A combined theoretical and experimental study of the low temperature properties of BaZrO3

Low temperature properties of BaZrO3 are revealed by combining experimental techniques (X-ray diffraction, neutron scattering and dielectric measurements) with theoretical first-principles-based methods (total energy and linear response calculations within density functional theory, and effective Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations). Unlike most of the perovskite systems, BaZrO3 does not undergo any (long-range-order) structural phase transition and thus remains cubic and paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On the other hand, these latter pure quantum effects lead to a negligible thermal dependency of the cubic lattice parameter below ~ 40 K. They also affect the dielectricity of BaZrO3 by inducing an overall saturation of the real part of the dielectric response, for temperatures below ~ 40 K. Two fine structures in the real part, as well as in the imaginary part, of dielectric response are further observed around 50-65 K and 15 K, respectively. Microscopic origins (e.g., unavoidable defects and oxygen octahedra rotation occurring at a local scale) of such anomalies are suggested. Finally, possible reasons for the facts that some of these dielectric anomalies have not been previously reported in the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also discussed.Comment: 8 pages, 5 figures, submitted to Physical Review

First principles based atomistic modeling of phase stability in PMN-xPT

We have performed molecular dynamics simulations using a shell model potential developed by fitting first principles results to describe the behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at all compositions. Our simulations give the slope of the morphotropic phase boundaries, crucial for high temperature applications. We find that the phase diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure

Effects of hydrostatic stress on the density of impurity states and donor-related optical absorption spectra in GaAs–(Ga,Al)As quantum wells

ABSTARCT: The effects of hydrostatic stress on the density of donor impurity states and donor-related optical absorption spectra in a GaAs–(Ga,Al)As quantum well are investigated. The shallow-donor binding energy for different well widths and different values of the hydrostatic stress has been calculated. It has been found that for wider well widths the binding energy increases slowly with hydrostatic stress contrary to the behavior of the binding energy for wells with smaller widths. In particular, it has been found that the binding energy does not change appreciably with the impurity position when the width of the well is small and for large values of hydrostatic stress. Two structures in both the density of states and the optical absorption spectra, associated with impurities located close to the center and to the edges of the structure, are obtained. Also, it has been observed that the density of states and the optical absorption spectra depend strongly on the applied hydrostatic stress

Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics

Infrared reflectivity and time-domain terahertz transmission spectra of EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is fully responsible for the recently observed quantum paraelectric behaviour. Even if our EuTiO3 ceramics show lower permittivity than the single crystal due to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7 phase, we confirmed the magnetic field dependence of the permittivity, also slightly smaller than in single crystal. Attempt to reveal the soft phonon dependence at 1.8K on the magnetic field up to 13T remained below the accuracy of our infrared reflectivity experiment

Cohabitation in Brazil : historical legacy and recent evolution

The availability of the micro data in the IPUMS samples for several censuses spanning a period of 40 years permits a detailed study of differentials and trends in cohabitation in Brazil than has hitherto been the case. The gist of the story is that the historical race/class and religious differentials and the historical spatial contrasts have largely been maintained, but are now operating at much higher levels than in the 1970s. During the last 40 years cohabitation has dramatically increased in all strata of the Brazilian population, and it has spread geographically to all areas in tandem with further expansions in the regions that had historically higher levels to start with. Moreover, the probability of cohabiting depends not only on individual-level characteristics but also on additional contextual effects operating at the level of meso-regions. The rise of cohabitation in Brazil fits the model of the "Second demographic transition", but it is grafted onto a historical pattern which is still manifesting itself in a number of ways

Territorial Tactics: The Socio-spatial Significance of Private Policing Strategies in Cape Town

This paper analyses the policing strategies of private security companies operating in urban space. An existing literature has considered the variety of ways that territory becomes of fundamental importance in the work of public police forces. However, this paper examines territory in the context of private security companies. Drawing on empirical research in Cape Town, it examines how demarcated territories become key subjects in private policing. Private security companies are responsible for a relatively small section of the city, while in contrast the public police ultimately have to see city space as a whole. Hence, private policing strategy becomes one of displacement, especially of so-called undesirables yielding a patchworked public space associated with private enclaves of consumption. The conclusions signal the historical resonances and comparative implications of these political-legal-security dynamics. © 2013 Urban Studies Journal Limited

Semantic cues in language learning: an artificial language study with adult and child learners

Grammatical regularities may correlate with semantics; e.g., grammatical gender is often partially predictable from the noun’s semantics. We explore whether learners generalise over semantic cues, and whether extent of exposure (1 versus 4 sessions) and number of exemplars for each semantic class (type-frequency) affect this. Six-year-olds and adults were exposed to semi-artificial languages where nouns co-occurred with novel particles, with particle usage fully or partially determined by the semantics of nouns. Both adults and children generalised to novel nouns when semantic cues were fully consistent. Adults (but not children) also generalised when cues were partially consistent. Generalisation increased with exposure, however there was no evidence that increasing type-frequency (i.e. more nouns per semantic class) increased generalisation. Post-experiment interviews also suggested that successful generalisation depended on explicit awareness. These results suggest that semantic cues are particularly difficult for children to exploit during the early stages of language acquisition

