314 research outputs found
Measuring Electric Fields From Surface Contaminants with Neutral Atoms
In this paper we demonstrate a technique of utilizing magnetically trapped
neutral Rb-87 atoms to measure the magnitude and direction of stray electric
fields emanating from surface contaminants. We apply an alternating external
electric field that adds to (or subtracts from) the stray field in such a way
as to resonantly drive the trapped atoms into a mechanical dipole oscillation.
The growth rate of the oscillation's amplitude provides information about the
magnitude and sign of the stray field gradient. Using this measurement
technique, we are able to reconstruct the vector electric field produced by
surface contaminants. In addition, we can accurately measure the electric
fields generated from adsorbed atoms purposely placed onto the surface and
account for their systematic effects, which can plague a precision
surface-force measurement. We show that baking the substrate can reduce the
electric fields emanating from adsorbate, and that the mechanism for reduction
is likely surface diffusion, not desorption.Comment: 7 pages, 6 figures, published in Physical Review
Thermally Induced Losses in Ultra-Cold Atoms Magnetically Trapped Near Room-Temperature Surfaces
We have measured magnetic trap lifetimes of ultra-cold Rb87 atoms at
distances of 5-1000 microns from surfaces of conducting metals with varying
resistivity. Good agreement is found with a theoretical model for losses
arising from near-field magnetic thermal noise, confirming the complications
associated with holding trapped atoms close to conducting surfaces. A
dielectric surface (silicon) was found in contrast to be so benign that we are
able to evaporatively cool atoms to a Bose-Einstein condensate by using the
surface to selectively adsorb higher energy atoms.Comment: Improved theory curve eliminates discrepancy. JLTP in pres
The TheLMA project: Multi-GPU Implementation of the Lattice Boltzmann Method
International audienceIn this paper, we describe the implementation of a multi-graphical processing unit (GPU) fluid flow solver based on the lattice Boltzmann method (LBM). The LBM is a novel approach in computational fluid dynamics, with numerous interesting features from a computational, numerical, and physical standpoint. Our program is based on CUDA and uses POSIX threads to manage multiple computation devices. Using recently released hardware, our solver may therefore run eight GPUs in parallel, which allows us to perform simulations at a rather large scale. Performance and scalability are excellent, the speedup over sequential implementations being at least of two orders of magnitude. In addition, we discuss tiling and communication issues for present and forthcoming implementations
Measurement of the Temperature Dependence of the Casimir-Polder Force
We report on the first measurement of a temperature dependence of the
Casimir-Polder force. This measurement was obtained by positioning a nearly
pure 87-Rb Bose-Einstein condensate a few microns from a dielectric substrate
and exciting its dipole oscillation. Changes in the collective oscillation
frequency of the magnetically trapped atoms result from spatial variations in
the surface-atom force. In our experiment, the dielectric substrate is heated
up to 605 K, while the surrounding environment is kept near room temperature
(310 K). The effect of the Casimir-Polder force is measured to be nearly 3
times larger for a 605 K substrate than for a room-temperature substrate,
showing a clear temperature dependence in agreement with theory.Comment: 4 pages, 4 figures, published in Physical Review Letter
Einfluss von Betonzusatzmitteln auf die Energieund CO2-Bilanz von Beton
The energy consumption and the C02 emissions during the production of two different concrete mixes with identical concrete properties were compared. The two concrete samples were prepared with and without the addition of a superplasticizer. Less energy was consumed and less C02 was emitted for the case of concrete production including a superplasticizer as compared to the case of concrete production without a superplasticizer. This is due to the fact, that when a superplasticizer is used, less cement is needed to produce concrete with the same properties as those of concrete without superplasticizers. However, the relatively small dijferences in the energy consumption and in the C02 emissions become insignificant, if the entire life cycle of a building is considered. Effects like durability and the need for maintenance are expected to have a much higher influence on the life cycle assessment of concrete building
UNSUPERVISED SIGNAL SEGMENTATION BASED ON TEMPORAL SPECTRAL CLUSTERING
ABSTRACT This paper presents an approach for applying spectral clustering to time series data. We define a novel similarity measure based on euclidean distance and temporal proximity between vectors. This metric is useful for conditioning matrices needed to perform spectral clustering, and its application leads to the detection of abrupt changes in a sequence of vectors. It defines a temporal segmentation of the signal. When the input to the algorithm is a speech signal, we further process the segments and achieve their labeling in one of three phonetic classes: silence, consonant or vowel. When the input signal is a video stream, the algorithm detects scene changes in the sequence of images. Our results are compared against classic unsupervised and supervised techniques, and evaluated with the phonetically labeled multi-language corpus OGI-MLTS and the video database of the french video indexing campaign ARGOS
Alkali Adsorbate Polarization on Conducting and Insulating Surfaces Probed with Bose-Einstein Condensates
A magnetically trapped 87Rb Bose-Einstein condensate is used as a sensitive probe of short-range electrical forces. In particular, the electric polarization of, and the subsequent electric field generated by, 87Rb adsorbates on conducting and insulating surfaces is measured by characterizing perturbations to the magnetic trapping potential using high quality factor condensate excitations. The nature of the alterations to the electrical properties of Rb adsorbates is studied on titanium (metal) and silicon (semiconductor) surfaces, which exhibit nearly identical properties, and on glass (insulator), which displays a smaller transitory electrical effect. The limits of this technique in detecting electrical fields and ramifications for measurements of short-range forces near surfaces are discussed
Weather Factors Influencing Winter Roosts of American Crows in Central Iowa
We investigated the effects of season and weather on the winter roosting behavior of American Crows (Corvus brachyrhynchos) in central Iowa from January through March 2006. The peak number of birds recorded at a roost in Ames, Story County was 9,000+ in early February, and the number of individuals entering the roost decreased through February and March. Crows tended to enter the roost later in the day as winter neared completion. High wind speed and low light intensity caused individual to enter the roost earlier in the day. Temperature did not affect arrival time of crows. Wind speed, light intensity, and temperature did not significantly affect total number of individuals entering the roost (P \u3e 0.05). However, all three factors were negatively correlated to total number of birds on the roost (-0.20, -0.25, and -0.18 respectively). This research improves our understanding of the roost dynamics of an abundant urban wildlife species, which may eventually be used to manage interactions between humans and wildlife in an urban setting
Recent progress in the discovery of macrocyclic compounds as potential anti-infective therapeutics
Novel therapeutic strategies are urgently needed for the treatment of serious diseases caused by viral, bacterial and parasitic infections, because currently used drugs are facing the problem of rapidly emerging resistance. There is also an urgent need for agents that act on novel pathogen-specific targets, in order to expand the repertoire of possible therapies. The high throughput screening of diverse small molecule compound libraries has provided only a limited number of new lead series, and the number of compounds acting on novel targets is even smaller. Natural product screening has traditionally been very successful in the anti-infective area. Several successful drugs on the market as well as other compounds in clinical development are derived from natural products. Amongst these, many are macrocyclic compounds in the 1-2 kDa size range. This review will describe recent advances and novel drug discovery approaches in the anti-infective area, focusing on synthetic and natural macrocyclic compounds for which in vivo proof of concept has been established. The review will also highlight the Protein Epitope Mimetics (PEM) technology as a novel tool in the drug discovery process. Here the structures of naturally occurring antimicrobial and antiviral peptides and proteins are used as starting points to generate novel macrocyclic mimetics, which can be produced and optimized efficiently by combinatorial synthetic methods. Several recent examples highlight the great potential of the PEM approach in the discovery of new anti-infective agents
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