2,924 research outputs found
Quantum dynamics in high codimension tilings: from quasiperiodicity to disorder
We analyze the spreading of wavepackets in two-dimensional quasiperiodic and
random tilings as a function of their codimension, i.e. of their topological
complexity. In the quasiperiodic case, we show that the diffusion exponent that
characterizes the propagation decreases when the codimension increases and goes
to 1/2 in the high codimension limit. By constrast, the exponent for the random
tilings is independent of their codimension and also equals 1/2. This shows
that, in high codimension, the quasiperiodicity is irrelevant and that the
topological disorder leads in every case, to a diffusive regime, at least in
the time scale investigated here.Comment: 4 pages, 5 EPS figure
Analytical probabilistic approach to the ground state of lattice quantum systems: exact results in terms of a cumulant expansion
We present a large deviation analysis of a recently proposed probabilistic
approach to the study of the ground-state properties of lattice quantum
systems. The ground-state energy, as well as the correlation functions in the
ground state, are exactly determined as a series expansion in the cumulants of
the multiplicities of the potential and hopping energies assumed by the system
during its long-time evolution. Once these cumulants are known, even at a
finite order, our approach provides the ground state analytically as a function
of the Hamiltonian parameters. A scenario of possible applications of this
analyticity property is discussed.Comment: 26 pages, 5 figure
Rigidity percolation on aperiodic lattices
We studied the rigidity percolation (RP) model for aperiodic (quasi-crystal)
lattices. The RP thresholds (for bond dilution) were obtained for several
aperiodic lattices via computer simulation using the "pebble game" algorithm.
It was found that the (two rhombi) Penrose lattice is always floppy in view of
the RP model. The same was found for the Ammann's octagonal tiling and the
Socolar's dodecagonal tiling. In order to impose the percolation transition we
used so c. "ferro" modification of these aperiodic tilings. We studied as well
the "pinwheel" tiling which has "infinitely-fold" orientational symmetry. The
obtained estimates for the modified Penrose, Ammann and Socolar lattices are
respectively: , , . The bond RP threshold of the pinwheel tiling was estimated to
. It was found that these results are very close to the
Maxwell (the mean-field like) approximation for them.Comment: 9 LaTeX pages, 3 PostScript figures included via epsf.st
Semi-autonomous competency assessment of powered mobility device users
This paper describes a stand-alone sensor package and algorithms for aiding the assessment by an occupational therapist whether a person has the capacity to safely and effectively operate a powered mobility device such as a walking aid or a wheelchair. The sensor package employed consists of a laser range finder, an RGB camera and an inertial measurement unit that can be attached to any mobility device with minimal modifications. Algorithms for capturing the data received by the sensor package and for generating the map of the environment as well as the trajectory of the mobility device have been developed. Such information presents occupational therapists with the capability to provide a quantitative assessment of whether patients are ready to be safely deployed with mobile aids for their daily activities. Preliminary evaluation of the sensor package and associated algorithms based on experiments, conducted at the premises of the Prince of Wales Hospital in Sydney, are presented. © 2011 IEEE
What is a crystal?
Almost 25 years have passed since Shechtman discovered quasicrystals, and 15
years since the Commission on Aperiodic Crystals of the International Union of
Crystallography put forth a provisional definition of the term crystal to mean
``any solid having an essentially discrete diffraction diagram.'' Have we
learned enough about crystallinity in the last 25 years, or do we need more
time to explore additional physical systems? There is much confusion and
contradiction in the literature in using the term crystal. Are we ready now to
propose a permanent definition for crystal to be used by all? I argue that time
has come to put a sense of order in all the confusion.Comment: Submitted to Zeitschrift fuer Kristallographi
Desiccation Responses and Survival of \u3ci\u3eSinorhizobium meliloti\u3c/i\u3e USDA 1021 in Relation to Growth Phase, Temperature, Chloride and Sulfate Availability
Aims: To identify physical and physiological conditions that affect the survival of Sinorhizobium meliloti USDA 1021 during desiccation. Methods and Results: An assay was developed to study desiccation response of S. meliloti USDA 1021 over a range of environmental conditions. We determined the survival during desiccation in relation to (i) matrices and media, (ii) growth phase, (iii) temperature, and (iv) chloride and sulfate availability. Conclusions: This study indicates that survival of S. meliloti USDA 1021 during desiccation is enhanced: (i) when cells were dried in the stationary phase, (ii) with increasing drying temperature at an optimum of 37°C, and (iii) during an increase of chloride and sulfate, but not sodium or potassium availability. In addition, we resolved that the best matrix to test survival was nitrocellulose filters. Significance and Impact of the Study: The identification of physical and physiological factors that determine the survival during desiccation of S. meliloti USDA 1021 may aid in (i) the strategic development of improved seed inocula, (ii) the isolation, and (iii) the development of rhizobial strains with improved ability to survive desiccation. Furthermore, this work may provide insights into the survival of rhizobia under drought conditions. © 2006 The Society for Applied Microbiology
On the design of an image compression scheme based upon a priori knowledge about imaging system and image statistics
This contribution is about the design of an image compression scheme for near loss-less image compression of a restricted class of images and a specific application. The images are digital diagnostic X-ray images of the coronary vessels of the human heart. This paper proposes a novel compression scheme with a compression ratio of 8-10 with preservation of the diagnostic image quality. Central in our approach is the amount of information a trained and highly skilled observer i.e. the cardiologist is able discern at a given exposure and thus quantum noise level. The physics of the image detection process together with the a priori knowledge of the imaging system are the basis of the image statistics. Relevant elements of the human visual system complete the stochastic characterization of imaging process whereon the compression scheme is based. 1
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