30,751 research outputs found
Exchange coupling between magnetic layers across non-magnetic superlattices
The oscillation periods of the interlayer exchange coupling are investigated
when two magnetic layers are separated by a metallic superlattice of two
distinct non-magnetic materials. In spite of the conventional behaviour of the
coupling as a function of the spacer thickness, new periods arise when the
coupling is looked upon as a function of the number of cells of the
superlattice. The new periodicity results from the deformation of the
corresponding Fermi surface, which is explicitly related to a few controllable
parameters, allowing the oscillation periods to be tuned.Comment: 13 pages; 5 figures; To appear in J. Phys.: Cond. Matte
Qualitative analysis of a scalar-tensor theory with exponential potential
A qualitative analysis of a scalar-tensor cosmological model, with an
exponential potential for the scalar field, is performed. The phase diagram for
the flat case is constructed. It is shown that solutions with an initial and
final inflationary behaviour appear. The conditions for which the scenario
favored by supernova type Ia observations becomes an attractor in the space of
the solutions are established.Comment: Latex file, 9 pages, 1 figur
Temperature effect on (2+1) experimental Kardar-Parisi-Zhang growth
We report on the effect of substrate temperature (T) on both local structure
and long-wavelength fluctuations of polycrystalline CdTe thin films deposited
on Si(001). A strong T-dependent mound evolution is observed and explained in
terms of the energy barrier to inter-grain diffusion at grain boundaries, as
corroborated by Monte Carlo simulations. This leads to transitions from
uncorrelated growth to a crossover from random-to-correlated growth and
transient anomalous scaling as T increases. Due to these finite-time effects,
we were not able to determine the universality class of the system through the
critical exponents. Nevertheless, we demonstrate that this can be circumvented
by analyzing height, roughness and maximal height distributions, which allow us
to prove that CdTe grows asymptotically according to the Kardar-Parisi-Zhang
(KPZ) equation in a broad range of T. More important, one finds positive
(negative) velocity excess in the growth at low (high) T, indicating that it is
possible to control the KPZ non-linearity by adjusting the temperature.Comment: 6 pages, 5 figure
Algoritmo genético para construção de ensembles de redes neurais: aplicação à lÃngua eletrônica.
bitstream/CNPDIA-2009-09/11846/1/CiT34_2006.pd
Dynamic RKKY interaction between magnetic moments in graphene nanoribbons
Graphene has been identified as a promising material with numerous
applications, particularly in spintronics. In this paper we investigate the
peculiar features of spin excitations of magnetic units deposited on graphene
nanoribbons and how they can couple through a dynamical interaction mediated by
spin currents. We examine in detail the spin lifetimes and identify a pattern
caused by vanishing density of states sites in pristine ribbons with armchair
borders. Impurities located on these sites become practically invisible to the
interaction, but can be made accessible by a gate voltage or doping. We also
demonstrate that the coupling between impurities can be turned on or off using
this characteristic, which may be used to control the transfer of information
in transistor-like devices.Comment: 10 pages, 10 figure
The stability of cosmological scaling solutions
We study the stability of cosmological scaling solutions within the class of
spatially homogeneous cosmological models with a perfect fluid subject to the
equation of state p_gamma=(gamma-1) rho_gamma (where gamma is a constant
satisfying 0 < gamma < 2) and a scalar field with an exponential potential. The
scaling solutions, which are spatially flat isotropic models in which the
scalar field energy density tracks that of the perfect fluid, are of physical
interest. For example, in these models a significant fraction of the current
energy density of the Universe may be contained in the scalar field whose
dynamical effects mimic cold dark matter. It is known that the scaling
solutions are late-time attractors (i.e., stable) in the subclass of flat
isotropic models. We find that the scaling solutions are stable (to shear and
curvature perturbations) in generic anisotropic Bianchi models when gamma <
2/3. However, when gamma > 2/3, and particularly for realistic matter with
gamma >= 1, the scaling solutions are unstable; essentially they are unstable
to curvature perturbations, although they are stable to shear perturbations. We
briefly discuss the physical consequences of these results.Comment: AMSTeX, 7 pages, re-submitted to Phys Rev Let
Disaster management in smart cities
The smart city concept, in which data from different systems are available, contains a multitude of critical infrastructures. This data availability opens new research opportunities in the study of the interdependency between those critical infrastructures and cascading effects solutions and focuses on the smart city as a network of critical infrastructures. This paper proposes an integrated resilience system linking interconnected critical infrastructures in a smart city to improve disaster resilience. A data-driven approach is considered, using artificial intelligence and methods to minimize cascading effects and the destruction of failing critical infrastructures and their components (at a city level). The proposed approach allows rapid recovery of infrastructures’ service performance levels after disasters while keeping the coverage of the assessment of risks, prevention, detection, response, and mitigation of consequences. The proposed approach has the originality and the practical implication of providing a decision support system that handles the infrastructures that will support the city disaster management system—make the city prepare, adapt, absorb, respond, and recover from disasters by taking advantage of the interconnections between its various critical infrastructures to increase the overall resilience capacity. The city of Lisbon (Portugal) is used as a case to show the practical application of the approach.info:eu-repo/semantics/publishedVersio
Stochastic Modelling Approach to the Incubation Time of Prionic Diseases
Transmissible spongiform encephalopathies like the bovine spongiform
encephalopathy (BSE) and the Creutzfeldt-Jakob disease (CJD) in humans are
neurodegenerative diseases for which prions are the attributed pathogenic
agents. A widely accepted theory assumes that prion replication is due to a
direct interaction between the pathologic (PrPsc) form and the host encoded
(PrPc) conformation, in a kind of an autocatalytic process. Here we show that
the overall features of the incubation time of prion diseases are readily
obtained if the prion reaction is described by a simple mean-field model. An
analytical expression for the incubation time distribution then follows by
associating the rate constant to a stochastic variable log normally
distributed. The incubation time distribution is then also shown to be log
normal and fits the observed BSE data very well. The basic ideas of the
theoretical model are then incorporated in a cellular automata model. The
computer simulation results yield the correct BSE incubation time distribution
at low densities of the host encoded protein
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