1,679 research outputs found
Collective Coordinates Theory for Discrete Soliton Ratchets in the sine-Gordon Model
A collective coordinate theory is develop for soliton ratchets in the damped
discrete sine-Gordon model driven by a biharmonic force. An ansatz with two
collective coordinates, namely the center and the width of the soliton, is
assumed as an approximated solution of the discrete non-linear equation. The
evolution of these two collective coordinates, obtained by means of the
Generalized Travelling Wave Method, explains the mechanism underlying the
soliton ratchet and captures qualitatively all the main features of this
phenomenon. The theory accounts for the existence of a non-zero depinning
threshold, the non-sinusoidal behaviour of the average velocity as a function
of the difference phase between the harmonics of the driver, the non-monotonic
dependence of the average velocity on the damping and the existence of
non-transporting regimes beyond the depinning threshold. In particular it
provides a good description of the intriguing and complex pattern of subspaces
corresponding to different dynamical regimes in parameter space
Anomalies of ac driven solitary waves with internal modes: Nonparametric resonances induced by parametric forces
We study the dynamics of kinks in the model subjected to a
parametric ac force, both with and without damping, as a paradigm of solitary
waves with internal modes. By using a collective coordinate approach, we find
that the parametric force has a non-parametric effect on the kink motion.
Specifically, we find that the internal mode leads to a resonance for
frequencies of the parametric driving close to its own frequency, in which case
the energy of the system grows as well as the width of the kink. These
predictions of the collective coordinate theory are verified by numerical
simulations of the full partial differential equation. We finally compare this
kind of resonance with that obtained for non-parametric ac forces and conclude
that the effect of ac drivings on solitary waves with internal modes is exactly
the opposite of their character in the partial differential equation.Comment: To appear in Phys Rev
Soliton ratchets in homogeneous nonlinear Klein-Gordon systems
We study in detail the ratchet-like dynamics of topological solitons in
homogeneous nonlinear Klein-Gordon systems driven by a bi-harmonic force. By
using a collective coordinate approach with two degrees of freedom, namely the
center of the soliton, , and its width, , we show, first, that
energy is inhomogeneously pumped into the system, generating as result a
directed motion; and, second, that the breaking of the time shift symmetry
gives rise to a resonance mechanism that takes place whenever the width
oscillates with at least one frequency of the external ac force. In addition,
we show that for the appearance of soliton ratchets, it is also necesary to
break the time-reversal symmetry. We analyze in detail the effects of
dissipation in the system, calculating the average velocity of the soliton as a
function of the ac force and the damping. We find current reversal phenomena
depending on the parameter choice and discuss the important role played by the
phases of the ac force. Our analytical calculations are confirmed by numerical
simulations of the full partial differential equations of the sine-Gordon and
systems, which are seen to exhibit the same qualitative behavior. Our
results are in agreement with recent experimental work on dissipation induced
symmetry breaking.Comment: Minor corrections, several references added, accepted for publication
in Chao
Internal mode mechanism for collective energy transport in extended systems
We study directed energy transport in homogeneous nonlinear extended systems
in the presence of homogeneous ac forces and dissipation. We show that the
mechanism responsible for unidirectional motion of topological excitations is
the coupling of their internal and translation degrees of freedom. Our results
lead to a selection rule for the existence of such motion based on resonances
that explains earlier symmetry analysis of this phenomenon. The direction of
motion is found to depend both on the initial and the relative phases of the
two harmonic drivings, even in the presence of noise.Comment: Final version, to appear in Physical Review Letter
Anomalous resonance phenomena of solitary waves with internal modes
We investigate the non-parametric, pure ac driven dynamics of nonlinear
Klein-Gordon solitary waves having an internal mode of frequency . We
show that the strongest resonance arises when the driving frequency
, whereas when the resonance is weaker,
disappearing for nonzero damping. At resonance, the dynamics of the kink center
of mass becomes chaotic. As we identify the resonance mechanism as an {\em
indirect} coupling to the internal mode due to its symmetry, we expect similar
results for other systems.Comment: 4 pages, 4 figures, to appear in Phys Rev Let
On the existence of internal modes of sine-Gordon kinks
We study whether or not sine-Gordon kinks exhibit internal modes or
``quasimodes.'' By considering the response of the kinks to ac forces and
initial distortions, we show that neither intrinsic internal modes nor
``quasimodes'' exist in contrast to previous reports. However, we do identify a
different kind of internal mode bifurcating from the bottom edge of the phonon
band which arises from the discretization of the system in the numerical
simulations, thus confirming recent predictions.Comment: 4 pages, 2 figures, REVTeX, to appear as a Rapid Communication in
Phys Rev E (July 1st
Overdamped sine-Gordon kink in a thermal bath
We study the sine-Gordon kink diffusion at finite temperature in the
overdamped limit. By means of a general perturbative approach, we calculate the
first- and second-order (in temperature) contributions to the diffusion
coefficient. We compare our analytical predictions with numerical simulations.
