2,289 research outputs found
Dissipative Transport of a Bose-Einstein Condensate
We investigate the effects of impurities, either correlated disorder or a
single Gaussian defect, on the collective dipole motion of a Bose-Einstein
condensate of Li in an optical trap. We find that this motion is damped at
a rate dependent on the impurity strength, condensate center-of-mass velocity,
and interatomic interactions. Damping in the Thomas-Fermi regime depends
universally on the disordered potential strength scaled to the condensate
chemical potential and the condensate velocity scaled to the peak speed of
sound. The damping rate is comparatively small in the weakly interacting
regime, and the damping in this case is accompanied by strong condensate
fragmentation. \textit{In situ} and time-of-flight images of the atomic cloud
provide evidence that this fragmentation is driven by dark soliton formation.Comment: 14 pages, 20 figure
Phase Coherence and Superfluid-Insulator Transition in a Disordered Bose-Einstein Condensate
We have studied the effects of a disordered optical potential on the
transport and phase coherence of a Bose-Einstein condensate (BEC) of 7Li atoms.
At moderate disorder strengths (V_D), we observe inhibited transport and
damping of dipole excitations, while in time-of-flight images, random but
reproducible interference patterns are observed. In-situ images reveal that the
appearance of interference is correlated with density modulation, without
complete fragmentation. At higher V_D, the interference contrast diminishes as
the BEC fragments into multiple pieces with little phase coherence.Comment: 4 pages, 5 figures, distortions in figures 1 and 4 have been fixed in
version 3. This paper has been accepted to PR
Dual Geometric Worm Algorithm for Two-Dimensional Discrete Classical Lattice Models
We present a dual geometrical worm algorithm for two-dimensional Ising
models. The existence of such dual algorithms was first pointed out by
Prokof'ev and Svistunov \cite{ProkofevClassical}. The algorithm is defined on
the dual lattice and is formulated in terms of bond-variables and can therefore
be generalized to other two-dimensional models that can be formulated in terms
of bond-variables. We also discuss two related algorithms formulated on the
direct lattice, applicable in any dimension. These latter algorithms turn out
to be less efficient but of considerable intrinsic interest. We show how such
algorithms quite generally can be "directed" by minimizing the probability for
the worms to erase themselves. Explicit proofs of detailed balance are given
for all the algorithms. In terms of computational efficiency the dual
geometrical worm algorithm is comparable to well known cluster algorithms such
as the Swendsen-Wang and Wolff algorithms, however, it is quite different in
structure and allows for a very simple and efficient implementation. The dual
algorithm also allows for a very elegant way of calculating the domain wall
free energy.Comment: 12 pages, 6 figures, Revtex
Knowledge Sharing: Exploring Institutional Policy and Educator Practice Through Eportfolios In Music And Writing
Many higher education institutions have embraced e-Learning and urge, or make compulsory, engagement by academics. Despite this, it is often the educators themselves who take the initiative to engage with innovative e-learning approaches. These approaches, in turn, can influence both peer- and institution-wide thinking about e-Learning. This paper focuses on the introduction or extension of ePortfolios within the creative arts at four Australian universities. Each educator adopted the ePortfolio for a different purpose, and in doing so has influenced, or is at least being monitored by, their university. All four studies have resulted in the growth, development and enrichment of teaching and learning because of the ePortfolio's facility to engage students in such activities as reflection, ongoing student-teacher dialogue, collaborative essay writing, peer evaluation, identity formation, and the documentation of skills, competencies and graduate attributes for career awareness and employability. In sharing this knowledge the studies have also influenced curriculum design and e-learning policy. The academic literature notes institutional interest in ePortfolios in relation to career preparation, demonstrating and assessing student learning, academic advising, and addressing public accountability concerns by facilitating internal and external departmental review and accreditation. Within this paper we discuss the bi-directional impact and sharing of knowledge about ePortfolio use as it occurs between institution and educator. The study findings inform future development of curriculum, policy and practice for creative arts students and academics in a variety of higher education settings. Further, the findings suggest that ePortfolios provide an efficient and transparent means to archive and access student work, and that they facilitate internal and external departmental review and broader institutional assessment. © ACPI
Dimension Spectra of Lines
This paper investigates the algorithmic dimension spectra of lines in the
Euclidean plane. Given any line L with slope a and vertical intercept b, the
dimension spectrum sp(L) is the set of all effective Hausdorff dimensions of
individual points on L. We draw on Kolmogorov complexity and geometrical
arguments to show that if the effective Hausdorff dimension dim(a, b) is equal
to the effective packing dimension Dim(a, b), then sp(L) contains a unit
interval. We also show that, if the dimension dim(a, b) is at least one, then
sp(L) is infinite. Together with previous work, this implies that the dimension
spectrum of any line is infinite
The Thresher : lucky imaging without the waste
JAH acknowledges funding from the Science and Technology Facilities Council of the United Kingdom.In traditional lucky imaging (TLI), many consecutive images of the same scene are taken with a high frame-rate camera, and all but the sharpest images are discarded before constructing the final shift-and-add image. Here, we present an alternative image analysis pipeline – The Thresher – for these kinds of data, based on online multi-frame blind deconvolution. It makes use of all available data to obtain the best estimate of the astronomical scene in the context of reasonable computational limits; it does not require prior estimates of the point-spread functions in the images, or knowledge of point sources in the scene that could provide such estimates. Most importantly, the scene it aims to return is the optimum of a justified scalar objective based on the likelihood function. Because it uses the full set of images in the stack, The Thresher outperforms TLI in signal-to-noise ratio; as it accounts for the individual-frame PSFs, it does this without loss of angular resolution. We demonstrate the effectiveness of our algorithm on both simulated data and real Electron-Multiplying CCD images obtained at the Danish 1.54-m telescope (hosted by ESO, La Silla). We also explore the current limitations of the algorithm, and find that for the choice of image model presented here, non-linearities in flux are introduced into the returned scene. Ongoing development of the software can be viewed at https://github.com/jah1994/TheThresher.Publisher PDFPeer reviewe
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