18,193 research outputs found
A Circuit Model for Domain Walls in Ferromagnetic Nanowires: Application to Conductance and Spin Transfer Torques
We present a circuit model to describe the electron transport through a
domain wall in a ferromagnetic nanowire. The domain wall is treated as a
coherent 4-terminal device with incoming and outgoing channels of spin up and
down and the spin-dependent scattering in the vicinity of the wall is modelled
using classical resistances. We derive the conductance of the circuit in terms
of general conductance parameters for a domain wall. We then calculate these
conductance parameters for the case of ballistic transport through the domain
wall, and obtain a simple formula for the domain wall magnetoresistance which
gives a result consistent with recent experiments. The spin transfer torque
exerted on a domain wall by a spin-polarized current is calculated using the
circuit model and an estimate of the speed of the resulting wall motion is
made.Comment: 10 pages, 5 figures; submitted to Physical Review
Aspects of Galilean and Relativistic Particle Mechanics with Dirac's Constraints
Relevant physical models are described by singular Lagrangians, so that their
Hamiltonian description is based on the Dirac theory of constraints. The
qualitative aspects of this theory are now understood, in particular the role
of the Shanmugadhasan canonical transformation in the determination of a
canonical basis of Dirac's observables allowing the elimination of gauge
degrees of freedom from the classical description of physical systems. This
programme was initiated by Dirac for the electromagnetic field with charged
fermions. Now Dirac's observables for Yang-Mills theory with fermions (whose
typical application is QCD) have been found in suitable function spaces where
the Gribov ambiguity is absent. Also the ones for the Abelian Higgs model are
known and those for the electroweak theory with fermions
are going to be found with the same method working for the Abelian case. The
main task along these lines will now be the search of Dirac's observables for
tetrad gravity in the case of asymptotically flat 3-manifolds. The philosophy
behind this approach is ``first reduce, then quantize": this requires a global
symplectic separation of the physical variables from the gauge ones so that the
role of differential geometry applied to smooth field configurations is
dominating, in contrast with the standard approach of ``first quantizing, then
reducing", where, in the case of gauge field theory, the reduction process
takes place on distributional field configurations, which dominate in quantum
measures. This global separation has been accomplished till now, at least at a
heuristic level, and one is going to have a classical (pseudoclassical for the
fermion) variables basis for the physical description of the standard model; instead, with tetrad gravity one expects toComment: Talk given at the Conference ``Theories of Fundamental Interactions",
Maynooth (Ireland), May 1995. (LaTeX file
Pseudorandom number generators revisited
Statistical Methods;mathematische statistiek
Research and Education in Computational Science and Engineering
Over the past two decades the field of computational science and engineering
(CSE) has penetrated both basic and applied research in academia, industry, and
laboratories to advance discovery, optimize systems, support decision-makers,
and educate the scientific and engineering workforce. Informed by centuries of
theory and experiment, CSE performs computational experiments to answer
questions that neither theory nor experiment alone is equipped to answer. CSE
provides scientists and engineers of all persuasions with algorithmic
inventions and software systems that transcend disciplines and scales. Carried
on a wave of digital technology, CSE brings the power of parallelism to bear on
troves of data. Mathematics-based advanced computing has become a prevalent
means of discovery and innovation in essentially all areas of science,
engineering, technology, and society; and the CSE community is at the core of
this transformation. However, a combination of disruptive
developments---including the architectural complexity of extreme-scale
computing, the data revolution that engulfs the planet, and the specialization
required to follow the applications to new frontiers---is redefining the scope
and reach of the CSE endeavor. This report describes the rapid expansion of CSE
and the challenges to sustaining its bold advances. The report also presents
strategies and directions for CSE research and education for the next decade.Comment: Major revision, to appear in SIAM Revie
Collaborative Uploading in Heterogeneous Networks: Optimal and Adaptive Strategies
Collaborative uploading describes a type of crowdsourcing scenario in
networked environments where a device utilizes multiple paths over neighboring
devices to upload content to a centralized processing entity such as a cloud
service. Intermediate devices may aggregate and preprocess this data stream.
Such scenarios arise in the composition and aggregation of information, e.g.,
from smartphones or sensors. We use a queuing theoretic description of the
collaborative uploading scenario, capturing the ability to split data into
chunks that are then transmitted over multiple paths, and finally merged at the
destination. We analyze replication and allocation strategies that control the
mapping of data to paths and provide closed-form expressions that pinpoint the
optimal strategy given a description of the paths' service distributions.
Finally, we provide an online path-aware adaptation of the allocation strategy
that uses statistical inference to sequentially minimize the expected waiting
time for the uploaded data. Numerical results show the effectiveness of the
adaptive approach compared to the proportional allocation and a variant of the
join-the-shortest-queue allocation, especially for bursty path conditions.Comment: 15 pages, 11 figures, extended version of a conference paper accepted
for publication in the Proceedings of the IEEE International Conference on
Computer Communications (INFOCOM), 201
A model-based multithreshold method for subgroup identification
Thresholding variable plays a crucial role in subgroup identification for personalizedmedicine. Most existing partitioning methods split the sample basedon one predictor variable. In this paper, we consider setting the splitting rulefrom a combination of multivariate predictors, such as the latent factors, principlecomponents, and weighted sum of predictors. Such a subgrouping methodmay lead to more meaningful partitioning of the population than using a singlevariable. In addition, our method is based on a change point regression modeland thus yields straight forward model-based prediction results. After choosinga particular thresholding variable form, we apply a two-stage multiple changepoint detection method to determine the subgroups and estimate the regressionparameters. We show that our approach can produce two or more subgroupsfrom the multiple change points and identify the true grouping with high probability.In addition, our estimation results enjoy oracle properties. We design asimulation study to compare performances of our proposed and existing methodsand apply them to analyze data sets from a Scleroderma trial and a breastcancer study
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