Modelling spin waves in noncollinear antiferromagnets: spin-flop states, spin spirals, skyrmions and antiskyrmions

Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $\alpha$--$\text{Cu}_2\text{V}_2\text{O}_7$ utilizing inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119}, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.Comment: 26 pages, 9 figure

First-principles investigation of spin wave dispersions in surface-reconstructed Co thin films on W(110)

We computed spin wave dispersions of surface-reconstructed Co films on the W(110) surface in the adiabatic approximation. The magnetic exchange interactions are obtained via first-principles electronic structure calculations using the Korringa-Kohn-Rostoker Green function method. We analyze the strength and oscillatory behavior of the intralayer and interlayer magnetic interactions and investigate the resulting spin wave dispersions as a function of the thickness of Co films. In particular, we highlight and explain the strong impact of hybridization of the electronic states at the Co-W interface on the magnetic exchange interactions and on the spin wave dispersions. We compare our results to recent measurements based on electron energy loss spectroscopy [E. Michel, H. Ibach, and C.M. Schneider, Phys. Rev. B 92, 024407 (2015)]. Good overall agreement with experimental findings can be obtained by considering the possible overestimation of the spin splitting, stemming from the local spin density approximation, and adopting an appropriate correction.Comment: 14 pages, 14 figure

Aprendiendo funciones complejas con GeoGebra

In this paper we describe a teaching experiment targeting with students of Complex Analysis attending an undergraduate course of a Portuguese university. Our main goal is the understanding of the GeoGebra role with respect to visualization and as a technological mediator, according to Vygotsky theory, in the teaching and learning process of complex functions. The first step of our study was the conception of a sequence of didactical tasks and the development of GeoGebra tools related to the target didactical objectives. Here we will describe the procedure related to the tasks implementation in a classroom environment and the achieved results based on the collected data composed by written assignments produced by students, video recording the student performance during the experiment, and the student constructions with GeoGebra. All the collected data were analysed from a qualitative and interpretative paradigm.publishe

Nonreciprocity of spin waves in noncollinear magnets due to the Dzyaloshinskii-Moriya interaction

Broken inversion symmetry in combination with the spin-orbit interaction generates a finite Dzyaloshinskii-Moriya interaction (DMI), which can induce noncollinear spin textures of chiral nature. The DMI is characterized by an interaction vector whose magnitude, direction and symmetries are crucial to determine the stability of various spin textures, such as skyrmions and spin spirals. The DMI can be measured from the nonreciprocity of spin waves in ferromagnets, which can be probed via inelastic scattering experiments. In a ferromagnet, the DMI can modify the spin-wave dispersion, moving its minimum away from the $\Gamma$ point. Spin waves propagating with opposite wavevectors are then characterized by different group velocities, energies and lifetimes, defining their nonreciprocity. Here, we address the case of complex spin textures, where the manifestation of DMI-induced chiral asymmetries remains to be explored. We discuss such nonreciprocal effects and propose ways of accessing the magnitude and direction of the DMI vectors in the context of spin-polarized or spin-resolved inelastic scattering experiments. We show that only when a periodic magnetic system has finite net magnetization, that is, when the vector sum of all magnetic moments is nonzero, can it present a total nonreciprocal spin-wave spectrum. However, even zero-net-magnetization systems, such as collinear antiferromagnets and cycloidal spin spirals, can have spin-wave modes that are individually nonreciprocal, while the total spectrum remains reciprocal.Comment: 12 figure

GeoGebra as a learning mathematical environment

GeoGebra, a software system for dynamic geometry and algebra in the plane, since its inception in 2001, has gone from a dynamic geometry software (DGS), to a powerful computational tool in several areas of mathematics. Powerful algebraic capabilities have joined GeoGebra, an efficient spreadsheet that can deal with many kinds of objects, an algebraic and symbolic calculation system and several graphical views that expand the possibility of multidimensional representations, namely, by using colouring domain techniques, expanded to representations in the Riemann sphere, making this DGS a powerful research tool in mathematics. On the other hand, GeoGebra can create applications easily and export to HTML, and the possibility to quickly integrating these applets in several web platforms provides this DGS with an excellent way to create strong collaborative environments to teach and learn mathematics. Recently was added to GeoGebra powerful capabilities that transform this software a real Learning Mathematical Environment, using the GeoGebraBooks and GeoGebraGroups, plain of collaborative functionality between students and teachers

