3,175 research outputs found
On the Complexity of List Ranking in the Parallel External Memory Model
We study the problem of list ranking in the parallel external memory (PEM)
model. We observe an interesting dual nature for the hardness of the problem
due to limited information exchange among the processors about the structure of
the list, on the one hand, and its close relationship to the problem of
permuting data, which is known to be hard for the external memory models, on
the other hand.
By carefully defining the power of the computational model, we prove a
permuting lower bound in the PEM model. Furthermore, we present a stronger
\Omega(log^2 N) lower bound for a special variant of the problem and for a
specific range of the model parameters, which takes us a step closer toward
proving a non-trivial lower bound for the list ranking problem in the
bulk-synchronous parallel (BSP) and MapReduce models. Finally, we also present
an algorithm that is tight for a larger range of parameters of the model than
in prior work
Smooth vortex precession in superfluid 4He
We have measured a precessing superfluid vortex line, stretched from a wire
to the wall of a cylindrical cell. By contrast to previous experiments with a
similar geometry, the motion along the wall is smooth. The key difference is
probably that our wire is substantially off center. We verify several numerical
predictions about the motion, including an asymmetry in the precession
signature, the behavior of pinning events, and the temperature dependence of
the precession.Comment: 8 pages, 8 figure
Effect of Iodine Doping on BiSrCaCuO: Charge Transfer or Interlayer Coupling?
A comparative study has been made of iodine-intercalated
BiSrCaCuO single crystal and 1 atm O
annealed BiSrCaCuO single crystal using AC
susceptibility measurement, X-ray photoemission (XPS) and angle-resolved
ultraviolet photoemission spectroscopy (ARUPS). AC susceptibility measurement
indicates that O-doped samples studied have T of 84 K,
whereas T of Iodine-doped samples studied are 80 K. XPS Cu 2p core
level data establish that the hole concentration in the CuO planes are
essentially the same for these two kinds of samples. ARUPS measurements show
that electronic structure of the normal states near the Fermi level has been
strongly affected by iodine intercalation. We conclude that the dominant effect
of iodine doping is to alter the interlayer coupling.Comment: LBL 9 pages, APS_Revtex. 5 Figures, available upon request.
UW-Madison preprin
On the transverse mode of an atom laser
The transverse mode of an atom laser beam that is outcoupled from a
Bose-Einstein condensate is investigated and is found to be strongly determined
by the mean--field interaction of the laser beam with the condensate. Since for
repulsive interactions the geometry of the coupling scheme resembles an
interferometer in momentum space, the beam is found show filamentation.
Observation of this effect would prove the transverse coherence of an atom
laser beam.Comment: 4 pages, 4 figure
Bistability and macroscopic quantum coherence in a BEC of ^7Li
We consider a Bose-Einstein condensate (BEC) of in a situation where
the density undergoes a symmetry breaking in real space. This occurs for a
suitable number of condensed atoms in a double well potential, obtained by
adding a standing wave light field to the trap potential. Evidence of
bistability results from the solution of the Gross-Pitaevskii equation. By
second quantization, we show that the classical bistable situation is in fact a
Schr\"odinger cat (SC) and evaluate the tunneling rate between the two SC
states. The oscillation between the two states is called MQC (macroscopic
quantum coherence); we study the effects of losses on MQC.Comment: 8 pages, 11 figures. e-mail: [email protected]
Superfluid Spin-down, with Random Unpinning of the Vortices
The so-called ``creeping'' motion of the pinned vortices in a rotating
superfluid involves ``random unpinning'' and ``vortex motion'' as two
physically separate processes. We argue that such a creeping motion of the
vortices need not be (biased) in the direction of an existing radial Magnus
force, nor should a constant microscopic radial velocity be assigned to the
vortex motion, in contradiction with the basic assumptions of the ``vortex
creep'' model. We point out internal inconsistencies in the predictions of this
model which arise due to this unjustified foundation that ignores the role of
the actual torque on the superfluid. The proper spin-down rate of a pinned
superfluid is then calculated and turns out to be much less than that suggested
in the vortex creep model, hence being of even less observational significance
for its possible application in explaining the post-glitch relaxations of the
radio pulsars.Comment: To be published in J. Low Temp. Phys., Vol. 139, May 2005 [Eqs 11,
15-17 here, have been revised and, may be substituted for the corresponding
ones in that paper
Energies and collapse times of symmetric and symmetry-breaking states of finite systems with a U(1) symmetry
We study quantum systems of volume V, which will exhibit the breaking of a
U(1) symmetry in the limit of V \to \infty, when V is large but finite. We
estimate the energy difference between the `symmetric ground state' (SGS),
which is the lowest-energy state that does not breaks the symmetry, and a `pure
phase vacuum' (PPV), which approaches a symmetry-breaking vacuum as V \to
\infty. Under some natural postulates on the energy of the SGS, it is shown
that PPVs always have a higher energy than the SGS, and we derive a lower bound
of the excess energy. We argue that the lower bound is O(V^0), which becomes
much larger than the excitation energies of low-lying excited states for a
large V. We also discuss the collapse time of PPVs for interacting many bosons.
It is shown that the wave function collapses in a microscopic time scale,
because PPVs are not energy eigenstates. We show, however, that for PPVs the
expectation value of any observable, which is a finite polynomial of boson
operators and their derivatives, does not collapse for a macroscopic time
scale. In this sense, the collapse time of PPVs is macroscopically long.Comment: In the revised manuscript, Eq. (22), Ref. [8], and Notes [13], [15]
and [17] have been adde
Conductivity sum rule, implication for in-plane dynamics and c-axis response
Recently observed -axis optical sum rule violations indicate non-Fermi
liquid in-plane behavior. For coherent -axis coupling, the observed flat,
nearly frequency independent -axis conductivity implies
a large in-plane scattering rate around and therefore any
pseudogap that might form at low frequency in the normal state will be smeared.
On the other hand incoherent -axis coupling places no restriction on the
value of and gives a more consistent picture of the observed sum rule
violation which, we find in some cases, can be less than half.Comment: 3 figures. To appear in PR
Evaluation of the Water Film Weber Number in Glaze Icing Scaling
Icing scaling tests were performed in the NASA Glenn Icing Research Tunnel to evaluate a new scaling method, developed and proposed by Feo for glaze icing, in which the scale liquid water content and velocity were found by matching reference and scale values of the nondimensional water-film thickness expression and the film Weber number. For comparison purpose, tests were also conducted using the constant We(sub L) method for velocity scaling. The reference tests used a full-span, fiberglass, 91.4-cm-chord NACA 0012 model with velocities of 76 and 100 knot and MVD sizes of 150 and 195 microns. Scale-to-reference model size ratio was 1:2.6. All tests were made at 0deg AOA. Results will be presented for stagnation point freezing fractions of 0.3 and 0.5
Fluctuations of Matrix Entries of Regular Functions of Wigner Matrices
We study the fluctuations of the matrix entries of regular functions of
Wigner random matrices in the limit when the matrix size goes to infinity. In
the case of the Gaussian ensembles (GOE and GUE) this problem was considered by
A.Lytova and L.Pastur in J. Stat. Phys., v.134, 147-159 (2009). Our results are
valid provided the off-diagonal matrix entries have finite fourth moment, the
diagonal matrix entries have finite second moment, and the test functions have
four continuous derivatives in a neighborhood of the support of the Wigner
semicircle law.Comment: minor corrections; the manuscript will appear in the Journal of
Statistical Physic
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