controllable by external factors, namely an
applied electric field. We propose that this control over impurity segregation
may allow manipulation and fine-tuning of the magnetic and transport properties
of GNRs.Comment: 5 pages, 4 figures, submitte
Exponential behavior of the interlayer exchange coupling across non-magnetic metallic superlattices
It is shown that the coupling between magnetic layers separated by
non-magnetic metallic superlattices can decay exponentially as a function of
the spacer thickness , as opposed to the usual decay. This effect
is due to the lack of constructive contributions to the coupling from extended
states across the spacer. The exponential behavior is obtained by properly
choosing the distinct metals and the superlattice unit cell composition.Comment: To appear in Phys. Rev.
Large Eddy Simulation Combined With Equivalent Diameter For Turbulent Jet Modelling And Gas Dispersion
CFD modelling combines transport phenomena and numerical approaches to solve physical problems. Although numerical modelling of flow scenarios is the cutting edge of flow modelling, there seems to be room for improvement. This paper proposes an approach for jet modelling in a low Mach number computational code. The methodology is based on the equivalent diameter and velocity profile calculated downstream from the jet leak orifice. The novel model DESQr (Diameter of Equivalent Simulation for Quicker Run) is combined with LES (Large Eddy Simulation) to calculate the gas jet profile due to accidental releases. The model is implemented in the framework of FDS (Fire Dynamics Simulator) and the open source code is modified to handle gas dispersion scenarios. Numerical findings for jet modelling and gas dispersion are compared with experimental data. The results are also compared with a commercial CFD tool. Good agreement is observed. Significant computational time reduction is achieved. A free open source CFD tool emerges and the results presented in this work are promising.33352554020th Brazilian Congress of Chemical Engineering (COBEQ)OCT 19-22, 2014Florianopolis, BRAZI
Analytical results for long time behavior in anomalous diffusion
We investigate through a Generalized Langevin formalism the phenomenon of
anomalous diffusion for asymptotic times, and we generalized the concept of the
diffusion exponent. A method is proposed to obtain the diffusion coefficient
analytically through the introduction of a time scaling factor . We
obtain as well an exact expression for for all kinds of diffusion.
Moreover, we show that is a universal parameter determined by the
diffusion exponent. The results are then compared with numerical calculations
and very good agreement is observed. The method is general and may be applied
to many types of stochastic problem
Selling a Single Item with Negative Externalities
We consider the problem of regulating products with negative externalities to
a third party that is neither the buyer nor the seller, but where both the
buyer and seller can take steps to mitigate the externality. The motivating
example to have in mind is the sale of Internet-of-Things (IoT) devices, many
of which have historically been compromised for DDoS attacks that disrupted
Internet-wide services such as Twitter. Neither the buyer (i.e., consumers) nor
seller (i.e., IoT manufacturers) was known to suffer from the attack, but both
have the power to expend effort to secure their devices. We consider a
regulator who regulates payments (via fines if the device is compromised, or
market prices directly), or the product directly via mandatory security
requirements.
Both regulations come at a cost---implementing security requirements
increases production costs, and the existence of fines decreases consumers'
values---thereby reducing the seller's profits. The focus of this paper is to
understand the \emph{efficiency} of various regulatory policies. That is,
policy A is more efficient than policy B if A more successfully minimizes
negatives externalities, while both A and B reduce seller's profits equally.
We develop a simple model to capture the impact of regulatory policies on a
buyer's behavior. {In this model, we show that for \textit{homogeneous}
markets---where the buyer's ability to follow security practices is always high
or always low---the optimal (externality-minimizing for a given profit
constraint) regulatory policy need regulate \emph{only} payments \emph{or}
production.} In arbitrary markets, by contrast, we show that while the optimal
policy may require regulating both aspects, there is always an approximately
optimal policy which regulates just one
Two-component mixture of charged particles confined in a channel: melting
The melting of a binary system of charged particles confined in a {\it
quasi}-one-dimensional parabolic channel is studied through Monte Carlo
simulations. At zero temperature the particles are ordered in parallel chains.
The melting is anisotropic and different melting temperatures are obtained
according to the spatial direction, and the different types of particles
present in the system. Melting is very different for the single-, two- and
four-chain configurations. A temperature induced structural phase transition is
found between two different four chain ordered states which is absent in the
mono-disperse system. In the mixed regime, where the two types of particles are
only slightly different, melting is almost isotropic and a thermally induced
homogeneous distribution of the distinct types of charges is observed.Comment: To appear in Journal of Physics: condensed matter ; (13 pages, 12
figures
Retraction methods and fixed point free maps with null minimal displacements on unit ball
In this paper we consider the class of H\"older-Lipschitz maps on unit ball
of a Banach space , and the question we deal with is whether for any
and there exists a fixed-point free map such that for all and . We show that if has a spreading
Schauder basis then such a map can always be built, answering a question posed
by the first author in \cite{Bar}. In the general case, using a recent approach
of R. Medina \cite{M} concerning H\"older retractions of -flat closed
convex sets, we show that for any decreasing null sequence , there exists a fixed-point free mapping on so that
for all and
. New results related to Lipschitz maps are also obtained.Comment: Comments welcom
A Review of 'Human Insecurities in Southeast Asia'
Debates on human insecurities are crucial in a changing world that witnesses high social inequality, degradation of environment, social tensions and a growing violation of human rights. Unfortunately, all these issues permeate the social structures of Southeast Asian countries in different ways. In that region civil society faces problems that are diverse, as seen in the political tensions in Thailand, the deterritorialization of indigenous peoples in Philippines and Malaysia, human rights violations in Myanmar, and numerous other challenges. Such setting demands different approaches from institutions and communities to overcome pending risks threatening their populations
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