Job quality of Portuguese health professionals: 2011 vs 2015

The present research uses data from European Union Labour Force Survey to analyse job quality of Portuguese health professionals, comparing data from the years 2011 and 2015. The data from both years were processed separately through Fuzzy Clustering Analysis, which enabled the identification of employment profiles within each year. The empirical findings identified sets of three and four profiles to 2011 and 2015, respectively. In these sets, the duality of bad and good jobs is distinguishable. The bad jobs are characterized by affecting mainly young professionals and are identified by the existence of precarious situations such as limited duration contracts, part-time jobs and having low income. Good jobs have the opposite characteristics. The good jobs are mainly occupied by the older professionals. These professionals have permanent and full-time jobs and are taking home wages placed within the highest deciles of income in the Portuguese society. Medical Doctors is the professional category with better contractual and income indicators, though is the category to be clearly identified as having more weekly working hours. The category Other Health Professionals is the one more affected by the precariousness in the early years of practice, which indicates the need of a deeper research on who these other professionals are, and if there are other relevant differences within the category. Apart from the found duality, this research also found a duality within the duality, which is the existence of atypical work within the profiles with better jobs, creating a distinction between professionals.A presente pesquisa utiliza dados do "European Union Labour Force Survey" para analisar a qualidade do emprego dos profissionais de saúde portugueses, comparando dados dos anos de 2011 e 2015. Os dados de cada ano foram tratados através de "Fuzzy Clustering Analysis", que permitiu a identificação de conjuntos de perfis de emprego dentro de cada ano. Os resultados empíricos identificaram conjuntos de três e quatro perfis de emprego para 2011 e 2015, respetivamente. Nestes conjuntos observa-se a existência da dualidade dos maus e bons empregos. Os maus empregos caracterizam-se por afetar jovens profissionais e são identificados pela existência de situações precárias, como contratos de tempo limitado, empregos a tempo parcial e salários de baixo valor. Os bons empregos são ocupados principalmente pelos profissionais de saúde mais antigos. Estes profissionais têm empregos com contratos sem termo e a tempo inteiro, levando para casa salários colocados nos níveis mais altos de rendimento em Portugal. Os médicos são a categoria profissional com melhores indicadores contratuais e de rendimento mensal, embora seja a categoria que é claramente identificada como tendo mais horas semanais de trabalho. A categoria dos outros profissionais da saúde é a mais afetada pela precariedade nos primeiros anos de prática, o que indicia a necessidade de uma pesquisa aprofundada sobre quem são esses outros profissionais, e se existem outras diferenças relevantes mesmo dentro da categoria. Encontrou-se, ainda, uma dualidade dentro da dualidade, que é a existência de trabalho atípico em alguns dos perfis dos bons empregos, criando separação entre os profissionais

Phase Composition and Transport Properties of oxide ion conductors based on Sr1-xKxGeO3-x/2

