89,075 research outputs found
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
Effect of Interactions on Molecular Fluxes and Fluctuations in the Transport Across Membrane Channels
Transport of molecules across membrane channels is investigated theoretically
using exactly solvable one-dimensional discrete-state stochastic models. An
interaction between molecules and membrane pores is modeled via a set of
binding sites with different energies. It is shown that the interaction
potential strongly influences the particle currents as well as fluctuations in
the number of translocated molecules. For small concentration gradients the
attractive sites lead to largest currents and fluctuations, while the repulsive
interactions yield the largest fluxes and dispersions for large concentration
gradients. Interaction energies that lead to maximal currents and maximal
fluctuations are the same only for locally symmetric potentials, while they
differ for the locally asymmetric potentials. The conditions for the most
optimal translocation transport with maximal current and minimal dispersion are
discussed. It is argued that in this case the interaction strength is
independent of local symmetry of the potential of mean forces. In addition, the
effect of the global asymmetry of the interaction potential is investigated,
and it is shown that it also strongly affects the particle translocation
dynamics. These phenomena can be explained by analyzing the details of the
particle entering and leaving the binding sites in the channel.Comment: submitted to J. Chem. Phy
The Static Dielectric Constant of a Colloidal Suspension
We derive an expression for the static dielectric constant of the colloidal
susp ensions based on the electrokinetic equations. The analysis assumes that
the ions have the same diffusivity, and that the double layer is much thinner
than the radius of curvature of the particles. It is shown that the dielectric
increment of the double layer polarization mechanism is originated from the
free energy stored in the salt concentration inhomogeniety. We also show that
the dominant polarization charges in the theory are at the electrodes, rather
than close to the particles.Comment: 15 pages, 1 figur
Intersecting non-SUSY -brane with chargeless 0-brane as black -brane
Unlike BPS -brane, non-supersymmetric (non-susy) -brane could be either
charged or chargeless. As envisaged in [hep-th/0503007], we construct an
intersecting non-susy -brane with chargeless non-susy -brane by taking
T-dualities along the delocalized directions of the non-susy -brane solution
delocalized in transverse directions (where ). In general
these solutions are characterized by four independent parameters. We show that
when the intersecting charged as well as chargeless non-susy -brane
with chargeless 0-brane can be mapped by a coordinate transformation to black
-brane when two of the four parameters characterizing the solution take some
special values. For definiteness we restrict our discussion to space-time
dimensions . We observe that parameters characterizing the black brane
and the related dynamics are in general in a different branch of the parameter
space from those describing the brane-antibrane annihilation process. We
demonstrate this in the two examples, namely, the non-susy D0-brane and the
intersecting non-susy D4 and D0-branes, where the solutions with the explicit
microscopic descriptions are known.Comment: 25 page
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Miniature Silicon Nanobeam Resonator Tuned by GST Phase Change Material
We report a silicon optical nanobeam resonator with central hole infiltrated with a thin layer of Ge2Sb2Te5 (GST) material. The resonances can be tuned when the GST changes its phases between the amorphous and crystalline states
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