Rules and mechanisms governing octahedral tilts in perovskites under pressure

The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this work, we discover several new mechanisms and formulate a set of simple rules that allow to understand how pressure affects oxygen octahedral tiltings, via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B-ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A-ion, by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure, by indirectly controlling the amplitude of octahedral rotations.Comment: Submitted to Phys. Re

Coeficientes de rozamiento estático y dinámico en la madera de pino silvestre (Pinus sylvestris L.), según las direcciones paralela y perpendicular a la fibra

In this study the static (µe) and kinetic (µd) coefficients of friction were obtained for Pinus sylvestis L. sawn timber of Spanish origin. Friction between transverse surfaces sliding perpendicular to the grain (tangential direction) and radial surfaces sliding parallel to the grain was analyzed. A specifically designed device was used for tests, which makes it possible to apply contact pressure and measure displacements and applied loads simultaneously. Coefficients of friction between transverse surfaces (µe = 0,24; µd = 0,17) were about twice of the coefficients of friction between radial surfaces (µe = 0,12; µd = 0,08). Furthermore, these values are located within normal values of those commonly reported for softwood. The results are considered preliminary due to the small number of specimens.En este estudio se determinaron los coeficientes de rozamiento, estático (µe) y dinámico (µd), en madera aserrada de Pinus sylvestris L. de procedencia española, diferenciando si se produce el contacto entre secciones de corte transversal con deslizamiento en dirección perpendicular a la fibra (en dirección tangencial), o entre secciones de corte radial con deslizamiento paralelo a la fibra. Para la realización de los ensayos se ha utilizado un dispositivo, diseñado específicamente, que posibilita la aplicación de una presión de contacto y la medición del desplazamiento y de la fuerza aplicada de manera simultánea, permitiendo la obtención de los coeficientes de rozamiento estático y dinámico. Los coeficientes de rozamiento obtenidos entre secciones transversales (µe = 0.24; µd = 0.17) fueron del orden del doble de los coeficientes de rozamiento entre secciones radiales (µe = 0.12; µd = 0.08). Además, estos valores se encuentran dentro de los valores que aparecen habitualmente en la bibliografía para madera de coníferas. Debido al escaso tamaño de la muestra los resultados se consideran preliminares

The effect of IFRS adoption on balance-sheet conservatism: the Spanish case

El presente estudio tiene por objeto analizar la evolución del conservadurismo contable en España tras la implantación en 2005 de las Normas Internacionales de Información Financiera. Es de esperar, a la vista de los estudios previos, que el cambio hacia una normativa de corte más anglosajón a la que teníamos previamente, haya producido un descenso del conservadurismo de balance. Mediante una muestra de empresas españolas y del Reino Unido durante el periodo 2000-2009, encontramos evidencia de que la entrada en vigor de las normas internacionales de contabilidad ha provocado una reducción del conservadurismo de balance en el mercado español. En cuanto a la brecha de conservadurismo existente entre España y el Reino Unido, nuestro análisis sugiere que las diferencias de conservadurismo se han reducido de manera significativa. No obstante, nuestro trabajo pone de manifiesto las limitaciones existentes a la hora de medir el conservadurismo abriendo la puerta a futuras investigaciones sobre el tema.This study aims to analyze the effect on accounting conservatism of IFRS adoption in Spain. Following evidence found in previous studies we expect a decline in balance-sheet accounting conservatism due to this change towards a more Anglo-Saxon accounting system. Using a sample of Spanish and UK firms during the period 2000-2009, we find evidence that the adoption of IFRS has led to a decrease of the balance-sheet conservatism in the Spanish market. Regarding the conservatism gap between these two countries, our analysis suggests that the conservatism difference between Spain and the UK has been reduced significantly after the IFRS adoption. Nevertheless, we show the limitations that exist in order to measure balance-sheet conservatism opening the door to new future research

Optimized local modes for lattice dynamical applications

We present a new scheme for the construction of highly localized lattice Wannier functions. The approach is based on a heuristic criterion for localization and takes the symmetry constraints into account from the start. We compare the local modes thus obtained with those generated by other schemes and find that they also provide a better description of the relevant vibrational subspace.Comment: 6 pages, ReVTeX, plus four postscript files for figure

