13,339 research outputs found
Spin Freezing in Geometrically Frustrated Antiferromagnets with Weak Disorder
We investigate the consequences for geometrically frustrated antiferromagnets
of weak disorder in the strength of exchange interactions. Taking as a model
the classical Heisenberg antiferromagnet with nearest neighbour exchange on the
pyrochlore lattice, we examine low-temperature behaviour. We show that random
exchange generates long-range effective interactions within the extensively
degenerate ground states of the clean system. Using Monte Carlo simulations, we
find a spin glass transition at a temperature set by the disorder strength.
Disorder of this type, which is generated by random strains in the presence of
magnetoelastic coupling, may account for the spin freezing observed in many
geometrically frustrated magnets.Comment: 4 pages, 5 figure
Mars: Seasonally variable radar reflectivity
Since reflectivity is a quantity characteristic of a given target at a particular geometry, the same (temporally unchanging) target examined by radar on different occasions should have the same reflectivity. Zisk and Mouginis-Mark noted that the average reflectivities in the Goldstone Mars data increased as the planet's S hemisphere passed from the late spring into early summer. The same data set was re-examined and the presence of the phenomenon of the apparent seasonal variability of radar reflectivity was confirmed. Two objections to these findings are addressed: (1) reflectivity variations may be present in the Goldstone Mars data as a result of an instrument/calibration error; and (2) the variations were introduced into the analysis through comparing reflectivities from two incompatible subsets of the data
Critical phenomena in a highly constrained classical spin system: Neel ordering from the Coulomb phase
Many classical, geometrically frustrated antiferromagnets have
macroscopically degenerate ground states. In a class of three-dimensional
systems, the set of degenerate ground states has power-law correlations and is
an example of a Coulomb phase. We investigate Neel ordering from such a Coulomb
phase, induced by weak additional interactions that lift the degeneracy. We
show that the critical point belongs to a universality class that is different
from the one for the equivalent transition out of the paramagnetic phase, and
that it is characterised by effective long-range interactions; alternatively,
ordering may be discontinuous. We suggest that a transition of this type may be
realised by applying uniaxial stress to a pyrochlore antiferromagnet.Comment: 4 pages, 3 figure
The energy efficient engine project
The Energy Efficient Engine Project is directed at providing, by 1984, the advanced technologies which could be used for a generation of fuel conservative turbofan engines. The project is conducted through contracts with the General Electric Company and Pratt and Whitney Aircraft. The scope of the entire project and the current status of these efforts are summarized. A description of the preliminary designs of the fully developed engines is included and the potential benefits of these advanced engines, as well as highlights of some of the component technology efforts conducted to date, are discussed
Design approaches to more energy efficient engines
The status of NASA's Energy Efficient Engine Project, a comparative government-industry effort aimed at advancing the technology base for the next generation of large turbofan engines for civil aircraft transports is summarized. Results of recently completed studies are reviewed. These studies involved selection of engine cycles and configurations that offer potential for at least 12% lower fuel consumption than current engines and also are economically attractive and environmentally acceptable. Emphasis is on the advancements required in component technologies and systems design concepts to permit future development of these more energy efficient engines
The average magnetic field draping and consistent plasma properties of the Venus magnetotail
A new technique has been developed to determine the average structure of the Venus magnetotail (in the range from â8 Rv to â12 Rv) from the Pioneer Venus magnetometer observations. The spacecraft position with respect to the cross-tail current sheet is determined from an observed relationship between the field-draping angle and the magnitude of the field referenced to its value in the nearby magnetosheath. This allows us statistically to remove the effects of tail flapping and variability of draping for the first time and thus to map the average field configuration in the Venus tail. From this average configuration we calculate the cross-tail current density distribution and J Ă B forces. Continuity of the tangential electric field is utilized to determine the average variations of the X-directed velocity which is shown to vary from â250 km/s at â8 Rv to â470 km/s at â12 Rv. From the calculated J Ă B forces, plasma velocity, and MHD momentum equation the approximate plasma acceleration, density, and temperature in the Venus tail are determined. The derived ion density is approximately âŒ0.07 p+/cmÂł (0.005 O+/cmÂł) in the lobes and âŒ0.9 p+/cmÂł (0.06 O+/cmÂł) in the current sheet, while the derived approximate average plasma temperature for the tail is âŒ6Ă106 K for a hydrogen plasma or âŒ9Ă107 K for an oxygen plasma
The role of dopamine in the accumbens core in the expression of Pavlovianâconditioned responses
The role of dopamine in reward is a topic of debate. For example, some have argued that phasic dopamine signaling provides a predictionâerror signal necessary for stimulusâreward learning, whereas others have hypothesized that dopamine is not necessary for learning per se , but for attributing incentive motivational value (âincentive salienceâ) to reward cues. These psychological processes are difficult to tease apart, because they tend to change together. To disentangle them we took advantage of natural individual variation in the extent to which reward cues are attributed with incentive salience, and asked whether dopamine (specifically in the core of the nucleus accumbens) is necessary for the expression of two forms of Pavlovianâconditioned approach behavior â one in which the cue acquires powerful motivational properties (signâtracking) and another closely related one in which it does not (goalâtracking). After acquisition of these conditioned responses (CRs), intraâaccumbens injection of the dopamine receptor antagonist flupenthixol markedly impaired the expression of a signâtracking CR, but not a goalâtracking CR. Furthermore, dopamine antagonism did not produce a gradual extinctionâlike decline in behavior, but maximally impaired expression of a signâtracking CR on the very first trial, indicating the effect was not due to new learning (i.e. it occurred in the absence of new predictionâerror computations). The data support the view that dopamine in the accumbens core is not necessary for learning stimulusâreward associations, but for attributing incentive salience to reward cues, transforming predictive conditional stimuli into incentive stimuli with powerful motivational properties. Ongoing debate exists about dopamineâs exact role in rewardârelated processes. We took advantage of natural individual variation in the degree to which reward cues are attributed with motivational value, and asked whether dopamine in the core of the nucleus accumbens is necessary for the performance of two forms of Pavlovian conditioned approach behavior â one in which the cue acquires powerful motivational properties (signâtracking) and another related one in which it does not (goalâtracking). We found that blocking dopamine transmission within the core impaired the expression of signâtracking responses, but not goalâtracking responses.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/93510/1/j.1460-9568.2012.08217.x.pd
Optimal placement of a limited number of observations for period searches
Robotic telescopes present the opportunity for the sparse temporal placement
of observations when period searching. We address the best way to place a
limited number of observations to cover the dynamic range of frequencies
required by an observer. We show that an observation distribution geometrically
spaced in time can minimise aliasing effects arising from sparse sampling,
substantially improving signal detection quality. The base of the geometric
series is however a critical factor in the overall success of this strategy.
Further, we show that for such an optimal distribution observations may be
reordered, as long as the distribution of spacings is preserved, with almost no
loss of quality. This implies that optimal observing strategies can retain
significant flexibility in the face of scheduling constraints, by providing
scope for on-the-fly adaptation. Finally, we present optimal geometric
samplings for a wide range of common observing scenarios, with an emphasis on
practical application by the observer at the telescope. Such a sampling
represents the best practical empirical solution to the undersampling problem
that we are aware of. The technique has applications to robotic telescope and
satellite observing strategies, where target acquisition overheads mean that a
greater total target exposure time (and hence signal-to-noise) can often in
practice be achieved by limiting the number of observations.Comment: 8 pages with 16 figure
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