28,072 research outputs found
Magneto-optical response enhanced by Mie resonances in nanoantennas
Control of light by an external magnetic field is one of the important
methods for modulation of its intensity and polarisation. Magneto-optical
effects at the nanoscale are usually observed in magnetophotonic crystals,
nanostructured hybrid materials or magnetoplasmonic crystals. An indirect
action of an external magnetic field (e.g. through the Faraday effect) is
explained by the fact that natural materials exhibit negligible magnetism at
optical frequencies. However, the concept of metamaterials overcome this
limitation imposed by nature by designing artificial subwavelength meta-atoms
that support a strong magnetic response, usually termed as optical magnetism,
even when they are made of nonmagnetic materials. The fundamental question is
what would be the effect of the interaction between an external magnetic field
and an optically-induced magnetic response of metamaterial structures. Here we
make the first step toward answering this fundamental question and demonstrate
the multifold enhancement of the magneto-optical response of nanoantenna
lattices due to the optical magnetism.Comment: 7 pages, 5 figure
Patterns of link reciprocity in directed networks
We address the problem of link reciprocity, the non-random presence of two
mutual links between pairs of vertices. We propose a new measure of reciprocity
that allows the ordering of networks according to their actual degree of
correlation between mutual links. We find that real networks are always either
correlated or anticorrelated, and that networks of the same type (economic,
social, cellular, financial, ecological, etc.) display similar values of the
reciprocity. The observed patterns are not reproduced by current models. This
leads us to introduce a more general framework where mutual links occur with a
conditional connection probability. In some of the studied networks we discuss
the form of the conditional connection probability and the size dependence of
the reciprocity.Comment: Final version accepted for publication on Physical Review Letter
Effects of Noise in a Cortical Neural Model
Recently Segev et al. (Phys. Rev. E 64,2001, Phys.Rev.Let. 88, 2002) made
long-term observations of spontaneous activity of in-vitro cortical networks,
which differ from predictions of current models in many features. In this paper
we generalize the EI cortical model introduced in a previous paper (S.Scarpetta
et al. Neural Comput. 14, 2002), including intrinsic white noise and analyzing
effects of noise on the spontaneous activity of the nonlinear system, in order
to account for the experimental results of Segev et al.. Analytically we can
distinguish different regimes of activity, depending from the model parameters.
Using analytical results as a guide line, we perform simulations of the
nonlinear stochastic model in two different regimes, B and C. The Power
Spectrum Density (PSD) of the activity and the Inter-Event-Interval (IEI)
distributions are computed, and compared with experimental results. In regime B
the network shows stochastic resonance phenomena and noise induces aperiodic
collective synchronous oscillations that mimic experimental observations at 0.5
mM Ca concentration. In regime C the model shows spontaneous synchronous
periodic activity that mimic activity observed at 1 mM Ca concentration and the
PSD shows two peaks at the 1st and 2nd harmonics in agreement with experiments
at 1 mM Ca. Moreover (due to intrinsic noise and nonlinear activation function
effects) the PSD shows a broad band peak at low frequency. This feature,
observed experimentally, does not find explanation in the previous models.
Besides we identify parametric changes (namely increase of noise or decreasing
of excitatory connections) that reproduces the fading of periodicity found
experimentally at long times, and we identify a way to discriminate between
those two possible effects measuring experimentally the low frequency PSD.Comment: 25 pages, 10 figures, to appear in Phys. Rev.
