1,790 research outputs found
The exchange bias phenomenon in uncompensated interfaces: Theory and Monte Carlo simulations
We performed Monte Carlo simulations in a bilayer system composed by two thin
films, one ferromagnetic (FM) and the other antiferromagnetic (AFM). Two
lattice structures for the films were considered: simple cubic (sc) and a body
center cubic (bcc). In both lattices structures we imposed an uncompensated
interfacial spin structure, in particular we emulated a FeF2-FM system in the
case of the (bcc) lattice. Our analysis focused on the incidence of the
interfacial strength interactions between the films J_eb and the effect of
thermal fluctuations on the bias field H_EB. We first performed Monte Carlo
simulations on a microscopic model based on classical Heisenberg spin
variables. To analyze the simulation results we also introduced a simplified
model that assumes coherent rotation of spins located on the same layer
parallel to the interface. We found that, depending on the AFM film anisotropy
to exchange ratio, the bias field is either controlled by the intrinsic pinning
of a domain wall parallel to the interface or by the stability of the first AFM
layer (quasi domain wall) near the interface.Comment: 18 pages, 11 figure
Large-Scale Detection of Non-Technical Losses in Imbalanced Data Sets
Non-technical losses (NTL) such as electricity theft cause significant harm
to our economies, as in some countries they may range up to 40% of the total
electricity distributed. Detecting NTLs requires costly on-site inspections.
Accurate prediction of NTLs for customers using machine learning is therefore
crucial. To date, related research largely ignore that the two classes of
regular and non-regular customers are highly imbalanced, that NTL proportions
may change and mostly consider small data sets, often not allowing to deploy
the results in production. In this paper, we present a comprehensive approach
to assess three NTL detection models for different NTL proportions in large
real world data sets of 100Ks of customers: Boolean rules, fuzzy logic and
Support Vector Machine. This work has resulted in appreciable results that are
about to be deployed in a leading industry solution. We believe that the
considerations and observations made in this contribution are necessary for
future smart meter research in order to report their effectiveness on
imbalanced and large real world data sets.Comment: Proceedings of the Seventh IEEE Conference on Innovative Smart Grid
Technologies (ISGT 2016
Stability of adversarial Markov chains, with an application to adaptive MCMC algorithms
We consider whether ergodic Markov chains with bounded step size remain
bounded in probability when their transitions are modified by an adversary on a
bounded subset. We provide counterexamples to show that the answer is no in
general, and prove theorems to show that the answer is yes under various
additional assumptions. We then use our results to prove convergence of various
adaptive Markov chain Monte Carlo algorithms.Comment: Published at http://dx.doi.org/10.1214/14-AAP1083 in the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Deformed vortices in (4+1)-dimensional Einstein-Yang-Mills theory
We study vortex-type solutions in a (4+1)-dimensional
Einstein-Yang-Mills-SU(2) model. Assuming all fields to be independent on the
extra coordinate, these solutions correspond in a four dimensional picture to
axially symmetric multimonopoles, respectively monopole-antimonopole solutions.
By boosting the five dimensional purely magnetic solutions we find new
configurations which in four dimensions represents rotating regular nonabelian
solutions with an additional electric charge.Comment: 11 pages, including 5 eps files; reference added, discussion
extended; typos correcte
Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets
We use series expansions to study the excitation spectra of spin-1/2
antiferromagnets on anisotropic triangular lattices. For the isotropic
triangular lattice model (TLM) the high-energy spectra show several anomalous
features that differ strongly from linear spin-wave theory (LSWT). Even in the
Neel phase, the deviations from LSWT increase sharply with frustration, leading
to roton-like minima at special wavevectors. We argue that these results can be
interpreted naturally in a spinon language, and provide an explanation for the
previously observed anomalous finite-temperature properties of the TLM. In the
coupled-chains limit, quantum renormalizations strongly enhance the
one-dimensionality of the spectra, in agreement with experiments on Cs_2CuCl_4.Comment: 4 pages, 5 figures. New Fig. 3 with higher-order series data, paper
shortened, references updated, one added (Ref. 28), minor changes otherwise.
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