The Mediating Role of Bullying and Victimisation on the Relationship Between Problematic Internet Use and Substance Abuse Among Adolescents in the UK: The Parent–Child Relationship as a Moderator

Over the last decade, research into the negative effects of problematic internet use has greatly increased. The current study adopted a mediation-moderation model in exploring the relationship between problematic internet use and substance abuse (drinking, drug use, and smoking tobacco cigarettes) among 1,613 adolescents (aged 10–16) in the UK. The findings of the study revealed a significant positive correlation between problematic internet use and substance abuse, which is mediated by traditional and cyber bullying and victimisation. Furthermore, the parent–child relationship was found to be a protective factor that moderated the correlation between problematic internet use and substance abuse and the correlation between problematic internet use and traditional bullying. The study emphasises the critical need to reduce problematic internet use among adolescents as a risk factor for involvement in bullying as perpetrators and victims, in addition to substance abuse. Furthermore, the findings of the study highlight the importance of a good parent–child relationship as a protective factor among adolescents. In light of the findings of the study, interventions for reducing problematic internet use taking into account bullying and the parent–child relationship are needed among adolescents

Comparison of stroke knowledge before and after education in Ethiopia (The ask project)

Background: In order to address the growing burden of stroke in Africa, it is essential to have teaching tools that are accessible to a wide variety of communities. Resources for stroke, recognition of symptoms, steps to assist a stroke victim, or any educational tools are scarce. In order to increase education about stroke, we introduced a stroke education intervention in Bahir Dar, Ethiopia in the Amharic language. The aim of the intervention was to see if stroke knowledge and recognition of stroke symptoms increased post-education. Methods: An anonymous pre-test questionnaire was given to identify their baseline stroke knowledge, and then participants viewed the video about stroke. The Amharic BE-FAST acronym was introduced in the video to evaluate participants’ retention of stroke symptoms, while emphasizing the importance of rapid response. Participants were given a survey to post-test survey of their short-term memory recall, along with another satisfaction survey. Results: Over 50% of participants remembered all five letters of the Amharic BE-FAST acronym, nearly 80% answered that they learned about stroke prevention, 90% were satisfied with the video, and 98% thought it should be available widely to the general public. The data presented showed that there was an improvement in stroke education retention with the use of the stroke education video. Conclusions: The Amharic BE-FAST acronym demonstrated to be an effective tool in communicating the symptoms of stroke from English to Amharic. Stroke education was well received, which provided insight on how to design and implement new medical concepts in native languages.   French title:Connaissances sur les accidents vasulaires cerebraux avant et apres une education en Éthiopie (Projet ask)   Introduction: Afin de faire face au fardeau croissant des AVC en Afrique, il est essentiel de disposer d'outils pédagogiques accssibles à une grande variété de communautés. Les ressources pour l'AVC, la reconnaissance des symptômes, les étapes pour aider une victime d'AVC ou tout autre outil éducatif sont rares. Afin d'accroître l'éducation sur l'AVC, nous avons introduit une intervention d'éducation sur l'AVC à Bahir Dar, en Ethiopie, en langue amharique. Le but de l'intervention était de voir si la connaissance et la reconnaissance des symptômes de l'AVC augmentaient après l'éducation. Méthodes: Un questionnaire pré-test anonyme a été donné pour identifier leurs connaissances de base sur l'AVC, puis les participants ont visionné la vidéo sur l'AVC. L'acronyme amharique BE-FAST a été introduit dans la vidéo pour évaluer la rétention des symptômes d'AVC par les participants, tout en soulignant l'importance d'une réponse rapide. Les participants ont reçu une enquête pour post-test de leur mémoire à court terme, ainsi qu'une autre enquête de satisfaction. Résultats: Plus de 50% des participants se sont souvenus des cinq lettres de l'acronyme amharique BE-FAST, près de 80% ont répondu avoir appris la prévention des AVC, 90% étaient satisfaits de la vidéo et 98% pensaient qu'elle devrait être largement accessible au grand public. Les données présentées ont montré qu'il y avait une amélioration de la rétention de l'éducation sur l'AVC grâce à l'utilisation de la vidéo d'éducation sur l'AVC. Conclusion: L'acronyme amharique BE-FAST s'est révélé être un outil efficace pour communiquer les symptômes de l'AVC de l'anglais à l'amharique. L'éducation sur l'AVC a été bien accueillie, ce qui a permis de comprendre comment concevoir et mettre en oeuvre de nouveaux concepts médicaux dans les langues maternelles