4,039 research outputs found
Current-induced magnetic vortex core switching in a Permalloy nanodisk
We report on the switching of a magnetic vortex core in a sub-micron
Permalloy disk, induced by a short current pulse applied in the film plane.
Micromagnetic simulations including the adiabatic and non-adiabatic spin-torque
terms are used to investigate the current-driven magnetization dynamics. We
predict that a core reversal can be triggered by current bursts a tenth of a
nanosecond long. The vortex core reversal process is found to be the same as
when an external field pulse is applied. The control of a vortex core's
orientation using current pulses introduces the technologically relevant
possibility to address individual nanomagnets within dense arrays.Comment: 3 pages, 3 figure
Ensemble inequivalence: A formal approach
Ensemble inequivalence has been observed in several systems. In particular it
has been recently shown that negative specific heat can arise in the
microcanonical ensemble in the thermodynamic limit for systems with long-range
interactions. We display a connection between such behaviour and a mean-field
like structure of the partition function. Since short-range models cannot
display this kind of behaviour, this strongly suggests that such systems are
necessarily non-mean field in the sense indicated here. We further show that a
broad class of systems with non-integrable interactions are indeed of
mean-field type in the sense specified, so that they are expected to display
ensemble inequivalence as well as the peculiar behaviour described above in the
microcanonical ensemble.Comment: 4 pages, no figures, given at the NEXT2001 conference on
non-extensive thermodynamic
A Path to the Direct Detection of sub-GeV Dark Matter Using Calorimetric Readout of a Superfluid He Target
A promising technology concept for sub-GeV dark matter detection is
described, in which low-temperature microcalorimeters serve as the sensors and
superfluid He serves as the target material. A superfluid helium target has
several advantageous properties, including a light nuclear mass for better
kinematic matching with light dark matter particles, copious production of
scintillation light, extremely good intrinsic radiopurity, a high impedance to
external vibration noise, and a unique mechanism for observing phonon-like
modes via liberation of He atoms into a vacuum (`quantum evaporation'). In
this concept, both scintillation photons and triplet excimers are detected
using calorimeters, including calorimeters immersed in the superfluid. Kinetic
excitations of the superfluid medium (rotons and phonons) are detected using
quantum evaporation and subsequent atomic adsorption onto a microcalorimeter
suspended in vacuum above the target helium. The energy of adsorption amplifies
the phonon/roton signal before calorimetric sensing, producing a gain mechanism
that can reduce the techonology's recoil energy threshold below the calorimeter
energy threshold. We describe signal production and signal sensing
probabilities, and estimate electron recoil discrimination. We then simulate
radioactive backgrounds from gamma rays and neutrons. Dark matter - nucleon
elastic scattering cross-section sensitivities are projected, demonstrating
that even very small (sub-kg) target masses can probe wide regions of as-yet
untested dark matter parameter space
PROJECTING WORLD FOOD DEMAND USING ALTERNATIVE DEMAND SYSTEMS
Computable General Equilibrium (CGE) models are increasingly being used to project world food markets in order to support forward-looking policy analysis. Such projections hinge critically on the underlying functional form for representing consumer demand. Simple functional forms can lead to unrealistic projections by failing to capture changes in income elasticities of demand. We adopt as our benchmark the recently introduced AIDADS demand system and compare it with several alternaive demand systems currently in widespread use in CGE models. This comparison is conducted in the context of projections for disaggregated global food demand using a global CGE model. We find that AIDADS represents a substantial improvement, particularly for the rapidly growing developing countries. For these economies, the most widely used demand systems tend to over-predict future food demands, and hence overestimate future production and import requirements for agricultural products.food demand, agricultural trade, functional form, demand system, CGE modeling, Demand and Price Analysis, D12, C68, F17, Q18,
Recalling in a State of Natural or Experimental Depression
