Magnetism in tetragonal manganese-rich Heusler compounds

A comprehensive study of the total energy of manganese-rich Heusler compounds using density functional theory is presented. Starting from a large set of cubic parent systems, the response to tetragonal distortions is studied in detail. We single out the systems that remain cubic from those that most likely become tetragonal. The driving force of the tetragonal distortion and its effect on the magnetic properties, especially where they deviate from the Slater--Pauling rule, as well as the trends in the Curie temperatures, are highlighted. By means of partial densities of states, the electronic structural changes reveal the microscopic origin of the observed trends. We focus our attention on the magnetocrystalline anisotropy and find astonishingly high values for tetragonal Heusler compounds containing heavy transition metals accompanied by low magnetic moments, which indicates that these materials are promising candidates for spin-transfer torque magnetization-switching applications

Topological insulators and thermoelectric materials

Topological insulators (TIs) are a new quantum state of matter which have gapless surface states inside the bulk energy gap. Starting with the discovery of two dimensional TIs, the HgTe-based quantum wells, many new topological materials have been theoretically predicted and experimentally observed. Currently known TI materials can possibly be classified into two families, the HgTe family and the Bi2Se family. The signatures found in the electronic structure of a TI also cause these materials to be excellent thermoelectric materials. On the other hand, excellent thermoelectric materials can be also topologically trivial. Here we present a short introduction to topological insulators and thermoelectrics, and give examples of compound classes were both good thermoelectric properties and topological insulators can be found.Comment: Phys. Status Solidi RRL, accepte

Lone Pair Effect, Structural Distortions and Potential for Superconductivity in Tl Perovskites

Drawing the analogy to BaBiO3, we investigate via ab-initio electronic structure calculations potential new superconductors of the type ATlX3 with A = Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on chemical reasoning, supported by the calculations, we show that Tl-based perovskites have structural and charge instabilities driven by the lone pair effect, similar to the case of BaBiO3, effectively becoming A2Tl1+Tl3+X6. We find that upon hole doping of RbTlCl3, structures without Tl1+, Tl3+ charge disproportionation become more stable, although the ideal cubic perovskite, often viewed as the best host for superconductivity, should not be the most stable phase in the system. The known superconductor (Sr,K)BiO3 and hole doped RbTlCl3, predicted to be most stable in the same tetragonal structure, display highly analogous calculated electronic band structures.Comment: 5 pages, 5 figure

Heusler 4.0: Tunable Materials

Heusler compounds are a large family of binary, ternary and quaternary compounds that exhibit a wide range of properties of both fundamental and potential technological interest. The extensive tunability of the Heusler compounds through chemical substitutions and structural motifs makes the family especially interesting. In this article we highlight recent major developments in the field of Heusler compounds and put these in the historical context. The evolution of the Heusler compounds can be described by four major periods of research. In the latest period, Heusler 4.0 has led to the observation of a variety of properties derived from topology that includes: topological metals with Weyl and Dirac points; a variety of non-collinear spin textures including the very recent observation of skyrmions at room temperature; and giant anomalous Hall effects in antiferromagnetic Heuslers with triangular magnetic structures. Here we give a comprehensive overview of these major achievements and set research into Heusler materials within the context of recent emerging trends in condensed matter physics

Emerging chiral edge states from the confinement of a magnetic Weyl semimetal in Co3_3Sn2_2S2_2

The quantum anomalous Hall effect (QAHE) and magnetic Weyl semimetals (WSMs) are topological states induced by intrinsic magnetic moments and spin-orbit coupling. Their similarity suggests the possibility of achieving the QAHE by dimensional confinement of a magnetic WSM along one direction. In this study, we investigate the emergence of the QAHE in the two-dimensional (2D) limit of magnetic WSMs due to finite size effects in thin films and step-edges. We demonstrate the feasibility of this approach with effective models and real materials. To this end, we have chosen the layered magnetic WSM Co$_3$Sn$_2$S$_2$, which features a large anomalous Hall conductivity and anomalous Hall angle in its 3D bulk, as our material candidate. In the 2D limit of Co$_3$Sn$_2$S$_2$ two QAHE states exist depending on the stoichiometry of the 2D layer. One is a semimetal with a Chern number of 6, and the other is an insulator with a Chern number of 3. The latter has a band gap of 0.05 eV, which is much larger than that in magnetically doped topological insulators. Our findings naturally explain the existence of chiral states in step edges of bulk Co$_3$Sn$_2$S$_2$ which habe been reported in a recent experiment at $T = 4K$ and present a realistic avenue to realize QAH states in thin films of magnetic WSMs.Comment: Revised 3rd version of the manuscrip

