Positive effective Q12 electrostrictive coefficient in perovskites

It is demonstrated that for classical perovskites such as BaTiO3, SrTiO3 and PbTiO3 electrostrictive strain induced by an electric field may not obey traditionally considered "extension along the field, contraction perpendicular to it" behavior if a sample is cut obliquely to the cubic crystallographic directions

Vortex-to-Polarization Phase Transformation Path in Pb(ZrTi)O3_3 Nanoparticles

Phase transformation in finite-size ferroelectrics is of fundamental relevance for understanding collective behaviors and balance of competing interactions in low-dimensional systems. We report a first-principles effective Hamiltonian study of vortex-to-polarization transformation in Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ nanoparticles, caused by homogeneous electric fields normal to the vortex plane. The transformation is shown to (1) follow an unusual {\it macroscopic} path that is symmetry non-conforming and characterized by the occurrence of a previously unknown structure as the bridging phase; (2) lead to the discovery of a striking collective phenomenon, revealing how ferroelectric vortex is annihilated {\it microscopically}. Interactions underlying these behaviors are discussed

Optimal configuration of microstructure in ferroelectric materials by stochastic optimization

An optimization procedure determining the ideal configuration at the microstructural level of ferroelectric (FE) materials is applied to maximize piezoelectricity. Piezoelectricity in ceramic FEs differ significantly from that of single crystals because of the presence of crystallites (grains) possessing crystallographic axes aligned imperfectly. The piezoelectric properties of a polycrystalline (ceramic) FE is inextricably related to the grain orientation distribution (texture). The set of combination of variables, known as solution space, which dictates the texture of a ceramic is unlimited and hence the choice of the optimal solution which maximizes the piezoelectricity is complicated. Thus a stochastic global optimization combined with homogenization is employed for the identification of the optimal granular configuration of the FE ceramic microstructure with optimum piezoelectric properties. The macroscopic equilibrium piezoelectric properties of polycrystalline FE is calculated using mathematical homogenization at each iteration step. The configuration of grains characterised by its orientations at each iteration is generated using a randomly selected set of orientation distribution parameters. Apparent enhancement of piezoelectric coefficient $d_{33}$ is observed in an optimally oriented BaTiO$_3$ single crystal. A configuration of crystallites, simultaneously constraining the orientation distribution of the c-axis (polar axis) while incorporating ab-plane randomness, which would multiply the overall piezoelectricity in ceramic BaTiO$_{3}$ is also identified. The orientation distribution of the c-axes is found to be a narrow Gaussian distribution centred around ${45^\circ}$. The piezoelectric coefficient in such a ceramic is found to be nearly three times as that of the single crystal.Comment: 11 pages, 7 figure

Derived bracket construction and Manin products

We will extend the classical derived bracket construction to any algebra over a binary quadratic operad. We will show that the derived product construction is a functor given by the Manin white product with the operad of permutation algebras. As an application, we will show that the operad of prePoisson algebras is isomorphic to Manin black product of the Poisson operad with the preLie operad. We will show that differential operators and Rota-Baxter operators are, in a sense, Koszul dual to each other.Comment: This is the final versio

Structural relaxation of E' gamma centers in amorphous silica

We report experimental evidence of the existence of two variants of the E' gamma centers induced in silica by gamma rays at room temperature. The two variants are distinguishable by the fine features of their line shapes in paramagnetic resonance spectra. These features suggest that the two E' gamma differ for their topology. We find a thermally induced interconversion between the centers with an activation energy of about 34 meV. Hints are also found for the existence of a structural configuration of minimum energy and of a metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let

High-throughput screening of perovskite alloys for piezoelectric performance and thermodynamic stability