Oxide ion conductors have been increasingly studied because of their potential applications in different electrochemical devices, such as, oxygen sensors, membranes for oxygen separation and components of fuel cells. Solid Oxide Fuel Cells (SOFCs) are electrochemical devices that operate at high temperatures, 600-1000 ºC, with higher efficiency for electrical generation than conventional systems based on fuel combustion. The high operating temperatures of the SOFC is mainly due to the limited ionic conductivity of the electrolyte. Zr0.84Y0.16O1.92 (YSZ) is the electrolyte most widely used in commercial systems due to its high stability and oxide ion conductivity at elevated temperatures (900-1000 ºC). However, there is a great interest in the development of devices with lower operation temperatures (600-800 ºC) to overcome collateral problems like difficulties in cell sealing or shorter lifetime of the components caused by the high operation temperature of YSZ. The high oxide ion conductivities recently reported in Na- and K-doped strontium silicates and germanates, make them potentially suitable for SOFC electrolytes. In this work, the structure, microstructure and electrical properties of Sr1-xKxGeO3-x/2 (x = 0.0, 0.1, 0.15 and 0.2) compounds have been re-investigated. The materials have been prepared by conventional ceramic and freeze-drying precursor methods. Different phases are stabilized depending on the synthetic method and the sintering temperature. Samples prepared by freeze-drying at 700 ºC exhibit a triclinic structure, which transforms to a mixture of monoclinic and trigonal related phases on heating at 1000 ºC. The presence of some broad diffractions peaks, which are not fitted in the Rietveld analysis, indicates the existence of an amorphous or low-crystalline phase (ACn) that have been quantified by an external standard procedure (G-factor approach). The homogeneity and chemical composition of the samples were checked by scanning electron microscopy combined with energy dispersive spectroscopy (EDX). The total conductivity of these materials was studied by impedance spectroscopy.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Fan culture e mods: a dimensão do multiverso cultural dos games na internet

Comunidades de mods, produções coletivas wikia, canaisde vídeos de análise, gameplay e notícias no YouTube eTwitch. Estas são apenas algumas das facetas com as quaisos jogadores se podem envolver hoje para além da simplesparticipação nos jogos digitais. Ao mergulhar nestasrealidades alternativas e exercer o princípio do fan, estesmesmos jogadores auxiliam na manutenção edesenvolvimento de um universo cultural alternativo,partilhado e que muitas vezes se apropria do universo deprodução cultural tradicional. Este ensaio tem comoobjetivo discutir o universo alternativo da fan culture nosgames. A partir das comunidades Steam Workshop e NexusMods, e com a apresentação de três mods da série de RPGThe Elder Scrolls (Helgen Reborn, Agent of RighteousMight, Falskaar), pretendemos analisar os pontos deintersecção em que as realidades da produção tradicional ealternativa se misturam, criando assim o que pretendemoschamar de um multiverso cultural

Quais as soluções da equação (x^2-5x+5)^(x^2-9x+20)

Este trabajo presenta los procesos de resolución de una ecuación por estudiantes del último año de educación secundaria, de un curso profesional, en Portugal. Con base en el constructivismo social, como marco teórico para el aprendizaje, y el uso de los mediadores como sea el grupo de alumnos y la tecnología, en particular el GeoGebra 3.0, se eligieron las tareas propuestas a los estudiantes y se gestionó el trabajo en el aula. Los datos fueron analizados e interpretados a través de un paradigma cualitativo interpretativo. La tarea se sitúa en el contexto de la utilización de la metodología de Aprendizaje Basado en Problemas. El profesor-investigador estaba interesado en aprender cómo los estudiantes respondieron a estas tareas, así como entender las estrategias desarrolladas y el tipo de tecnología por ellos utilizada para resolver el problema. A partir de estas premisas se analizan e interpretan los resultados obtenidos

Introducción al GeoGebra 3D

Con GeoGebra 5.0 el desarrollo de aplicaciones para la geometría en el espacio son, cada vez, más accesibles y rápidas de elaborar en el aula. Así, este taller tiene como objetivo introducir los procedimientos básicos para utilizar las capacidades 3D de la versión de GeoGebra 5.0 Beta. Se mostrará la construcción de algunos ejemplos de aplicaciones que permiten el estudio de: geometría analítica en el espacio; de situaciones relacionadas con modulación 3D y el análisis de gráficas de funciones; las secciones de un plano en un doble cono; y otras situaciones útiles para el desarrollo de habilidades de visualización para la enseñanza y el aprendizaje de las matemáticas. El taller se desarrollará a partir de secuencias de comandos o herramientas, ayudando a los participantes para realizar las tareas, cada una de ellas ejemplificadas durante la sesión