19,517 research outputs found
Kink stability, propagation, and length scale competition in the periodically modulated sine-Gordon equation
We have examined the dynamical behavior of the kink solutions of the
one-dimensional sine-Gordon equation in the presence of a spatially periodic
parametric perturbation. Our study clarifies and extends the currently
available knowledge on this and related nonlinear problems in four directions.
First, we present the results of a numerical simulation program which are not
compatible with the existence of a radiative threshold, predicted by earlier
calculations. Second, we carry out a perturbative calculation which helps
interpret those previous predictions, enabling us to understand in depth our
numerical results. Third, we apply the collective coordinate formalism to this
system and demonstrate numerically that it accurately reproduces the observed
kink dynamics. Fourth, we report on a novel occurrence of length scale
competition in this system and show how it can be understood by means of linear
stability analysis. Finally, we conclude by summarizing the general physical
framework that arises from our study.Comment: 19 pages, REVTeX 3.0, 24 figures available from A S o
Semiclassical and Quantum Black Holes and their Evaporation, de Sitter and Anti-de Sitter Regimes, Gravitational and String Phase Transitions
An effective string theory in physically relevant cosmological and black hole
space times is reviewed. Explicit computations of the quantum string entropy,
partition function and quantum string emission by black holes (Schwarzschild,
rotating, charged, asymptotically flat, de Sitter dS and AdS space times) in
the framework of effective string theory in curved backgrounds provide an
amount of new quantum gravity results as: (i) gravitational phase transitions
appear with a distinctive universal feature: a square root branch point
singularity in any space time dimensions. This is of the type of the de Vega -
Sanchez transition for the thermal self-gravitating gas of point particles.
(ii) There are no phase transitions in AdS alone. (iii) For background,
upper bounds of the Hubble constant H are found, dictated by the quantum string
phase transition.(iv) The Hawking temperature and the Hagedorn temperature are
the same concept but in different (semiclassical and quantum) gravity regimes
respectively. (v) The last stage of black hole evaporation is a microscopic
string state with a finite string critical temperature which decays as usual
quantum strings do in non-thermal pure quantum radiation (no information
loss).(vi) New lower string bounds are given for the Kerr-Newman black hole
angular momentum and charge, which are entirely different from the upper
classical bounds. (vii) Semiclassical gravity states undergo a phase transition
into quantum string states of the same system, these states are duals of each
other in the precise sense of the usual classical-quantum (wave-particle)
duality, which is universal irrespective of any symmetry or isommetry of the
space-time and of the number or the kind of space-time dimensions.Comment: review paper, no figures. to appear in Int Jour Mod Phys
Hysteresis Switching Loops in Ag-manganite memristive interfaces
Multilevel resistance states in silver-manganite interfaces are studied both
experimentally and through a realistic model that includes as a main ingredient
the oxygen vacancies diffusion under applied electric fields. The switching
threshold and amplitude studied through Hysteresis Switching Loops are found to
depend critically on the initial state. The associated vacancy profiles further
unveil the prominent role of the effective electric field acting at the
interfaces. While experimental results validate main assumptions of the model,
the simulations allow to disentangle the microscopic mechanisms behind the
resistive switching in metal-transition metal oxide interfaces.Comment: 14 pages, 3 figures, to be published in Jour. of Appl. Phy
Tip Splittings and Phase Transitions in the Dielectric Breakdown Model: Mapping to the DLA Model
We show that the fractal growth described by the dielectric breakdown model
exhibits a phase transition in the multifractal spectrum of the growth measure.
The transition takes place because the tip-splitting of branches forms a fixed
angle. This angle is eta dependent but it can be rescaled onto an
``effectively'' universal angle of the DLA branching process. We derive an
analytic rescaling relation which is in agreement with numerical simulations.
The dimension of the clusters decreases linearly with the angle and the growth
becomes non-fractal at an angle close to 74 degrees (which corresponds to eta=
4.0 +- 0.3).Comment: 4 pages, REVTex, 3 figure
Form invariance symmetry generates a large set of FRW cosmologies
We show that Einstein's field equations for spatially flat
Friedmann-Robertson-Walker (FRW) space times have a form invariance symmetry
(FIS) realized by the form invariance transformations (FIT) which are indeed
generated by an invertible function of the source energy density. These
transformations act on the Hubble expansion rate, the energy density, and
pressure of the cosmic fluid; likewise such transformations are endowed with a
Lie group structure. Each representation of this group is associated with a
particular fluid and consequently a determined cosmology, so that, the FIS
defines a set of equivalent cosmological models. We focus our seek in the FIT
generated by a linear function because it provides a natural framework to
express the duality and also produces a large sets of cosmologies, starting
from a seed one, in several contexts as for instance in the cases of a perfect
fluid source and a scalar field driven by a potential depending linearly on the
scalar field kinetic energy density.Comment: 11 pages, 3 figures. Accepted for publication in Modern Physics
Letters A (2012
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
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