We screen a large chemical space of perovskite alloys for systems with optimal properties to accommodate a morphotropic phase boundary (MPB) in their composition-temperature phase diagram, a crucial feature for high piezoelectric performance. We start from alloy end points previously identified in a high-throughput computational search. An interpolation scheme is used to estimate the relative energies between different perovskite distortions for alloy compositions with a minimum of computational effort. Suggested alloys are further screened for thermodynamic stability. The screening identifies alloy systems already known to host an MPB and suggests a few others that may be promising candidates for future experiments. Our method of investigation may be extended to other perovskite systems, e.g., (oxy-)nitrides, and provides a useful methodology for any application of high-throughput screening of isovalent alloy systems

Associations of ambivalent leadership with distress and cortisol secretion

Ambivalent social ties, i.e., whereby a relationship is evaluated simultaneously in positive and negative terms, are a potential source of distress and can perturb health-relevant biological functions. Social interactions at the workplace, in particular with supervisors, are often described in ambivalent terms, but the psychological and psychobiological impact of such interactions has received little scientific attention. The current study examined associations between ambivalent attitudes towards one's supervisor, perceived distress (general and work-related), and diurnal dynamics of the stress hormone cortisol. 613 employees evaluated their supervisor in terms of positive and negative behaviors, which was combined into an ambivalent index. Higher ambivalence was associated with higher perceived distress and work-related stress (p < .001), and with a larger cortisol awakening response and higher day-time secretion post-awakening (p < .01). The present study is the first to identify ambivalence towards supervisors as a predictor of employee distress and stress-related endocrine dysregulation. In consequence, focusing solely on positive or negative leader behavior may insufficiently capture the true complexity of workplace interactions and attempts to compensate negative behaviors with positive are unlikely to reduce distress-but quite the opposite-by increasing ambivalence

Trauma electives in South Africa provide valuable training for international surgeons

Background. Trauma electives in South Africa (SA) are common and many foreign-based surgeons have undertaken such electives over the past 3 decades. Despite this, little academic attention has been paid to these electives, which remain largely informal and unstructured. This project aims to redress this deficit.Objectives. To investigate and document the extent of trauma clinical electives and to assess their impact on the careers of foreign surgeons who have undertaken such electives.Methods. A mixed methods-style questionnaire was compiled, which sought to document the demographics of surgeons undertaking an SA trauma clinical elective, the trauma clinical experience they had prior to the elective, as well as the volume of experience they acquired during the elective.Results. Sixty questionnaires were sent out and 21 were completed. There were 16 male and 5 female respondents. Only 17 had undertaken a formal trauma rotation before their elective in SA. The mean number of major resuscitations managed prior to rotating through surgery departments in SA was 15, and the mean number managed during a 12-month rotation in SA was 204. It would take each respondent 14 years in their country of origin to acquire an equivalent level of exposure to major resuscitation. During the year before their elective, each surgeon had been exposed to a mean number of the following: 0.5 gunshot wounds (GSWs), 2 stab wounds (SWs), 0.1 blast injuries and 19 road traffic accidents (RTAs). The equivalent mean number for their year in SA was 106 GSWs, 153 SWs, 4 blast injuries and 123 RTAs. The time necessary to achieve a similar level of exposure to their SA experience if they had remained in their country of origin was 213 years for GSWs, 73 years for SWs, 41 years for blast injuries and 7 years for RTAs. Compared with their SA elective, it would take each respondent 3 years to insert as many central venous lines, 9 years to perform the same number of tube thoracostomies, 9 years to manage as many surgical airways, 18 years to explore as many SWs of the neck and 93 years to explore as many GSWs of the neck. Furthermore, it would take 33 years to see and perform as many laparotomies for SWs to the abdomen, 374 years to perform an equivalent number of GSWs to the abdomen and 34 years of experience to perform as many damage-control laparotomies in their countries of origin. In terms of vascular trauma, it would take 23 years to see as many vascular injuries secondary to SWs and 77 years to see an equivalent number of vascular injuries secondary to GSWs.Conclusions. A trauma clinical elective in SA provides an unparalleled exposure to almost all forms of trauma in conjunction with a welldeveloped academic support programme. Formalising these trauma electives might allow for the development of exchange programmes for SA trainees who wish to acquire international exposure to advanced general surgical training

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