71,263 research outputs found
Selfdecomposability of Weak Variance Generalised Gamma Convolutions
Weak variance generalised gamma convolution processes are multivariate
Brownian motions weakly subordinated by multivariate Thorin subordinators.
Within this class, we extend a result from strong to weak subordination that a
driftless Brownian motion gives rise to a self-decomposable process. Under
moment conditions on the underlying Thorin measure, we show that this condition
is also necessary. We apply our results to some prominent processes such as the
weak variance alpha-gamma process, and illustrate the necessity of our moment
conditions in some cases
Domain wall switching: optimizing the energy landscape
It has recently been suggested that exchange spring media offer a way to
increase media density without causing thermal instability
(superparamagnetism), by using a hard and a soft layer coupled by exchange.
Victora has suggested a figure of merit xi = 2 E_b/mu_0 m_s H_sw, the ratio of
the energy barrier to that of a Stoner-Wohlfarth system with the same switching
field, which is 1 for a Stoner-Wohlfarth (coherently switching) particle and 2
for an optimal two-layer composite medium. A number of theoretical approaches
have been used for this problem (e.g., various numbers of coupled
Stoner-Wohlfarth layers and continuum micromagnetics). In this paper we show
that many of these approaches can be regarded as special cases or
approximations to a variational formulation of the problem, in which the energy
is minimized for fixed magnetization. The results can be easily visualized in
terms of a plot of the energy as a function of magnetic moment m_z, in which
both the switching field [the maximum slope of E(m_z)] and the stability
(determined by the energy barrier E_b) are geometrically visible. In this
formulation we can prove a rigorous limit on the figure of merit xi, which can
be no higher than 4. We also show that a quadratic anistropy suggested by Suess
et al comes very close to this limit.Comment: Acccepted for proceedings of Jan. 2007 MMM Meeting, paper BE-0
The breakage prediction for hydromechanical deep drawing based on local bifurcation theory
A criterion of sheet metal localized necking under plane stress was established based on the bifurcation theory and the characteristics theory of differential equation. In order to be capable to incorporate the directional dependence of the plastic strain rate on stress rate, Ito-Goya’s constitutive equation which gave a one to one relationship between stress rate component and plastic strain rate component was employed. The hydromechanical deep drawing process of a cylindrical cup part was simulated using the commercial software ABAQUS IMPLICIT. The onset of breakage of the part during the forming process was predicted by combining the simulation results with the local necking criterion. The proposed method is applied to the hydro-mechanical deep drawing process for A2219 aluminum alloy sheet metal to predict the breakage of the cylindrical cup part. The proposed method can be applied to the prediction of breakage in the forming of the automotive bodies
Spectrums of Black Hole in de Sitter Spacetime with Highly Damped Quasinormal Modes: High Overtone Case
Motivated by recent physical interpretation on quasinormal modes presented by
Maggiore, the adiabatic quantity method given by Kunstatter is used to
calculate the spectrums of a non-extremal Schwarzschild de Sitter black hole in
this paper, as well as electrically charged case. According to highly damped
Konoplya and Zhidenko's numerical observational results for high overtone
modes\cite{Konoplya}, we found that the asymptotic non-flat spacetime structure
leads two interesting facts as followings: (i) near inner event horizon, the
area and entropy spectrums, which are given by ,
, are equally spaced accurately. (ii) However, near
outer cosmological horizon the spectrums, which are in the form of , , are not
markedly equidistant. Finally, we also discuss the electrically charged case
and find the black holes in de Sitter spacetime have similar quantization
behavior no matter with or without charge.Comment: 12 pages, 2 firures, published versio
Relationship between the gamma-ray burst pulse width and energy due to the Doppler effect of fireballs
We study in details how the pulse width of gamma-ray bursts is related with
energy under the assumption that the sources concerned are in the stage of
fireballs. Due to the Doppler effect of fireballs, there exists a power law
relationship between the two quantities within a limited range of frequency.
The power law range and the power law index depend strongly on the observed
peak energy as well as the rest frame radiation form, and the upper and
lower limits of the power law range can be determined by . It is found
that, within the same power law range, the ratio of the of the rising
portion to that of the decaying phase of the pulses is also related with energy
in the form of power laws. A platform-power-law-platform feature could be
observed in the two relationships. In the case of an obvious softening of the
rest frame spectrum, the two power law relationships also exist, but the
feature would evolve to a peaked one. Predictions on the relationships in the
energy range covering both the BATSE and Swift bands for a typical hard burst
and a typical soft one are made. A sample of FRED (fast rise and exponential
decay) pulse bursts shows that 27 out of the 28 sources belong to either the
platform-power-law-platform feature class or the peaked feature group,
suggesting that the effect concerned is indeed important for most of the
sources of the sample. Among these bursts, many might undergo an obvious
softening evolution of the rest frame spectrum.Comment: Accepted for publication in The Astrophysical Journa
- …