The good agreement allows us to conclude that, up to temperatures where
kink-antikink nucleation processes cannot be neglected, a diffusion constant
linear and quadratic in temperature gives a very accurate description of the
diffusive motion of the kink. The quadratic temperature dependence is shown to
stem from the interaction with the phonons. In addition, we calculate and
compute the average value of the wave function as a function of
time and show that its width grows with . We discuss the
interpretation of this finding and show that it arises from the dispersion of
the kink center positions of individual realizations which all keep their
width.Comment: REVTeX, 12 pages, 10 figures, to appear in Phys Rev
Sustancias tóxicas y antinutricionales de Amaranthus dubius Mart. ex Thell. Efecto de la parte de la planta y la época de recolecta
Amaranth species are gaining increasing interest for human and animal
nutrition, although its use is limited due to their toxic and antinutritional
substances contents. The contents of toxic and antinutritionals substances
(oxalates, phytates, total phenols, condensate tannins, hydrolysable tannins and
cyanide) in the leaves, stems, and panicles of Amaranthus dubius Mart. ex Thell.,
not yet investigated, were evaluated in both rainy and dry seasons. The plants
were cultivated in El Néctar Hacienda, located in Merecure, in the municipality
of Acevedo, Miranda State, Venezuela (10º31’38" N, 66º33’16" W). The
concentration of oxalates, phytates, total phenols, condensate and hydrolysable
tannins were determined by classic colorimetric techniques, the cyanide being
determined by silver nitrate titration. The content of toxic and antinutrients in A. dubius ranged from 169.6 to 368.5 mg of oxalates.kg-1 DM, 0.771 to 7.482 mg
of phytates.g-1 DM, 0.47 to 1.77 mg of total phenols.g-1 DM, and 0.22 to 1.20 mg of
condensate tannins.g-1 DM. Cyanide and hydrolysable tannins were not detected.
The values of most of these substances presented differences according to harvesting
seasons and plant part (P<0.001), as well as an interaction between both factors.
The content of toxic and antinutrients was generally higher in the dry than in
the rainy season for all parts of the plant. The harvesting season and the plant
part affect the content of toxic and antinutritional substances in A.dubius, whose
values were, however, below the maximum levels allowed by regulators for human
consumption. Therefore, as raw material it does not need to be processed in order
to guarantee its harmlessnessLas especies de amaranto están generando creciente interés para la nutrición
humana y animal, aunque su uso es limitado debido al contenido de sustancias
tóxicas y antinutricionales. El contenido de sustancias tóxicas y antinutrientes
(oxalatos, fitatos, fenoles totales, taninos condensados, taninos hidrolizables y
cianuro) en las hojas, tallos y panÃculas de Amaranthus dubius Mart. ex Thell.,
no ha sido investigado, por ello se evaluaron en época lluviosa y seca. Las plantas
fueron cultivadas en la hacienda El Néctar, localizada en Merecure, municipio
Acevedo, estado Miranda, Venezuela (10º31’38'’ N, 66º33’16'’ O). La concentración
de oxalatos, fitatos, fenoles totales, taninos condensados y taninos hidrolizables
se determinaron mediante técnicas colorimétricas clásicas y el cianuro se determinó
por titulación con nitrato de plata. El contenido de tóxicos y antinutrientes
en A. dubius osciló entre 169,6-368,5 mg de oxalatos.kg-1
en biomasa seca (BS);
0,771-7,482 mg de fitatos.g-1 BS; 0,47-1,77 mg de fenoles totales.g-1 BS y 0,22-1,20
mg de taninos condensados.g-1 BS. No se detectó cianuro ni taninos hidrolizables.
