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.

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 $3d$, $4d$, and $5d$ 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

Implications of the band gap problem on oxidation and hydration in acceptor-doped barium zirconate

Charge carrier concentrations in acceptor-doped proton-conducting perovskites are to a large extent determined by the hydration and oxidation of oxygen vacancies, which introduce protons and holes, respectively. First-principles modeling of these reactions involves calculation of formation energies of charged defects, which requires an accurate description of the band gap and the position of the band edges. Since density-functional theory (DFT) with local and semi-local exchange-correlation functionals (LDA and GGA) systematically fails to predict these quantities this can have serious implications on the modeling of defect reactions. In this study we investigate how the description of band gap and band edge positions affects the hydration and oxidation in acceptor-doped BaZrO$_3$. First-principles calculations are performed in combination with thermodynamic modeling in order to obtain equilibrium charge carrier concentrations at different temperatures and partial pressures. Three different methods have been considered: DFT with both semi-local (PBE) and hybrid (PBE0) exchange-correlation functionals, and many-body perturbation theory within the $G_0W_0$-approximation. All three methods yield similar results for the hydration reaction, which are consistent with experimental findings. For the oxidation reaction, on the other hand, there is a qualitative difference. PBE predicts the reaction to be exothermic while the two others predict an endothermic behavior. Results from thermodynamic modeling are compared with available experimental data, such as enthalpies, concentrations and conductivities, and only the results obtained with PBE0 and $G_0W_0$, with an endothermic oxidation behavior, give a satisfactory agreement with experiments.Comment: 15 pages, 12 figures + supplementary material (2 pages

The creation and utilization of a physical science tablet application in the primary classroom

In traditional science education, lecture-style presentation dominates. There is a call, however, to integrate a greater amount of inquiry and discovery into the science classroom, specifically utilizing new media and technology. To answer that call, an undergraduate team created a tablet application for primary students, which was designed to supplement instruction in the physical sciences. The app was then taken into the classroom to test for efficacy. The primary students’ test scores showed a significant increase after use of the tablet application. Grade level and school location did affect change in test score; whereas, gender, type of school, student IEP, and iPad use did not. The undergraduate team who designed the app also showed significant increase in concept knowledge after the project in specific topic areas.Department of ChemistryLiterature review -- Methodology and results for undergraduate study -- iPad application : the product -- Methodology and results for primary student study -- Implications and recommendations.Thesis (M.S.

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$_2$ as well as bulk nickel

Optimal monetary policy committee size: Theory and cross country evidence

Theoretical and empirical studies of different sciences suggest that an optimal committee consists of roughly 5-9 members, although it can swell mildly under specific circumstances. This paper develops a conceptual model in order to analyze the issue in case of monetary policy formulation. The optimal monetary policy committee (MPC) size varies according to the uncertainty of MPC members’ information influenced by the size of the monetary zone and overall economic stability. Our conceptual model is backed up with econometric evidence using a 2006 survey of 85 countries. The survey is available for further research and published on the web. The MPC size of large monetary zones (EMU, USA, Japan) is close to the estimated optimal level, but there exist several smaller countries with too many or too few MPC members.monetary policy committe, mpc size, decision making.

Efficacy of the DFT+U formalism for modeling hole polarons in perovskite oxides

We investigate the formation of self-trapped holes (STH) in three prototypical perovskites (SrTiO3, BaTiO3, PbTiO3) using a combination of density functional theory (DFT) calculations with local potentials and hybrid functionals. First we construct a local correction potential for polaronic configurations in SrTiO3 that is applied via the DFT+U method and matches the forces from hybrid calculations. We then use the DFT+U potential to search the configuration space and locate the lowest energy STH configuration. It is demonstrated that both the DFT+U potential and the hybrid functional yield a piece-wise linear dependence of the total energy on the occupation of the STH level suggesting that self-interaction effects have been properly removed. The DFT+U model is found to be transferable to BaTiO3 and PbTiO3, and formation energies from DFT+U and hybrid calculations are in close agreement for all three materials. STH formation is found to be energetically favorable in SrTiO3 and BaTiO3 but not in PbTiO3, which can be rationalized by considering the alignment of the valence band edges on an absolute energy scale. In the case of PbTiO3 the strong coupling between Pb 6s and O 2p states lifts the valence band minimum (VBM) compared to SrTiO3 and BaTiO3. This reduces the separation between VBM and STH level and renders the STH configuration metastable with respect to delocalization (band hole state). We expect that the present approach can be adapted to study STH formation also oxides with different crystal structures and chemical composition.Comment: 7 pages, 6 figure