Fitness-dependent topological properties of the World Trade Web
Among the proposed network models, the hidden variable (or good get richer)
one is particularly interesting, even if an explicit empirical test of its
hypotheses has not yet been performed on a real network. Here we provide the
first empirical test of this mechanism on the world trade web, the network
defined by the trade relationships between world countries. We find that the
power-law distributed gross domestic product can be successfully identified
with the hidden variable (or fitness) determining the topology of the world
trade web: all previously studied properties up to third-order correlation
structure (degree distribution, degree correlations and hierarchy) are found to
be in excellent agreement with the predictions of the model. The choice of the
connection probability is such that all realizations of the network with the
same degree sequence are equiprobable.Comment: 4 Pages, 4 Figures. Final version accepted for publication on
Physical Review Letter
Short term effects of irradiance on the growth of Pterocladiella capillacea (Gelidiales, Rhodophyta)
Pterocladiella capillacea has been economically exploited for agar extraction in the Azores for many years. Harvesting dropped to a full stop in the early 1990s due to a population collapse, but restarted in 2013. Since then it has been intensively harvested and overexploitation must be prevented, with both sustainable harvesting and effective cultivation practices. This study represents the first attempt to determine optimal conditions for P. capillacea production in the Azores, and evaluates its vegetative growth in two experiments using von Stosch’s medium designed to test entire thallus and tips portions response to different irradiances (30, 70 and 150 μmol photons m¯² s¯¹). The best relative growth rate (RGR) was recorded at 150 μmol photons m¯² s¯¹ for the entire thalli and tips after two-weeks and three-weeks, respectively, indicating that an acclimation period is necessary to assure the growth of this alga under experimental conditions. Higher RGR was obtained at higher irradiance (3.98 ± 2.10% fm day¯¹), but overall, growth rates were low or negative. Epiphytes were a serious problem towards the end of the entire thallus experiments, where Feldmannia irregularis proliferate at all irradiances. Future cultivation approaches complemented with other relevant environmental factors (e.g. pH, photoperiod, salinity), are recommended.FCT – Fundação para a Ciência e Tecnologia projects UID/BIA/00329/2013, 2015 - 2018 and UID/BIA/00329/2019, CIRN (Centro de Investigação de Recursos Naturais, University of the Azores), and CIIMAR (Interdisciplinary Centre of Marine and Environmental Research, Porto, Portugal). RFP was supported by a doctoral grant M3.1.2/F/024/2011, Fundo Regional para a Ciência e Tecnologia.info:eu-repo/semantics/publishedVersio
On Multidimensional Axisymmetric Oscillations of a Collisional Cold Plasma
We study the influence of the friction term on the radially symmetric
solutions of the repulsive Euler-Poisson equations with a non-zero background,
corresponding to cold plasma oscillations in many spatial dimensions. It is
shown that for any arbitrarily small constant non-negative constant friction
coefficient, there exists a neighborhood of the zero stationary solution in the
norm such that the solution of the Cauchy problem with initial data
belonging to this neighborhood remains globally smooth in time. Moreover, this
solution stabilizes to the zero as . This result contrasts with the
situation of zero friction, where any small deviation from the zero equilibrium
generally leads to a blow-up. Our method allows us to estimate the lifetime of
smooth solutions. Further, we prove that for any initial data, one can find
such friction coefficient that the respective solution to the Cauchy problem
keeps smoothness for all and stabilizes to zero. We also present the
results of a numerical experiment for a physically reasonable situation, which
allows us to estimate the value of the friction coefficient, which makes it
possible to suppress the formation of singularities of the solution.Comment: 16 pages, 2 figure
Pleijelius longae n. gen., n. sp., un poliqueto abisal sorprendente del atlántico noroccidental (Polychaeta: Hesionidae)
A new genus and species, Pleijelius longae n. gen., n. sp., is proposed for a small hesionid from deep water off Woods Hole, in the Northwestern Atlantic Ocean. The new genus can be separated from other members of the family by having six pairs of enlarged anterior cirri, abundant dorsally-oriented notochaetae, provided with a single row of 2-4 denti- cles, and 10 cushion-shaped marginal pharynx papillae. A cladistic analysis indicates that this new taxon falls within Hesionidae and is the sister group to Hesioninae. A key to all ‘hesionid’ genera is also included.Un nuevo género y especie, Pleijelius longae n. gen., n. sp., es propuesto para un pequeño hesiónido sorprendente abisal recogido frente a Woods Hole, en el Atlántico Noroccidental. El nuevo género puede sepa- rarse de los otros de la familia por contar con seis pares de cirros anteriores alargados, notochaetas abundantes dirigidas dor - salmente, provistas con una hilera de 2-4 dentículos, y faringe con 10 papilas redondeadas marginales. Un análsis cladístico indica que el nuevo género pertenece a Hesionidae y es el grupo hermano de los Hesioninae. También se presenta una clave para todos los géneros de “hesiónidos” del mundo.Fil: Salazar Vallejo, S. I.. El Colegio de la Frontera del Sur; MéxicoFil: Orensanz, Jose Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Nacional Patagónico; Argentin
Dipolar ground state of planar spins on triangular lattices
An infinite triangular lattice of classical dipolar spins is usually
considered to have a ferromagnetic ground state. We examine the validity of
this statement for finite lattices and in the limit of large lattices. We find
that the ground state of rectangular arrays is strongly dependent on size and
aspect ratio. Three results emerge that are significant for understanding the
ground state properties: i) formation of domain walls is energetically favored
for aspect ratios below a critical valu e; ii) the vortex state is always
energetically favored in the thermodynamic limit of an infinite number of
spins, but nevertheless such a configuration may not be observed even in very
large lattices if the aspect ratio is large; iii) finite range approximations
to actual dipole sums may not provide the correct ground sta te configuration
because the ferromagnetic state is linearly unstable and the domain wall energy
is negative for any finite range cutoff.Comment: Several short parts have been rewritten. Accepted for publication as
a Rapid Communication in Phys. Rev.
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