236 research outputs found
Metastable neon collisions: anisotropy and scattering length
In this paper we investigate the effective scattering length of
spin-polarized Ne*. Due to its anisotropic electrostatic interaction, its
scattering length is determined by five interaction potentials instead of one,
even in the spin-polarized case, a unique property among the Bose condensed
species and candidates. Because the interaction potentials of Ne* are not known
accurately enough to predict the value of the scattering length, we investigate
the behavior of as a function of the five phase integrals corresponding to
the five interaction potentials. We find that the scattering length has five
resonances instead of only one and cannot be described by a simple gas-kinetic
approach or the DIS approximation. However, the probability for finding a
positive or large value of the scattering length is not enhanced compared to
the single potential case. The complex behavior of is studied by comparing
a quantum mechanical five-channel numerical calculation to simpler two-channel
models. We find that the induced dipole-dipole interaction is responsible for
coupling between the different |\Omega> states, resulting in an inhomogeneous
shift of the resonance positions and widths in the quantum mechanical
calculation as compared to the DIS approach. The dependence of the resonance
positions and widths on the input potentials turns out to be rather
straightforward. The existence of two bosonic isotopes of Ne* enables us to
choose the isotope with the most favorable scattering length for efficient
evaporative cooling towards the Bose-Einstein Condensation transition, greatly
enhancing the feasibility to reach this transition.Comment: 13pages, 8 eps figures, analytical model in section V has been
remove
Collapse dynamics of trapped Bose-Einstein condensates
We analyze the implosion and subsequent explosion of a trapped condensate
after the scattering length is switched to a negative value. Our results
compare very well qualitatively and fairly well quantitatively with the results
of recent experiments at JILA.Comment: 4 pages, 3 figure
Dynamics of vortex tangle without mutual friction in superfluid He
A recent experiment has shown that a tangle of quantized vortices in
superfluid He decayed even at mK temperatures where the normal fluid was
negligible and no mutual friction worked. Motivated by this experiment, this
work studies numerically the dynamics of the vortex tangle without the mutual
friction, thus showing that a self-similar cascade process, whereby large
vortex loops break up to smaller ones, proceeds in the vortex tangle and is
closely related with its free decay. This cascade process which may be covered
with the mutual friction at higher temperatures is just the one at zero
temperature Feynman proposed long ago. The full Biot-Savart calculation is made
for dilute vortices, while the localized induction approximation is used for a
dense tangle. The former finds the elementary scenario: the reconnection of the
vortices excites vortex waves along them and makes them kinked, which could be
suppressed if the mutual friction worked. The kinked parts reconnect with the
vortex they belong to, dividing into small loops. The latter simulation under
the localized induction approximation shows that such cascade process actually
proceeds self-similarly in a dense tangle and continues to make small vortices.
Considering that the vortices of the interatomic size no longer keep the
picture of vortex, the cascade process leads to the decay of the vortex line
density. The presence of the cascade process is supported also by investigating
the classification of the reconnection type and the size distribution of
vortices. The decay of the vortex line density is consistent with the solution
of the Vinen's equation which was originally derived on the basis of the idea
of homogeneous turbulence with the cascade process. The obtained result is
compared with the recent Vinen's theory.Comment: 16 pages, 16 figures, submitted to PR
Random Convex Hulls and Extreme Value Statistics
In this paper we study the statistical properties of convex hulls of
random points in a plane chosen according to a given distribution. The points
may be chosen independently or they may be correlated. After a non-exhaustive
survey of the somewhat sporadic literature and diverse methods used in the
random convex hull problem, we present a unifying approach, based on the notion
of support function of a closed curve and the associated Cauchy's formulae,
that allows us to compute exactly the mean perimeter and the mean area enclosed
by the convex polygon both in case of independent as well as correlated points.
Our method demonstrates a beautiful link between the random convex hull problem
and the subject of extreme value statistics. As an example of correlated
points, we study here in detail the case when the points represent the vertices
of independent random walks. In the continuum time limit this reduces to
independent planar Brownian trajectories for which we compute exactly, for
all , the mean perimeter and the mean area of their global convex hull. Our
results have relevant applications in ecology in estimating the home range of a
herd of animals. Some of these results were announced recently in a short
communication [Phys. Rev. Lett. {\bf 103}, 140602 (2009)].Comment: 61 pages (pedagogical review); invited contribution to the special
issue of J. Stat. Phys. celebrating the 50 years of Yeshiba/Rutgers meeting
Integrated motor drives: state of the art and future trends
With increased need for high power density, high efficiency and high temperature capabilities in Aerospace and Automotive applications, Integrated Motor Drives (IMD) offers a potential solution. However, close physical integration of the converter and the machine may also lead to an increase in components temperature. This requires careful mechanical, structural and thermal analysis; and design of the IMD system.
This paper reviews existing IMD technologies and their thermal effects on the IMD system. The effects of the power electronics (PE) position on the IMD system and its respective thermal management concepts are also investigated. The challenges faced in designing and manufacturing of an IMD along with the mechanical and structural impacts of close physical integration is also discussed and potential solutions are provided. Potential converter topologies for an IMD like the Matrix converter, 2-level Bridge, 3-level NPC and Multiphase full bridge converters are also reviewed. Wide band gap devices like SiC and GaN and their packaging in power modules for IMDs are also discussed. Power modules components and packaging technologies are also presented
Particles-vortex interactions and flow visualization in He4
Recent experiments have demonstrated a remarkable progress in implementing
and use of the Particle Image Velocimetry (PIV) and particle tracking
techniques for the study of turbulence in He4. However, an interpretation of
the experimental data in the superfluid phase requires understanding how the
motion of tracer particles is affected by the two components, the viscous
normal fluid and the inviscid superfluid. Of a particular importance is the
problem of particle interactions with quantized vortex lines which may not only
strongly affect the particle motion, but, under certain conditions, may even
trap particles on quantized vortex cores. The article reviews recent
theoretical, numerical, and experimental results in this rapidly developing
area of research, putting critically together recent results, and solving
apparent inconsistencies. Also discussed is a closely related technique of
detection of quantized vortices negative ion bubbles in He4.Comment: To appear in the J Low Temperature Physic
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