3,431 research outputs found
Emphasizing Common Childhood Anxieties in Children’s Fantasy: An Analysis of the Illustrations in Matilda and Charlotte’s Web
In children’s literature, illustrations are just as important to story as a book’s text; illustrations contribute to the overall tone of the story and to the way readers interpret its events
Thermodynamics of mono and di-vacancies in barium titanate
The thermodynamic and kinetic properties of mono and di-vacancy defects in
cubic (para-electric) barium titanate are studied by means of
density-functional theory calculations. It is determined which vacancy types
prevail for given thermodynamic boundary conditions. The calculations confirm
the established picture that vacancies occur in their nominal charge states
almost over the entire band gap. For the dominating range of the band gap the
di-vacancy binding energies are constant and negative. The system, therefore,
strives to achieve a state in which under metal-rich (oxygen-rich) conditions
all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration
barriers are calculated for mono-vacancies in different charge states. Since
oxygen vacancies are found to readily migrate at typical growth temperatures,
di-vacancies can be formed at ease. The key results of the present study with
respect to the thermodynamic behavior of mono and di-vacancies influence the
initial defect distribution in the ferroelectric phases and therefore the
conditions for aging.Comment: 9 pages, 4 figures, 4 table
The launch of HUFONIA and the related international experience of overnight indexed swap (OIS) markets
In relation to the October 2010 launch of the HUFONIA Swap Index, we discuss the most important characteristics of the overnight indexed swap (OIS) market, one of the fastest-growing segments of advanced money markets. OIS contracts allow for the cost-effective management of short-term interest rate risks while also facilitating profitable investment strategies to foresee the central bank's interest rate decisions, which, according to international experience, have greatly contributed to their popularity. A further benefit of OIS contracts is that partners’ credit risk and counterparty limits only play a minor role in their pricing. Looking at the underlying motives of central banks in market development, empirical analyses show that OIS markets can provide one of the most accurate indicators of short-term interest rate expectations, and could furnish additional information in the preparation and evaluation of monetary policy decisions. In conjunction, the financial crisis brought attention to the significance of the so-called LIBOR-OIS spread, an indicator also suitable for assessing the solvency of the banking system. The essential conditions of market development, such as the availability of a reliable reference rate and the presence of foreign market makers, are ensured in Hungary. However, due to the limited market size the fixed costs of market development are somewhat higher than in major currency areas. Nearly half of market makers have prepared their trading and accounting systems for transactions until 2011 Q2. Market activity could be driven by strategies aimed at mitigating or converting banks’ exposure to interest rate risks in the future.OIS, overnight indexed swap, HUFONIA, libor-ois spread, interest rate risk, policy rate expectations, market building.
Formation and switching of defect dipoles in acceptor doped lead titanate: A kinetic model based on first-principles calculations
The formation and field-induced switching of defect dipoles in acceptor doped
lead titanate is described by a kinetic model representing an extension of the
well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)].
Energy barriers for defect association and reorientation of oxygen
vacancy-dopant (Cu and Fe) complexes are obtained from first-principles
calculations and serve as input data for the kinetic coefficients in the rate
equation model. The numerical solution of the model describes the time
evolution of the oxygen vacancy distribution at different temperatures and
dopant concentrations in the presence or absence of an alternating external
field. We predict the characteristic time scale for the alignment of all defect
dipoles with the spontanenous polarization of the surrounding matrix. In this
state the defect dipoles act as obstacles for domain wall motion and contribute
to the experimentally observed aging. Under cycling conditions the fully
aligned configuration is perturbed and a dynamic equilibrium is established
with defect dipoles in parallel and anti-parallel orientation relative to the
spontaneous polarization. This process can be related to the deaging behavior
of piezoelectric ceramics.Comment: 10 pages, 7 figure
Efficient construction of linear models in materials modeling and applications to force constant expansions
Linear models, such as force constant (FC) and cluster expansions, play a key
role in physics and materials science. While they can in principle be
parametrized using regression and feature selection approaches, the convergence
behavior of these techniques, in particular with respect to thermodynamic
properties is not well understood. Here, we therefore analyze the efficacy and
efficiency of several state-of-the-art regression and feature selection
methods, in particular in the context of FC extraction and the prediction of
different thermodynamic properties. Generic feature selection algorithms such
as recursive feature elimination with ordinary least-squares (OLS), automatic
relevance determination regression, and the adaptive least absolute shrinkage
and selection operator can yield physically sound models for systems with a
modest number of degrees of freedom. For large unit cells with low symmetry
and/or high-order expansions they come, however, with a non-negligible
computational cost that can be more than two orders of magnitude higher than
that of OLS. In such cases, OLS with cutoff selection provides a viable route
as demonstrated here for both second-order FCs in large low-symmetry unit cells
and high-order FCs in low-symmetry systems. While regression techniques are
thus very powerful, they require well-tuned protocols. Here, the present work
establishes guidelines for the design of protocols that are readily usable,
e.g., in high-throughput and materials discovery schemes. Since the underlying
algorithms are not specific to FC construction, the general conclusions drawn
here also have a bearing on the construction of other linear models in physics
and materials science.Comment: 15 pages, 12 figure
Finite-temperature properties of non-magnetic transition metals: Comparison of the performance of constraint-based semi and nonlocal functionals
We assess the performance of nonempirical, truly nonlocal and semi-local
functionals with regard to structural and thermal properties of , , and
non-magnetic transition metals. We focus on constraint-based functionals
and consider the new consistent-exchange van der Waals density functional
version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys.
Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100,
136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108
(2005)]. Using the quasi-harmonic approximation structural parameters, elastic
response, and thermal expansion at finite temperatures are computed and
compared to experimental data. We also compute cohesive energies explicitly
including zero-point vibrations. It is shown that overall vdW-DF-cx provides an
accurate description of thermal properties and retains a level of
transferability and accuracy that is comparable to or better than some of the
best constraint-based semi-local functionals. Especially, with regard to the
cohesive energies the consistent inclusion of spin polarization effects in the
atoms turns out to be crucial and it is important to use the rigorous
spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This
demonstrates that vdW-DF-cx has general-purpose character and can be used to
study systems that have both sparse and dense electron distributions.Comment: 10 pages, 5 figure
The hiphive package for the extraction of high-order force constants by machine learning
The efficient extraction of force constants (FCs) is crucial for the analysis
of many thermodynamic materials properties. Approaches based on the systematic
enumeration of finite differences scale poorly with system size and can rarely
extend beyond third order when input data is obtained from first-principles
calculations. Methods based on parameter fitting in the spirit of interatomic
potentials, on the other hand, can extract FC parameters from semi-random
configurations of high information density and advanced regularized regression
methods can recover physical solutions from a limited amount of data. Here, we
present the hiPhive Python package, that enables the construction of force
constant models up to arbitrary order. hiPhive exploits crystal symmetries to
reduce the number of free parameters and then employs advanced machine learning
algorithms to extract the force constants. Depending on the problem at hand
both over and underdetermined systems are handled efficiently. The FCs can be
subsequently analyzed directly and or be used to carry out e.g., molecular
dynamics simulations. The utility of this approach is demonstrated via several
examples including ideal and defective monolayers of MoS as well as bulk
nickel
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