4,787 research outputs found
Analysis of error growth and stability for the numerical integration of the equations of chemical kinetics
Error growth and stability analyzed for numerical integration of differential equations in chemical kinetic
Taming the Yukawa potential singularity: improved evaluation of bound states and resonance energies
Using the tools of the J-matrix method, we absorb the 1/r singularity of the
Yukawa potential in the reference Hamiltonian, which is handled analytically.
The remaining part, which is bound and regular everywhere, is treated by an
efficient numerical scheme in a suitable basis using Gauss quadrature
approximation. Analysis of resonance energies and bound states spectrum is
performed using the complex scaling method, where we show their trajectories in
the complex energy plane and demonstrate the remarkable fact that bound states
cross over into resonance states by varying the potential parameters.Comment: 8 pages, 2 tables, 1 figure. 2 mpg videos and 1 pdf table file are
available upon request from the corresponding Autho
Breathers in the weakly coupled topological discrete sine-Gordon system
Existence of breather (spatially localized, time periodic, oscillatory)
solutions of the topological discrete sine-Gordon (TDSG) system, in the regime
of weak coupling, is proved. The novelty of this result is that, unlike the
systems previously considered in studies of discrete breathers, the TDSG system
does not decouple into independent oscillator units in the weak coupling limit.
The results of a systematic numerical study of these breathers are presented,
including breather initial profiles and a portrait of their domain of existence
in the frequency-coupling parameter space. It is found that the breathers are
uniformly qualitatively different from those found in conventional spatially
discrete systems.Comment: 19 pages, 4 figures. Section 4 (numerical analysis) completely
rewritte
Lagrange-mesh calculations in momentum space
The Lagrange-mesh method is a powerful method to solve eigenequations written
in configuration space. It is very easy to implement and very accurate. Using a
Gauss quadrature rule, the method requires only the evaluation of the potential
at some mesh points. The eigenfunctions are expanded in terms of regularized
Lagrange functions which vanish at all mesh points except one. It is shown that
this method can be adapted to solve eigenequations written in momentum space,
keeping the convenience and the accuracy of the original technique. In
particular, the kinetic operator is a diagonal matrix. Observables in both
configuration space and momentum space can also be easily computed with a good
accuracy using only eigenfunctions computed in the momentum space. The method
is tested with Gaussian and Yukawa potentials, requiring respectively a small
or a great mesh to reach convergence.Comment: Extended versio
Existence of the Stark-Wannier quantum resonances
In this paper we prove the existence of the Stark-Wannier quantum resonances
for one-dimensional Schrodinger operators with smooth periodic potential and
small external homogeneous electric field. Such a result extends the existence
result previously obtained in the case of periodic potentials with a finite
number of open gaps.Comment: 30 pages, 1 figur
Generating functions for generalized binomial distributions
In a recent article a generalization of the binomial distribution associated
with a sequence of positive numbers was examined. The analysis of the
nonnegativeness of the formal expressions was a key-point to allow to give them
a statistical interpretation in terms of probabilities. In this article we
present an approach based on generating functions that solves the previous
difficulties: the constraints of nonnegativeness are automatically fulfilled, a
complete characterization in terms of generating functions is given and a large
number of analytical examples becomes available.Comment: PDFLaTex, 27 pages, 5 figure
Solitary wave dynamics in time-dependent potentials
We rigorously study the long time dynamics of solitary wave solutions of the
nonlinear Schr\"odinger equation in {\it time-dependent} external potentials.
To set the stage, we first establish the well-posedness of the Cauchy problem
for a generalized nonautonomous nonlinear Schr\"odinger equation. We then show
that in the {\it space-adiabatic} regime where the external potential varies
slowly in space compared to the size of the soliton, the dynamics of the center
of the soliton is described by Hamilton's equations, plus terms due to
radiation damping. We finally remark on two physical applications of our
analysis. The first is adiabatic transportation of solitons, and the second is
Mathieu instability of trapped solitons due to time-periodic perturbations.Comment: 38 pages, some typos corrected, one reference added, one remark adde
Path integral for a relativistic Aharonov-Bohm-Coulomb system
The path integral for the relativistic spinless Aharonov-Bohm-Coulomb system
is solved, and the energy spectra are extracted from the resulting amplitude.Comment: 6 pages, Revte
Crack Growth Studies in a welded Ni-base superalloy
It is well known that the introduction of sustained tensile loads during high-temperature
fatigue (dwell-fatigue) significantly increases the crack propagation rates in many superalloys. One
such superalloy is the Ni-Fe based Alloy 718, which is a high-strength corrosion resistant alloy used
in gas turbines and jet engines. As the problem is typically more pronounced in fine-grained
materials, the main body of existing literature is devoted to the characterization of sheets or forgings
of Alloy 718. However, as welded components are being used in increasingly demanding
applications, there is a need to understand the behavior. The present study is focused on the
interaction of the propagating crack with the complex microstructure in Alloy 718 weld metal
during cyclic and dwell-fatigue loading at 550 °C and 650 °C
Room temperature plasticity in sub-micrometer thermally grown oxide scales
Thermally grown oxides (TGOs) are generally considered to be brittle, capable of sustaining very limited plastic deformation before fracture. As they are prone to exhibit different forms of defects, the fracture toughness, typically measured to be some 1–2 MPa m1/2 [1], is typically reached well before sufficiently high stresses to induce plasticity can be applied [2]. This is particularly true at room temperature, where possible low-stress thermally activated creep mechanisms are suppressed. However, the occurrence of plasticity in e.g. Al2O3 single crystals at room temperature can occur for samples in the micrometer range [3]. Most measurements of the deformation of TGOs have been made on relatively thick scales, (\u3e1 micrometer), which are limited by the fracture originating from inherent defects. Furthermore, the studies have been limited in resolution and sensitivity, as the scales were adherent to the substrates and tested as a composite. Recently, micro-mechanical testing has been introduced as a method to evaluate mechanical behavior of TGOs on a ferritic/martensitic steel [4], where micro-cantilever bending was used to test specimen extracted from different layers in a 5–10 micrometers thick oxide. Still, the cantilever cross-section was typically several micrometers, and the very similar fracture stresses for notched and un-notched cantilevers seems to indicate that the deformation is still limited by inherent defects.
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