ValiTex -- a uniform validation framework for computational text-based measures of social science constructs

Guidance on how to validate computational text-based measures of social science constructs is fragmented. Whereas scholars are generally acknowledging the importance of validating their text-based measures, they often lack common terminology and a unified framework to do so. This paper introduces a new validation framework called ValiTex, designed to assist scholars to measure social science constructs based on textual data. The framework draws on a long-established tradition within psychometrics while extending the framework for the purpose of computational text analysis. ValiTex consists of two components, a conceptual model, and a dynamic checklist. Whereas the conceptual model provides a general structure along distinct phases on how to approach validation, the dynamic checklist defines specific validation steps and provides guidance on which steps might be considered recommendable (i.e., providing relevant and necessary validation evidence) or optional (i.e., useful for providing additional supporting validation evidence. The utility of the framework is demonstrated by applying it to a use case of detecting sexism from social media data

Enhancing K2 Students' Skills With Loose Parts at Kindergarten XYZ

This quasi-experimental study aims to measure the effectiveness of using loose parts as a media to enhance problem-solving, critical thinking, and creativity skills among K2 students at Kindergarten XYZ Jakarta. The research involved a control group of 26 students and an experimental group of 25 students, using pretest and posttest assessments to measure progress. The experimental group integrated loose parts into classroom activities, while the control group followed the standard curriculum. Next, the pretest and posttest data are analyzed for their n-Gain values to obtain the difference between the two groups. N-Gain is used to measure the effectiveness of an intervention or learning approach by comparing score improvements between pretest and posttest, providing an overview of the extent of improvement in the use of loose parts in the experimental group. The higher the n-Gain value, the greater the improvement achieved by that group. The results showed an increased average score in problem-solving and creativity for the experimental group compared to the control group in both pretest and posttest. However, critical thinking scores showed no significant difference between the two groups. This suggests that the use of loose parts positively impacts problem-solving and creativity in kindergarten students, but improving critical thinking may require further investigation or alternative strategies

Mixing it up: A general framework for Markovian statistics

Up to now, the nonparametric analysis of multidimensional continuous-time Markov processes has focussed strongly on specific model choices, mostly related to symmetry of the semigroup. While this approach allows to study the performance of estimators for the characteristics of the process in the minimax sense, it restricts the applicability of results to a rather constrained set of stochastic processes and in particular hardly allows incorporating jump structures. As a consequence, for many models of applied and theoretical interest, no statement can be made about the robustness of typical statistical procedures beyond the beautiful, but limited framework available in the literature. To close this gap, we identify $\beta$-mixing of the process and heat kernel bounds on the transition density as a suitable combination to obtain $\sup$-norm and $L^2$ kernel invariant density estimation rates matching the case of reversible multidimenisonal diffusion processes and outperforming density estimation based on discrete i.i.d. or weakly dependent data. Moreover, we demonstrate how up to $\log$-terms, optimal $\sup$-norm adaptive invariant density estimation can be achieved within our general framework based on tight uniform moment bounds and deviation inequalities for empirical processes associated to additive functionals of Markov processes. The underlying assumptions are verifiable with classical tools from stability theory of continuous time Markov processes and PDE techniques, which opens the door to evaluate statistical performance for a vast amount of Markov models. We highlight this point by showing how multidimensional jump SDEs with L\'evy driven jump part under different coefficient assumptions can be seamlessly integrated into our framework, thus establishing novel adaptive $\sup$-norm estimation rates for this class of processes