1,005 research outputs found
Experimental observation of the 'Tilting Mode' of an array of vortices in a dilute Bose-Einstein Condensate
We have measured the precession frequency of a vortex lattice in a
Bose-Einstein condensate of 87Rb atoms. The observed mode corresponds to a
collective motion in which all the vortices in the array are tilted by a small
angle with respect to the z-axis (the symmetry axis of the trapping potential)
and synchronously rotate about this axis. This motion corresponds to excitation
of a Kelvin wave along the core of each vortex and we have verified that it has
the handedness expected for such helical waves, i.e. precession in the opposite
sense to the rotational flow around the vortices. The experimental method used
to excite this collective mode closely resembles that used to study the
scissors mode and excitation of the scissors mode for a condensate containing a
vortex array was used to determine the angular momentum of the system. Indeed,
the collective tilting of the array that we have observed has previously been
referred to as an `anomalous' scissors mode.Comment: 5 pages, 7 figures to be published in PR
Experimental demonstration of painting arbitrary and dynamic potentials for Bose-Einstein condensates
There is a pressing need for robust and straightforward methods to create
potentials for trapping Bose-Einstein condensates which are simultaneously
dynamic, fully arbitrary, and sufficiently stable to not heat the ultracold
gas. We show here how to accomplish these goals, using a rapidly-moving laser
beam that "paints" a time-averaged optical dipole potential in which we create
BECs in a variety of geometries, including toroids, ring lattices, and square
lattices. Matter wave interference patterns confirm that the trapped gas is a
condensate. As a simple illustration of dynamics, we show that the technique
can transform a toroidal condensate into a ring lattice and back into a toroid.
The technique is general and should work with any sufficiently polarizable
low-energy particles.Comment: Minor text changes and three references added. This is the final
version published in New Journal of Physic
Seismic capacity comparison between square and circular plan adobe construction
Unreinforced adobe or mud-brick structures have in the past suffered severe damage from seismic forces and have caused a vast number of deaths. However, a number of adobe buildings located in seismic regions have performed well under several seismic events. Most of these traditional buildings' shapes are symmetrical which has significant bearing on the performance of the building during strong earthquakes. This paper presents an experimental comparison between two symmetrical shapes, i.e., a square and a circular model of unreinforced adobe walls. One-third scale models were built and tested using a static tilt test for seismic performance evaluation of both structures. The adobe house models were subjected to a constant acceleration when tilted on a tilt-up table. The lateral component of model weight was used as a parameter to quantify the maximum seismic force for each model. The paper describes the configuration of both specimens and testing method. The results of these comparative experiments indicated a better performance of the circular structure. There are simple and effective solutions for construction of new adobe buildings located in seismic hazard areas which can assist in decreasing damage and death
RF spectroscopy in a resonant RF-dressed trap
We study the spectroscopy of atoms dressed by a resonant radiofrequency (RF)
field inside an inhomogeneous magnetic field and confined in the resulting
adiabatic potential. The spectroscopic probe is a second, weak, RF field. The
observed line shape is related to the temperature of the trapped cloud. We
demonstrate evaporative cooling of the RF-dressed atoms by sweeping the
frequency of the second RF field around the Rabi frequency of the dressing
field.Comment: 7 figures, 8 pages; to appear in J. Phys.
Toward a model-based cognitive neuroscience of mind wandering
Published version also available at http://dx.doi.org/10.1016/j.neuroscience.2015.09.053People often ‘‘mind wander” during everyday
tasks, temporarily losing track of time, place, or current task
goals. In laboratory-based tasks, mind wandering is often
associated with performance decrements in behavioral
variables and changes in neural recordings. Such empirical
associations provide descriptive accounts of mind
wandering – howit affects ongoing task performance – but fail
to provide true explanatory accounts – why it affects task
performance. In this perspectives paper, we consider mind
wandering as a neural state or process that affects the
parameters of quantitative cognitive process models, which
in turn affect observed behavioral performance. Our
approach thus uses cognitive process models to bridge
the explanatory divide between neural and behavioral data.
We provide an overview of two general frameworks for
developing a model-based cognitive neuroscience of mind
wandering. The first approach uses neural data to segment
observed performance into a discrete mixture of latent
task-related and task-unrelated states, and the second
regresses single-trial measures of neural activity onto
structured trial-by-trial variation in the parameters of
cognitive process models. We discuss the relative merits of
the two approaches, and the research questions they can
answer, and highlight that both approaches allow neural data
to provide additional constraint on the parameters of cognitive
models, which will lead to a more precise account of the
effect of mind wandering on brain and behavior. We conclude
by summarizing prospects for mind wandering as conceived
within a model-based cognitive neuroscience framework,
highlighting the opportunities for its continued study and
the benefits that arise from using well-developed quantitative techniques to study abstract theoretical constructs
Dystroglycan Overexpression in Vivo Alters Acetylcholine Receptor Aggregation at the Neuromuscular Junction
AbstractDystroglycan is a member of the transmembrane dystrophin glycoprotein complex in muscle that binds to the synapse-organizing molecule agrin. Dystroglycan binding and AChR aggregation are mediated by two separate domains of agrin. To test whether dystroglycan plays a role in receptor aggregation at the neuromuscular junction, we overexpressed it by injecting rabbit dystroglycan RNA into one- or two-celled Xenopus embryos. We measured AChR aggregation in myotomes by labeling them with rhodamine–α-bungarotoxin followed by confocal microscopy and image analysis. Dystroglycan overexpression decreased AChR aggregation at the neuromuscular junction. This result is consistent with dystroglycan competition for agrin without signaling AChR aggregation. It also supports the hypothesis that dystroglycan is not the myotube-associated specificity component, (MASC) a putative coreceptor needed for agrin to activate muscle-specific kinase (MuSK) and signal AChR aggregation. Dystroglycan was distributed along the surface of muscle membranes, but was concentrated at the ends of myotomes, where AChRs normally aggregate at synapses. Overexpressed dystroglycan altered AChR aggregation in a rostral–caudal gradient, consistent with the sequential development of neuromuscular synapses along the embryo. Increasing concentrations of dystroglycan RNA did not further decrease AChR aggregation, but decreased embryo survival. Development often stopped during gastrulation, suggesting an essential, nonsynaptic role of dystroglycan during this early period of development
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