Los valores de la mayorÃa de estas sustancias presentaron diferencias de acuerdo
a la época de recolecta y partes de la planta (P<0,001), asà como interacción entre
ambos factores. El contenido de tóxicos y antinutrientes fue generalmente mayor
en la época seca que en la época lluviosa para todas las partes de la planta. La
época de recolecta y la parte de la planta afectaron el contenido de sustancias
tóxicas y antinutricionales de A. dubius; sin embargo, los valores estuvieron por
debajo de los niveles máximos permitidos regulados para el consumo humano.
Por lo tanto, la materia prima no necesita ser procesada para garantizar su
inocuida
Recombination reduction on lead halide perovskite solar cells based on low temperature synthesized hierarchical TiO2 nanorods
Intensive research on the electron transport material (ETM) has been pursued to improve the efficiency of perovskite solar cells (PSCs) and decrease their cost. More importantly, the role of the ETM layer is not yet fully understood, and research on new device architectures is still needed. Here, we report the use of three-dimensional (3D) TiO2 with a hierarchical architecture based on rutile nanorods (NR) as photoanode material for PSCs. The proposed hierarchical nanorod (HNR) films were synthesized by a two-step low temperature (180 °C) hydrothermal method, and consist of TiO2 nanorod trunks with optimal lengths of 540 nm and TiO2 nanobranches with lengths of 45 nm. Different device configurations were fabricated with TiO2 structures (compact layer, NR and HNR) and CH3NH3PbI3, using different synthetic routes, as the active material. PSCs based on HNR-CH3NH3PbI3 achieved the highest power conversion efficiency compared to PSCs with other TiO2 structures. This result can be ascribed mainly to lower charge recombination as determined by impedance spectroscopy. Furthermore, we have observed that the CH3NH3PbI3 perovskite deposited by the two-step route shows higher efficiency, surface coverage and infiltration within the structure of 3D HNR than the one-step CH3NH3PbI3−xClx perovskite.This work was supported by the Universitat Jaume I (project
12I361.01/1), the Spanish MINECO (project MAT2013-47192-
C3-1-R), CONACYT-México (project CB-2010/153270) and
UNAM (PAPIIT-IN1030
Resonances in the dynamics of kinks perturbed by ac forces
We study the dynamics of kinks perturbed by an ac force, both with
and without damping. We address this issue by using a collective coordinate
theory, which allows us to reduce the problem to the dynamics of the kink
center and width. We carry out a careful analysis of the corresponding ordinary
differential equations, of Mathieu type in the undamped case, finding and
characterizing the resonant frequencies and the regions of existence of
resonant solutions. We verify the accuracy of our predictions by numerical
simulation of the full partial differential equation, showing that the
collective coordinate prediction is very accurate. Numerical simulations for
the damped case establish that the strongest resonance is the one at half the
frequency of the internal mode of the kink. In the conclusion we discuss on the
possible relevance of our results for other systems, especially the sine-Gordon
equation. We also obtain additional results regarding the equivalence between
different collective coordinate methods applied to this problem.Comment: 23 pages, 7 figures, REVTeX, accepted for publication in Phys. Rev.
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