Multi-phonon scattering and Ti-induced hydrogen dynamics in sodium alanate

We use ab initio methods and neutron inelastic scattering (NIS) to study the structure, energetics, and dynamics of pure and Ti-doped sodium alanate (NaAlH_4), focusing on the possibility of substitutional Ti doping. The NIS spectrum is found to exhibit surprisingly strong and sharp two-phonon features. The calculations reveal that substitutional Ti doping is energetically possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al--H bond breaking. Our results hint at new ways of improving the hydrogen dynamics and storage capacity of the alanates.Comment: 5 pages, with 4 postscript figures embedded. Uses REVTEX4 and graphicx macro

Ab-initio design of perovskite alloys with predetermined properties: The case of Pb(Sc_{0.5} Nb_{0.5})O_{3}

A first-principles derived approach is combined with the inverse Monte Carlo technique to determine the atomic orderings leading to prefixed properties in Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements between Sc and Nb atoms result in drastic changes with respect to the disordered material, including ground states of new symmetries, large enhancement of electromechanical responses, and considerable shift of the Curie temperature. We discuss the microscopic mechanisms responsible for these unusual effects.Comment: 5 pages with 2 postscript figures embedde

Targeted Community Merging provides an efficient comparison between collaboration clusters and departmental partitions

Community detection theory is vital for the structural analysis of many types of complex networks, especially for human-like collaboration networks. In this work, we present a new community detection algorithm, the Targeted Community Merging algorithm, based on the well-known Girvan–Newman algorithm, which allows obtaining community partitions with high values of modularity and a small number of communities. We then perform an analysis and comparison between the departmental and community structure of scientific collaboration networks within the University of Zaragoza. Thus, we draw valuable conclusions from the inter- and intra-departmental collaboration structure that could be useful to take decisions on an eventual departmental restructuring

Emotional cuing to test attentional network functioning in trait anxiety

The Attention Networks Test (ANT) has been widely used to assess the three attentional networks proposed by Posner and his collaborators. Here we present a version of the ANT that uses emotionally laden words as cues to evaluate the functioning of the attention networks and their interactions. University students participated in the task and the results replicated those found in previous studies with the original version of the test. Then, those with extreme scores on a trait anxiety scale, STAI (State Trait Anxiety Inventory) were assigned to the low or the high anxiety group. The high anxiety group showed normal patterns in the functioning of the three attentional networks, but negative cues modulated the interaction between the orienting and the executive network. These participants failed to narrow the attention focus to cover the region containing the target, affecting conflict resolution in incongruent trials.El test de las redes atencionales (Attention Networks Test, ANT) ha sido muy utilizado para evaluar las tres redes atencionales propuestas por Posner y colaboradores. Aquí presentamos una versión del ANT que utiliza palabras cargadas emocionalmente como señales de orientación de la atención para evaluar el funcionamiento de las redes atencionales y sus interacciones. Estudiantes universitarios participaron en la tarea y los resultados replicaron aquellos obtenidos en estudios previos con la versión original del test. Aquellos participantes con puntuaciones extremas en la escala de ansiedad rasgo, STAI (State Trait Anxiety Inventory) fueron asignados al grupo de baja o alta ansiedad. El grupo de alta ansiedad mostró patrones normales de funcionamiento en las tres redes atencionales, pero las señales con carga emocional negativa modularon la interacción entre la red de orientación y la red ejecutiva. Concretamente, mostraron un déficit en el ajuste del foco atencional para abarcar justo la región espacial que contenía el estímulo objetivo, afectando a la resolución del conflicto en los ensayos incongruentes.This research was supported by the Spanish Ministry of Economy and Competitivity (grant PS2011-23340/PSIC), and by the Fundació Caixa Castelló- Bancaixa (grant E-2006-24

A First-Principles Approach to Insulators in Finite Electric Fields

We describe a method for computing the response of an insulator to a static, homogeneous electric field. It consists of iteratively minimizing an electric enthalpy functional expressed in terms of occupied Bloch-like states on a uniform grid of k points. The functional has equivalent local minima below a critical field E_c that depends inversely on the density of k points; the disappearance of the minima at E_c signals the onset of Zener breakdown. We illustrate the procedure by computing the piezoelectric and nonlinear dielectric susceptibility tensors of III-V semiconductors.Comment: 4 pages, with 1 postscript figure embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/is_ef/index.htm