In three experiments we attempted to extend the cognitive-effort account of depressive deficits in memory to naturally depressed college students. This account maintains that depression reduces attentional resources, thereby impairing performance on demanding tasks, and has received support through experimental inductions of depressed moods. Nondepressed, naturally depressed, and (in Experiment 2) experimentally depressed college students performed unannounced tests of free recall following learning tasks with two levels of difficulty and (in Experiment 2) two degrees of structure. In Experiments 1 and 2 we measured cognitive effort on those tasks via latencies on a secondary task. Latencies and subsequent recall increased with the structure and difficulty of the learning task for nondepressed and naturally depressed subjects, but these effects were reduced or absent for experimentally depressed subjects. When the secondary task was omitted (Experiment 3), naturally depressed students still recalled without a deficit. We discuss possible differences associated with the two types of depression and implications for the cognitive-effort account
Depressive Deficits in Memory: Focusing Attention Improves Subsequent Recall
58 Ss (aged up to 55 yrs) diagnosed as depressed, recovered from depression, or without a history of depression performed an unintentional learning task, followed by tests of free and forced recall. In the learning task, Ss decided whether a series of nouns sensibly completed corresponding sentence frames that varied in decision difficulty. For half of the Ss, the focus of attention was unconstrained by the demands of this task. The others, however, were required to repeat the targeted noun at the end of the trial as a means of focusing their attention on the task. Depressed Ss in the unfocused condition subsequently recalled fewer words than did both control groups, but this deficit disappeared in the focused condition. These results suggest that depression might not fundamentally impair the resources required for good performance on such tasks. The results\u27 relevance to resource-allocation, initiative, and inhibition accounts of depressive deficits in memory is discussed
Crosstalk between nanotube devices: contact and channel effects
At reduced dimensionality, Coulomb interactions play a crucial role in
determining device properties. While such interactions within the same carbon
nanotube have been shown to have unexpected properties, device integration and
multi-nanotube devices require the consideration of inter-nanotube
interactions. We present calculations of the characteristics of planar carbon
nanotube transistors including interactions between semiconducting nanotubes
and between semiconducting and metallic nanotubes. The results indicate that
inter-tube interactions affect both the channel behavior and the contacts. For
long channel devices, a separation of the order of the gate oxide thickness is
necessary to eliminate inter-nanotube effects. Because of an exponential
dependence of this length scale on dielectric constant, very high device
densities are possible by using high-k dielectrics and embedded contacts
Depressive Deficits in Recognition: Dissociation of Recollection and Familiarity
Dysphoric and nondysphoric students (48 women and 24 men) participated in an experiment that was designed to separate automatic and controlled uses of memory in a modified recognition paradigm. First, they judged the relation of target words to paired words. Later they made recognition decisions on target items alone or in the context of the original paired item. The use of L.L. Jacoby\u27s (1991) process dissociation procedure revealed depressive deficits in estimates of recollection but not in estimates of familiarity. The paired test improved recollection for all subjects and showed a trend in the direction of increased familiarity. These outcomes support approaches to depressive cognition that emphasize impaired cognitive control
Projecting World Food Demand: A Comparison of Alternative Demand Systems
Projections of world food demands hinge critically on the underlying functional form used to predict future demands. Simple functional forms can lead to unrealistic projections by failing to capture changes in income elasticities of demand as consumer becomes wealthier. This paper compares several demand systems in the projection of disaggregated food demand across a wide range of countries with different income levels using a global general equilibrium model. We find that the recently introduced AIDADS system represents a substantial improvement over existing demand systems currently in use in CGE modeling. In particular, our projection results show that for relatively poor regions experiencing rapid income growth, the widely used LES and CDE demand systems tend to over-predict growth in consumer demand, and hence import and output requirements for food products and under-predict that for non-food products, compared to the AIDADS system. On the other hand, for high-income regions with modest income growth, the choice of functional form is less critical.food demand, agricultural trade, functional form, demand system, CGE modeling, Demand and Price Analysis,
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