1,259 research outputs found
Balanced Allocation on Graphs: A Random Walk Approach
In this paper we propose algorithms for allocating sequential balls into
bins that are interconnected as a -regular -vertex graph , where
can be any integer.Let be a given positive integer. In each round
, , ball picks a node of uniformly at random and
performs a non-backtracking random walk of length from the chosen node.Then
it allocates itself on one of the visited nodes with minimum load (ties are
broken uniformly at random). Suppose that has a sufficiently large girth
and . Then we establish an upper bound for the maximum number
of balls at any bin after allocating balls by the algorithm, called {\it
maximum load}, in terms of with high probability. We also show that the
upper bound is at most an factor above the lower bound that is
proved for the algorithm. In particular, we show that if we set , for every constant , and
has girth at least , then the maximum load attained by the
algorithm is bounded by with high probability.Finally, we
slightly modify the algorithm to have similar results for balanced allocation
on -regular graph with and sufficiently large girth
Loop structure of the lowest Bloch band for a Bose-Einstein condensate
We investigate analytically and numerically Bloch waves for a Bose--Einstein
condensate in a sinusoidal external potential. At low densities the dependence
of the energy on the quasimomentum is similar to that for a single particle,
but at densities greater than a critical one the lowest band becomes
triple-valued near the boundary of the first Brillouin zone and develops the
structure characteristic of the swallow-tail catastrophe. We comment on the
experimental consequences of this behavior.Comment: 4 pages, 7 figure
Gravity-induced Wannier-Stark ladder in an optical lattice
We discuss the dynamics of ultracold atoms in an optical potential
accelerated by gravity. The positions and widths of the Wannier-Stark ladder of
resonances are obtained as metastable states. The metastable Wannier-Bloch
states oscillate in a single band with the Bloch period. The width of the
resonance gives the rate transition to the continuum.Comment: 5 pages + 8 eps figures, submitted to Phys. Rev.
A mapping approach to synchronization in the "Zajfman trap": stability conditions and the synchronization mechanism
We present a two particle model to explain the mechanism that stabilizes a
bunch of positively charged ions in an "ion trap resonator" [Pedersen etal,
Phys. Rev. Lett. 87 (2001) 055001]. The model decomposes the motion of the two
ions into two mappings for the free motion in different parts of the trap and
one for a compressing momentum kick. The ions' interaction is modelled by a
time delay, which then changes the balance between adjacent momentum kicks.
Through these mappings we identify the microscopic process that is responsible
for synchronization and give the conditions for that regime.Comment: 12 pages, 9 figures; submitted to Phys Rev
Scaling property of the critical hopping parameters for the Bose-Hubbard model
Recently precise results for the boundary between the Mott insulator phase
and the superfluid phase of the homogeneous Bose-Hubbard model have become
available for arbitrary integer filling factor g and any lattice dimension d >
1. We use these data for demonstrating that the critical hopping parameters
obey a scaling relationship which allows one to map results for different g
onto each other. Unexpectedly, the mean-field result captures the dependence of
the exact critical parameters on the filling factor almost fully. We also
present an approximation formula which describes the critical parameters for d
> 1 and any g with high accuracy.Comment: 5 pages, 5 figures. to appear in EPJ
Random Scattering by Atomic Density Fluctuations in Optical Lattices
We investigate hitherto unexplored regimes of probe scattering by atoms
trapped in optical lattices: weak scattering by effectively random atomic
density distributions and multiple scattering by arbitrary atomic
distributions. Both regimes are predicted to exhibit a universal semicircular
scattering lineshape for large density fluctuations, which depend on
temperature and quantum statistics.Comment: 4 pages, 2 figure
GP attitudes and self-reported behaviour in primary care consultations for low back pain
Background. The implementation of guideline recommendations in primary care has become widespread. The treatment of low back pain (LBP) has followed suite. Research shows that the use of LBP guidelines is influenced by the believability of the underlying evidence, the GPs consultation style and uncertainties surrounding diagnosis and treatment
Superfluid Dynamics of a Bose-Einstein Condensate in a Periodic Potential
We investigate the superfluid properties of a Bose-Einstein condensate (BEC)
trapped in a one dimensional periodic potential. We study, both analytically
(in the tight binding limit) and numerically, the Bloch chemical potential, the
Bloch energy and the Bogoliubov dispersion relation, and we introduce {\it two}
different, density dependent, effective masses and group velocities. The
Bogoliubov spectrum predicts the existence of sound waves, and the arising of
energetic and dynamical instabilities at critical values of the BEC
quasi-momentum which dramatically affect its coherence properties. We
investigate the dependence of the dipole and Bloch oscillation frequencies in
terms of an effective mass averaged over the density of the condensate. We
illustrate our results with several animations obtained solving numerically the
time-dependent Gross-Pitaevskii equation.Comment: 13 pages, 7 figures, movies and published paper available at
http://www.iop.org/EJ/abstract/1367-2630/5/1/11
Periodically-dressed Bose-Einstein condensates: a superfluid with an anisotropic and variable critical velocity
Two intersecting laser beams can produce a spatially-periodic coupling
between two components of an atomic gas and thereby modify the dispersion
relation of the gas according to a dressed-state formalism. Properties of a
Bose-Einstein condensate of such a gas are strongly affected by this
modification. A Bogoliubov transformation is presented which accounts for
interparticle interactions to obtain the quasiparticle excitation spectrum in
such a condensate. The Landau critical velocity is found to be anisotropic and
can be widely tuned by varying properties of the dressing laser beams.Comment: 5 pages, 4 figure
Bloch oscillations and mean-field effects of Bose-Einstein condensates in 1-D optical lattices
We have loaded Bose-Einstein condensates into one-dimensional, off-resonant
optical lattices and accelerated them by chirping the frequency difference
between the two lattice beams. For small values of the lattice well-depth,
Bloch oscillations were observed. Reducing the potential depth further,
Landau-Zener tunneling out of the lowest lattice band, leading to a breakdown
of the oscillations, was also studied and used as a probe for the effective
potential resulting from mean-field interactions as predicted by Choi and Niu
[Phys. Rev. Lett. {\bf 82}, 2022 (1999)]. The effective potential was measured
for various condensate densities and trap geometries, yielding good qualitative
agreement with theoretical calculations.Comment: 5 pages, 3 figures; accepted for publication in Physical Review
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