51,946 research outputs found
Non- Mn-driven ferroelectricity in antiferromagnetic BaMnO
Using first-principles density functional theory we predict a ferroelectric
ground state -- driven by off-centering of the magnetic Mn ion -- in
perovskite-structure BaMnO.
Our finding is surprising, since the competition between energy-lowering
covalent bond formation, and energy-raising
Coulombic repulsions usually only favors off-centering on the perovskite
-site for non-magnetic ions.
We explain this tendency for ferroelectric off-centering by analyzing the
changes in electronic structure between the centrosymmetric and polar states,
and by calculating the Born effective charges; we find anomalously large values
for Mn and O consistent with our calculated polarization of 12.8 C/cm.
Finally, we suggest possible routes by which the perovskite phase may be
stabilized over the usual hexagonal phase, to enable a practical realization of
a single-phase multiferroic.Comment: 6 pages, 3 figure
Representation of Functional Data in Neural Networks
Functional Data Analysis (FDA) is an extension of traditional data analysis
to functional data, for example spectra, temporal series, spatio-temporal
images, gesture recognition data, etc. Functional data are rarely known in
practice; usually a regular or irregular sampling is known. For this reason,
some processing is needed in order to benefit from the smooth character of
functional data in the analysis methods. This paper shows how to extend the
Radial-Basis Function Networks (RBFN) and Multi-Layer Perceptron (MLP) models
to functional data inputs, in particular when the latter are known through
lists of input-output pairs. Various possibilities for functional processing
are discussed, including the projection on smooth bases, Functional Principal
Component Analysis, functional centering and reduction, and the use of
differential operators. It is shown how to incorporate these functional
processing into the RBFN and MLP models. The functional approach is illustrated
on a benchmark of spectrometric data analysis.Comment: Also available online from:
http://www.sciencedirect.com/science/journal/0925231
Polarization Enhancement in Short Period Superlattices via Interfacial Intermixing
The effect of intermixing at the interface of short period
PbTiO/SrTiO superlattices is studied using first-principles density
functional theory. The results indicate that interfacial intermixing
significantly enhances the polarization within the superlattice. This
enhancement is directly related to the off-centering of Pb and Sr cations and
can be explained through a discussion of interacting dipoles. This picture
should be general for a wide range of multicomponent superlattices and may have
important consequences for the design of ferroelectric devices.Comment: 4 pages, 6 figure
A functional central limit theorem for a Markov-modulated infinite-server queue
The production of molecules in a chemical reaction network is modelled as a
Poisson process with a Markov-modulated arrival rate and an exponential decay
rate. We analyze the distributional properties of , the number of molecules,
under specific time-scaling; the background process is sped up by ,
the arrival rates are scaled by , for large. A functional central limit
theorem is derived for , which after centering and scaling, converges to an
Ornstein-Uhlenbeck process. A dichotomy depending on is observed. For
the parameters of the limiting process contain the deviation
matrix associated with the background process.Comment: 4 figure
Density functionals, with an option-pricing application
We present a method of estimating density-related functionals, without prior knowledge of the density’s functional form. The approach revolves around the specification of an explicit formula for a new class of distributions that encompasses many of the known cases in statistics, including the normal, gamma, inverse gamma, and mixtures thereof. The functionals are based on a couple of hypergeometric functions. Their parameters can be estimated, and the estimates then reveal both the functional form of the density and the parameters that determine centering, scaling, etc. The function to be estimated always leads to a valid density, by design, namely, one that is nonnegative everywhere and integrates to 1. Unlike fully nonparametric methods, our approach can be applied to small datasets. To illustrate our methodology, we apply it to finding risk-neutral densities associated with different types of financial options. We show how our approach fits the data uniformly very well. We also find that our estimated densities’ functional forms vary over the dataset, so that existing parametric methods will not do uniformly well
Centering in-the-large: Computing referential discourse segments
We specify an algorithm that builds up a hierarchy of referential discourse
segments from local centering data. The spatial extension and nesting of these
discourse segments constrain the reachability of potential antecedents of an
anaphoric expression beyond the local level of adjacent center pairs. Thus, the
centering model is scaled up to the level of the global referential structure
of discourse. An empirical evaluation of the algorithm is supplied.Comment: LaTeX, 8 page
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